Structural tubular pile having an internal flow reducing device

Abstract

Structural tubular pile having an internal flow reducing device for reducing a flow of water through an inner space of said pile, wherein said internal flow reducing device is arranged inside the structural tubular pile and is connected to the structural tubular pile, in particular to a circumferential wall of the structural tubular pile; wherein said internal flow reducing device comprises a sheet of flexible material, having a one or more throughflow openings therein, that is arranged to cooperate with the circumferential wall in such a way that said sheet of flexible material spans the largest portion of, in particular substantially the full, surface area of the inner space of said structural tubular pile, as determined in a plane substantially perpendicular to a longitudinal axis of the structural tubular pile; wherein said internal flow reducing device is arranged such that, at least in a deployed state wherein a column of water is pushed through the inner space in a longitudinal direction from a tip to a top of said structural tubular pile, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile.

Description

[0001] STRUCTURAL TUBULAR PILE HAVING AN INTERNAL FLOW REDUCING DEVICE

[0002] Sudden pile drop is a phenomenon that can occur during the installation of tubular piles, particularly in the field of geotechnical and offshore engineering. It involves a sudden, unexpected decrease in the resistance encountered by the tubular pile as it is being driven into the soil or seabed, leading, for instance, to a rapid penetration of the pile into seabed without the anticipated level of resistance. Additionally, when the pile is suspended from a crane during lowering of the pile onto the seabed, an accidental drop, for instance due to a failure of the rigging connecting the pile to the crane, can also occur. As both events can jeopardize both the pile itself, the installation equipment and the people working thereon, mitigation measures are needed.

[0003] Sudden pile drop is most commonly associated with driven piles, where a pile is forcefully driven into the ground or seabed using impact hammers, vibratory methods, pushing in the pile and / or any other pile installation method. The occurrence of sudden pile drop is often linked to variations in soil conditions. For example, if a pile encounters a softer layer of soil or if the soil suddenly yields, the resistance against the pile may decrease abruptly, leading to the unexpected drop.

[0004] The risk of sudden pile drop is not only limited to effects on the pile itself, as it could for instance drop too deep in the seabed, or in the extreme cases even fully sinking into the seabed, and / or lose its (verticality) tolerance, but there is also a risk for the installation equipment and the personnel working on, or with the installation equipment. In many cases a heavy pile driver, or hammer, is sitting on top of the pile, while also being hooked to the crane of the installation vessel. A sudden drop of the pile also causes a sudden drop of the hammer, that is caught by the installation crane, thereby causing a sudden impulse to the crane and the installation vessel whereon the crane is fixed. This sudden impulse cases an extreme loading event in the crane structure, but can, in case of a jack-up installation vessel also impact the jackup legs, or, when using a heavy lifting vessel, also can affect the stability of the vessel. The heavy lifting vessel may experience shifts in its center of gravity, potentially leading to instability if not properly managed.

[0005] To address the risks associated with sudden pile drop, various strategies exist. This can include conducting thorough site investigations to understand the soil conditions, using appropriate pile types and designs, and employing monitoring systems to detect changes in resistance during the installation. Crane hook damping systems may be employed for damping the impulse due to the falling hammer, thereby mitigating some of the effects on the installation equipment. These solutions however, do not reduce the effect of a sudden pile drop on the pile itself.

[0006] The current invention is aimed at preventing, or at least reducing, both the negative effects of a pile drop on the pile itself, as well as the negative effects on the installation equipment. This goal is achieved by providing a structural tubular pile having an internal flow reducing device for reducing a longitudinal free-falling speed of said pile in a body of water, wherein said internal flow reducing device is arranged inside the structural tubular pile and is connected to the structural tubular pile, in particular to a circumferential wall of the structural tubular pile; wherein said internal flow reducing device comprises a sheet of flexible material, having a one or more throughflow openings therein, that is arranged to cooperate with the circumferential wall in such a way that the internal flow reducing device spans the largest portion of, in particular substantially the full, surface area of the inner space of said structural tubular pile, as determined in a plane substantially perpendicular to a longitudinal axis of the structural tubular pile; wherein said internal flow reducing device is arranged such that, at least in a deployed state wherein a column of water is pushed through the inner space in a longitudinal direction from a tip to a top of said structural tubular pile, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile.

[0007] During a lifting and up-ending process, during which a structural pile is arranged vertically and lowered onto the bottom underneath a body of water (e.g. a seabed), the pile typically becomes, at least partly, filed with water. Upon touching and / or an initial penetration of the bottom, the therein retained column of water is trapped in the inner space of the structural tubular pile on the sides, and the bottom of the body of water on the bottom, such that the contained water is enclosed on all sides, except on the top of the pile. During a sudden pile drop, the pile slides through the water and the bottom, causing a relative motion between the bottom and the structural tubular pile, such that the retained column of water also moves relative to the pile in the direction of the top of the pile. By arranging the pile with an internal flow reducing device, water column has to pass an additional resistance, that mostly acts as a damper and thereby slows the relative speed of the water column with respect to the pile. This causes a pressure increase, that, in turn, leads to an upwards oriented reaction force onto the falling pile, thereby decelerating the suddenly falling pile. The one or more throughflow openings are preferably sized, in dependence of the structural properties of the structural tubular pile, such as the mass and internal diameter of the pile, such that, as determined in steady-state, a minimum (longitudinal) lowering speed of the pile is obtained that is in the range of 0.5 m / min - 20 m / min, preferably in the range of 2 m / min - 15 m / min, more preferably in the range of 3 m / min - 10 m / min. Hence, although the internal flow reducing device preferably spans the full surface area of (cross-section of) the inner space, it does not fully obstruct the full surface area of the inner space, as it will always have one or more throughflow openings.

[0008] However by spanning the full surface area, an improved control over the allowed throughflow is obtained, when compared to a device that would not span the full surface and allow for uncontrollable “leaks”.

[0009] The internal flow reducing device comprises a sheet of flexible material (which may be a flexible permeable or non-permeable textile), having a one or more throughflow openings therein, causing for a relatively high flow-through resistance, while being relatively lightweight and flexible at the same time. This is important, as for instance during piling successive hammer blows of a pile driver cause for high accelerations in the structural tubular pile. Auxiliary components arranged on the pile, such as the internal flow reducing device, need to be lightweight as otherwise large inertia forces are caused that can lead to deformations and / or increased fatigue loading of the structural tubular pile. Additionally, by increasing the weight of the pile, the installation itself also requires more energy. By arranging the sheet of flexible material such that is curves outwardly, the forces due to the increased pressure can effectively be taken up as tensile forces in the fabric that can effectively be transferred to the structural tubular pile. In short, a sudden pile drop parachute is thereby obtained that will effectively brake the pile during a sudden pile drop.

[0010] If said flexible material is a permeable material, it is preferably selected on the basis of, at least, the geometrical properties of the pile, as these define the weight and surface area of the inner space of the structural tubular pile (i.e. internal diameter of the structural pile). Said permeable material is preferably a permeable fabric having a plurality of throughflow openings therein. Such permeable fabrics are often characterized by having a large number of pores therein (i.e. more than several hundreds of (small) openings per m2). Parameter of the permeable material that may be vaned and / or selected may contain the permeability of the material, the average pore (often also referred to as mesh) size of the fabric and / or the tensile strength of the fabric. Optionally, the internal flow reducing device further comprises one or more additional openings having a relatively large size, that is, larger than an average size of the one or more throughflow openings (e.g. average pore size) in the permeable material. By arranging additional holes, i.e. additional flow-through openings, the permeability of the internal flow reducing device may be increased (thereby effectively relaxing the flow restriction of the internal flow reducing device) without having to select an different permeable material. Reasons to do so may include to limit a maximum tensile stress in the permeable material that may arise during a sudden pile drop.

[0011] It is however preferred that said internal flow reducing device comprises one, preferably only one, substantially centrally arranged through hole that is arranged in the sheet of flexible material, wherein said through hole is preferably substantially oval or circular. Such a central hole is relatively easy to arrange in the sheet of flexible material. Additionally, its performance (in terms of a generated drag force for limiting the drop speed of the pile) is easier to predict using relatively simple first-principle models, as is explained below.

[0012] The additional openings are then preferably formed as one or more additional holes in the sheet of flexible material and / or formed as one or more bypass openings between an outer edge of the sheet of flexible material and the circumferential wall. Additional holes may be arranged in the fabric, for instance in the center of (i.e. at the top of the curvature) of the sheet of flexible material, as such alterations are easily made. Bypass openings between an outer edge of the sheet of flexible material and the circumferential wall have the benefit that the amount of discontinuities in the fabric are prevented as much as possible, which will typically improve the overall tensile strength of the sheet of flexible material.

[0013] In a preferred embodiment, the total surface area of all throughflow openings of the internal flow reducing device is approximated by an equivalent circular through flow hole having an equivalent throughflow diameter de, the ratio between the equivalent throughflow diameter and the inner diameter di of the structural tubular pile, as determined from de / di, is in the range of 0.01 - 0.2, preferably 0.02 - 0.15 times, more preferably 0.03 — 0.11, most preferably 0.05 - 0.08. A parameter for tuning the flow-through resistance of the internal flow reducing device is the ratio between the sum of the surface areas of all throughflow openings in the internal flow reducing device and the surface area of (cross-section of) the inner space of the structural tubular pile, which can be given in equivalent diameters. If the ratio is too high, the hole is too large and the effectively induced additional flow-through resistance is too low to lead to an effective braking effect on the speed due to a sudden pile drop. If the ratio is too low, the hole is too small such that the resistance is too high, such that this leads to too slow drop speed of the pile during normal installation, such that the installation time is negatively affected. A favorable ratio, is therefore obtained using the above given ranges of ratios.

[0014] In a preferred embodiment, said internal flow reducing device comprises a grid-like reinforcement structure and wherein said sheet of flexible material is arranged onto, and connected to, the grid-like reinforcement structure at a plurality of different points; wherein said sheet of flexible material is coupled to the structural tubular pile by means of the grid-like reinforcement structure. It is noted that the flexible material is preferably arranged onto the underside of the grid-like reinforcement.

[0015] The grid-like reinforcement structure is for instance formed from a plurality of substantially parallel arranged first reinforcement elements that extend, onto the sheet of permeably fabric, along first direction and a plurality of substantially parallel arranged second reinforcement elements that extend, onto the sheet of flexible material, along a second direction, wherein the first and second direction are substantially perpendicular, such that the first and second reinforcement elements intersect each other and are, preferably interconnected, at the respective intersections. The sheet of permeably fabric may then be connected, at a plurality of sheet connection points, to each of the first and second reinforcement elements. This enables to reduce the tensile forces in the sheet of flexible material, as the forces are transferred to the grid-like reinforcement structure. Preferably, the grid-like reinforcement structure is arranged on top (as seen in the direction from the tip to the top of the pile) of the sheet of flexible material, such that, during a sudden pile drop, the sheet is pressed against the grid-like reinforcement structure, thereby preventing excessive forces in the sheet connection points. It is then further preferred that the grid-like reinforcement structure is a flexible grid-like reinforcement structure, preferably comprising flexible reinforcement means, such as cables, chains, flexible rods and / or beams, reinforced ropes and / or inflatable elements. This enable to obtain a structure that is at least to some extend elastically deformable and / or compactible to allow it to be brought into, or out of, the pile in a more compact (e.g. retracted) state. Mounting and dismounting of the internal flow reducing device can thereby be done more easily. Additionally, a flexible grid-like reinforcement structure is allowed to deform under the pressure of the water column that tries to push through, such that a redistribution of internal forces and stresses in the internal flow reducing device is enabled.

[0016] Preferably, the internal flow reducing device is removably connected to the structural tubular pile. This allows for recycling said internal flow reducing device on a next tubular structural pile that is to be installed and / or does not hinder the placement of auxiliary internal structures, such as for instance internal platforms in a monopile for a wind turbine.

[0017] It is then preferred that the internal flow reducing device comprises a release mechanism that is operable from outside of the structural tubular pile (for instance from above by arranging a mechanical release cable through the top opening of the pile) for releasing the removable connection between the internal flow reducing device and the structural tubular pile. This allows to safely detach the device. Additionally, or alternatively, it is preferred that the internal flow reducing device is arranged to, after being disconnected from the structural tubular pile, be movable into a retracted state wherein at least one dimension of the internal flow reducing devices is smaller than in the deployed and / or installed state; preferably wherein, in the retracted state, a width of the internal flow reducing device is smaller than 0.9, more preferably smaller than 0.75, most preferably smaller than 0.6, times the width of the internal flow reducing device in the deployed state. Many monopiles for wind turbines are fitted with a reduced diameter pile tip above the waterline, as this leads to a decreased wave loading on the structure and to allow a connection to a bottom tower section of s wind turbine. By reducing the size of the internal flow reducing device in the retracted state, the device can be lifted out of the top of the monopile. It is noted that is some embodiments, the internal flow reducing device is not removed, it may therefore be unremovable and permanently fixed to the structural tubular pile.

[0018] It is preferred that the flexible material is permeable fabric, such as a geosynthetic fabric, such as a geotextile, a geogrid or a geocomposite, preferably comprising geotextile and / or geogrid, wherein said geosynthetic fabric comprises polymer materials. Geosynthetic materials are permeable materials that are available in a range of permeabilities, such as 15 - 80 L / m2s (as determined using a 50mm water column) having high tensile strength, for instance, in the range of 2000 - 3000kN / m. These geosynthetic fabrics are therefore suitable for coping with the induced forces during a sudden pile drop, while having a permeability in, or near, the desired ranges. The permeable material may also be a chain mail fabric consisting of (small) metal, plastic and / or rubber rings that are linked together in a pattern to form a mesh. Alternatively, or additionally, the permeable material may be obtained by creating the one or more throughflow openings in a non-permeable material, such as for instance a reinforced sheet of plastic material. Such a reinforced sheet of plastic material may be formed by arranging (e.g. welding) a plastic layer (e g. elastomeric foil) onto, or around, a sheet formed from reinforcing fibers and / or wire material, such as for instance a nylon-wire, metal -wire (e.g. steel wire), glass-fiber, carbon-fiber basis. Alternatively, a woven sheet of fiber material may for instance be covered by a layer of an elastomeric material (through a casting process or the like). A sheet of plastic material is hereby obtained that is reinforced by the therein arranged internal fibers and / or wires.

[0019] In a preferred embodiment, the sheet of flexible material comprises a plurality of layers of flexible material. The layers are preferably interconnected by stitching said layers such that the plurality of stacked layers effectively acts as a single sheet of flexible material. By arranging the sheet as a stack of a plurality of different and / or the same layers, one is able to tune the permeability and / or tensile strength of the sheet of flexible material.

[0020] In case a material is used having a high tensile strength in only one direction, the sheet of flexible material is preferably formed from at least two layers of the plurality of layers have a higher tensile strength along a primary axis and a lower tensile strength along a secondary axis, and wherein the two layers of the plurality of layers are arranged such that the respective primary axis are at a non-zero angle, preferably at 90°, with respect to each other. As for instance high-strength geotextiles typically have a bidirectional strength, wherein the tensile strength along a primary axis is higher than along a secondary, perpendicular, axis, a stacking of at least two layers, wherein the layers are arranged at a non-zero angle, in particular a 90° angle, allows to obtain a sheet of flexible material having an omnidirectional tensile strength.

[0021] Additionally, or alternatively, the plurality of layers of flexible material may comprise at least one secondary layer having a different, preferably lower, permeability when compared to at least one primary layer of said plurality of layers of flexible material, preferably wherein the at least one secondary layer is arranged in between two primary layers. This allows to combine, for instance, one or more low strength, low permeability layers with one or more high strength, high permeability layers for obtaining a sheet of flexible material having both advantages.

[0022] In a preferred embodiment, the internal flow reducing device is arranged such that, at least in an installed state wherein the internal flow reducing device is connected to the structural tubular pile and not subject to a column of water that is pushed through, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile, or in other words: said internal flow reducing device is arranged such that, at least in an installed and submerged state wherein the internal flow reducing device is connected to the structural tubular pile, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile. A center of the sheet of flexible material may be biased towards the top of the pile. This may be by arranging a wire, rope and / or cable between the sheet of flexible material, in particular its center, and a biasing connection point on the structural tubular pile at a location closer to the top of the pile. Alternatively, or additionally, the grid-like reinforcement structure may form a pre-shaped skeleton whereto the sheet of flexible material is connected to; and / or the grid-like reinforcement structure may comprise biasing means, such as flexible rods and / or beams, that, upon connecting the grid-like reinforcement structure, cause the center to be pushed in an upward direction towards the top of the pile. The shape of the installed state is hereby similar to, or at least closely resembles, the shape of the deployed state. Thereby the internal flow reducing device is able to directly respond to a sudden pile drop, without losing time during an initializing process wherein the shape of deployed state needs to formed.

[0023] Preferably, wherein the sheet of flexible material curves, when in the deployed and / or installed state, outwardly towards the top of the structural tubular pile, as seen in a first plane, wherein the first plane runs substantially parallel to the longitudinal axis, in particularly wherein the sheet of flexible material curves in the first plane only, such that the sheet of flexible matenal has a single curvature only; or wherein the sheet of flexible material curves, when in the deployed and / or installed state, outwardly towards the top of the structural tubular pile, as seen in the first and a second plane, such that the sheet of flexible material has a double curvature, wherein said first and second planes run substantially parallel to the longitudinal axis and are substantially orthogonal with respect to each other, and, preferably, wherein a radii of curvature of the first plane and second plane are substantially equal for forming the sheet of flexible material into a dome-shape.

[0024] Alternatively, a radius of curvature of the first plane is smaller than the radius of curvature in the second plane; and, preferably, wherein the radius of curvature, as seen in the first plane is in the range of 0.8 - 1.2 times, preferably in the range of 0.9 - 1.1 times, more preferably substantially equal to, an inner diameter of the structural tubular pile as determined at the location where the internal flow reducing device is arranged in the structural tubular pile.

[0025] By arranging the curvature in at least the first plane in an outwardly direction towards to the pile top, the forces due to the increased pressure can effectively be taken up as tensile forces in the fabric and / or gridlike reinforcement structure, as was discussed above. A double curvature, in particular the dome-shape, leads to an improved force distribution, for instance due to the self-canceling thrust forces (i.e. forces substantially perpendicular to the longitudinal axis) , but is more difficult to manufacture from a planer sheet of flexible material. The manufacturing difficulties can be somewhat reduced by giving the sheet of flexible material only a slight curvature (i.e. having a larger radius of curvature) in a second direction, although this also reduces some of the benefits associated with a substantial equal double curvature (i.e. a dome shape). Manufacturing of the double curvature, even wherein the radius of the second curvature is substantially increased, requires further internal stitching in the sheet of flexible material, which may be more susceptible to failure. By arranging only a single curvature, as is described above, the sheet of flexible material can be manufactured more easily from a planar sheet, while also enabling an sufficient load bearing capacity.

[0026] It is further preferred that said sheet of flexible material is preformed (for instance using any of the processed described above, to have a, in a resting or uninstalled state, a substantially hollow semi- spherical shape, in particular thereby forming a flexible structure having a hollow semi-spherical shape. The sheet of flexible material is thereby formed such that, prior to mounting the sheet of flexible material to the internal flow reducing device, it already has the substantially hollow semi-spherical shape. However, as the material is flexible, gravity may cause it, at least in state separate from the internal flow reducing device and / or the tubular structural pile, to deform, thereby altering its shape, when oriented such that the curvature of the hollow semi-spherical shape is directed upwards. When orienting it such that the curvature is directed downwards, it is expected to be shaped in its substantially hollow semi- sphencal shape.

[0027] It is preferred that the sheet and / or at least one of the respective stacked layers may also be manufactured into its designed (i.e. curved, single curved or double curved) shape from, preferably a single strand of material (i.e. yam, fiber and / or longitudinal strip of material), by, for instance, a weaving process. This would allow for directly forming the sheet of flexible material in the shape corresponding to its installed state, preferably into a double curvature having, in particular a semi-spherical shape (i.e. dome shape). As the dome-shape is optimal from a force-transfer point of view, this dome shaped could be manufactured by such a process, wherein the reinforcing fibers of the sheet of flexible material are formed in the domeshape directly (for instance by means of a weaving process), whereafter a non-porous layer of a plastic and / or elastomeric material is heat-welded, and or melted around the dome-shaped reinforcement fibers.

[0028] The primary radius of curvature (i.e. in case of a double curvature: the smallest ratio) is preferably in the same order of size, more preferably in the ranges as indicated above, as this enables an effective transfer of the tensile forces in the internal flow reducing device, in particular in the sheet of flexible material and / or the grid-like reinforcement structure. It is noted that the radius of curvature may be a generalized (e.g. averaged) radius of curvature in case the curvature is not formed by an exact circular sector. The generalize (e.g. averaged) radius, may be defined as halve of a generalized (i.e. averaged) diameter, which is defined as a distance, when one would extrapolate the curvature until respective sections of the curve become substantially parallel to longitudinal axis, between the respective parallel sections of the curve.

[0029] It is preferred that, in a separated state wherein the sheet of flexible material is not arranged in the structural tubular pile, said sheet of flexible material is substantially planar and / or wherein the shape of said, preferably planar, sheet of flexible material is substantially defined by two curved edges that intersect each other at both outer ends of the curved edges. The shape of the, preferably substantially planar, sheet of flexible material thereby resembles the shape of a convex lens. The outer edges, or at least the longest outer edges, of the sheet of flexible material are substantially convex and the outer ends of the convex lines intersect. Note that the point of intersection may be rounded. Preferably, the two curved outer edges have a continuous curvature, that may be defined by a single radius of curvature.

[0030] In a preferred embodiment, outer edges of internal flow reducing device, in particular outer edges of the sheet of flexible material abut the inner surface of the circumferential wall, in particular wherein the internal flow reducing device comprises a radial biasing mechanism for urging the outer edges in an at least radial direction towards the inner surface of the circumferential wall. This enables to span substantially the full surface (of the cross-section) of the inner space of the structural tubular pile.

[0031] In a (further) preferred embodiment, the outer edges of the internal flow reducing device, in particular the outer edges of the sheet of flexible material, are formed as reinforced edges for transferring the forces of a column of water pushing to said internal flow reducing device and / or comprise sealing means for forming, at the locations where they abut each other, a sealed interface between said outer edges and the inner surface.

[0032] The reinforced edges may be obtained by creating a seam at the outer ends of the sheet of flexible material, which may comprise flexible reinforcement bars, beams and / or rods, cables, ropes and / or reinforces fibers for further reinforcement. The internal flow reducing device may also be arranged with a substantially ring-shaped body, wherein the sheet of flexible material is arranged. The ring-shaped body is then preferably flexible and / or retractable / foldable to allow it to move to the retracted position. This enables to connect the internal flow reducing device at a limited number of discrete connection points, as the resulting stress concentrations can be effectively taken up by the reinforced edges of the sheet of flexible material. A sealed interface prevents, or at least reduces, undesired bypassing water, as this may reduce the effectiveness of the internal flow reducing device.

[0033] It is preferred that the internal flow reducing device, in particular the outer edges of the sheet of flexible material, are connected to the circumferential wall by means of mechanical fastening elements, such as bolts, clamps, shear keys, hooks, retaining clips, and / or carabines. Mechanical fastening elements are readily available, easily installable and removable and are able to transmit large forces, such that they are suitable for connecting the internal flow reducing device to the structural tubular pile.

[0034] In a preferred embodiment, the circumferential wall comprises a one or more connection points for connecting the internal flow reducing device, such as through-holes, shear keys, one or more internal flanges, pad-eyes, half pad-eyes. This enables to securely connect the internal flow reducing device to the pile.

[0035] Alternatively, or additionally, the internal flow reducing device is connected by means of a normal force based connection between the inner circumferential surface of the structural pile and the internal flow reducing device and / or be means of a glued and / or welded connection. Although a glued or welded connection is not (easily) removable, it does provide for a secure coupling that is able to transfer large forces. A normal force based connection can be obtained by applying a normal force (pressing force) to press the circumferential wall of the structural pile. This may be formed as a friction based connection, wherein the outer edge, which may be fitted with high friction material, is pressed against the circumferential wall of the pile. This may be achieved by means of a suitable biasing mechanism, comprising for instance inflatable members, pre-stressed springs and / or active components, such as hydraulic or pneumatic cylinders. A fnction based connection is easily removable, by removing the applied normal force.

[0036] In a second aspect, the invention relates to a method of installing an structural tubular pile in a bottom underneath a body of water, such as a seabed, wherein the method comprises, in sequential order, the steps of: providing for a structural tubular pile according to any of the preceding claims; in any suitable order, lowering the structural tubular pile into a body of water and arranging the structural tubular pile in a vertical orientation, such that longitudinal axis is substantially parallel to the direction of gravity and the internal flow reducing device is submerged in the body of water; placing said structural tubular pile onto the bottom such that the tip of the structural tubular pile abuts the bottom; causing said tip to penetrate the bottom until a predefined depth is reached; preferably, removing the internal flow reducing device.

[0037] The method allows to install the structural tubular piles, such as monopiles and pin piles, at offshore sites having an increased risk of sudden pile drop, as the internal flow reducing device effectively slows downs the speed of the pile, such that pile loss, sudden impacts to the crane and / or installation vessel during piling are prevented, or at least reduced.

[0038] In a third aspect, the invention relates to an internal flow reducing device as applied in a structural tubular pile according to any of the embodiments of the first aspect of the invention.

[0039] The present invention is further illustrated by the following figures, which show preferred embodiments of the structural tubular pile and / or the internal flow reducing device according to the present disclosure, and are not intended to limit the scope of the invention in any way, wherein: - Figure 1 schematically shows, in a three-dimensional perspective view, a monopile foundation, comprising a dome-shaped internal flow reducing device, that is suspended from a crane of an installation vessel.

[0040] - Figure 2A schematically shows a cross-sectional view of the monopile foundation comprising the domeshaped internal flow reducing device;

[0041] - Figure 2B schematically shows, in a three-dimensional perspective view, the monopile foundation of figure 2A, wherein a more detailed view is given of the dome-shaped internal flow reducing device whereby the sheet of flexible material is not shown;

[0042] - Figure 2C schematically shows, in a three-dimensional perspective view, the dome-shaped internal flow reducing device in a separated state from the monopile and whereby the sheet of flexible material is not shown;

[0043] - Figure 3 schematically shows a cross-sectional view of a monopile foundation comprising a an alternative embodiment of the internal flow reducing device;

[0044] - Figure 4 schematically shows a perspective view of the internal flow reducing device of figure 3,

[0045] - Figures 5A & 5B schematically show a first and second connection embodiments for connecting the internal flow reducing device to the circumferential wall of the monopile foundation.

[0046] - Figure 6 schematically shows a third connection embodiment for connecting the internal flow reducing device to the circumferential wall of the monopile foundation.

[0047] - Figure 7 schematically shows a fourth connection embodiment for connecting the internal flow reducing device to the circumferential wall of the monopile foundation.

[0048] - Figure 8 schematically shows a fourth connection embodiment for connecting the internal flow reducing device to the circumferential wall of the monopile foundation.

[0049] - Figure 9 schematically shows a fifth connection embodiment for connecting the internal flow reducing device to the circumferential wall of the monopile foundation.

[0050] - Figure 10 schematically shows a sealing arrangement for creating a sealed interface between the internal flow reducing device and the circumferential wall of the monopile foundation.

[0051] - Figures 11A - 11C schematically show further embodiments of the internal flow reducing device.

[0052] - Figures 12A - 12C show simulation results predicting the effects of the internal flow reducing device on the steady-state drop speed of a structural tubular pile.

[0053] Figure 1 schematically shows, in a three-dimensional perspective view, a monopile foundation 1 (in a cross-sectional view that is taken along a plane substantially parallel to the longitudinal axis I), comprising a dome-shaped internal flow reducing device 100, that is suspended from a crane 3 of an installation vessel 2. The installation vessel 2 is a jack-up vessel comprising a hull 22 that is supported on a seabed by means of the jack-up legs 21. On top of the hull 22 a crane 3 is arranged, having a hook 31 that is suspended from a lifting cable 32 that is, in turn, suspended from the crane boom 33. The monopile foundation 1, which is a structural tubular pile, is suspended by the crane hook 31 by means of a suitable rigging arrangement 34. In the current situation, the monopile 1 lowered in a body of water, in particular the sea, and thereby penetrates the waterline 16. The monopile 1 is seen to comprise atop section 12, a bottom section 11 and a tapering section 13 that is arranged to taper from a bottom section diameter of the bottom section to a top section diameter of the top section 12. The monopile 1 is a tubular pile formed, mostly, by the circumferential wall 14 that surrounds the longitudinal axis I and an interior space 15. Athroughflow opening of the monopile is thereby defined by in inner diameter of the (i.e. diameter of the interior space 15) of the monopile 1.

[0054] Figures 2A- 2C shows the dome-shaped (i.e. double curved) internal flow reducing device 100 arranged inside of the monopile 1. At a lower end 110, which is substantially ring-shaped in a manner corresponding to the inner diameter of the monopile 1, the internal flow reducing device 100 is connected to the circumferential wall 14 at a plurality of connection points 111, as will be discussed in more detail in relation to figures 5A - 9. In the current embodiment, the ring-shaped lower end 110 is provided with a plurality of connecting loops 133 that are divided over the circumference of the ring-shaped lower end 110. These connecting loops 133 are then received in suitable connection points 111, such as pad-eyes having connecting holes therein. In case the connecting holes have an open circumference, the pay-eyes are typically referred to as half-pad-eyes.

[0055] The internal flow reducing device 100 spans substantially the full throughflow opening of the monopile 1 by means of a flexible material 101 comprising one or more throughflow openings therein. The flexible material 101 is arranged below, and thereby supported by a grid-like reinforcement structure 130 that comprises a plurality of reinforcement ribs 131 that extend in a curved arrangement from a central top section 122 of the internal flow reducing device 100 to the ring-shaped lower end 110. The reinforcement ribs 131 may be formed from a flexible beam, or rod, for instance made of a metal (e.g. steel) and / or composite material. Alternatively, the reinforcement ribs, which are designed to withstand primarily tensile forces, may also be formed from reinforced ropes, (metal; e.g. steel) cables and / or strands of fiber- reinforced material. In the current embodiment the reinforcement ribs 131, which are effectively reinforcement cables, interconnect the ring-shaped lower end 110 with a ring-shaped upper end 132 (as best seen in figure 2C, where the sheet of flexible fabric is not shown for). A central through opening 123 is thus obtainable by not covering the internal part of the ring-shaped upper end 132. The size of the central through opening 123 can be thus be tuned by applying a ring-shaped upper end 132 having a suitable inner diameter.

[0056] In order to improve the response time of the internal flow reducing device 100 in case of a sudden drop, the installed state of the internal flow reducing device 100 is substantially similar to the deployed state of the internal flow reducing device 100 (i.e. a state wherein a water column pressure is acting in an upward direction onto the underside of the internal flow reducing device 100, in particular on the underside of the flexible material 101. The current embodiment is proved with secondary connecting cables 140 that are (semi-spherically) arranged between the flexible material 101 and / or the grid-like reinforcement structure 130, in particular at the central top section 122 by connecting them to the ring-shaped upper end 132., on the one end, and the monopile 1, in particular the circumferential wall 14 thereof, on the other end. The wall 14 may be provided with suitable connecting points 144 for this purpose. The secondary connecting cables 140 thereby retain the internal flow reducing device 100 in its installed state. It is noted that, alternatively, or additionally, the grid-like reinforcement structure 130, in particular the reinforcement ribs

[0057] 131, may be sufficiently stiff as to retain the dome shape 120 in its installed state, and / or buoyant elements (i.e. floatation devices, such as air inflated bags) may be coupled to the flexible material 101 and / or the grid-like reinforcement structure 130, in particular at the central top section 122, to create an upward buoyancy force once the internal flow reducing device 100 is in a (water) submerged state.

[0058] Figure 3 shows alternative embodiment of an internal flow reducing device 200 that spans substantially the fall throughflow opening of the monopile 1 by means of a flexible material 201 comprising one or more throughflow openings therein. The internal flow reducing device 200 comprises only a single curvature (i.e. having the shape of barrel roof), such that the sheet of flexible material 201 as such, i.e. in a non-installed / non-deployed state, may be a simple planar sheet of flexible material 201.

[0059] Figure 4 shows the internal flow reducing device 200 having the barrel roof shape 220 of figure 3 in more details. The internal flow reducing device 200 is shaped to have only a single curvature. Hence, in a cross-sectional view taken in the plane substantially perpendicular to the secondary axis II, the internal flow reducing device 200 is curved, whereas it is not curved in a cross-sectional view taken in the plane substantially perpendicular to the tertiary axis III that is substantially perpendicular to the with respect to the longitudinal axis I and to secondary axis II.

[0060] The internal flow reducing device 200 comprises a grid-like reinforcement structure 230 comprising a plurality of first ribs 231 that are arranged in a longitudinal direction along the sheet of flexible material 201, such that they follow the single curvature, and a plurality of second ribs 232 that are arranged substantially perpendicular to the first ribs 231 and are, at least in a state at rest, substantially straight. The ribs 231, 232 thereby form a grid that the having substantially rectangular and / or square compartments. The individual ribs 231, 232 may be formed in ways substantially similar to the ribs 131. The sheet of flexible material 201 is again arranged at an underside of the respective grid-like reinforcement structure 230. The edge 210 is arranged to be connected to the monopile in ways substantially similar to lower end 110. The edge 210 may therefore also comprise a reinforcement member.

[0061] Figures 5A- 9 shows various embodiments for connecting the internal flow reducing device 100, 200 to the monopile 1, in particular to the circumferential wall 14 thereof. In figures 5A and 5B different embodiments are shown wherein the a number of pad-eyes 141 (which may also comprise half pad-eyes) are arranged on in inside of the circumferential wall 14 (for instance by means of welding). In the first pad-eye based connection 310, an sideways extending pin 311 is arranged in the pad-eye 141. A looped end 301 of, for instance, a coupling belt and / or cable is arranged around tbe extending part(s) of the sideways extending pin 311. In order to retain the pin 311 and / or looped end 301, respective end flange(s) 312 may be arranged at the outer ends of the sideways extending pin 311. At one end, or both ends, the end flange(s) 312 may be removable, for instance by means of a bolted connection. Figure 5B shows a second pad-eye based connection 320, wherein the pad-eyes 141 and looped end 301 are coupled by means of a shackle 321, such as a bow-, threaded- and / or D-shackle.

[0062] Figure 6 shows a hook-type connection 330, wherein a (plurality of) hook-ended member(s) 142 comprising a hooked end 143 is / are connected (e g. welded) to the inside of the circumferential wall 14. The respective looped end(s) 301 may then be arranged inside of the hooked end 143, thereby connecting the internal flow reducing device 100, 200 to the monopile 1.

[0063] Figure 7 shows a round-wire connection 340, wherein the internal flow reducing device 100 (or 200), is connected to a plurality of pad-eyes, hook-ended members, and / or similar connecting members, by means a, preferably single, round-wire 341 that is alternately strung through the internal flow reducing device 100, in particular through connection points arranged at the radial edge 110 thereof, and one of the respective plurality of pad-eyes, hook-ended members, and / or similar connecting members. Such a connection has the benefit of being more easily decouplable, also remotely, by for instance pulling the round-wire 341 out of the respective plurality of pad-eyes, hook-ended members, and / or similar connecting members, and / or by means of cuting the wire.

[0064] Figure 8 shows an internal ring connection 350, wherein an internal ring-shaped member 351 is connected (e.g. welded) in substantially spaced-apart manner to the inner side of the circumferential wall 14 at a number of discrete internal ring-shaped member connecting points 352. The looped ends 301 may than be arranged around the internal ring-shaped member 351, in particular to the ring-sector sections that span the distance between adjacent discrete internal ring-shaped member connecting points 352.

[0065] Figure 9 shows an further connection 360, wherein the circumferential wall 14 is arranged with a number of through holes 143, wherein, for instance, a threaded pad-eye 361 comprising a pad-eye at one end and a threaded rod 362 at the other end, can be arranged. The threaded rod 362 is arranged through the through hole 143 and mounted with a suitable nut 363. The internal flow reducing device 100 (or 200) can be directly mounted to the threaded pad-eye 361, and / or by using the looped ends 301.

[0066] To have an improved control on the overall permeability of the internal flow reducing device 100 (or 200), an edge sealing arrangement 370, as is shown in figures 10A & 10B) may be arranged, comprising sealing member 371, preferably made of a rubber and / or elastomeric material, that seal the interface between the inner side of the circumferential wall 14 and the internal flow reducing device 100 (or 200). A reliable sealing of the interface ensures that any bypassing water, during a sudden drop event, are minimized, or at least reduced, thereby ensuring that the internal flow reducing device 100 (or 200) functions as designed for.

[0067] Figures 11A - 11C show, in cross-sectional views, various further embodiments of the internal flow reducing devices 1100, 1200, 1300. These may be arranged with sheets of flexible material having a single, or double curvature. The main difference between the internal flow reducing devices 1100, 1200, 1300 is the flexible material used. Internal flow reducing devices 1100 features a permeable fabric 1101, such as a geotextile, geofabric or chain mail fabric. Such permeable fabrics are often characterized by having a large number of pores 1102 therein. Internal flow reducing devices 1200 features a sheet of flexible material 1201 that is made permeable by adding one or more, preferably centrally arranged, through hole(s) 1202 therein. As such, the sheet 1201 may comprise a non-permeable fabric. Internal flow reducing devices 1300 features a sheet of flexible material 1301 that is made permeable by adding one or more periphencal arranged, through hole(s) 1302 therein. As such, the sheet 1301 may then also comprise anon-permeable fabric. Further embodiments can be envisioned by various combinations of materials and additional through holes, in other words the embodiments of internal flow reducing devices 1100, 1200, 1300 may be combined for defining further embodiments.

[0068] Figures 12A - 12C show simulation results predicting the effects of the internal flow reducing device on the steady-state drop speed of a structural tubular pile. The simulations are performed using a model that is based a sudden flow-restriction in a pipe, which mimics the flow obstruction that the internal flow limiting device causes in the structural tubular pile. On the basis of the Bernoulli equation, while taking into account the effect of the Vena Contracta and a steady-state drop speed of the pile (wherein the inertial forces are thus neglected), an estimate of the force on the parachute can be estimated to be: wherein Ai is the surface area of the inner space of the structural tubular pile, p is the density of water, C2 is the Vena Contract coefficient, A2is the surface area of the throughflow opening through the internal flow reducing device, and Vi is the flow speed (i.e. the steady state drop speed of the pile).

[0069] A number of simulations for different sized pin- and monopiles have been performed, of which an overview is given below. Figure 12A shows the resulting drag force (in metric tons) for a steady-state drop speed of 3 m / min, which is typically considered a minimum (practical) speed for lowering a pile (although lower steadystate drop speeds are also possible), figure 12B for a steady-state drop speed of 5 m / min, which is an average speed, and figure 12C for a steady-state drop speed of 10 m / min, which is already a relatively high drop-speed (although higher pile drop-speeds are also possible). The x-axis of all graphs indicates the ratio between the equivalent throughflow diameter de(= D2) and the inner diameter di (=D 1) of the structural tubular pile, as determined from de / di. The points indicate the various load cases from the list given above, indicating the range is typically from 3% to 11%. Nonetheless, a broader range (on both ends) is also feasible, but may result in drop-speeds above 10 m / min, or below 3 m / min, which, depending on the circumstances, typically does not pose a problem.

[0070] The hereabove presented embodiments in terms of shaping of the internal reducing device, in particular of the grid-shaped reinforcement structure and / or the sheet of flexible material, the various embodiments for connecting the device to the monopile, and so on, can be combined for forming further embodiments. Additionally, the present invention is not limited to the embodiments shown, but also extends to other embodiments falling within the scope of the appended claims.

Claims

Claims1. Structural tubular pile having an internal flow reducing device for reducing a flow of water through an inner space of said pile, wherein said internal flow reducing device is arranged inside of the structural tubular pile and is connected to the structural tubular pile, in particular to a circumferential wall of the structural tubular pile; wherein said internal flow reducing device comprises a sheet of flexible material, having one or more throughflow openings therein, that is arranged to cooperate with the circumferential wall in such a way that said internal flow reducing device spans the largest portion of, in particular substantially the full, surface area of the inner space of said structural tubular pile, as determined in a plane substantially perpendicular to a longitudinal axis of the structural tubular pile; wherein said internal flow reducing device is arranged such that, at least in a deployed state wherein the sheet of flexible matenal is exposed to upwards pressure from a column of water that is pushed through the inner space in a longitudinal direction from a tip to a top of said structural tubular pile, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile.

2. Structural tubular pile according to claim 1, wherein said internal flow reducing device comprises one, preferably only one, substantially centrally arranged through hole that is arranged in the sheet of flexible material, wherein said through hole is preferably substantially oval or circular.

3. Structural tubular pile according to claim 1 or 2, wherein, when the total surface area of all throughflow openings of the internal flow reducing device is approximated by an equivalent circular through flow hole having an equivalent throughflow diameter de, the ratio between the equivalent throughflow diameter and the inner diameter di of the structural tubular pile, as determined from de / di, is in the range of 0.01 - 0.2, preferably 0.02 - 0.15 times, more preferably 0.03 - 0.11, most preferably 0.05 - 0.08.

4. Structural tubular pile according to any of the preceding claims, wherein the sheet of flexible material curves, when in the deployed and / or installed state, outwardly towards the top of the structural tubular pile, as seen in a first plane, wherein the first plane runs substantially parallel to the longitudinal axis, in particularly wherein the sheet of flexible material curves in the first plane only, such that the sheet of flexible material has a single curvature only; or wherein the sheet of flexible material curves, when in the deployed and / or installed state, outwardly towards the top of the structural tubular pile, as seen in the first and a second plane, such that the sheet of flexible material has a double curvature, wherein said first and second planes run substantially parallel to the longitudinal axis and are substantially orthogonal with respect to each other,and, preferably, wherein a radii of curvature of the first plane and second plane are substantially equal for forming the sheet of flexible material into a dome-shape.

5. Structural tubular pile according to any of the preceding claims, wherein said sheet of flexible material is preformed to have a, in a resting or uninstalled state, a substantially hollow semi-spherical shape.

6. Structural tubular pile according to any of the preceding claims, wherein said internal flow reducing device comprises a grid-like reinforcement structure and wherein said sheet of flexible material is connected to the grid-like reinforcement structure at a plurality of different points; wherein said sheet of flexible material is coupled to the structural tubular pile by means of the grid-like reinforcement structure; preferably, wherein the grid-like reinforcement structure is a flexible grid-like reinforcement structure, preferably comprising flexible reinforcement means, such as cables, chains, flexible rods and / or beams, reinforced ropes and / or inflatable elements.

7. Structural tubular pile according to any of the preceding claims, wherein the internal flow reducing device is removably connected to the structural tubular pile.

8. Structural tubular pile according to claim 7, wherein the internal flow reducing device comprises a release mechanism that is operable from outside of the structural tubular pile for releasing the removable connection between the internal flow reducing device and the structural tubular pile.

9. Structural tubular pile according to claim 7 or 8, wherein the internal flow reducing device is arranged to, after being disconnected from the structural tubular pile, be movable into a retracted state wherein at least one dimension of the internal flow reducing devices is smaller than in the deployed and / or installed state; preferably wherein, in the retracted state, a width of the internal flow reducing device is smaller than 0.9, more preferably smaller than 0.75, times the width of the internal flow reducing device in the deployed state.

10. Structural tubular pile according to any of the preceding claims, wherein the flexible material comprises a geosynthetic fabric, such as a geotextile, a geognd or a geocomposite, preferably comprising geotextile and / or geogrid, wherein said geosynthetic fabric comprises polymer materials; and / or wherein the flexible material compnses a chainmail fabric consisting of metal, plastic and / or rubber nngs that are linked together to form a mesh pattern.

11. Structural tubular pile according to any of the preceding claims, wherein the flexible material comprises a non-permeable material, such as a reinforced sheet of plastic material, wherein one or more throughflow openings are arranged.

12. Structural tubular pile according to any of the preceding claims, wherein said internal flow reducing device is arranged such that, at least in an installed and unsubmerged state wherein the internal flow reducing device is connected to the structural tubular pile, the sheet of flexible material curves outwardly in the direction of the top of the structural tubular pile.

13. Structural tubular pile according to any of the preceding claims, wherein outer edges of internal flow reducing device, in particular outer edges of the sheet of flexible material, abut the inner surface of the circumferential wall, in particular wherein the internal flow reducing device comprises a radial biasing mechanism for urging the outer edges in an at least radial direction towards the inner surface of the circumferential wall.

14. Structural tubular pile according to any of the preceding claims, in particular according to claim 13, wherein the outer edges of the internal flow reducing device, in particular the outer edges of the sheet of flexible material, are formed as reinforced edges for transferring the forces of a column of water pushing to said internal flow reducing device and / or comprise sealing means for forming, at the locations where they abut each other, a sealed interface between said outer edges and the inner surface.

15. Structural tubular pile according to any of the preceding claims, wherein the internal flow reducing device, in particular the outer edges of the sheet of flexible material, are connected to the circumferential wall by means of mechanical fastening elements, such as bolts, clamps, splines, hooks, retaining clips, shackles and / or carabines.

16. Structural tubular pile according to any of the preceding claims, wherein the circumferential wall comprises one or more connection points for connecting the internal flow reducing device, such as through-holes, shear keys, one or more internal flanges, pad-eyes, half pad-eyes.

17. Structural tubular pile according to any of the preceding claims, wherein the internal flow reducing device is connected by means of a normal force based connection, such as a friction based connection, between the inner circumferential surface of the structural pile and .the internal flow reducing device and / or be means of a glued and / or welded connection18. Method of installing an structural tubular pile in a bottom underneath a body of water, such as a seabed, wherein the method comprises, in sequential order, the steps of: providing a structural tubular pile according to any of the preceding claims;in any suitable order, lowering the structural tubular pile into a body of water and arranging the structural tubular pile in a vertical orientation, such that longitudinal axis is substantially parallel to the direction of gravity and the internal flow reducing device is submerged in the body of water; placing said structural tubular pile onto the bottom such that the tip of the structural tubular pile abuts the bottom; causing said tip to penetrate the bottom until a predefined depth is reached; preferably, removing the internal flow reducing device.

19. Device for use as an internal flow reducing device as applied in a structural tubular pile according to any of the preceding claims 1 - 17, wherein the structural tubular pile comprises a circumferential wall delimiting an inner space, the device comprising: a sheet of flexible material, having one or more throughflow openings therein; wherein said device comprises a grid-like reinforcement structure and wherein said sheet of flexible material is connected to the grid-like reinforcement structure at a plurality of different points; wherein said sheet of flexible material is arranged to be coupled to the structural tubular pile by means of the grid-like reinforcement structure; wherein said device is arranged to be connected to the circumferential wall by means of mechanical fastening elements, such as bolts, clamps, splines, hooks, retaining clips, shackles and / or carabines; wherein the device is arranged such that, when in use, the sheet of flexible material is arranged to cooperate with the circumferential wall in such a way that the device spans the largest portion of, in particular substantially the full, surface area of the inner space, as determined in a plane substantially perpendicular to a longitudinal axis of the structural tubular pile; wherein said device is arranged such that at least when the sheet of flexible material is exposed to upwards pressure, the sheet of flexible material curves outwardly in the upwards direction.

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