Short footprint mooring buoy and method for installing such a buoy

Abstract

A mooring buoy configured for mooring a floating body on water, including a buoyancy having a first surface as top facing upwards from the water, a second surface as bottom facing downwards and an intermediate tubular shape extending along a body axis between the first and second surfaces; the buoyancy being coupled to an anchoring device below the water by a pair of anchoring chains, and provided with two parallel linear through-holes, the through-holes extending through the buoyancy from bottom to top; the top of the buoyancy provided with chain ratchets for connecting each of the anchoring chains to one of the chain ratchets through a respective through-hole; the through-holes being spaced apart in the buoyancy and defining a chain guiding plane between the through-holes extending from the bottom to the top, the buoyancy having a centre of buoyancy with a location thereof displaced out of the chain guiding plane.

Description

[0001] Short footprint mooring buoy and method for installing such a buoy

[0002] Field of the invention

[0003] The invention relates to a mooring buoy, a method for installation of such a mooring buoy and a method for manufacturing such a mooring buoy.

[0004] Background art

[0005] For mooring vessels in shallow and congested waters by mooring buoys, such mooring buoys have a relatively short mooring line or chain. During movement of the buoy in shallow water, a twist of the mooring line can occur which may lead to damage and failure by breaking.

[0006] It is an object of the invention to overcome or mitigate the disadvantage from the prior art.

[0007] Summary of the invention

[0008] The invention of the Short Footprint Buoy Mooring (SFBM) relates to generally a floating storage unit vessel such as a FSRLI project for use of mooring buoys in shallow & congested waters.

[0009] The SFBM buoy system is designed as a permanent mooring system taking into account today’s certification requirement and also how to install it in cost effective way.

[0010] In the design process it was recognized to consider, a design case, one chain broken below any of the SFBM buoys which would reduce the mooring strength of the SFB buoys system. To avoid doubling the number of SFBM buoys, the doubling of the chain below the SFBM buoys was required. Also, in such a case, the transient analysis of the buoy behaviour just after a chain failure below an SFBM buoy can lead to large twist angles that could break the second chain. To avoid such a damaging twist in the second chain, the invention comprises applicability of an eccentricity of the Center of Buoyancy of the SFBM buoy compared to the axis of the mooring load path, so that the mooring buoy’s behaviour is stabilized by the buoyancy, keeping the twist well within the capacity of the remaining second chain.

[0011] Also, to further help in distributing the twist over more chain links, the bottom chain goes through the SFBM buoy and is connected via a rachet at the top of the SFBM buoy.

[0012] In accordance with the above, in an embodiment the invention provides a mooring buoy configured for mooring a floating body on a body of water, comprising a buoyancy volume having a first surface for during use as top surface facing upwards from the water, a second surface for during use as bottom surface facing downwards and an intermediate tubular shape extending along a body axis between the first and second surfaces; the buoyancy volume configured to be coupled to an anchoring device [pile] on a floor of the body of water by a pair of anchoring chains to be mounted on the anchoring device, the buoyancy volume being provided with a pair of parallel and linear through-holes, one for each anchoring chain, the through-holes extending through the buoyancy volume from bottom surface to top surface; the top surface of the buoyancy volume being provided with a pair of chain ratchets for connecting each of the anchoring chains to a respective one of the chain ratchets through the respective through-hole for the corresponding anchoring chain; the through-holes being spaced apart in the buoyancy volume and defining a chain guiding plane between the through-holes extending from the bottom surface to the top surface, the buoyancy volume having a centre of buoyancy with a location thereof displaced out of the chain guiding plane.

[0013] Also the invention relates to an installation methodology and the arrangement that allows it. The SFBM buoys are designed such that they are stable, before the pretension in the bottom chains, with the top to the buoy having chain rachets above water line which allows to use removable chain jacks - sized to the bottom chain pretension - that can be mounted when the SFBM buoy is horizontal and recover when the bottom chains are at target pretension (with the SFBM buoy almost vertical in nominal position).

[0014] In accordance with this methodology, the invention provides method for installing a mooring buoy as described above, in which the method comprises: allowing the buoy to float in the body of water in a horizontal position with the body axis substantially parallel to the water surface, and having a section of the intermediate tubular shape comprising the centre of buoyancy facing upwards from the water surface; mounting on the first surface above the water surface a pair of chain jacks, one at each through-hole.

[0015] According to a further aspect, the method comprises: preceding allowing the buoy to float, providing at least one anchoring device in the bed of the body of water, connecting on the at least one anchoring device a pair of anchoring chains.

[0016] According to yet a further aspect, the method comprises: providing on a free end of each of the pair of anchoring chains a messenger line, and picking up and pulling each messenger line through a respective one of the through- holes in the buoyancy volume, such that the free end of the anchoring chains extends through the respective through-hole; using each chain jack to pre-tension the anchoring chain extending through the respective through-hole at its free end.

[0017] The invention relates to design features that are required for a SFBM system designed as permanent mooring system. Indeed, if the SFBM system can be designed with a single chain below the SFBM buoys, then the track records of SALM (Single Angle Leg Mooring) design and installation method will apply.

[0018] Further, SFBM could be designed with any hawser / rope material (nylon, polyester, dyneema, or even steel ropes, and others), depending on project specifics.

[0019] The invention also relates to an alternative method for installing a mooring buoy as described above on an anchoring device in a body of water as shown in the figure here below.

[0020] The method comprises: allowing the buoy to float in the body of water in a horizontal position with the body axis substantially parallel to the water surface; subsequently connecting a clump weight to the second surface and allowing the clump weight to bring the buoy into an upright position, with the second surface directed downwards to the seabed; providing at least one anchoring device in the bed of the body of water, connecting on the at least one anchoring device a pair of anchoring chains; pulling the free end of each anchoring chain through a corresponding one of the through holes with the free end of each anchoring chain reaching to the top surface; and after each free end of the anchoring chains having reached the top surface, securing each free end to the top surface by engaging on the anchoring chain a chain connecting device located at the respective through-hole on the top surface.

[0021] In an embodiment the clump weight is connected to the buoy by means of one or more soft slings or lines.

[0022] After securing each free end of the anchoring chains, the connection of the clump weight to the mooring buoy is released.

[0023] After release from the mooring buoy, the clump weight is pulled up from the water. In the method an installation vessel is provided to carry out services relating to the method. In summary, - for SFBM systems requiring double chains below a SFBM buoy - the following aspects of the invention are considered:

[0024] Eccentricity of the Center of Buoyancy from mooring load path axis is such that the design twist angle in the remaining bottom chain after failure of the adjacent chain (in any transient analysis) is within the allowable interlink twist as defined / guaranteed by the chain manufacturer and accepted by a Certifying Authority.

[0025] Connection of the bottom chains at the top of the SFBM buoy, the chain is only guided through the vertical of the SFBM buoy, this increases the number of chain links on which the overall chain twist is distributed into incremental interlink twist. to avoid too large interlink angle variations at SFBM buoy bottom - the chain is guided with a trumpet shape in accordance with (predetermined) allowable interlink angle as defined / guaranteed by the chain manufacturer and accepted by the Certifying Authority.

[0026] The SFBM buoy is designed such that its top is stable with bottom chain rachets above water line during installation so that typical (e.g. removable rental type) chain jack equipment can be mounted and operated during the various steps to move from horizontal to vertical SFBM buoy position by activating the two chain jacks up to the target bottom chain pretension (which will lead to the nominal vertical position of the SFBM buoy). Once the SFBM buoy is installed, the chain jacks can be removed and used for a next SFBM buoy installation.

[0027] Diverless installation: when the bottom chains can be connected to the pile / foundation before its installation, the bottom chains can be pulled through the SFBM buoy using messenger lines until the installation chains (which is the extra length of chain corresponding to the number of links that the chain jacks will pull) are engaged into the chain jacks.

[0028] The installation chain lengths can be either cut away once installation complete or recovered if using a detachable link. The latter may enable a further saving, to be confirmed during detailed project procedure.

[0029] - Alternatively, Installation of the SFBM buoy with the assistance of a clump weight. The clump weight enables to sink the buoy such that the chain can be pulled through the through holes by only lifting the chain weight (as the buoy buoyancy is neutralized by the clump weight). To do that it is only needed to add a substantially round bar at the lower section of the steel structure part, i.e., the frame of the buoy. Soft slings can be used to connect the clump to the buoy which allow easy release without intervention by a diver when the clump is recovered after having transferred the buoyancy load to the permanent chains sitting on their respective rachet

[0030] The SFBM design in accordance with the invention has the following advantages:

[0031] Lower EPCI CAPEX by reducing the number of SFBM buoys while satisfying Certification requirements,

[0032] Functional and safe to operate,

[0033] Diverless installation,

[0034] - Ability to permanently moor a vessel in shallow & congested waters thanks to the short footprint of the proposed mooring arrangement.

[0035] Brief description of the drawings

[0036] The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are not necessarily on scale and are intended exclusively for illustrative purposes, not as a restriction of the inventive concept. The scope of the invention is only limited by the definitions presented in the appended claims.

[0037] Figures 1A - 1C schematically show a first and second perspective view and a cross- sectional view respectively of a mooring buoy in accordance with an embodiment of the invention;

[0038] Figures 2A, 2B schematically show isometric views of the mooring buoy in a first floating position;

[0039] Figures 3A - 3C schematically show a first and second perspective view and a cross- sectional view respectively of a mooring buoy in accordance with an embodiment of the invention;

[0040] Figures 4A, 4B schematically show isometric views of the mooring buoy in a second floating position;

[0041] Figures 5A - 5L schematically show a method for installing a mooring buoy in accordance with an embodiment. Description of embodiments

[0042] Figures 1A - 1C schematically show a first and second perspective view and a cross- sectional view respectively of a mooring buoy in accordance with an embodiment of the invention.

[0043] The mooring buoy 100 is depicted in an installed state in a body of water. On an anchoring device 10, e.g. a monopile, the mooring buoy is attached by a pair of anchoring chains 20. The chains 20 each run through a respective through-hole 30 in a bottom surface 105 and are attached to a top surface 110 of the mooring buoy. According to the invention, the mooring buoy is arranged to have a center of buoyancy that is eccentric relative to the mooring load path (along the chains). As explained above, the buoyancy force counteracts rotation of the buoy around the longitudinal axis and can thus reduce twisting of the one or more chains during use.

[0044] The center of buoyancy is determined by the shape of the buoyancy volume 120 comprising buoyancy bodies 125 that are arranged around the through-holes 30. Figures 2A, 2B schematically show isometric views of the mooring buoy in a first floating position. The figures 2A and 2B show an example of a construction of the mooring buoy 100. As shown in Figure 2B the mooring buoy 100 comprises a frame 130 in which a bottom portion 105a and a top portion 110a are connected by conduits 30 that function as through holes. On the top surface bottom chain rachets 150 are arranged that during use will be above water line.

[0045] Along the length of the conduits, a number of collars 132 are arranged with surfaces that extend perpendicular to the conduits. On the collar surfaces buoyancy bodies 125 are mounted in a manner (see Fig 2A) that the resulting center of buoyancy volume 120 is eccentric relative to the plane defined between the parallel through-holes 30. In a free buoyant position the eccentricity results in a tilted orientation of the mooring buoy 100.

[0046] In an embodiment, each buoyancy body 125 has an eccentric shape.

[0047] Figures 3A - 3C schematically show a first and second perspective view and a cross- sectional view respectively of a mooring buoy in accordance with an embodiment of the invention.

[0048] Figures 4A, 4B schematically show isometric views of the mooring buoy in a second floating position.

[0049] Figures 3A-3C and 4A, 4B show a mooring buoy 100 similar to that shown in Figures 1 A-1C and 2A, 2B, but in a second position without slack of the anchor chains 20 and where a buoyancy upward force is acting fully on the mooring buoy, resulting in an upright position of it. Figures 5A - 5L schematically show a method for installing a mooring buoy 100 in a body of water such as a sea, in accordance with an embodiment.

[0050] As shown in figure 5A, the method comprises: providing at least one anchoring device 10 in the bed of the body of water, connecting on the at least one anchoring device 10 a pair of anchoring chains 20, and allowing the buoy 100 to float in the body of water in a horizontal position with the body axis substantially parallel to the water surface, and having a section of the intermediate tubular shape comprising the centre of buoyancy facing upwards from the water surface; mounting on the first surface above the water surface a pair of chain jacks, one at each through-hole.

[0051] As shown in figures 5B-5D, the method further comprises: providing on a free end of each of the pair of anchoring chains 20 a messenger line 25 , and picking up and pulling each messenger line 25 through a respective one of the through-holes 30 in the buoyancy volume 120 of the mooring buoy 100;

[0052] As shown in figure 5E the method further comprises: connecting a clump weight 210 to the second surface (bottom surface) 105 and allowing the clump weight to bring the mooring buoy 100 into an upright position, with the second surface directed downwards to the seabed.

[0053] The method further comprises, as shown in figures 5F-5I, by means of the respective messenger line 25 pulling the free end of each anchoring chain 20 through a corresponding one of the through holes 30 with the free end of each anchoring chain reaching to the top surface 110 of the mooring buoy 100; and after each free end of the anchoring chains having reached the top surface, securing each free end to the top surface 110 by engaging on the anchoring chain 20 a chain connecting device 150 e.g. a chain ratchet, located at the respective through- hole 30 on the top surface, while using a chain jack to pre-tension the anchoring chain 20 extending through the respective through-hole 30 at its free end, such that the load is transferred from the clump weight 210 to the anchoring chain 20.

[0054] Figures 5J - 5L finally show that after securing each free end of the anchoring chains 20 on the top surface of the morning buoy 100, the connection of the clump weight 210 to the mooring buoy 100 is released, and after release from the mooring buoy, the clump weight 210 is pulled up from the water by the installation vessel 200.

[0055] The method is carried out by an installation vessel 200 and optionally for some steps relating to pulling the messenger lines 25 and anchoring chains 20 by an auxiliary vessel 220. In an embodiment the mooring buoy 100 as described above is attached to an auxiliary chain (not shown) which is either separately anchored to the seabed or also anchored to the anchoring device 30. Should one of the anchoring chains 20 fail, then the auxiliary chain reduces the freedom of the mooring buoy to let the remaining anchoring chain rotate and twist. Such auxiliary chain may be a chain with relatively low strength requirements as it only requires to reduce / avoid twist of the remaining anchoring chain.

[0056] In another embodiment, the mooring buoy is additionally attached to the anchoring device 30 by means of a third anchoring chain (not shown) similar to any one of the pair of anchoring chains 20.

[0057] In yet another embodiment, the anchoring chains 20 are oversized with respect to the nominal required strength to increase the resistance to torque induced by the twist of the remaining anchoring chain after one of the anchoring chains has broken. For example the anchoring chains may be designed to at least a 10% oversize. Alternatively the invention may be reduced to practice by one or more of the following embodiments.

[0058] In an embodiment the invention relates to a mooring buoy 100 which is arranged with a pair of anchoring chains 20 that are provided with a chain swivel (not shown).

[0059] In an embodiment, the mooring buoy 100 is a spring buoy, wherein for each anchoring chain, a mooring hawser (not shown) is connected to the anchoring chain.

[0060] The invention has been described with reference to the preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims

Claims1 . A mooring buoy configured for mooring a floating body on a body of water, comprising a buoyancy volume having a first surface for during use as top surface facing upwards from the water, a second surface for during use as bottom surface facing downwards and an intermediate tubular shape extending along a body axis between the first and second surfaces; the buoyancy volume configured to be coupled to an anchoring device [pile] on a floor of the body of water by a pair of anchoring chains to be mounted on the anchoring device, the buoyancy volume being provided with a pair of parallel and linear through-holes, one for each anchoring chain, the through-holes extending through the buoyancy volume from bottom surface to top surface; the top surface of the buoyancy volume being provided with a pair of chain ratchets for connecting each of the anchoring chains to a respective one of the chain ratchets through the respective through-hole for the corresponding anchoring chain; the through-holes being spaced apart in the buoyancy volume and defining a chain guiding plane between the through-holes extending from the bottom surface to the top surface, the buoyancy volume having a centre of buoyancy with a location thereof displaced out of the chain guiding plane.

2. A mooring buoy configured for mooring a floating body on a body of water, comprising a buoyancy volume having a first surface for during use as top surface facing upwards and a second surface for during use as bottom surface facing downwards and an intermediate tubular shape extending along a body axis between the first and second surfaces, the buoyancy volume being coupled to an anchoring device [pile] on a floor of the body of water by a pair of anchoring chains mounted on the anchoring device, the buoyancy volume being provided with a pair of parallel and linear through-holes, one for each anchoring chain, the through-holes extending through the buoyancy volume from bottom surface to top surface, with each of the anchoring chains running to a respective one of the through-holes; the top surface of the buoyancy volume being provided with a pair of chain ratchets each connecting one of the anchoring chains; the through-holes being spaced apart in the buoyancy volume and defining a chain guiding plane extending between the through-holes from the bottom surface to the top surface,the buoyancy volume having a centre of buoyancy with a location thereof displaced out of the chain guiding plane.

3. The mooring buoy according to claim 1 or claim 2, wherein the location of the centre of buoyancy is displaced perpendicular to the chain guiding plane.

4. The mooring buoy according to claim 1 or 2 or 3, wherein in projection on the chain guiding plane the location of the centre of buoyancy is located on a centre line between the through-holes.

5. The mooring buoy according to any of claims 1 - 4, wherein the buoyancy volume is asymmetric relative to the chain guiding plane.

6. The mooring buoy according to any one of the preceding claims, wherein the through-holes at the bottom surface of the buoyancy volume each comprise a funnel opening for receiving the respective one to the anchoring chains.

7. The mooring buoy according to any one of the preceding claims 1 - 6, wherein the top surface of the mooring buoy is configured with at least one connector for either a hawser or a mooring line from a floating structure.

8. The mooring buoy according to any one of the preceding claims, wherein the at least one connector for either the hawser or the mooring line comprises a unijoint.

9. The mooring buoy according to any one of the preceding claims, wherein the buoyancy volume comprises at least a body made from a synthetic materialbased foam, for example a polyurethane foam.

10. The mooring buoy according to claim 2, wherein the anchoring chain is connected to the anchoring device by means of a uni-joint connector.

11. The mooring buoy according to any one of the preceding claims, wherein the top surface is configured to have a chain jack mounted at each of the through-holes, each chain jack configured to pretension the anchoring chain running through the respective through-hole.

12. A method for installing a mooring buoy on a body of water, the buoy comprising a buoyancy volume having a first surface for during use as top surface facing upwards and a second surface for during use as a bottom surface facing downwards and an intermediate tubular shape extending along a body axis between the first and second sides, the buoyancy volume configured for coupling to an anchoring device or pile on a floor of the body of water by a pair of anchoring chains mounted on the anchoring device, the buoyancy volume being provided with a pair of parallel and linear through- holes, one for each anchoring chain, the through-holes extending through the buoyancy volume from bottom surface to top surface; the through-holes being spaced apart in the buoyancy volume and defining a chain guiding plane extending between the through-holes from the bottom surface to the top surface, the buoyancy volume having a centre of buoyancy with a location thereof displaced from the chain guiding plane, the method comprising: allowing the buoy to float in the body of water in a horizontal position with the body axis substantially parallel to the water surface, and having a section of the intermediate tubular shape comprising the centre of buoyancy facing upwards from the water surface; mounting on the first surface above the water surface a pair of chain jacks, one at each through-hole.

13. The method according to claim 12, comprising: preceding allowing the buoy to float, providing at least one anchoring device in the bed of the body of water, connecting on the at least one anchoring device a pair of anchoring chains.

14. The method according to claim 13, comprising: providing on a free end of each of the pair of anchoring chains a messenger line, and picking up and pulling each messenger line through a respective one of the through-holes in the buoyancy volume, such that the free end of the anchoring chains extends through the respective through-hole; using each chain jack to pre-tension the anchoring chain extending through the respective through-hole at its free end.

15. The method according to claim 14, comprising: tilting the buoyancy volume from the horizontal position towards a vertical position by means of the pre-tensioning of the anchoring chains.

16. The method according to claim 15, comprising: after reaching the vertical position, securing each pre-tensioned anchoring chain by engaging on the anchoring chain a chain ratchet at the respective through-hole on the top surface.

17. The method according to any one of claims 1 - 16, further comprising: providing an installation vessel; the installation vessel providing services for carrying out the method.

18. A method for installing a mooring buoy on a body of water, the buoy comprising a buoyancy volume having a first surface for during use as top surface facing upwards and a second surface for during use as a bottom surface facing downwards and an intermediate tubular shape extending along a body axis between the first and second sides, the buoyancy volume configured for coupling to an anchoring device [pile] on a floor of the body of water by a pair of anchoring chains mounted on the anchoring device, the buoyancy volume being provided with a pair of parallel and linear through- holes, one for each anchoring chain, the through-holes extending through the buoyancy volume from bottom surface to top surface; the through-holes being spaced apart in the buoyancy volume and defining a chain guiding plane extending between the through-holes from the bottom surface to the top surface, the buoyancy volume having a centre of buoyancy with a location thereof displaced from the chain guiding plane, the method comprising: allowing the buoy to float in the body of water in a horizontal position with the body axis substantially parallel to the water surface; subsequently connecting a clump weight to the second surface and allowing the clump weight to bring the buoy into an upright position, with the second surface directed downwards to the seabed; providing at least one anchoring device in the bed of the body of water, connecting on the at least one anchoring device a pair of anchoring chains;pulling the free end of each anchoring chains through a corresponding one of the through holes with the free end of each anchoring chain reaching to the top surface; and after each free end of the anchoring chains having reached the top surface, securing each free end to the top surface by engaging on the anchoring chain a chain connecting device located at the respective through-hole on the top surface.

19. The method according to claim 18, wherein after securing each free end of the anchoring chains, releasing the connection of the clump weight from the second surface of the buoy.

20. The method according to any one of claims 18 - 19, wherein the step of connecting the clump weight is carried out by means of one or more soft slings or lines.

21. The method according to claim 19, further comprising pulling up the clump weight from the water.

22. The method according to any one of claims 18 - 21 , further comprising: providing an installation vessel; the installation vessel providing services for carrying out the method.

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