System for free floating of a suction pile

Abstract

System (101) for free floating of a suction pile (100), comprising: - a suction pile (100) comprising an inner cavity (105), adapted to float with the open end (108) submerged in water; - an arrangement (110) adapted to provide a constant air supply (304) towards the cavity (105); - a venting element (104) adapted to establish a fluid connection from a height level Hv (103) in the inner cavity (105) towards the outside, such that during floating: - upon an increasing air volume (305), the venting element (104) forms a fluid connection between the air volume (305) and the outside, resulting in an air venting flow (308) from the air volume (305), and - upon a decreasing air volume (300), said air venting flow is blocked, thereby allowing the air volume (300) to be controlled to a target value, while maintaining a constant air supply (304) towards the cavity (105).

Description

Field of the Invention

[0001] The present invention generally relates to a solution for free-floating transport of a suction pile, the suction pile e.g. serving as a suction pile anchor or making part of a suction pile foundation. In particular, a solution is presented that ensures hydrostatic stability of the suction piles during transport, e.g. by towing, and still allowing for a swift offshore installation.Background of the Invention

[0002] A suction pile is an open-bottomed steel cylinder closed with a cap at the top, wherein the pile may be embedded into an underwater sea bottom. The suction pile may serve as a suction pile anchor, or may make part of a suction pile foundation e.g. for an oil platform or for an offshore wind turbine. A suction pile foundation may for example be provided as a Suction Pile Jacket, comprising a lattice structure mounted to three suction piles, or as a Tri Suction Pile Caisson (TSPC). The latter is composed of a central column similar to a monopile, and a base comprising three suction piles.

[0003] A suction pile is installed by lowering the pile towards the seafloor by means of a crane, after which it first penetrates into the underwater bottom under its own weight. The remainder of embedment is achieved through suction: water is pumped out of the pile, thereby generating suction that forces the pile deeper into the subsea floor. After embedment, a remaining gap between the top cap of the pile and soil inside the suction pile may be filled, e.g. with grout. Installation of the foundation can thus be performed by a noiseless suction-based process, instead of requiring a pile driving process as for installation of traditional monopile and jacket foundations. Moreover, at the end of life, suction piles are removable by reversing the installation process, i.e. applying an overpressure inside the suction pile, thereby allowing for recycling and even circularity.

[0004] The suction pile foundation like a TSPC may be transported to the offshore installation location by a barge, and installed using a Heavy Lift Vessel. Alternatively, advantage can be taken of the free-floating capacity of the TSPC base. Indeed, due to the buoyancy of the TSPC base, transport may be done by towing the base to the offshore site using tugs, thereby eliminating the need for a barge. Transport may also be done while the suction pile foundation is suspended by a crane, and partly submerged in the water, e.g. for relocation of the foundation after de-installation at a first location. Moreover, buoyancy of the base allows for a ballast-controlled lowering to the seabed, thereby allowing for a reduced offshore crane capacity.

[0005] However, free-floating transport of the foundation is not straightforward in practice, as hydrostatic stability of the floating base must be maintained during transport. Since the suction piles are floating on the air entrapped in the respective piles, the air volume in the respective piles needs to be controlled well. The latter is challenging, due to the fact that during floating, some air may be dissolved in the water or may escape underneath the pile tip. For this reason, solutions have been proposed in the prior art, wherein the suction piles are closed off at the bottom with a lid during towing. However, besides the purchase cost of such lids, the latter also complicates the installation process, as both releasing the lids from the piles, and recovering them after falling to the seafloor are cumbersome.

[0006] It is an objective of the present invention to disclose a solution for free-floating transport of suction piles, that resolves one or more of the above-described shortcomings of the prior art solutions. More particularly, it is an objective to present a solution that ensures hydrostatic stability during transport, and still allowing for a swift offshore installation.Summary of the Invention

[0007] According to a first aspect of the present invention, the above identified objectives are realized by a system for free floating of a suction pile, defined by claim 1, the system comprising: a suction pile comprising a tubular side wall provided with a top cover, the suction pile comprising an inner cavity extending in a height direction between the open bottom end of the side wall and the top cover, wherein a height level in the cavity is measured from the bottom end, the suction pile being adapted to float with the open end submerged in water, wherein water inside the cavity reaches a water level and buoyancy is created by an air volume enclosed in a top portion of the cavity; an arrangement adapted to provide a constant air supply towards the inner cavity of the suction pile while floating; a venting element arranged at the suction pile, adapted to establish a fluid connection starting from a height level Hv in the inner cavity towards the outside of the suction pile, such that during floating: upon an increasing enclosed air volume, wherein the actual air volume extends from the top cover to below the height level Hv, the venting element forms a fluid connection between the enclosed air volume and the outside, resulting in an air venting flow from the enclosed air volume, and upon a decreasing enclosed air volume, wherein the water level inside the cavity increases to above the height level Hv, said air venting flow is blocked, thereby allowing the enclosed air volume to be controlled to a target value corresponding to a target draught of the suction pile, while maintaining a constant air supply towards the inner cavity.

[0008] Thus, the invention concerns a system for free floating of a suction pile. A suction pile is a pile which allows to be embedded into an underwater bottom, wherein at least a part of the embedment is achieved through suction. In a typical installation process of such suction pile, water is pumped out or the top part of the pile, thereby generating suction that forces the pile deeper into the subsea floor. In various embodiments, the suction pile may have different functions. For example, the suction pile may serve as a suction pile anchor, or may make part of a suction pile foundation, e.g. a foundation for an oil platform or for an offshore wind turbine. A suction pile foundation may for example be provided as a Suction Pile Jacket, comprising a lattice structure mounted to three, four or a plurality of suction piles, or as a Tri Suction Pile Caisson (TSPC). The latter is composed of a central column similar to a monopile, and a base comprising three suction piles.

[0009] The suction pile comprises a tubular side wall extending according to a length or height direction. The tubular side wall is a sleeve-like elongated element, delimiting an internal space, and extending between a bottom end and a top end. Typically, the tubular side wall is cylindrical, i.e. having a circular cross section, but a cross section of any other shape is possible as well. The bottom end of the tubular side wall is open, while the top end is closed off by means of a cover, cap or lid. Typically, the tubular side wall and the top cap are made of steel, but other materials are possible too. The horizontal direction, or a horizontal plane, are defined as being perpendicular, or substantially perpendicular, to the height direction. The horizontal plane located at the bottom end of the tubular side wall is defined as the bottom face of the suction pile. Whenever a direction of an element is mentioned, being substantially in a particular direction refers to a deviation of at most 20%, more preferably at most 10%, still more preferably at most 5%, and most preferably at most 0,5% with respect to the indicated direction.

[0010] The tubular side wall and top cover together define a hollow body, wherein the space inside the hollow body is referred to as the inner cavity of the suction pile. The inner cavity thus is delimited by the inner sides of the tubular wall and the top cap on the one hand, and by the bottom face of the suction pile on the other hand, the latter not representing a physical border. The outside of the suction pile is defined as the environment located at the outer sides of tubular wall and the top cap, and located at the side of the bottom face not facing the internal cavity. The inner cavity thus extends in height direction between the open bottom end of the side wall and the top cover, and any height level in the cavity is measured from the bottom end.

[0011] The suction pile is adapted to float with its open end submerged in water, i.e. the bottom end of the suction pile is not closed off. Thus, during floating, a bottom portion of the inner cavity is filled with water, up to a certain water level, and the remaining top portion is filled with air. Due to the air volume enclosed in the top portion of the cavity, buoyancy is created. In particular, the amount of air volume enclosed in the suction pile determines - for a given suction pile - the resulting draught of the pile, i.e. how deep the pile is submerged under the water line. Free floating implies that the pile is able to float on its own, thus not being carried by another floating body such as a barge, vessel or platform.

[0012] The system further comprises an arrangement adapted to provide a constant air supply towards the inner cavity of the suction pile while floating. For example, the system may comprise an air compressor, and one or more hoses or tubes connecting the air compressor to the suction pile. The arrangement allows to supply a continuous flow of air towards the inner cavity of the suction pile, at a constant flow rate. For this purpose, typically an aperture is available in the top cover or the tubular side wall, for connecting a hose or tube.

[0013] The system further comprises a venting element arranged at the suction pile, adapted to establish a fluid connection starting from a height level Hv in the inner cavity towards the outside of the suction pile. This implies that, when considering the system when not being in use, the venting element is provided as an element that connects the inner cavity of the pile to the outside environment of the pile. This connection is such that a fluid connection may be established, i.e. the connection allows, when considering the system not being in use, for a fluid flow via the venting element. In other words, the venting element connects a first spot, located inside the cavity, to a second spot, located at the outside of the suction pile.

[0014] Various embodiments are possible for the venting element. For example, the venting element may be provided as a venting pipe extending though the top cover. In this case, the first spot of the fluid connection corresponds to the first end of the pipe, located inside the cavity, and the second spot corresponds to the second end of the pipe, located above the top cover. In another embodiment, the venting element may be provided as a snorkel-shaped pipe, wherein the first end of the pipe is located inside the cavity, the second end is located next to the tubular wall, and both ends are connected via the open bottom side of the suction pile. In yet another embodiment, the venting element is provided as a venting hole through the tubular wall, such that a fluid connection is provided between a first spot inside the cavity and a second spot at the outside via the hole in the wall.

[0015] In any of the embodiments, the fluid connection established by the venting element starts at a height level Hv inside the cavity. This means that the first spot of the connection is located inside the cavity, at a height level Hv, measured from the bottom side of the suction pile. For example, if the venting element is provided as a venting pipe, of which the first end has a horizontal end face, this horizontal end face is at a distance Hv measured from the bottom side. In another embodiment, the venting element may be provided as a pipe of which the first end has a chamfered or angled end face, i.e. being inclined with respect to the horizontal plane. In this case, Hv is defined as the height level of the highest edge of the first pipe end, i.e. the edge of the first pipe end being at the largest distance from the bottom side of the suction pile. In yet another embodiment, wherein the venting element is provided as a venting hole through the tubular side wall, Hv is defined as the upper edge of the hole, i.e. the edge of the hole being at the largest distance from the bottom side of the suction pile.

[0016] The system is such that during use, i.e. while the pile floats on the enclosed air volume, it ensures an automatic, continuous adjustment of the enclosed air volume. Indeed, the system is such that, upon an increasing enclosed air volume, wherein the actual air volume extends from the top cover to below the height level Hv, the venting element forms a fluid connection between the enclosed air volume and the outside, resulting in an air venting flow from the enclosed air volume. An increasing enclosed air volume may e.g. occur when the air losses, due to air dissolving in the water or escaping from the cavity, are temporarily low compared to the continuous constant air supply towards the cavity. Without any further control, this would result in an increased buoyancy and a draught of the suction pile deviating from the target draught. However, due to the presence of the venting element, the increasing air volume will instantly result in air escaping from the cavity via the venting element. Indeed, as soon as the water level inside the suction pile reaches below Hv, the venting element connects the enclosed air volume to the outside. Thus, in this condition, the first spot of the fluid connection is found in air, and air may freely flow towards the second spot at the outside, thus resulting in an air venting flow from the enclosed air volume via the venting element. For example, if the venting element is provided as a venting pipe with a horizontal first end face, a venting flow arises as soon as the water level in the pile is underneath that first end face. In case of a chamfered or inclined first end face, the venting flow arises as soon as the water level is lower than the highest edge of the first pipe end. In an embodiment wherein the venting element is provided as a hole thought the tubular side wall, a venting flow towards the water environment outside the pile arises as soon as the water level in the pile is lower than the upper edge of the hole.

[0017] In any of the embodiments, the air venting flow via the venting element instantly results in a decreasing enclosed air volume, such that the enclosed air volume is automatically steered back towards the target value, and only small variations in the enclosed air volume will occur. In particular, the system is such that if the water level inside the pile rises above Hv, the air venting flow is blocked again. Indeed, in such condition, the first spot of the fluid connection established by the venting element is not located in air anymore, and the water present at the first spot prevents air to further escape from the enclosed air volume, thereby preventing any further air venting flow.

[0018] Thus, the system is such that in a condition wherein the water level inside the pile is sufficiently high, i.e. above Hv, the venting flow is blocked, thus being zero or substantially zero. In certain embodiments, this results in a type of on-off control, wherein the venting flow is either at a maximum value or zero, and quick transitions occur between both states. This may for example be the case when the venting element is a pipe with a horizontal first end face. In other embodiments, however, a control may be obtained wherein the venting flow may gradually change from zero towards a maximum value, such that also some intermediate state with a modest air venting flow may occur. This may for example be the case when the venting element is a pipe with a chamfered or angled first end face.

[0019] In any of the embodiments, the system allows the enclosed air volume to be controlled to a target value corresponding to a target draught of the suction pile, while maintaining a constant air supply towards the inner cavity. Indeed, as soon as a the enclosed air volume starts increasing, such that the draught of the suction pile would change, some air is vented towards the environment, and the enclosed air volume returns to the target value. The enclosed air volume is thus continuously adjusted, such that the buoyancy is continuously controlled to target value, and the pile maintains at the target draught.

[0020] In this, controlling the enclosed air volume happens in an automatic, passive way without requiring any measurements or active control. The invention therefore brings the advantage that only minor adjustments to the suction pile design are needed, thus contributing to a reduced cost. Moreover, hydrostatic stability of the suction pile is obtained while not relying on adaptations of the air supply towards the cavity. Indeed, controlling the enclosed air volume by continuously adapting the air supply would be infeasible in practice, due to small occurring pressure values. Instead, the venting element allows to continuously adjust the air volume in a passive way, such that it is kept in a small acceptable range, and hydrostatic stability is maintained. Consequently, upon free floating of e.g. a foundation structure with a three-pile base, each of the suction piles continuously maintains substantially the same draught, thereby enabling minimum inclination of the foundation structure during transport, e.g. by towing.

[0021] In this way, the invention makes free-floating transport of suction pile foundations and anchors technically feasible in practice, without the additional cost and complexity related to using lids for closing off the open pile end during transport. Transport may e.g. happen by towing the suction pile foundation to an offshore installation location by means of a tug, or transport it while floating and being suspended from a crane, e.g. for relocation of a previously installed foundation structure. It therefore provides an advantageous alternative over transport by barge, as the need for such barges is eliminated. Moreover, advantage may be taken of the buoyancy of the pile(s) during lowering towards the seabed, thereby allowing for a ballast-controlled lowering and a reduced offshore crane capacity. Also in applications such as a self-installing platform, i.e. a four-legged platform founded on suction piles, the free-floating capacity of the platform legs, and optimal control thereof, may be leveraged during the platform installation process.

[0022] Optionally, the venting element is provided as a venting pipe, the venting pipe extending between a first end positioned inside the inner cavity at the height level Hv, and a second end positioned outside the suction pile. The pipe may be provided as a pipe, tube, duct, hose or the like, and may have any shape e.g. straight, curved, S-shaped, etc. Moreover, the connection between the first end and the second end may run through the top cover of the suction pile, or through the tubular side wall, or may run via the open bottom end of the suction pile. Typically, the first end is directed downwards and the second end is directed upwards, such that the venting air flows vertically in upward direction when flowing through the first end and when flowing through the second end. Between the first and second end, however, the venting pipe may comprise curves, angles, etc, such that between the first and second end the venting air flows in a direction deviating from the vertical direction.

[0023] Optionally, the end face of the venting pipe at the first end is in a horizontal plane, wherein the height level of the horizontal end face corresponds to the height level Hv of the venting element. This implies that the border of the first end of the venting pipe, which is located inside the cavity, lies in a horizontal plane.

[0024] Optionally, the end face of the venting pipe at the first end is chamfered or angled with respect to a horizontal plane, the edge of the first end being at highest height level defining the highest edge of the first end, wherein the height level of the highest edge corresponds to the height level Hv of the venting element. The edge may refer to a single point in the border of the first end, or may be a portion of the border, e.g. a straight edge. When the pipe has a chamfered or angled first end, edges comprised in the border are at different height positions, and the highest edge defines the height level Hv of the venting pipe. Having a chamfered or angled first end may be advantageous with respect to controlling the air venting flow, as the latter may progressively increase with the extent the actual air volume deviates from the target value. This differs from a venting pipe with a horizontal end face, wherein a pure on-off control of the air venting flow is obtained.

[0025] Optionally, the venting pipe extends upwards starting from the height level Hv, through a hole in the top cover. Thus, the venting pipe runs through the top cover of the suction pile. The venting pipe may be of any shape, e.g. straight, curved, ... For example, the hole in the top cover is positioned centrally, e.g. at the centre point of a disc-shaped cover plate.

[0026] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, and a second portion positioned above the top cover. Thus, the venting pipe partially protrudes above the top cover of the suction pile.

[0027] Optionally, the venting pipe is a straight pipe.

[0028] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, the first portion starting from the first end at height level Hv and extending upwards, a second portion positioned outside the suction pile, next to the side wall, the second portion ending at the second end of the venting pipe, and an intermediate portion connecting the first and the second portion, via the open bottom end of the suction pile. Thus, the first end of the venting pipe and the second end of the venting pipe are connected via the open bottom end of the suction pile. For example, the venting pipe may be S-shaped or snorkel-shaped. Typically, the first end is directed downwards and the second end is directed upwards, such that the venting air flows in upward direction when flowing through the first end and when flowing through the second end. Between the first and second end, however, when flowing through the intermediate portion, the air venting flow changes direction.

[0029] Optionally, the venting pipe is S-shaped or snorkel-shaped. For example, the intermediate portion of the venting pipe comprises two curved parts or turns, and a straight part therebetween.

[0030] Optionally, the venting pipe, or a portion thereof, is adjustable, thereby allowing to adjust the height level Hv of the venting element. This implies that the first end of the venting pipe, being located inside the cavity has an adjustable height position. For example, the venting pipe may be slidable as a whole, or a bottom portion of the venting pipe may slide over an upper portion. In this way, the height level Hv of the venting pipe may be adapted. This may be applied to any shape of venting pipe, e.g. a straight pipe or a snorkel-shaped pipe.

[0031] By changing the height position of the venting pipe, the enclosed air volume will automatically be controlled to another value, thereby changing the buoyancy of the suction pile. This may be useful in different situations. Firstly, during free-floating transport, adjusting the height position of the venting pipe allows to change the draught of the suction pile to another desired value. Thus, depending on the desired draught in a certain situation, another target draught may be set, by simply retracting or inserting the venting pipe. For example, when towing the suction pile to the offshore installation location, the draught of the suction pile may be adapted after leaving the harbour. Secondly, after arriving at the offshore installation location, the suction pile is connected to the crane, for lowering the pile towards the sea bottom. While lowering the suction pile through the water, advantage can be taken of the buoyancy of the pile; when an air volume remains entrapped in the pile during lowering, the crane load will be lower than when the complete weight of the pile needs to be carried by the crane, thereby allowing for a smaller crane capacity. However, in view of a controlled lowering by the crane, the buoyancy of the suction pile needs to be lower than during free-floating transport. Therefore, by adjusting the height position of the venting pipe to a larger Hv, i.e. the pipe is retracted somewhat from the cavity, the enclosed air volume is reduced, thereby decreasing the buoyancy of the suction pile. The same can be applied when decommissioning the suction pile at a later time, wherein a retracted venting pipe with larger Hv allows for a controlled lifting by the crane when raising the pile through the water.

[0032] Finally, the height-adjustable venting pipe allows to retract the venting pipe somewhat from the cavity before installation of the suction pile, i.e. before embedment into the seafloor. In this way, it may be prevented that the venting pipe would hamper the suction process, or would get damaged. Retracting the venting pipe before embedment of the pile also avoids that the pipe would be filled with sand over a large length; the latter may be disadvantageous in view of reusing the venting pipe in a later stage as a decommissioning pipe, wherein the previously installed venting pipe is reused for inserting a pressurized fluid through the grout plug upon removing the suction pile from the seabed.

[0033] Optionally, the venting pipe is slidable in vertical direction, thereby allowing to adjust the height level Hv of the venting element. Thus, the venting pipe as a whole may be moved in vertical direction, thereby changing the height position of both the first and second pile end.

[0034] Optionally, the venting pipe is slidable in vertical direction, wherein the venting pipe is connected to the top cover via a gas-tight sealing, thereby allowing for sliding of the venting pipe, while sealing the inner cavity of the suction pile. For example, grease may be used to allow for both sliding and sealing.

[0035] Optionally, the venting pipe is removable, thereby allowing to detach the venting pipe from the suction pile. This implies that the venting pipe is connected to the suction pile in a releasable way, i.e. in such a way that it can be removed or detached without causing any damage. This brings the advantage that the venting pipe can be removed from the suction pile before embedding the suction pile into the seafloor, thereby avoiding that the venting pipe would hamper the suction process.

[0036] Optionally, the venting pipe is provided with a valve adapted to adjust and / or block a fluid flow through the pipe. For example, a ball valve or gate valve may be arranged at the venting pipe, which e.g. may be operated remotely via an umbilical signal, or by means of a Remotely Operated Vehicle (ROV). In another example, a one-way valve may be used, which is automatically closed when lowering the pile into the water. The valve allows to adjust or block the air venting flow through the venting pipe, which may have different functions. A first function of the valve may be to adapt the outlet opening of the venting pipe during free-floating transport, such that the flow rate of the air venting flow may be controlled. This allows for a better control of the venting flow during free floating, in particular in view of avoiding overshoots. A second function of the valve may be to close off the opening of the venting pipe before starting installation of the suction pile; closing off the opening allows to create an underpressure or vacuum inside the cavity during the suction phase of the installation process. A third function of the valve may be to close off the opening of the venting pipe during decommissioning of the suction pile, i.e. when removing the suction pile from the seabed at a later time.

[0037] Optionally, the valve is a ball valve or gate valve or butterfly valve or one-way valve.

[0038] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, and a second portion positioned above the top cover, the valve being placed in the second portion.

[0039] Optionally, the venting element is provided as a venting aperture through the side wall, wherein the height position of the upper edge of the aperture corresponds to the height level Hv of the venting element. In such embodiment, the venting element is not provided by a pipe, but by a hole through the side wall of the suction pile. As soon as the water level inside the pile reaches below the upper edge of the hole, some air may escape from the enclosed air volume towards the outside.

[0040] Optionally, the venting aperture is closable by means of a hatch or valve, for example a one-way valve. For example, upon installing the suction pile in the seabed, the venting aperture may be closed off by the hatch or valve, in view of creating an underpressure or vacuum inside the cavity during the suction phase. Different types of valves or hatches may be used. E.g. a one-way valve may be used, thereby allowing for an automatic closing of the valve when lowering the pile into the water.

[0041] Optionally, the system comprises one or more additional venting elements arranged at the suction pile, each of the additional venting elements adapted to establish a fluid connection starting from a respective height level Hva in the inner cavity towards the outside of the suction pile, wherein each of the additional venting elements defines a different height level Hva, Hva being, for any of the additional venting elements, higher than the height level Hv. This implies that at least two venting elements are arranged at the suction pile. The venting element defining the height level Hv is referred to as the first venting element. The additional venting elements, additional to the first venting element, are referred to as the second, third, etc. venting element. Each of the additional venting elements defines a respective height level Hva, wherein Hva is different for each of the additional venting elements, and Hva is different from Hv defined by the first venting element. In particular Hva is higher than Hv, for any of the additional venting elements. In an embodiment, venting pipes of different lengths are provided, any of them extending through the top cover, wherein the first end of each of the venting pipes is at a different height level. In such a case, the longest venting pipe defines height level Hv, while the additional venting pipe(s) are shorter and define respective height level(s) Hva. In another embodiment, the venting elements are provided as snorkel-shaped pipes, of which the first ends are at a different height level. In yet another embodiment, the venting elements are provided as apertures in the tubular side wall, the apertures being at different heights.

[0042] Each or some of the venting elements may e.g. be provided with a valve, thereby allowing to select an active venting element of which the valve is opened, and air venting towards the outside occurs via the selected venting element. Activating a specific venting element may happen by opening the valve of that venting element, and closing the valve(s) of all the other venting element(s). In another embodiment, activating a specific venting element with height level Hv or Hva may happen by opening the valve of that venting element, while the valve(s) of venting elements with a shorter Hva may remain opened, as the latter automatically will not allow for venting anymore.

[0043] By switching to another venting element, the enclosed air volume will automatically be controlled to another value, corresponding to the heigh level defined by the selected venting element, thereby changing the buoyancy of the suction pile. This may be useful in different situations. Firstly, during free-floating transport, selecting a venting pipe with a different height position allows to change the draught of the suction pile to another desired value. Thus, depending on the desired draught in a certain situation, another target draught may be set, by activating another venting pipe. For example, when towing the suction pile to the offshore installation location, the draught of the suction pile may be adapted after leaving the harbour. Secondly, after arriving at the offshore installation location, the suction pile is connected to the crane, for lowering the pile towards the sea bottom. While lowering the suction pile through the water, advantage can be taken of the buoyancy of the pile; when an air volume remains entrapped in the pile during lowering, the crane load will be lower than when the complete weight of the pile needs to be carried by the crane, thereby allowing for a smaller crane capacity. However, in view of a controlled lowering by the crane, the buoyancy of the suction pile needs to be lower than during free-floating transport. Therefore, by selecting another venting element with a higher Hva, the enclosed air volume is reduced, thereby decreasing the buoyancy of the suction pile. The same can be applied when decommissioning the suction pile at a later time, wherein selecting a venting element with higher Hva allows for a controlled lifting by the crane when raising the pile through the water.

[0044] Optionally, the additional venting elements are provided as additional venting pipes, each of the additional venting pipes extending between a first end positioned inside the inner cavity at the heigh level Hva, and a second end positioned outside the suction pile.

[0045] Optionally, each of the additional venting pipes is a straight pipe extending upwards through a respective hole in the top cover, wherein the length of each additional venting pipe is smaller than the length of the venting pipe, and is different from the length of any other additional venting pipe. Thus, each of the available venting pipes has a different length, and has a first end at a different height position. The second end may be at the same height for all the venting pipes, or may be at different heights.

[0046] Optionally, each of the additional venting pipes is provided as a pipe branching off from the venting pipe. Instead of providing multiple venting pipes being as separate components, the additional venting pipes may be provided as branches. For example, the first venting pipe extends through the top cover, having a second end located at the outside and a first end located in the inner cavity. An additional venting pipe is provided as a branch to the first venting pipe, such that the additional venting has a first end inside the cavity, and a second end connected to the first venting pipe. Each or some of the venting elements may e.g. be provided with a valve, thereby allowing to select an active venting element of which the valve is opened, and air venting towards the outside occurs via the selected venting element. Activating a specific venting element may happen by opening the valve of that venting element, and closing the valve(s) of all the other venting element(s). In another embodiment, activating a specific venting element with height level Hv or Hva may happen by opening the valve of that venting element, while the valve(s) of venting elements with a shorter Hva may remain opened, as the latter automatically will not allow for venting anymore.

[0047] Optionally, air is supplied towards the inner cavity of the suction pile, via an opening through the top cover and / or the tubular side wall.

[0048] Optionally, the arrangement comprises an air compressor, and at least one hose or tube for connecting the air compressor to the suction pile. For example, an air compressor may be installed at the towing vessel, or onto the floating foundation structure.

[0049] Optionally, the suction pile serves as a suction pile anchor or makes part of a suction pile foundation.

[0050] Optionally, the suction pile makes part of a suction pile foundation, the suction pile foundation being a Suction Pile Jacket or a Tri Suction Pile Caisson.

[0051] According to a second aspect of the present invention, there is provided a method for free-floating of a suction pile, the method comprising providing a system according to the first aspect of the invention; floating the suction pile with the open end submerged in water, wherein buoyancy is created by an air volume enclosed in a top portion of the inner cavity; while providing a constant air supply towards the inner cavity of the suction pile, controlling the enclosed air volume by the venting element, to a target value corresponding to a target draught of the suction pile.

[0052] Optionally, a method is provided for transporting a suction pile towards an installation location, the method comprising: free-floating the suction pile according to the second aspect of the invention; transporting the suction pile in floating condition to the installation location, while controlling the enclosed air volume by the venting element. Transport of the suction pile in free-floating condition may e.g. happen when the structure comprising the suction pile(s) is transported from a harbour towards an offshore installation location, by towing with a tug. In another example, the structure comprising the suction pile(s) is transported while the structure is suspended from a crane, the crane being installed at a vessel and the suction pile(s) being in floating condition. The latter way of transport may e.g. be used when a previously installed foundation structure needs to be relocated: after pressing the suction pile foundation structure out of the seabed, and hoisting it towards the sea level, the structure remains coupled to the crane while being transported to a new installation location.

[0053] Optionally, the height level Hv of the venting element is adjusted during transport, thereby adjusting the draught of the floating suction pile.

[0054] Optionally, the outflow opening of the venting pipe is adjusted during transport, by means of a valve arranged at the venting pipe.

[0055] Optionally, a method is provided for installing a suction pile in a seabed, the method comprising: free-floating the suction pile on water according to the second aspect of the invention, before lowering the suction pile towards the seabed; after lowering the suction pile towards the seabed, installing the suction pile in the seabed, wherein embedment of the suction pile in the seabed is at least partially achieved through suction created inside the suction pile, and wherein prior to creating suction ∘ the venting element is sealed for blocking a fluid flow through the venting element, or ∘ the venting element is removed from the suction pile, and the top cover and / or side wall is sealed at the position of the removed venting element.

[0056] This implies that in a first phase, the venting element is used for controlling the enclosed air volume during free-floating of the suction pile, e.g. during towing towards the offshore installation location. Next, upon arrival at the offshore installation location, the suction pile is lowered to the seabed, thus being lowered through the water, by means of a crane. After reaching the seabed, the suction pile is embedded into the seabed. Typically, the pile first penetrates into the seabed under its own weight, and the remainder of embedment is achieved through suction: water is pumped out of the top part of the pile, thereby generating suction that forces the pile deeper into the subsea floor. In order to create an underpressure or vacuum inside the cavity during the suction phase, the cavity needs to be sealed. For this purpose, the venting element may be sealed, e.g. by closing the valve arranged at the venting pipe, or by closing off the venting aperture by means of a valve or hatch. In another embodiment, the venting element may be removed from the suction pile before the suction process. In this case, the opening originating from the removed venting element needs to be sealed before starting the suction process.

[0057] Optionally, the method comprises: free-floating the suction pile on water according to the second aspect of the invention, wherein the height level Hv of the venting element is at a first value; lowering the suction pile towards the seabed while being connected to a crane, wherein before lowering, the height level Hv of the venting element is adjusted to a second value, the second value being higher than the first value, thereby obtaining a reduced enclosed air volume and a decreased buoyancy of the suction pile than during free-floating.

[0058] For example, the venting element is provided as a venting pipe, of which the height position is adjustable. Before lowering the suction pile towards the seabed, thus being lowered through the water, the free-floating suction pile is first connected to the crane. Next, the venting pipe is pulled up, i.e. retracted somewhat from the cavity, such that its first end is at a higher height level Hv. Accordingly, the enclosed air volume is reduced, thus obtaining a reduced buoyancy compared to the free-floating condition. Such reduced buoyancy results in the required crane load that allows for a controlled lowering of the pile towards the seabed.

[0059] Optionally, the method comprises: free-floating the suction pile on water according to the second aspect of the invention, wherein during free-floating each of the one or more additional venting elements is closed, lowering the suction pile towards the seabed while being connected to a crane, wherein before lowering, at least one of the additional venting elements is opened, thereby obtaining a reduced enclosed air volume and a decreased buoyancy of the suction pile than during free-floating.

[0060] For example, the venting elements are provided as venting pipes of different lengths, wherein the first venting pipe defines the height level Hv, and an additional venting pipe defines a heigh level Hva being higher than Hv. The first venting pipe thus corresponds to the longest pipe. During free-floating, the first venting pipe is active, i.e. the valve of the first venting pipe is opened, and the additional shorter pipes are closed off. Before lowering the suction pile towards the seabed, thus being lowered through the water, the free-floating suction pile is first connected to the crane. Next, the valve of an additional, shorter venting pipe is opened. The valve of the first venting pipe may be closed or may remain opened. As a shorter venting pipe becomes active, the enclosed air volume is reduced, thus obtaining a reduced buoyancy compared to the free-floating condition. Such reduced buoyancy results in the required crane load that allows for a controlled lowering of the pile towards the seabed.

[0061] Optionally, there is provided a method for decommissioning an installed suction pile, the method comprising: installing the suction pile in the seabed as defined above, wherein the venting element is provided as a venting pipe, the venting pipe extending upwards starting from the height level Hv, through a hole in the top cover, and wherein the venting pipe is not removed from the installed suction pile; at a later stage, removing the previously installed suction pile from the seabed, wherein removing the suction pile from the seabed is at least partially achieved through pressure created inside the suction pile, wherein during pressing the suction pile out of the seabed, the venting pipe is used for supplying a pressurized fluid to the inside of the suction pile, thereby reusing the venting pipe as a decommissioning pipe.

[0062] During installation of the suction pile, the pile is embedded into the seabed through suction. After embedment, typically a gap remains between the top cover of the pile and soil inside the suction pile. This gap may be filled, e.g. with grout, in view of enlarging the bearing strength of the pile. At the end of life, the suction pile may be removed from the seabed, by pressing it out of the seabed. For this purpose, a fluid needs to be pumped into the cavity below the grout plug. If a venting pipe is arranged at the suction pile, that e.g. previously was used for venting during free-floating transport of the suction pile, this venting pipe may at a later stage be reused as a decommissioning pipe: upon pressing the pile out of the seabed, the available venting pipe may be used for pumping fluid into the cavity below the grout plug. It is thus required that the portion of the venting pipe inside the cavity is long enough, such that it extends through the complete depth of the grout plug. By reusing the already available venting pipe as a decommissioning pipe, it is avoided that a hole needs to be drilled through the grout plug during the decommissioning process.

[0063] Optionally, the method comprises: after pressing the suction pile out of the seabed, lifting the suction pile towards the sea level by means of a crane, thereby raising the suction pile though the water, wherein during raising, the height level Hv of the venting element is at a third value; after reaching the sea level, free-floating the suction pile on the water, wherein after reaching the seal level, the height level Hv of the venting element is adjusted to a fourth value, the fourth value being lower than the third value, thereby obtaining an enlarged enclosed air volume and an increased buoyancy of the suction pile during free-floating than during raising.

[0064] For example, the venting element is provided as a venting pipe, of which the height position is adjustable. After pressing the suction pile out of the seabed, a crane lifts the foundation to the water surface. During this phase, the venting pipe is sufficiently retracted from the cavity, such that buoyancy is low enough to allow for a controlled lifting by the crane. Once the suction pile is at the surface, the venting pipe is inserted more in the cavity, thereby obtaining an increased buoyancy. This results in zero crane load, and the pile to free-float.

[0065] Optionally, the method comprises: after pressing the suction pile out of the seabed, lifting the suction pile towards the sea level by means of a crane, thereby raising the suction pile though the water, wherein during raising, venting occurs via an active venting element, selected from the venting element and the one or more additional venting elements, wherein Hv of the active venting element corresponds to a third value; after reaching the sea level, free-floating the suction pile on the water, wherein after reaching the sea level, another venting element is activated, Hv of the newly activated venting element corresponding to a fourth value being lower than the third value, thereby obtaining an enlarged enclosed air volume and an increased buoyancy of the suction pile during free-floating than during raising.

[0066] For example, the venting elements are provided as venting pipes of different lengths, wherein the first venting pipe defines the height level Hv, and an additional venting pipe defines a heigh level Hva being higher than Hv. After pressing the suction pile out of the seabed, a crane lifts the foundation to the water surface. During this phase, a short venting pipe is used, such that buoyancy is low enough to allow for a controlled lifting by the crane. Once the suction pile is at the surface, a longer venting pipe is opened while the shorter ones are closed off. This results in zero crane load, and the pile to free-float.Brief Description of the Drawings

[0067] Fig. 1 and Fig. 2 show a system for free floating of a suction pile, according to four different embodiments of the invention. Fig. 3 illustrates how the air volume enclosed in the suction pile may be controlled, by means of a system according to an embodiment of the invention. Fig. 4 illustrates a system according to an embodiment of the invention, wherein the height position of the venting element is adjustable. Fig. 5 illustrates a system according to an embodiment of the invention, wherein multiple venting pipes with different lengths are provided. Fig. 6 and Fig. 7 respectively show a Suction Pile Anchor and a Tri Suction Pile Caisson (TSPC). Fig. 8 illustrates use of the system according to an embodiment of the invention, during free-floating tow-out of a Tri Suction Pile Caisson. Fig. 9 and Fig. 10 illustrate the lowering of a Tri Suction Pile Caisson towards the seabed. Fig. 11 illustrates an installation process of a suction pile, according to an embodiment of the invention. Detailed Description of Embodiment(s)

[0068] Fig. 1(a) shows a first embodiment of a system 101 according to the invention, wherein the system is represented in a schematic way. The system 101 comprises a suction pile 100, a venting element 104, and an arrangement for air supply 110. The suction pile 100 comprises a tubular side wall 107, provided as a cylindrical hollow sleeve, and a top cover 106. The top cover 106 is arranged at the top side of the tubular side wall 107. The opposite bottom side 108 of the tubular side wall is open, thereby providing access to the inner cavity 105 of the suction pile. The inner cavity 105 extends according to a height direction 109, also referred to as the vertical direction, or the length direction of the suction pile 100. The arrangement 110 is adapted to provide a constant air supply towards the inner cavity 105. For example, the arrangement 100 comprises an air compressor, and a hose connecting the air compressor to a hole in the top cover 106 of the suction pile 100.

[0069] The system 101 comprises a venting element 104, which, in the first embodiment, is provided as a venting pipe 104. The venting pipe 104 is a straight tube extending in vertical direction, through an aperture in the top cover 106. The venting pipe 104 extends between a first end 111, located in the inner cavity 105, and a second end 112, located at the outside of the suction pile 100. The second end 112 is positioned above the top cover 106, such that the venting pipe 104 protrudes with respect to the suction pile. The first end 111 has a horizontal end face, positioned at a height level Hv 103, the latter being measured in height direction 109, starting from the bottom end 108. In this way, the venting pipe 104 is adapted to establish a fluid connection starting from the first 111 end at height level Hv, towards the second end 112 at the outside. The straight venting pipe 104 thus provides a passageway through which a fluid such as air may flow, between the inner cavity 105 and the outside, provided that free passage of such fluid through the pipe is enabled.

[0070] Fig. 1(b) shows a system 102, representing a second embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the first embodiment of Fig. 1. However, the venting element 114 is now provided as a snorkel-shaped venting pipe 114. The venting pipe 114 comprises a first end 121, located in the inner cavity 105, and a second end 122, located at the outside of the suction pile 100. The second end 122 is positioned next to the tubular side wall 107. The snorkel-shaped pipe 114 connects the first end 121 to the second end 122 via the open bottom side 108 of the suction pile 100. For this purpose, the pipe 114 comprises a first straight portion 115 starting at the first end 121, connected to a first curved portion 116. A second straight portion 119, ending at the second end 122, is connected to a second curved portion 118. Both curved portions 118 and 116 are connected via an intermediate straight portion 117. Similar to the first embodiment, the first end 121 has a horizontal end face, positioned at a height level Hv 113, the latter being measured in height direction 109, starting from the bottom end 108. In this way, the snorkel-shaped venting pipe 114 is adapted to establish a fluid connection starting from the first 121 end at height level Hv, towards the second end 122 at the outside, thereby allowing to connect the inner cavity 105 to the outside of the suction pile 100.

[0071] Fig. 2(a) shows a system 201, representing a third embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the first and second embodiment. Just like in the first embodiment of Fig. 1(a), the venting element 204 is provided as a straight pipe 204, extending vertically through the top cover 106, between a first end 205 inside the cavity 105 and a second end 206 at the outside. However, different from the first embodiment, the first end 205 is chamfered, the end face thus being angled with respect to a horizontal plane. The first end 205 comprises a highest edge 207, being at the largest distance from the bottom side 108 of the suction pile, and a lowest edge 208, being at the smallest distance from the bottom side 108. Hv of the chamfered venting pipe 205 is defined as the height position 203 of the highest edge 207. In this way, the chamfered venting pipe 204 is adapted to establish a fluid connection starting from the first end 205 at height level Hv 203, towards the second end 206 at the outside.

[0072] Fig. 2(b) shows a system 202, representing a fourth embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the previous embodiments. Different from the previous embodiments, the venting element 212 is now provided as a venting aperture 212 or venting hole 212 through the side wall 107. The aperture 212 comprises an upper edge 210, at the largest distance from the bottom side 108 of the suction pile, and a lower edge 209, being at the smallest distance from the bottom side 108. Hv of the venting aperture 212 is defined as the height position 211 of the upper edge 210. In this way, the venting aperture 212 is adapted to establish a fluid connection between the inner cavity 105 of the suction pile 100 and the outside.

[0073] As is illustrated in Fig. 3(a), the suction pile 100 is adapted to float with its open end 108 submerged in water, the open end 108 not being closed by a lid. The system 101 corresponds to the first embodiment of Fig. 1(a), wherein the venting element 104 is a straight venting pipe 104 extending through the top cover 106, and a constant air supply 304 is supplied to the inner cavity 105. A water volume 301 inside the cavity 105 reaches up to a water level Hw 302, measured from the bottom side of the suction pile 100. The remaining portion of the cavity 105 is filled with air, thereby defining an enclosed air volume 300. The enclosed air volume 300 thus extends between the top cover 106 and the inner water level Hw 302. The suction pile 100 floats on the water, due to buoyancy created by the air volume 300 enclosed in the top portion of the cavity. The suction pile 100 partly protrudes above the sea level 303, corresponding to a draught 309 of the suction pile 100. Inside the venting pipe 104, the water level is at the same height as the outside sea level 303.

[0074] In Fig. 3(a), the suction pile 100 is in an equilibrium condition or target state, wherein the pile 100 floats at a target draught 309. In this condition, the water level Hw 103 inside the cavity 105 reaches up to the height level Hv 103 of the first end 111 of the venting pipe 104. Accordingly, no air can escape from the enclosed air volume 300 via the venting pipe 104. Fig. 3(b) illustrates how the enclosed air volume is controlled by the venting element 104 during floating of the suction pile 100. In the condition of Fig. 3(b), the enclosed air volume 305 has temporarily increased compared to the target state of Fig. 3(a), due to the air losses, e.g. by air dissolving in the water or escaping from the cavity 105, being smaller than the constant air supply 304. Such an increased enclosed air volume 305 would result in a draught of the pile being smaller than the target draught 309. However, Fig. 3(b) shows that instantly, the water level Hw 307 inside the pile 100 is lower than the height position Hv 103 defined by the venting pipe 104. Accordingly, the first end 111 of the venting pipe 104 now ends in the air volume 305, thereby connecting the enclosed air volume 305 with the outside via the venting pipe 104. An air venting flow 308 thus occurs via the venting pipe 104, leading to a decreasing air volume enclosed in the suction pile 100.

[0075] As a result, the inside water level Hw 307 will rise up to the value of Fig. 3(a), wherein it reaches Hv 103 again, such that any further air venting flow is blocked. In this way, the enclosed air volume is automatically steered back towards the target value 300, corresponding to the target draught 309 of Fig. 3(a). Remark that in practice, as soon as a small deviation in the air volume occurs, immediately some venting to the outside follows, thus allowing for continuous small adjustments of the air volume; the drawings therefore need to be interpreted as mere conceptual drawings, wherein shown deviations may be represented exaggerated compared to reality.

[0076] A similar control is obtained when using the system according to any of the other embodiments represented in Fig. 1(b), Fig. 2(a) and Fig. 2(b). When the system 102 of Fig. 1(a) is used during floating of the suction pile 100, with a snorkel-shaped venting pipe 114, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 113. The latter corresponds to the height level of the horizontal end face of the first end 121, and an on-off control similar to the first embodiment will be obtained.

[0077] When the system 201 of Fig. 2(a) is used during floating of the suction pile 100, with a chamfered venting pipe 204, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 203 of the highest edge 207. Remark that depending on how low the enclosed air volume actually reaches, another flow rate of the venting flow may be obtained in this embodiment, wherein a maximal value applies if the air volume reaches to below the lowest edge 208, and a moderate value applies if the air volume reaches between the highest edge 207 and lowest edge 208. When the system 202 of Fig. 2(b) is used during floating of the suction pile 100, with a venting aperture 212, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 211 of the upper edge 210.

[0078] Fig. 4 illustrates how the target draught of the suction pile 100 can be set to another value, by means of a height-adjustable venting element 104. In Fig. 4, the venting element 104 is provided as the straight venting pipe 104 of Fig. 1(a), but a similar concept can be applied to other embodiments too. The venting pipe 104 extends through a hole in the top cover 106, and is connected via a gas-tight sealing, e.g. by using grease. Moreover, the connection allows the venting pipe 104 to slide in vertical direction, thereby allowing to change the height position Hv of the venting pipe 104. Indeed, in Fig. 4(a), the venting pipe 104 is at its original height position Hv 103, such that the target draught is set at a first value 309. While floating, the system 101 allows to control the enclosed air volume 300, thereby substantially maintaining the draught of the pile 100 at the first target value 309. In Fig. 4(b), the venting pipe 104 has been pulled upwards, such the height position Hv 401 has increased. Due to pulling up the venting pipe 104, automatically air will be vented from the enclosed air volume, thereby leading to a decreased air volume 400 compared to Fig. 4(a). As a result, buoyancy of the pile is decreased, thereby obtaining an increased draught 402. The latter corresponds to a new target value, and any deviations therefrom will automatically be adjusted by the venting element 104.

[0079] Fig. 5 illustrates two embodiments wherein multiple venting elements are used. In the embodiment of Fig. 5(a), the venting elements consist of a first venting pipe 500, and an additional second venting pipe 501. The pipes are provided as separate straight pipes, each extending through the top cover 106 of the suction pile 100, and each having a horizontal end face. The second ends of the two venting pipes, i.e. the ends located above the top cover 106, are at the same height level. The first end, however, i.e. the ends located inside the cavity 105, are at a different height level: the first venting pipe 501 is the longest pipe and ends at a height level Hv 503, while the additional venting pipe 500 is the shortest pipe and ends at a height level Hva 502. A valve 504 is arranged at the first venting pipe 501, and a valve 505 is arranged at the additional venting pipe 500, thereby allowing to close off the respective pipes. The presence of the venting pipes of different length allows to set the target draught at another value, similar to the height-adjustable venting pipe of Fig. 4. Indeed, in a first state, the first venting pipe 501 may be opened, while the valve of the additional venting pipe 500 is closed. In this condition, the enclosed air volume will reach to height level Hv 503, and a first target draught will result. Upon opening the additional shorter venting pipe, the enclosed air volume will be reduced, thereby obtaining an enlarged draught of the suction pile. Switching from the longer pipe 501 to the shorter pipe 500 thus has a similar effect as pulling up the venting pipe 104 of Fig. 4. Remark that upon opening the shorter additional venting pipe 500, the longer first venting pipe 501 may be closed off by means of valve 504. Alternatively, since the longer venting pipe 501 will not allow for venting anyway, the longer venting pipe 501 may remain opened, i.e. valve 504 may remain opened or no such valve may be present.

[0080] Fig. 5(b) shows another variant wherein multiple venting elements are used. In this embodiment, the additional venting pipe 510 branches off from the first venting pipe 509. Indeed, the first venting pipe 509 is provided as a straight pipe extending through the top cover 106, and ending at a height level Hv 508 inside the cavity 105. The second venting pipe 510 does not extend through the top cover 106, but branches off from the first venting pipe 509. The second venting pipe 510 has a second end with a horizontal end face, positioned at a height level Hva 507, Hva being higher than Hv. Each of the venting pipes 509, 510 is provided with a respective valve 511, 512, thereby allowing to close off the corresponding pipe. Thus, similar to the embodiment of Fig. 5(a), by switching from the longer venting pipe 509 to the shorter venting pipe 510, the target draught may be increased.

[0081] Fig. 6 and Fig. 7 show two different applications wherein one or more suction piles 100 are used. Fig. 6 shows a Suction Pile Anchor 600, comprising a suction pile 601 and a chain 602. The suction pile 601 may be embedded into the seafloor via a suction process, thereby anchoring a vessel or other floating object. In anchored condition, the chain 602 is connected to the mooring line towards the vessel or object. Fig. 7 shows a Tri Suction Pile Caisson (TSPC) 700, which may be embedded into the seafloor for providing a suction pile foundation, e.g. a foundation for an offshore wind turbine. The TSPC 700 comprises a central column 705, similar to a monopile foundation, and a base 704. The base 704 comprises three suction piles 701, 702, 703, which may be embedded into the seafloor for anchoring the foundation structure.

[0082] Fig. 8 illustrates the use of the invented system during free-floating tow-out of a TSPC 700. While being towed towards an offshore installation location by vessels 800, 801, the TSPC 700 floats on the suction piles 701, 702, 703. During floating, the air volume enclosed in the respective suction piles is controlled by means of the invented system 101. For this purpose, one or more air compressors may be installed at the vessel 800, or at the base of the TSPC, for providing the required air supply to the suction piles 701-703. The system 101 allows the three piles 701-703 to float at a target draught, while keeping any deviations within an acceptable range, thereby maintaining hydrostatic stability of the TSPC during floating. In a typical example, given a suction pile of height 13m, and a venting pipe with height level Hv of 4m, the pile protrudes about 3m above the sea surface during free floating, corresponding to a draught of about 10m.

[0083] Although not shown in Fig. 1 to 3, a valve may be arranged at the venting element 104, such that the outflow opening of the venting pipe 104 may be adjusted during free-floating transport. Adapting the outflow opening may be done to adapt the venting flow rate, and in particular to prevent any overshoots in the control. Changing the valve position may be done remotely, from the tug, e.g. via an umbilical, or by means of a Remotely Operated Vehicle (ROV). Moreover, depending on the desired draught, wherein e.g. another desired draught may apply in the harbour than at open sea, the heigh position of the venting pipe may be adjusted, as was illustrated in Fig. 4. Alternatively, another target draught may be obtained by selecting a venting pipe of appropriate length, as was illustrated in Fig. 5.

[0084] After arriving at the offshore installation location, the free-floating TSPC 700 is connected to a crane 900, installed at a Heavy Lift Vessel, as is illustrated in Fig. 9. Next, the venting pipe 104 of the respective suction piles 701-703 may be pulled up, as was illustrated in Fig. 4, such that corresponding buoyancy reduces compared to the free-floating condition. This results in the required crane load that is needed for controlled lowering of the TSPC 700 towards the seabed 903. Fig. 10 further illustrates that the TSPC 700 is lowered through the water 902, by means of crane cables 901, until it reaches the seafloor 903.

[0085] After reaching the seafloor 903, the suction piles 701-703 are embedded into the seabed. Such installation process, of a suction pile 100, is illustrated in Fig. 11. In a first stage, the suction pile 100 penetrates into the seabed 1100 under its own weight, as is shown in Fig. 11(a). Next, as is shown in Fig. 11(b), a suction process is applied, wherein water is pumped out of the top part 1104, such that suction forces the pile 100 deeper into the subsea floor. In order to create an underpressure or vacuum, the cavity 1104 needs to be sealed, e.g. by closing valve 1103 arranged at the venting pipe 104. At the end of the suction process, typically some gap 1102 remains between the top cover 106 and the soil 1101 inside the pile 100. As is shown in Fig. 11(c), the gap 1102 may be filled with grout, resulting in a grout plug 1105.

[0086] Fig. 11 shows that the pipe 104, which was previously used as a venting pipe 104 during free-floating transport, may remain in place during and after the installation process, wherein the venting pipe 104 penetrates throughout the grout plug 1105. In this way, the pipe 104 may be reused afterwards as a decommissioning pipe. Indeed, at the end of life, typically after years, the suction pile 100 may be removed by reversing the installation process: an overpressure is created inside the suction pile, to press the pile 100 out of the seabed. For this purpose, a pressurized fluid needs to be pumped into the space below the grout plug 1105. Instead of drilling a hole through the grout plug 1105, or pre-installation of a dedicated decommissioning pipe, the pipe 104, previously serving as a venting pipe, may be reused for supplying the pressurized fluid during the decommissioning process.

[0087] After pressing out the suction piles 701-703 out of the seabed, a crane 900 lifts the TSPC 700 to the surface. During this phase, the shorter vent pipe 500, 510 of Fig. 5 is to be used, or the venting pipe 104 is pulled up according to Fig. 4(b), to allow for a controlled lifting operation by the crane 900. Once the TSPC 700 is at the surface, the longer vent pipe 501, 509 of Fig. 5 is to be used, or the venting pipe 104 is pushed in according to Fig. 4(a), thereby resulting in zero crane and bringing the TSPC 700 in free-floating condition.

[0088] Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. In other words, it is contemplated to cover any and all modifications, variations or equivalents that fall within the scope of the basic underlying principles and whose essential attributes are claimed in this patent application. It will furthermore be understood by the reader of this patent application that the words "comprising" or "comprise" do not exclude other elements or steps, that the words "a" or "an" do not exclude a plurality, and that a single element, such as a computer system, a processor, or another integrated unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the respective claims concerned. The terms "first", "second", third", "a", "b", "c", and the like, when used in the description or in the claims are introduced to distinguish between similar elements or steps and are not necessarily describing a sequential or chronological order. Similarly, the terms "top", "bottom", "over", "under", and the like are introduced for descriptive purposes and not necessarily to denote relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and embodiments of the invention are capable of operating according to the present invention in other sequences, or in orientations different from the one(s) described or illustrated above.

Examples

first embodiment

[0068]Fig. 1(a) shows a system 101 according to the invention, wherein the system is represented in a schematic way. The system 101 comprises a suction pile 100, a venting element 104, and an arrangement for air supply 110. The suction pile 100 comprises a tubular side wall 107, provided as a cylindrical hollow sleeve, and a top cover 106. The top cover 106 is arranged at the top side of the tubular side wall 107. The opposite bottom side 108 of the tubular side wall is open, thereby providing access to the inner cavity 105 of the suction pile. The inner cavity 105 extends according to a height direction 109, also referred to as the vertical direction, or the length direction of the suction pile 100. The arrangement 110 is adapted to provide a constant air supply towards the inner cavity 105. For example, the arrangement 100 comprises an air compressor, and a hose connecting the air compressor to a hole in the top cover 106 of the suction pile 100.

[0069]The system 101 comprises a ve...

fourth embodiment

[0072]Fig. 2(b) shows a system 202, representing a fourth embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the previous embodiments. Different from the previous embodiments, the venting element 212 is now provided as a venting aperture 212 or venting hole 212 through the side wall 107. The aperture 212 comprises an upper edge 210, at the largest distance from the bottom side 108 of the suction pile, and a lower edge 209, being at the smallest distance from the bottom side 108. Hv of the venting aperture 212 is defined as the height position 211 of the upper edge 210. In this way, the venting aperture 212 is adapted to establish a fluid connection between the inner cavity 105 of the suction pile 100 and the outside.

[0073]As is illustrated in Fig. 3(a), the suction pile 100 is adapted to float with its open end 108 submerged in water, the open end 108 not being closed by a lid. The system 101 corresponds to the first embod...

Field of the Invention

[0001] The present invention generally relates to a solution for free-floating transport of a suction pile, the suction pile e.g. serving as a suction pile anchor or making part of a suction pile foundation. In particular, a solution is presented that ensures hydrostatic stability of the suction piles during transport, e.g. by towing, and still allowing for a swift offshore installation.Background of the Invention

[0002] A suction pile is an open-bottomed steel cylinder closed with a cap at the top, wherein the pile may be embedded into an underwater sea bottom. The suction pile may serve as a suction pile anchor, or may make part of a suction pile foundation e.g. for an oil platform or for an offshore wind turbine. A suction pile foundation may for example be provided as a Suction Pile Jacket, comprising a lattice structure mounted to three suction piles, or as a Tri Suction Pile Caisson (TSPC). The latter is composed of a central column similar to a monopile, and a base comprising three suction piles.

[0003] A suction pile is installed by lowering the pile towards the seafloor by means of a crane, after which it first penetrates into the underwater bottom under its own weight. The remainder of embedment is achieved through suction: water is pumped out of the pile, thereby generating suction that forces the pile deeper into the subsea floor. After embedment, a remaining gap between the top cap of the pile and soil inside the suction pile may be filled, e.g. with grout. Installation of the foundation can thus be performed by a noiseless suction-based process, instead of requiring a pile driving process as for installation of traditional monopile and jacket foundations. Moreover, at the end of life, suction piles are removable by reversing the installation process, i.e. applying an overpressure inside the suction pile, thereby allowing for recycling and even circularity.

[0004] The suction pile foundation like a TSPC may be transported to the offshore installation location by a barge, and installed using a Heavy Lift Vessel. Alternatively, advantage can be taken of the free-floating capacity of the TSPC base. Indeed, due to the buoyancy of the TSPC base, transport may be done by towing the base to the offshore site using tugs, thereby eliminating the need for a barge. Transport may also be done while the suction pile foundation is suspended by a crane, and partly submerged in the water, e.g. for relocation of the foundation after de-installation at a first location. Moreover, buoyancy of the base allows for a ballast-controlled lowering to the seabed, thereby allowing for a reduced offshore crane capacity.

[0005] However, free-floating transport of the foundation is not straightforward in practice, as hydrostatic stability of the floating base must be maintained during transport. Since the suction piles are floating on the air entrapped in the respective piles, the air volume in the respective piles needs to be controlled well. The latter is challenging, due to the fact that during floating, some air may be dissolved in the water or may escape underneath the pile tip. For this reason, solutions have been proposed in the prior art, wherein the suction piles are closed off at the bottom with a lid during towing. However, besides the purchase cost of such lids, the latter also complicates the installation process, as both releasing the lids from the piles, and recovering them after falling to the seafloor are cumbersome.

[0006] It is an objective of the present invention to disclose a solution for free-floating transport of suction piles, that resolves one or more of the above-described shortcomings of the prior art solutions. More particularly, it is an objective to present a solution that ensures hydrostatic stability during transport, and still allowing for a swift offshore installation.Summary of the Invention

[0007] According to a first aspect of the present invention, the above identified objectives are realized by a system for free floating of a suction pile, defined by claim 1, the system comprising: a suction pile comprising a tubular side wall provided with a top cover, the suction pile comprising an inner cavity extending in a height direction between the open bottom end of the side wall and the top cover, wherein a height level in the cavity is measured from the bottom end, the suction pile being adapted to float with the open end submerged in water, wherein water inside the cavity reaches a water level and buoyancy is created by an air volume enclosed in a top portion of the cavity; an arrangement adapted to provide a constant air supply towards the inner cavity of the suction pile while floating; a venting element arranged at the suction pile, adapted to establish a fluid connection starting from a height level Hv in the inner cavity towards the outside of the suction pile, such that during floating: upon an increasing enclosed air volume, wherein the actual air volume extends from the top cover to below the height level Hv, the venting element forms a fluid connection between the enclosed air volume and the outside, resulting in an air venting flow from the enclosed air volume, and upon a decreasing enclosed air volume, wherein the water level inside the cavity increases to above the height level Hv, said air venting flow is blocked, thereby allowing the enclosed air volume to be controlled to a target value corresponding to a target draught of the suction pile, while maintaining a constant air supply towards the inner cavity.

[0008] Thus, the invention concerns a system for free floating of a suction pile. A suction pile is a pile which allows to be embedded into an underwater bottom, wherein at least a part of the embedment is achieved through suction. In a typical installation process of such suction pile, water is pumped out or the top part of the pile, thereby generating suction that forces the pile deeper into the subsea floor. In various embodiments, the suction pile may have different functions. For example, the suction pile may serve as a suction pile anchor, or may make part of a suction pile foundation, e.g. a foundation for an oil platform or for an offshore wind turbine. A suction pile foundation may for example be provided as a Suction Pile Jacket, comprising a lattice structure mounted to three, four or a plurality of suction piles, or as a Tri Suction Pile Caisson (TSPC). The latter is composed of a central column similar to a monopile, and a base comprising three suction piles.

[0009] The suction pile comprises a tubular side wall extending according to a length or height direction. The tubular side wall is a sleeve-like elongated element, delimiting an internal space, and extending between a bottom end and a top end. Typically, the tubular side wall is cylindrical, i.e. having a circular cross section, but a cross section of any other shape is possible as well. The bottom end of the tubular side wall is open, while the top end is closed off by means of a cover, cap or lid. Typically, the tubular side wall and the top cap are made of steel, but other materials are possible too. The horizontal direction, or a horizontal plane, are defined as being perpendicular, or substantially perpendicular, to the height direction. The horizontal plane located at the bottom end of the tubular side wall is defined as the bottom face of the suction pile. Whenever a direction of an element is mentioned, being substantially in a particular direction refers to a deviation of at most 20%, more preferably at most 10%, still more preferably at most 5%, and most preferably at most 0,5% with respect to the indicated direction.

[0010] The tubular side wall and top cover together define a hollow body, wherein the space inside the hollow body is referred to as the inner cavity of the suction pile. The inner cavity thus is delimited by the inner sides of the tubular wall and the top cap on the one hand, and by the bottom face of the suction pile on the other hand, the latter not representing a physical border. The outside of the suction pile is defined as the environment located at the outer sides of tubular wall and the top cap, and located at the side of the bottom face not facing the internal cavity. The inner cavity thus extends in height direction between the open bottom end of the side wall and the top cover, and any height level in the cavity is measured from the bottom end.

[0011] The suction pile is adapted to float with its open end submerged in water, i.e. the bottom end of the suction pile is not closed off. Thus, during floating, a bottom portion of the inner cavity is filled with water, up to a certain water level, and the remaining top portion is filled with air. Due to the air volume enclosed in the top portion of the cavity, buoyancy is created. In particular, the amount of air volume enclosed in the suction pile determines - for a given suction pile - the resulting draught of the pile, i.e. how deep the pile is submerged under the water line. Free floating implies that the pile is able to float on its own, thus not being carried by another floating body such as a barge, vessel or platform.

[0012] The system further comprises an arrangement adapted to provide a constant air supply towards the inner cavity of the suction pile while floating. For example, the system may comprise an air compressor, and one or more hoses or tubes connecting the air compressor to the suction pile. The arrangement allows to supply a continuous flow of air towards the inner cavity of the suction pile, at a constant flow rate. For this purpose, typically an aperture is available in the top cover or the tubular side wall, for connecting a hose or tube.

[0013] The system further comprises a venting element arranged at the suction pile, adapted to establish a fluid connection starting from a height level Hv in the inner cavity towards the outside of the suction pile. This implies that, when considering the system when not being in use, the venting element is provided as an element that connects the inner cavity of the pile to the outside environment of the pile. This connection is such that a fluid connection may be established, i.e. the connection allows, when considering the system not being in use, for a fluid flow via the venting element. In other words, the venting element connects a first spot, located inside the cavity, to a second spot, located at the outside of the suction pile.

[0014] Various embodiments are possible for the venting element. For example, the venting element may be provided as a venting pipe extending though the top cover. In this case, the first spot of the fluid connection corresponds to the first end of the pipe, located inside the cavity, and the second spot corresponds to the second end of the pipe, located above the top cover. In another embodiment, the venting element may be provided as a snorkel-shaped pipe, wherein the first end of the pipe is located inside the cavity, the second end is located next to the tubular wall, and both ends are connected via the open bottom side of the suction pile. In yet another embodiment, the venting element is provided as a venting hole through the tubular wall, such that a fluid connection is provided between a first spot inside the cavity and a second spot at the outside via the hole in the wall.

[0015] In any of the embodiments, the fluid connection established by the venting element starts at a height level Hv inside the cavity. This means that the first spot of the connection is located inside the cavity, at a height level Hv, measured from the bottom side of the suction pile. For example, if the venting element is provided as a venting pipe, of which the first end has a horizontal end face, this horizontal end face is at a distance Hv measured from the bottom side. In another embodiment, the venting element may be provided as a pipe of which the first end has a chamfered or angled end face, i.e. being inclined with respect to the horizontal plane. In this case, Hv is defined as the height level of the highest edge of the first pipe end, i.e. the edge of the first pipe end being at the largest distance from the bottom side of the suction pile. In yet another embodiment, wherein the venting element is provided as a venting hole through the tubular side wall, Hv is defined as the upper edge of the hole, i.e. the edge of the hole being at the largest distance from the bottom side of the suction pile.

[0016] The system is such that during use, i.e. while the pile floats on the enclosed air volume, it ensures an automatic, continuous adjustment of the enclosed air volume. Indeed, the system is such that, upon an increasing enclosed air volume, wherein the actual air volume extends from the top cover to below the height level Hv, the venting element forms a fluid connection between the enclosed air volume and the outside, resulting in an air venting flow from the enclosed air volume. An increasing enclosed air volume may e.g. occur when the air losses, due to air dissolving in the water or escaping from the cavity, are temporarily low compared to the continuous constant air supply towards the cavity. Without any further control, this would result in an increased buoyancy and a draught of the suction pile deviating from the target draught. However, due to the presence of the venting element, the increasing air volume will instantly result in air escaping from the cavity via the venting element. Indeed, as soon as the water level inside the suction pile reaches below Hv, the venting element connects the enclosed air volume to the outside. Thus, in this condition, the first spot of the fluid connection is found in air, and air may freely flow towards the second spot at the outside, thus resulting in an air venting flow from the enclosed air volume via the venting element. For example, if the venting element is provided as a venting pipe with a horizontal first end face, a venting flow arises as soon as the water level in the pile is underneath that first end face. In case of a chamfered or inclined first end face, the venting flow arises as soon as the water level is lower than the highest edge of the first pipe end. In an embodiment wherein the venting element is provided as a hole thought the tubular side wall, a venting flow towards the water environment outside the pile arises as soon as the water level in the pile is lower than the upper edge of the hole.

[0017] In any of the embodiments, the air venting flow via the venting element instantly results in a decreasing enclosed air volume, such that the enclosed air volume is automatically steered back towards the target value, and only small variations in the enclosed air volume will occur. In particular, the system is such that if the water level inside the pile rises above Hv, the air venting flow is blocked again. Indeed, in such condition, the first spot of the fluid connection established by the venting element is not located in air anymore, and the water present at the first spot prevents air to further escape from the enclosed air volume, thereby preventing any further air venting flow.

[0018] Thus, the system is such that in a condition wherein the water level inside the pile is sufficiently high, i.e. above Hv, the venting flow is blocked, thus being zero or substantially zero. In certain embodiments, this results in a type of on-off control, wherein the venting flow is either at a maximum value or zero, and quick transitions occur between both states. This may for example be the case when the venting element is a pipe with a horizontal first end face. In other embodiments, however, a control may be obtained wherein the venting flow may gradually change from zero towards a maximum value, such that also some intermediate state with a modest air venting flow may occur. This may for example be the case when the venting element is a pipe with a chamfered or angled first end face.

[0019] In any of the embodiments, the system allows the enclosed air volume to be controlled to a target value corresponding to a target draught of the suction pile, while maintaining a constant air supply towards the inner cavity. Indeed, as soon as a the enclosed air volume starts increasing, such that the draught of the suction pile would change, some air is vented towards the environment, and the enclosed air volume returns to the target value. The enclosed air volume is thus continuously adjusted, such that the buoyancy is continuously controlled to target value, and the pile maintains at the target draught.

[0020] In this, controlling the enclosed air volume happens in an automatic, passive way without requiring any measurements or active control. The invention therefore brings the advantage that only minor adjustments to the suction pile design are needed, thus contributing to a reduced cost. Moreover, hydrostatic stability of the suction pile is obtained while not relying on adaptations of the air supply towards the cavity. Indeed, controlling the enclosed air volume by continuously adapting the air supply would be infeasible in practice, due to small occurring pressure values. Instead, the venting element allows to continuously adjust the air volume in a passive way, such that it is kept in a small acceptable range, and hydrostatic stability is maintained. Consequently, upon free floating of e.g. a foundation structure with a three-pile base, each of the suction piles continuously maintains substantially the same draught, thereby enabling minimum inclination of the foundation structure during transport, e.g. by towing.

[0021] In this way, the invention makes free-floating transport of suction pile foundations and anchors technically feasible in practice, without the additional cost and complexity related to using lids for closing off the open pile end during transport. Transport may e.g. happen by towing the suction pile foundation to an offshore installation location by means of a tug, or transport it while floating and being suspended from a crane, e.g. for relocation of a previously installed foundation structure. It therefore provides an advantageous alternative over transport by barge, as the need for such barges is eliminated. Moreover, advantage may be taken of the buoyancy of the pile(s) during lowering towards the seabed, thereby allowing for a ballast-controlled lowering and a reduced offshore crane capacity. Also in applications such as a self-installing platform, i.e. a four-legged platform founded on suction piles, the free-floating capacity of the platform legs, and optimal control thereof, may be leveraged during the platform installation process.

[0022] Optionally, the venting element is provided as a venting pipe, the venting pipe extending between a first end positioned inside the inner cavity at the height level Hv, and a second end positioned outside the suction pile. The pipe may be provided as a pipe, tube, duct, hose or the like, and may have any shape e.g. straight, curved, S-shaped, etc. Moreover, the connection between the first end and the second end may run through the top cover of the suction pile, or through the tubular side wall, or may run via the open bottom end of the suction pile. Typically, the first end is directed downwards and the second end is directed upwards, such that the venting air flows vertically in upward direction when flowing through the first end and when flowing through the second end. Between the first and second end, however, the venting pipe may comprise curves, angles, etc, such that between the first and second end the venting air flows in a direction deviating from the vertical direction.

[0023] Optionally, the end face of the venting pipe at the first end is in a horizontal plane, wherein the height level of the horizontal end face corresponds to the height level Hv of the venting element. This implies that the border of the first end of the venting pipe, which is located inside the cavity, lies in a horizontal plane.

[0024] Optionally, the end face of the venting pipe at the first end is chamfered or angled with respect to a horizontal plane, the edge of the first end being at highest height level defining the highest edge of the first end, wherein the height level of the highest edge corresponds to the height level Hv of the venting element. The edge may refer to a single point in the border of the first end, or may be a portion of the border, e.g. a straight edge. When the pipe has a chamfered or angled first end, edges comprised in the border are at different height positions, and the highest edge defines the height level Hv of the venting pipe. Having a chamfered or angled first end may be advantageous with respect to controlling the air venting flow, as the latter may progressively increase with the extent the actual air volume deviates from the target value. This differs from a venting pipe with a horizontal end face, wherein a pure on-off control of the air venting flow is obtained.

[0025] Optionally, the venting pipe extends upwards starting from the height level Hv, through a hole in the top cover. Thus, the venting pipe runs through the top cover of the suction pile. The venting pipe may be of any shape, e.g. straight, curved, ... For example, the hole in the top cover is positioned centrally, e.g. at the centre point of a disc-shaped cover plate.

[0026] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, and a second portion positioned above the top cover. Thus, the venting pipe partially protrudes above the top cover of the suction pile.

[0027] Optionally, the venting pipe is a straight pipe.

[0028] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, the first portion starting from the first end at height level Hv and extending upwards, a second portion positioned outside the suction pile, next to the side wall, the second portion ending at the second end of the venting pipe, and an intermediate portion connecting the first and the second portion, via the open bottom end of the suction pile. Thus, the first end of the venting pipe and the second end of the venting pipe are connected via the open bottom end of the suction pile. For example, the venting pipe may be S-shaped or snorkel-shaped. Typically, the first end is directed downwards and the second end is directed upwards, such that the venting air flows in upward direction when flowing through the first end and when flowing through the second end. Between the first and second end, however, when flowing through the intermediate portion, the air venting flow changes direction.

[0029] Optionally, the venting pipe is S-shaped or snorkel-shaped. For example, the intermediate portion of the venting pipe comprises two curved parts or turns, and a straight part therebetween.

[0030] Optionally, the venting pipe, or a portion thereof, is adjustable, thereby allowing to adjust the height level Hv of the venting element. This implies that the first end of the venting pipe, being located inside the cavity has an adjustable height position. For example, the venting pipe may be slidable as a whole, or a bottom portion of the venting pipe may slide over an upper portion. In this way, the height level Hv of the venting pipe may be adapted. This may be applied to any shape of venting pipe, e.g. a straight pipe or a snorkel-shaped pipe.

[0031] By changing the height position of the venting pipe, the enclosed air volume will automatically be controlled to another value, thereby changing the buoyancy of the suction pile. This may be useful in different situations. Firstly, during free-floating transport, adjusting the height position of the venting pipe allows to change the draught of the suction pile to another desired value. Thus, depending on the desired draught in a certain situation, another target draught may be set, by simply retracting or inserting the venting pipe. For example, when towing the suction pile to the offshore installation location, the draught of the suction pile may be adapted after leaving the harbour. Secondly, after arriving at the offshore installation location, the suction pile is connected to the crane, for lowering the pile towards the sea bottom. While lowering the suction pile through the water, advantage can be taken of the buoyancy of the pile; when an air volume remains entrapped in the pile during lowering, the crane load will be lower than when the complete weight of the pile needs to be carried by the crane, thereby allowing for a smaller crane capacity. However, in view of a controlled lowering by the crane, the buoyancy of the suction pile needs to be lower than during free-floating transport. Therefore, by adjusting the height position of the venting pipe to a larger Hv, i.e. the pipe is retracted somewhat from the cavity, the enclosed air volume is reduced, thereby decreasing the buoyancy of the suction pile. The same can be applied when decommissioning the suction pile at a later time, wherein a retracted venting pipe with larger Hv allows for a controlled lifting by the crane when raising the pile through the water.

[0032] Finally, the height-adjustable venting pipe allows to retract the venting pipe somewhat from the cavity before installation of the suction pile, i.e. before embedment into the seafloor. In this way, it may be prevented that the venting pipe would hamper the suction process, or would get damaged. Retracting the venting pipe before embedment of the pile also avoids that the pipe would be filled with sand over a large length; the latter may be disadvantageous in view of reusing the venting pipe in a later stage as a decommissioning pipe, wherein the previously installed venting pipe is reused for inserting a pressurized fluid through the grout plug upon removing the suction pile from the seabed.

[0033] Optionally, the venting pipe is slidable in vertical direction, thereby allowing to adjust the height level Hv of the venting element. Thus, the venting pipe as a whole may be moved in vertical direction, thereby changing the height position of both the first and second pile end.

[0034] Optionally, the venting pipe is slidable in vertical direction, wherein the venting pipe is connected to the top cover via a gas-tight sealing, thereby allowing for sliding of the venting pipe, while sealing the inner cavity of the suction pile. For example, grease may be used to allow for both sliding and sealing.

[0035] Optionally, the venting pipe is removable, thereby allowing to detach the venting pipe from the suction pile. This implies that the venting pipe is connected to the suction pile in a releasable way, i.e. in such a way that it can be removed or detached without causing any damage. This brings the advantage that the venting pipe can be removed from the suction pile before embedding the suction pile into the seafloor, thereby avoiding that the venting pipe would hamper the suction process.

[0036] Optionally, the venting pipe is provided with a valve adapted to adjust and / or block a fluid flow through the pipe. For example, a ball valve or gate valve may be arranged at the venting pipe, which e.g. may be operated remotely via an umbilical signal, or by means of a Remotely Operated Vehicle (ROV). In another example, a one-way valve may be used, which is automatically closed when lowering the pile into the water. The valve allows to adjust or block the air venting flow through the venting pipe, which may have different functions. A first function of the valve may be to adapt the outlet opening of the venting pipe during free-floating transport, such that the flow rate of the air venting flow may be controlled. This allows for a better control of the venting flow during free floating, in particular in view of avoiding overshoots. A second function of the valve may be to close off the opening of the venting pipe before starting installation of the suction pile; closing off the opening allows to create an underpressure or vacuum inside the cavity during the suction phase of the installation process. A third function of the valve may be to close off the opening of the venting pipe during decommissioning of the suction pile, i.e. when removing the suction pile from the seabed at a later time.

[0037] Optionally, the valve is a ball valve or gate valve or butterfly valve or one-way valve.

[0038] Optionally, the venting pipe comprises a first portion positioned inside the inner cavity, and a second portion positioned above the top cover, the valve being placed in the second portion.

[0039] Optionally, the venting element is provided as a venting aperture through the side wall, wherein the height position of the upper edge of the aperture corresponds to the height level Hv of the venting element. In such embodiment, the venting element is not provided by a pipe, but by a hole through the side wall of the suction pile. As soon as the water level inside the pile reaches below the upper edge of the hole, some air may escape from the enclosed air volume towards the outside.

[0040] Optionally, the venting aperture is closable by means of a hatch or valve, for example a one-way valve. For example, upon installing the suction pile in the seabed, the venting aperture may be closed off by the hatch or valve, in view of creating an underpressure or vacuum inside the cavity during the suction phase. Different types of valves or hatches may be used. E.g. a one-way valve may be used, thereby allowing for an automatic closing of the valve when lowering the pile into the water.

[0041] Optionally, the system comprises one or more additional venting elements arranged at the suction pile, each of the additional venting elements adapted to establish a fluid connection starting from a respective height level Hva in the inner cavity towards the outside of the suction pile, wherein each of the additional venting elements defines a different height level Hva, Hva being, for any of the additional venting elements, higher than the height level Hv. This implies that at least two venting elements are arranged at the suction pile. The venting element defining the height level Hv is referred to as the first venting element. The additional venting elements, additional to the first venting element, are referred to as the second, third, etc. venting element. Each of the additional venting elements defines a respective height level Hva, wherein Hva is different for each of the additional venting elements, and Hva is different from Hv defined by the first venting element. In particular Hva is higher than Hv, for any of the additional venting elements. In an embodiment, venting pipes of different lengths are provided, any of them extending through the top cover, wherein the first end of each of the venting pipes is at a different height level. In such a case, the longest venting pipe defines height level Hv, while the additional venting pipe(s) are shorter and define respective height level(s) Hva. In another embodiment, the venting elements are provided as snorkel-shaped pipes, of which the first ends are at a different height level. In yet another embodiment, the venting elements are provided as apertures in the tubular side wall, the apertures being at different heights.

[0042] Each or some of the venting elements may e.g. be provided with a valve, thereby allowing to select an active venting element of which the valve is opened, and air venting towards the outside occurs via the selected venting element. Activating a specific venting element may happen by opening the valve of that venting element, and closing the valve(s) of all the other venting element(s). In another embodiment, activating a specific venting element with height level Hv or Hva may happen by opening the valve of that venting element, while the valve(s) of venting elements with a shorter Hva may remain opened, as the latter automatically will not allow for venting anymore.

[0043] By switching to another venting element, the enclosed air volume will automatically be controlled to another value, corresponding to the heigh level defined by the selected venting element, thereby changing the buoyancy of the suction pile. This may be useful in different situations. Firstly, during free-floating transport, selecting a venting pipe with a different height position allows to change the draught of the suction pile to another desired value. Thus, depending on the desired draught in a certain situation, another target draught may be set, by activating another venting pipe. For example, when towing the suction pile to the offshore installation location, the draught of the suction pile may be adapted after leaving the harbour. Secondly, after arriving at the offshore installation location, the suction pile is connected to the crane, for lowering the pile towards the sea bottom. While lowering the suction pile through the water, advantage can be taken of the buoyancy of the pile; when an air volume remains entrapped in the pile during lowering, the crane load will be lower than when the complete weight of the pile needs to be carried by the crane, thereby allowing for a smaller crane capacity. However, in view of a controlled lowering by the crane, the buoyancy of the suction pile needs to be lower than during free-floating transport. Therefore, by selecting another venting element with a higher Hva, the enclosed air volume is reduced, thereby decreasing the buoyancy of the suction pile. The same can be applied when decommissioning the suction pile at a later time, wherein selecting a venting element with higher Hva allows for a controlled lifting by the crane when raising the pile through the water.

[0044] Optionally, the additional venting elements are provided as additional venting pipes, each of the additional venting pipes extending between a first end positioned inside the inner cavity at the heigh level Hva, and a second end positioned outside the suction pile.

[0045] Optionally, each of the additional venting pipes is a straight pipe extending upwards through a respective hole in the top cover, wherein the length of each additional venting pipe is smaller than the length of the venting pipe, and is different from the length of any other additional venting pipe. Thus, each of the available venting pipes has a different length, and has a first end at a different height position. The second end may be at the same height for all the venting pipes, or may be at different heights.

[0046] Optionally, each of the additional venting pipes is provided as a pipe branching off from the venting pipe. Instead of providing multiple venting pipes being as separate components, the additional venting pipes may be provided as branches. For example, the first venting pipe extends through the top cover, having a second end located at the outside and a first end located in the inner cavity. An additional venting pipe is provided as a branch to the first venting pipe, such that the additional venting has a first end inside the cavity, and a second end connected to the first venting pipe. Each or some of the venting elements may e.g. be provided with a valve, thereby allowing to select an active venting element of which the valve is opened, and air venting towards the outside occurs via the selected venting element. Activating a specific venting element may happen by opening the valve of that venting element, and closing the valve(s) of all the other venting element(s). In another embodiment, activating a specific venting element with height level Hv or Hva may happen by opening the valve of that venting element, while the valve(s) of venting elements with a shorter Hva may remain opened, as the latter automatically will not allow for venting anymore.

[0047] Optionally, air is supplied towards the inner cavity of the suction pile, via an opening through the top cover and / or the tubular side wall.

[0048] Optionally, the arrangement comprises an air compressor, and at least one hose or tube for connecting the air compressor to the suction pile. For example, an air compressor may be installed at the towing vessel, or onto the floating foundation structure.

[0049] Optionally, the suction pile serves as a suction pile anchor or makes part of a suction pile foundation.

[0050] Optionally, the suction pile makes part of a suction pile foundation, the suction pile foundation being a Suction Pile Jacket or a Tri Suction Pile Caisson.

[0051] According to a second aspect of the present invention, there is provided a method for free-floating of a suction pile, the method comprising providing a system according to the first aspect of the invention; floating the suction pile with the open end submerged in water, wherein buoyancy is created by an air volume enclosed in a top portion of the inner cavity; while providing a constant air supply towards the inner cavity of the suction pile, controlling the enclosed air volume by the venting element, to a target value corresponding to a target draught of the suction pile.

[0052] Optionally, a method is provided for transporting a suction pile towards an installation location, the method comprising: free-floating the suction pile according to the second aspect of the invention; transporting the suction pile in floating condition to the installation location, while controlling the enclosed air volume by the venting element. Transport of the suction pile in free-floating condition may e.g. happen when the structure comprising the suction pile(s) is transported from a harbour towards an offshore installation location, by towing with a tug. In another example, the structure comprising the suction pile(s) is transported while the structure is suspended from a crane, the crane being installed at a vessel and the suction pile(s) being in floating condition. The latter way of transport may e.g. be used when a previously installed foundation structure needs to be relocated: after pressing the suction pile foundation structure out of the seabed, and hoisting it towards the sea level, the structure remains coupled to the crane while being transported to a new installation location.

[0053] Optionally, the height level Hv of the venting element is adjusted during transport, thereby adjusting the draught of the floating suction pile.

[0054] Optionally, the outflow opening of the venting pipe is adjusted during transport, by means of a valve arranged at the venting pipe.

[0055] Optionally, a method is provided for installing a suction pile in a seabed, the method comprising: free-floating the suction pile on water according to the second aspect of the invention, before lowering the suction pile towards the seabed; after lowering the suction pile towards the seabed, installing the suction pile in the seabed, wherein embedment of the suction pile in the seabed is at least partially achieved through suction created inside the suction pile, and wherein prior to creating suction ∘ the venting element is sealed for blocking a fluid flow through the venting element, or ∘ the venting element is removed from the suction pile, and the top cover and / or side wall is sealed at the position of the removed venting element.

[0056] This implies that in a first phase, the venting element is used for controlling the enclosed air volume during free-floating of the suction pile, e.g. during towing towards the offshore installation location. Next, upon arrival at the offshore installation location, the suction pile is lowered to the seabed, thus being lowered through the water, by means of a crane. After reaching the seabed, the suction pile is embedded into the seabed. Typically, the pile first penetrates into the seabed under its own weight, and the remainder of embedment is achieved through suction: water is pumped out of the top part of the pile, thereby generating suction that forces the pile deeper into the subsea floor. In order to create an underpressure or vacuum inside the cavity during the suction phase, the cavity needs to be sealed. For this purpose, the venting element may be sealed, e.g. by closing the valve arranged at the venting pipe, or by closing off the venting aperture by means of a valve or hatch. In another embodiment, the venting element may be removed from the suction pile before the suction process. In this case, the opening originating from the removed venting element needs to be sealed before starting the suction process.

[0057] Optionally, the method comprises: free-floating the suction pile on water according to the second aspect of the invention, wherein the height level Hv of the venting element is at a first value; lowering the suction pile towards the seabed while being connected to a crane, wherein before lowering, the height level Hv of the venting element is adjusted to a second value, the second value being higher than the first value, thereby obtaining a reduced enclosed air volume and a decreased buoyancy of the suction pile than during free-floating.

[0058] For example, the venting element is provided as a venting pipe, of which the height position is adjustable. Before lowering the suction pile towards the seabed, thus being lowered through the water, the free-floating suction pile is first connected to the crane. Next, the venting pipe is pulled up, i.e. retracted somewhat from the cavity, such that its first end is at a higher height level Hv. Accordingly, the enclosed air volume is reduced, thus obtaining a reduced buoyancy compared to the free-floating condition. Such reduced buoyancy results in the required crane load that allows for a controlled lowering of the pile towards the seabed.

[0059] Optionally, the method comprises: free-floating the suction pile on water according to the second aspect of the invention, wherein during free-floating each of the one or more additional venting elements is closed, lowering the suction pile towards the seabed while being connected to a crane, wherein before lowering, at least one of the additional venting elements is opened, thereby obtaining a reduced enclosed air volume and a decreased buoyancy of the suction pile than during free-floating.

[0060] For example, the venting elements are provided as venting pipes of different lengths, wherein the first venting pipe defines the height level Hv, and an additional venting pipe defines a heigh level Hva being higher than Hv. The first venting pipe thus corresponds to the longest pipe. During free-floating, the first venting pipe is active, i.e. the valve of the first venting pipe is opened, and the additional shorter pipes are closed off. Before lowering the suction pile towards the seabed, thus being lowered through the water, the free-floating suction pile is first connected to the crane. Next, the valve of an additional, shorter venting pipe is opened. The valve of the first venting pipe may be closed or may remain opened. As a shorter venting pipe becomes active, the enclosed air volume is reduced, thus obtaining a reduced buoyancy compared to the free-floating condition. Such reduced buoyancy results in the required crane load that allows for a controlled lowering of the pile towards the seabed.

[0061] Optionally, there is provided a method for decommissioning an installed suction pile, the method comprising: installing the suction pile in the seabed as defined above, wherein the venting element is provided as a venting pipe, the venting pipe extending upwards starting from the height level Hv, through a hole in the top cover, and wherein the venting pipe is not removed from the installed suction pile; at a later stage, removing the previously installed suction pile from the seabed, wherein removing the suction pile from the seabed is at least partially achieved through pressure created inside the suction pile, wherein during pressing the suction pile out of the seabed, the venting pipe is used for supplying a pressurized fluid to the inside of the suction pile, thereby reusing the venting pipe as a decommissioning pipe.

[0062] During installation of the suction pile, the pile is embedded into the seabed through suction. After embedment, typically a gap remains between the top cover of the pile and soil inside the suction pile. This gap may be filled, e.g. with grout, in view of enlarging the bearing strength of the pile. At the end of life, the suction pile may be removed from the seabed, by pressing it out of the seabed. For this purpose, a fluid needs to be pumped into the cavity below the grout plug. If a venting pipe is arranged at the suction pile, that e.g. previously was used for venting during free-floating transport of the suction pile, this venting pipe may at a later stage be reused as a decommissioning pipe: upon pressing the pile out of the seabed, the available venting pipe may be used for pumping fluid into the cavity below the grout plug. It is thus required that the portion of the venting pipe inside the cavity is long enough, such that it extends through the complete depth of the grout plug. By reusing the already available venting pipe as a decommissioning pipe, it is avoided that a hole needs to be drilled through the grout plug during the decommissioning process.

[0063] Optionally, the method comprises: after pressing the suction pile out of the seabed, lifting the suction pile towards the sea level by means of a crane, thereby raising the suction pile though the water, wherein during raising, the height level Hv of the venting element is at a third value; after reaching the sea level, free-floating the suction pile on the water, wherein after reaching the seal level, the height level Hv of the venting element is adjusted to a fourth value, the fourth value being lower than the third value, thereby obtaining an enlarged enclosed air volume and an increased buoyancy of the suction pile during free-floating than during raising.

[0064] For example, the venting element is provided as a venting pipe, of which the height position is adjustable. After pressing the suction pile out of the seabed, a crane lifts the foundation to the water surface. During this phase, the venting pipe is sufficiently retracted from the cavity, such that buoyancy is low enough to allow for a controlled lifting by the crane. Once the suction pile is at the surface, the venting pipe is inserted more in the cavity, thereby obtaining an increased buoyancy. This results in zero crane load, and the pile to free-float.

[0065] Optionally, the method comprises: after pressing the suction pile out of the seabed, lifting the suction pile towards the sea level by means of a crane, thereby raising the suction pile though the water, wherein during raising, venting occurs via an active venting element, selected from the venting element and the one or more additional venting elements, wherein Hv of the active venting element corresponds to a third value; after reaching the sea level, free-floating the suction pile on the water, wherein after reaching the sea level, another venting element is activated, Hv of the newly activated venting element corresponding to a fourth value being lower than the third value, thereby obtaining an enlarged enclosed air volume and an increased buoyancy of the suction pile during free-floating than during raising.

[0066] For example, the venting elements are provided as venting pipes of different lengths, wherein the first venting pipe defines the height level Hv, and an additional venting pipe defines a heigh level Hva being higher than Hv. After pressing the suction pile out of the seabed, a crane lifts the foundation to the water surface. During this phase, a short venting pipe is used, such that buoyancy is low enough to allow for a controlled lifting by the crane. Once the suction pile is at the surface, a longer venting pipe is opened while the shorter ones are closed off. This results in zero crane load, and the pile to free-float.Brief Description of the Drawings

[0067] Fig. 1 and Fig. 2 show a system for free floating of a suction pile, according to four different embodiments of the invention. Fig. 3 illustrates how the air volume enclosed in the suction pile may be controlled, by means of a system according to an embodiment of the invention. Fig. 4 illustrates a system according to an embodiment of the invention, wherein the height position of the venting element is adjustable. Fig. 5 illustrates a system according to an embodiment of the invention, wherein multiple venting pipes with different lengths are provided. Fig. 6 and Fig. 7 respectively show a Suction Pile Anchor and a Tri Suction Pile Caisson (TSPC). Fig. 8 illustrates use of the system according to an embodiment of the invention, during free-floating tow-out of a Tri Suction Pile Caisson. Fig. 9 and Fig. 10 illustrate the lowering of a Tri Suction Pile Caisson towards the seabed. Fig. 11 illustrates an installation process of a suction pile, according to an embodiment of the invention. Detailed Description of Embodiment(s)

[0068] Fig. 1(a) shows a first embodiment of a system 101 according to the invention, wherein the system is represented in a schematic way. The system 101 comprises a suction pile 100, a venting element 104, and an arrangement for air supply 110. The suction pile 100 comprises a tubular side wall 107, provided as a cylindrical hollow sleeve, and a top cover 106. The top cover 106 is arranged at the top side of the tubular side wall 107. The opposite bottom side 108 of the tubular side wall is open, thereby providing access to the inner cavity 105 of the suction pile. The inner cavity 105 extends according to a height direction 109, also referred to as the vertical direction, or the length direction of the suction pile 100. The arrangement 110 is adapted to provide a constant air supply towards the inner cavity 105. For example, the arrangement 100 comprises an air compressor, and a hose connecting the air compressor to a hole in the top cover 106 of the suction pile 100.

[0069] The system 101 comprises a venting element 104, which, in the first embodiment, is provided as a venting pipe 104. The venting pipe 104 is a straight tube extending in vertical direction, through an aperture in the top cover 106. The venting pipe 104 extends between a first end 111, located in the inner cavity 105, and a second end 112, located at the outside of the suction pile 100. The second end 112 is positioned above the top cover 106, such that the venting pipe 104 protrudes with respect to the suction pile. The first end 111 has a horizontal end face, positioned at a height level Hv 103, the latter being measured in height direction 109, starting from the bottom end 108. In this way, the venting pipe 104 is adapted to establish a fluid connection starting from the first 111 end at height level Hv, towards the second end 112 at the outside. The straight venting pipe 104 thus provides a passageway through which a fluid such as air may flow, between the inner cavity 105 and the outside, provided that free passage of such fluid through the pipe is enabled.

[0070] Fig. 1(b) shows a system 102, representing a second embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the first embodiment of Fig. 1. However, the venting element 114 is now provided as a snorkel-shaped venting pipe 114. The venting pipe 114 comprises a first end 121, located in the inner cavity 105, and a second end 122, located at the outside of the suction pile 100. The second end 122 is positioned next to the tubular side wall 107. The snorkel-shaped pipe 114 connects the first end 121 to the second end 122 via the open bottom side 108 of the suction pile 100. For this purpose, the pipe 114 comprises a first straight portion 115 starting at the first end 121, connected to a first curved portion 116. A second straight portion 119, ending at the second end 122, is connected to a second curved portion 118. Both curved portions 118 and 116 are connected via an intermediate straight portion 117. Similar to the first embodiment, the first end 121 has a horizontal end face, positioned at a height level Hv 113, the latter being measured in height direction 109, starting from the bottom end 108. In this way, the snorkel-shaped venting pipe 114 is adapted to establish a fluid connection starting from the first 121 end at height level Hv, towards the second end 122 at the outside, thereby allowing to connect the inner cavity 105 to the outside of the suction pile 100.

[0071] Fig. 2(a) shows a system 201, representing a third embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the first and second embodiment. Just like in the first embodiment of Fig. 1(a), the venting element 204 is provided as a straight pipe 204, extending vertically through the top cover 106, between a first end 205 inside the cavity 105 and a second end 206 at the outside. However, different from the first embodiment, the first end 205 is chamfered, the end face thus being angled with respect to a horizontal plane. The first end 205 comprises a highest edge 207, being at the largest distance from the bottom side 108 of the suction pile, and a lowest edge 208, being at the smallest distance from the bottom side 108. Hv of the chamfered venting pipe 205 is defined as the height position 203 of the highest edge 207. In this way, the chamfered venting pipe 204 is adapted to establish a fluid connection starting from the first end 205 at height level Hv 203, towards the second end 206 at the outside.

[0072] Fig. 2(b) shows a system 202, representing a fourth embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the previous embodiments. Different from the previous embodiments, the venting element 212 is now provided as a venting aperture 212 or venting hole 212 through the side wall 107. The aperture 212 comprises an upper edge 210, at the largest distance from the bottom side 108 of the suction pile, and a lower edge 209, being at the smallest distance from the bottom side 108. Hv of the venting aperture 212 is defined as the height position 211 of the upper edge 210. In this way, the venting aperture 212 is adapted to establish a fluid connection between the inner cavity 105 of the suction pile 100 and the outside.

[0073] As is illustrated in Fig. 3(a), the suction pile 100 is adapted to float with its open end 108 submerged in water, the open end 108 not being closed by a lid. The system 101 corresponds to the first embodiment of Fig. 1(a), wherein the venting element 104 is a straight venting pipe 104 extending through the top cover 106, and a constant air supply 304 is supplied to the inner cavity 105. A water volume 301 inside the cavity 105 reaches up to a water level Hw 302, measured from the bottom side of the suction pile 100. The remaining portion of the cavity 105 is filled with air, thereby defining an enclosed air volume 300. The enclosed air volume 300 thus extends between the top cover 106 and the inner water level Hw 302. The suction pile 100 floats on the water, due to buoyancy created by the air volume 300 enclosed in the top portion of the cavity. The suction pile 100 partly protrudes above the sea level 303, corresponding to a draught 309 of the suction pile 100. Inside the venting pipe 104, the water level is at the same height as the outside sea level 303.

[0074] In Fig. 3(a), the suction pile 100 is in an equilibrium condition or target state, wherein the pile 100 floats at a target draught 309. In this condition, the water level Hw 103 inside the cavity 105 reaches up to the height level Hv 103 of the first end 111 of the venting pipe 104. Accordingly, no air can escape from the enclosed air volume 300 via the venting pipe 104. Fig. 3(b) illustrates how the enclosed air volume is controlled by the venting element 104 during floating of the suction pile 100. In the condition of Fig. 3(b), the enclosed air volume 305 has temporarily increased compared to the target state of Fig. 3(a), due to the air losses, e.g. by air dissolving in the water or escaping from the cavity 105, being smaller than the constant air supply 304. Such an increased enclosed air volume 305 would result in a draught of the pile being smaller than the target draught 309. However, Fig. 3(b) shows that instantly, the water level Hw 307 inside the pile 100 is lower than the height position Hv 103 defined by the venting pipe 104. Accordingly, the first end 111 of the venting pipe 104 now ends in the air volume 305, thereby connecting the enclosed air volume 305 with the outside via the venting pipe 104. An air venting flow 308 thus occurs via the venting pipe 104, leading to a decreasing air volume enclosed in the suction pile 100.

[0075] As a result, the inside water level Hw 307 will rise up to the value of Fig. 3(a), wherein it reaches Hv 103 again, such that any further air venting flow is blocked. In this way, the enclosed air volume is automatically steered back towards the target value 300, corresponding to the target draught 309 of Fig. 3(a). Remark that in practice, as soon as a small deviation in the air volume occurs, immediately some venting to the outside follows, thus allowing for continuous small adjustments of the air volume; the drawings therefore need to be interpreted as mere conceptual drawings, wherein shown deviations may be represented exaggerated compared to reality.

[0076] A similar control is obtained when using the system according to any of the other embodiments represented in Fig. 1(b), Fig. 2(a) and Fig. 2(b). When the system 102 of Fig. 1(a) is used during floating of the suction pile 100, with a snorkel-shaped venting pipe 114, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 113. The latter corresponds to the height level of the horizontal end face of the first end 121, and an on-off control similar to the first embodiment will be obtained.

[0077] When the system 201 of Fig. 2(a) is used during floating of the suction pile 100, with a chamfered venting pipe 204, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 203 of the highest edge 207. Remark that depending on how low the enclosed air volume actually reaches, another flow rate of the venting flow may be obtained in this embodiment, wherein a maximal value applies if the air volume reaches to below the lowest edge 208, and a moderate value applies if the air volume reaches between the highest edge 207 and lowest edge 208. When the system 202 of Fig. 2(b) is used during floating of the suction pile 100, with a venting aperture 212, a venting flow will occur as soon as the actual enclosed air volume extends to below the height level Hv 211 of the upper edge 210.

[0078] Fig. 4 illustrates how the target draught of the suction pile 100 can be set to another value, by means of a height-adjustable venting element 104. In Fig. 4, the venting element 104 is provided as the straight venting pipe 104 of Fig. 1(a), but a similar concept can be applied to other embodiments too. The venting pipe 104 extends through a hole in the top cover 106, and is connected via a gas-tight sealing, e.g. by using grease. Moreover, the connection allows the venting pipe 104 to slide in vertical direction, thereby allowing to change the height position Hv of the venting pipe 104. Indeed, in Fig. 4(a), the venting pipe 104 is at its original height position Hv 103, such that the target draught is set at a first value 309. While floating, the system 101 allows to control the enclosed air volume 300, thereby substantially maintaining the draught of the pile 100 at the first target value 309. In Fig. 4(b), the venting pipe 104 has been pulled upwards, such the height position Hv 401 has increased. Due to pulling up the venting pipe 104, automatically air will be vented from the enclosed air volume, thereby leading to a decreased air volume 400 compared to Fig. 4(a). As a result, buoyancy of the pile is decreased, thereby obtaining an increased draught 402. The latter corresponds to a new target value, and any deviations therefrom will automatically be adjusted by the venting element 104.

[0079] Fig. 5 illustrates two embodiments wherein multiple venting elements are used. In the embodiment of Fig. 5(a), the venting elements consist of a first venting pipe 500, and an additional second venting pipe 501. The pipes are provided as separate straight pipes, each extending through the top cover 106 of the suction pile 100, and each having a horizontal end face. The second ends of the two venting pipes, i.e. the ends located above the top cover 106, are at the same height level. The first end, however, i.e. the ends located inside the cavity 105, are at a different height level: the first venting pipe 501 is the longest pipe and ends at a height level Hv 503, while the additional venting pipe 500 is the shortest pipe and ends at a height level Hva 502. A valve 504 is arranged at the first venting pipe 501, and a valve 505 is arranged at the additional venting pipe 500, thereby allowing to close off the respective pipes. The presence of the venting pipes of different length allows to set the target draught at another value, similar to the height-adjustable venting pipe of Fig. 4. Indeed, in a first state, the first venting pipe 501 may be opened, while the valve of the additional venting pipe 500 is closed. In this condition, the enclosed air volume will reach to height level Hv 503, and a first target draught will result. Upon opening the additional shorter venting pipe, the enclosed air volume will be reduced, thereby obtaining an enlarged draught of the suction pile. Switching from the longer pipe 501 to the shorter pipe 500 thus has a similar effect as pulling up the venting pipe 104 of Fig. 4. Remark that upon opening the shorter additional venting pipe 500, the longer first venting pipe 501 may be closed off by means of valve 504. Alternatively, since the longer venting pipe 501 will not allow for venting anyway, the longer venting pipe 501 may remain opened, i.e. valve 504 may remain opened or no such valve may be present.

[0080] Fig. 5(b) shows another variant wherein multiple venting elements are used. In this embodiment, the additional venting pipe 510 branches off from the first venting pipe 509. Indeed, the first venting pipe 509 is provided as a straight pipe extending through the top cover 106, and ending at a height level Hv 508 inside the cavity 105. The second venting pipe 510 does not extend through the top cover 106, but branches off from the first venting pipe 509. The second venting pipe 510 has a second end with a horizontal end face, positioned at a height level Hva 507, Hva being higher than Hv. Each of the venting pipes 509, 510 is provided with a respective valve 511, 512, thereby allowing to close off the corresponding pipe. Thus, similar to the embodiment of Fig. 5(a), by switching from the longer venting pipe 509 to the shorter venting pipe 510, the target draught may be increased.

[0081] Fig. 6 and Fig. 7 show two different applications wherein one or more suction piles 100 are used. Fig. 6 shows a Suction Pile Anchor 600, comprising a suction pile 601 and a chain 602. The suction pile 601 may be embedded into the seafloor via a suction process, thereby anchoring a vessel or other floating object. In anchored condition, the chain 602 is connected to the mooring line towards the vessel or object. Fig. 7 shows a Tri Suction Pile Caisson (TSPC) 700, which may be embedded into the seafloor for providing a suction pile foundation, e.g. a foundation for an offshore wind turbine. The TSPC 700 comprises a central column 705, similar to a monopile foundation, and a base 704. The base 704 comprises three suction piles 701, 702, 703, which may be embedded into the seafloor for anchoring the foundation structure.

[0082] Fig. 8 illustrates the use of the invented system during free-floating tow-out of a TSPC 700. While being towed towards an offshore installation location by vessels 800, 801, the TSPC 700 floats on the suction piles 701, 702, 703. During floating, the air volume enclosed in the respective suction piles is controlled by means of the invented system 101. For this purpose, one or more air compressors may be installed at the vessel 800, or at the base of the TSPC, for providing the required air supply to the suction piles 701-703. The system 101 allows the three piles 701-703 to float at a target draught, while keeping any deviations within an acceptable range, thereby maintaining hydrostatic stability of the TSPC during floating. In a typical example, given a suction pile of height 13m, and a venting pipe with height level Hv of 4m, the pile protrudes about 3m above the sea surface during free floating, corresponding to a draught of about 10m.

[0083] Although not shown in Fig. 1 to 3, a valve may be arranged at the venting element 104, such that the outflow opening of the venting pipe 104 may be adjusted during free-floating transport. Adapting the outflow opening may be done to adapt the venting flow rate, and in particular to prevent any overshoots in the control. Changing the valve position may be done remotely, from the tug, e.g. via an umbilical, or by means of a Remotely Operated Vehicle (ROV). Moreover, depending on the desired draught, wherein e.g. another desired draught may apply in the harbour than at open sea, the heigh position of the venting pipe may be adjusted, as was illustrated in Fig. 4. Alternatively, another target draught may be obtained by selecting a venting pipe of appropriate length, as was illustrated in Fig. 5.

[0084] After arriving at the offshore installation location, the free-floating TSPC 700 is connected to a crane 900, installed at a Heavy Lift Vessel, as is illustrated in Fig. 9. Next, the venting pipe 104 of the respective suction piles 701-703 may be pulled up, as was illustrated in Fig. 4, such that corresponding buoyancy reduces compared to the free-floating condition. This results in the required crane load that is needed for controlled lowering of the TSPC 700 towards the seabed 903. Fig. 10 further illustrates that the TSPC 700 is lowered through the water 902, by means of crane cables 901, until it reaches the seafloor 903.

[0085] After reaching the seafloor 903, the suction piles 701-703 are embedded into the seabed. Such installation process, of a suction pile 100, is illustrated in Fig. 11. In a first stage, the suction pile 100 penetrates into the seabed 1100 under its own weight, as is shown in Fig. 11(a). Next, as is shown in Fig. 11(b), a suction process is applied, wherein water is pumped out of the top part 1104, such that suction forces the pile 100 deeper into the subsea floor. In order to create an underpressure or vacuum, the cavity 1104 needs to be sealed, e.g. by closing valve 1103 arranged at the venting pipe 104. At the end of the suction process, typically some gap 1102 remains between the top cover 106 and the soil 1101 inside the pile 100. As is shown in Fig. 11(c), the gap 1102 may be filled with grout, resulting in a grout plug 1105.

[0086] Fig. 11 shows that the pipe 104, which was previously used as a venting pipe 104 during free-floating transport, may remain in place during and after the installation process, wherein the venting pipe 104 penetrates throughout the grout plug 1105. In this way, the pipe 104 may be reused afterwards as a decommissioning pipe. Indeed, at the end of life, typically after years, the suction pile 100 may be removed by reversing the installation process: an overpressure is created inside the suction pile, to press the pile 100 out of the seabed. For this purpose, a pressurized fluid needs to be pumped into the space below the grout plug 1105. Instead of drilling a hole through the grout plug 1105, or pre-installation of a dedicated decommissioning pipe, the pipe 104, previously serving as a venting pipe, may be reused for supplying the pressurized fluid during the decommissioning process.

[0087] After pressing out the suction piles 701-703 out of the seabed, a crane 900 lifts the TSPC 700 to the surface. During this phase, the shorter vent pipe 500, 510 of Fig. 5 is to be used, or the venting pipe 104 is pulled up according to Fig. 4(b), to allow for a controlled lifting operation by the crane 900. Once the TSPC 700 is at the surface, the longer vent pipe 501, 509 of Fig. 5 is to be used, or the venting pipe 104 is pushed in according to Fig. 4(a), thereby resulting in zero crane and bringing the TSPC 700 in free-floating condition.

[0088] Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. In other words, it is contemplated to cover any and all modifications, variations or equivalents that fall within the scope of the basic underlying principles and whose essential attributes are claimed in this patent application. It will furthermore be understood by the reader of this patent application that the words "comprising" or "comprise" do not exclude other elements or steps, that the words "a" or "an" do not exclude a plurality, and that a single element, such as a computer system, a processor, or another integrated unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the respective claims concerned. The terms "first", "second", third", "a", "b", "c", and the like, when used in the description or in the claims are introduced to distinguish between similar elements or steps and are not necessarily describing a sequential or chronological order. Similarly, the terms "top", "bottom", "over", "under", and the like are introduced for descriptive purposes and not necessarily to denote relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and embodiments of the invention are capable of operating according to the present invention in other sequences, or in orientations different from the one(s) described or illustrated above.

Examples

first embodiment

[0068]Fig. 1(a) shows a system 101 according to the invention, wherein the system is represented in a schematic way. The system 101 comprises a suction pile 100, a venting element 104, and an arrangement for air supply 110. The suction pile 100 comprises a tubular side wall 107, provided as a cylindrical hollow sleeve, and a top cover 106. The top cover 106 is arranged at the top side of the tubular side wall 107. The opposite bottom side 108 of the tubular side wall is open, thereby providing access to the inner cavity 105 of the suction pile. The inner cavity 105 extends according to a height direction 109, also referred to as the vertical direction, or the length direction of the suction pile 100. The arrangement 110 is adapted to provide a constant air supply towards the inner cavity 105. For example, the arrangement 100 comprises an air compressor, and a hose connecting the air compressor to a hole in the top cover 106 of the suction pile 100.

[0069]The system 101 comprises a ve...

fourth embodiment

[0072]Fig. 2(b) shows a system 202, representing a fourth embodiment according to the invention. The design of the suction pile 100 and arrangement 110 are similar as in the previous embodiments. Different from the previous embodiments, the venting element 212 is now provided as a venting aperture 212 or venting hole 212 through the side wall 107. The aperture 212 comprises an upper edge 210, at the largest distance from the bottom side 108 of the suction pile, and a lower edge 209, being at the smallest distance from the bottom side 108. Hv of the venting aperture 212 is defined as the height position 211 of the upper edge 210. In this way, the venting aperture 212 is adapted to establish a fluid connection between the inner cavity 105 of the suction pile 100 and the outside.

[0073]As is illustrated in Fig. 3(a), the suction pile 100 is adapted to float with its open end 108 submerged in water, the open end 108 not being closed by a lid. The system 101 corresponds to the first embod...

Claims

1. System (101) for free floating of a suction pile (100), comprising: - a suction pile (100) comprising a tubular side wall (107) provided with a top cover (106), the suction pile (100) comprising an inner cavity (105) extending in a height direction (109) between the open bottom end (108) of the side wall (107) and the top cover (106), wherein a height level in the cavity (105) is measured from the bottom end (108), the suction pile (100) being adapted to float with the open end (108) submerged in water, wherein water inside the cavity (105) reaches a water level (302) and buoyancy is created by an air volume (300) enclosed in a top portion of the cavity (105); - an arrangement (110) adapted to provide a constant air supply (304) towards the inner cavity (105) of the suction pile (100) while floating; - a venting element (104) arranged at the suction pile (100), adapted to establish a fluid connection starting from a height level Hv (103) in the inner cavity (105) towards the outside of the suction pile (100), such that during floating: - upon an increasing enclosed air volume (305), wherein the actual air volume (305) extends from the top cover (106) to below the height level Hv (103), the venting element (104) forms a fluid connection between the enclosed air volume (305) and the outside, resulting in an air venting flow (308) from the enclosed air volume (305), and - upon a decreasing enclosed air volume (300), wherein the water level (302) inside the cavity increases to above the height level Hv (103), said air venting flow is blocked, thereby allowing the enclosed air volume (300) to be controlled to a target value corresponding to a target draught (309) of the suction pile (100), while maintaining a constant air supply (304) towards the inner cavity (105).

2. System (101) according to claim 1, wherein the venting element is provided as a venting pipe (104), the venting pipe (104) extending between a first end (111) positioned inside the inner cavity (105) at the height level Hv (103), and a second end (112) positioned outside the suction pile (100).

3. System (101) according to claim 2, wherein the venting pipe (104) extends upwards starting from the height level Hv (103), through a hole in the top cover (106).

4. System (102) according to claim 2, wherein the venting pipe (114) comprises: - a first portion (115) positioned inside the inner cavity (105), the first portion (115) starting from the first end (121) at height level Hv (113) and extending upwards, - a second portion (119) positioned outside the suction pile (100), next to the side wall (107), the second portion (119) ending at the second end (122) of the venting pipe (114), and - an intermediate portion (116, 117, 118) connecting the first (115) and the second portion (119), via the open bottom end (108) of the suction pile (100).

5. System (101) according to claim 2 to 4, wherein the venting pipe (104), or a portion thereof, is adjustable, thereby allowing to adjust the height level Hv (103) of the venting element (104).

6. System (101) according to claim 5, wherein the venting pipe (104) is slidable in vertical direction, thereby allowing to adjust the height level Hv (103) of the venting element (104).

7. System (101) according to claim 2 to 6, wherein the venting pipe (104) is provided with a valve adapted to adjust and / or block a fluid flow through the pipe (104).

8. System (202) according to claim 1, wherein the venting element is provided as a venting aperture (212) through the side wall (107), wherein the height position of the upper edge (210) of the aperture (212) corresponds to the height level Hv (211) of the venting element.

9. System according to any of the preceding claims, wherein the system comprises one or more additional venting elements (500) arranged at the suction pile (100), each of the additional venting elements (500) adapted to establish a fluid connection starting from a respective height level Hva (502) in the inner cavity (105) towards the outside of the suction pile (100), wherein each of the additional venting elements (500) defines a different height level Hva (502), Hva being, for any of the additional venting elements (500), higher than the height level Hv (503).

10. System (101) according to any of the preceding claims, wherein the arrangement (110) comprises an air compressor, and at least one hose or tube for connecting the air compressor to the suction pile (100).

11. System according to any of the preceding claims, wherein the suction pile (601, 701) serves as a Suction Pile Anchor (600) or makes part of a Suction Pile Foundation (700).

12. Method for free-floating of a suction pile (100), comprising: - providing a system (101) according to any of the preceding claims; - floating the suction pile (100) with the open end (108) submerged in water, wherein buoyancy is created by an air volume (300) enclosed in a top portion of the inner cavity (105); - while providing a constant air supply (304) towards the inner cavity (105) of the suction pile (100), controlling the enclosed air volume (300) by the venting element (104), to a target value corresponding to a target draught (309) of the suction pile (100).

13. Method for transporting a suction pile (100) towards an installation location, the method comprising: - free-floating the suction pile (100) according to claim 12; - transporting, for example by towing, the suction pile (100) in free-floating condition to the installation location, while controlling the enclosed air volume (300) by the venting element (104).

14. Method for installing a suction pile (100) in a seabed (1100), the method comprising: - free-floating the suction pile (100) on water according to claim 12, before lowering the suction pile (100) towards the seabed; - after lowering the suction pile (100) towards the seabed (1100), installing the suction pile (100) in the seabed (1100), wherein embedment of the suction pile (100) in the seabed (1100) is at least partially achieved through suction created inside the suction pile (100), and wherein prior to creating suction ∘ the venting element (104) is sealed for blocking a fluid flow through the venting element (104), or ∘ the venting element (104) is removed from the suction pile (100), and the top cover (106) and / or side wall (107) is sealed at the position of the removed venting element (104).

15. Method for decommissioning an installed suction pile (100), the method comprising: - installing the suction pile (100) in the seabed according to claim 14, wherein the venting element (104) is provided as a venting pipe (104), the venting pipe (104) extending upwards starting from the height level Hv (103), through a hole in the top cover (106), and wherein the venting pipe (104) is not removed from the installed suction pile (100); - at a later stage, removing the previously installed suction pile (100) from the seabed (1100), wherein removing the suction pile (100) from the seabed (1100) is at least partially achieved through pressure created inside the suction pile (100), wherein during pressing the suction pile out (100) of the seabed (1100), the venting pipe (104) is used for supplying a pressurized fluid to the inside of the suction pile (100), thereby reusing the venting pipe (104) as a decommissioning pipe.

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