Tether assembly

Abstract

The present invention relates to a tether assembly for use in securing a floating offshore platform to an offshore anchor pile. The tether assembly comprises: a tether comprising: a first end configured in use to be attached to a floating offshore platform; and a second opposed end configured in use to be attached to an anchor pile embedded within a borehole of a seabed. The tether assembly further comprises at least one radially extending member mounted on the tether located between the first end and second ends thereof. The radially extending member(s) is configured in use to engage with and to shed loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole.

Description

[0001] TETHER ASSEMBLY

[0002] The present invention relates to a tether assembly for use in connecting the mooring line of an offshore floating structure to an offshore foundation, for example anchor pile. The present invention also relates to a method for anchoring a floating offshore platform to an offshore foundation, for example anchor pile, in a borehole using a tether assembly.

[0003] BACKGROUND OF INVENTION

[0004] A variety of offshore pile foundation systems are known in the art, such as driven piles, drilled and grouted piles and suction piles. Floating Offshore Platforms are conventionally secured in position by a mooring line attached to an anchor pile within a bore hole.

[0005] After installation, the mooring line initially extends substantially parallel to the axis of the borehole and extends across the surface of the geological formation. Over time, the normal forces exerted by the platform cause the mooring line to progressively cut into the adjacent geological formations resulting in movement (lateral displacement) of the floating offshore platform away from the initial predetermined position and away from the anchor pile. As a result, the mooring line needs to be shortened, over time, in order to ensure that the Floating Offshore Platforms are retained in the predetermined position. Furthermore, normal forces exerted on the mooring line by movement of the platform may be passed on to the anchor pile within the bore hole which may cause the anchor pile to be displaced, for example to move laterally. This movement of the anchor pile can disadvantageously result in loosening of the mooring line and movement of the platform.

[0006] It is among the objectives of embodiments of the present invention to obviate or alleviate these and other disadvantages of known anchor piling systems.

[0007] SUMMARY OF INVENTION

[0008] According to a first aspect of the present invention, there is provided a tether assembly for use in securing a floating offshore platform to an offshore foundation, for example anchor pile, the tether assembly comprising: a tether comprising: a first end configured in use to be attached to a floating offshore platform or to a mooring line extending from a floating platform; and a second opposed end configured in use to be attached to an offshore foundation, for example anchor pile, embedded within a borehole of a seabed; and at least one radially extending member mounted on the tether located between the first end and second ends thereof, in which the radially extending member(s) is configured in use to engage with and to shed loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole.

[0009] The tether of the tether assembly may be configured in use to either be directly connected (at the first end thereof) to a floating offshore platform, or to be connected to, and extend between, the offshore foundation and a mooring line extending from a floating platform.

[0010] The tether of the tether assembly may be configured in use to provide a lower section of a mooring line, in which the tether extends between the offshore foundation and a location adjacent a seabed crossing point for connection to a mooring line extending from a floating offshore platform.

[0011] Once installed, movement of the floating offshore platform exerts loads onto the tether assembly.

[0012] A conventional mooring line (used to secure a floating offshore platform to an offshore foundation) progressively moves through the geological formations over time. Over time, the floating platform exerts loads on the mooring line which causes the mooring line to cut through the geological formations creating an increasingly shallower curve (i.e. having a decreasing angle of curvature) which approaches a caternary curve. As the curve of the mooring line becomes increasingly shallow, the displacement of the floating offshore platform from the original predetermined position increases (and the distance from the foundation increases). As a result, the mooring line will need to be shortened in order to return or maintain the platform in the predetermined position relative to the foundation. As the curve of the mooring line becomes shallower over time, normal forces exerted on the tether can be increasingly passed on to the foundation increasing the risk of lateral displacement of the foundation.

[0013] In comparison, the at least one radially extending member of the tether assembly of the present invention engages with and sheds loads into the adjacent geological formations. The loads transmitted to the offshore foundation as a result of the tether assembly of the present invention are therefore considerably reduced compared to the loads transmitted to an offshore foundation connected by a mooring line without radially extending member(s). Furthermore, the radially extending member(s) engage with the adjacent geological formations thereby reducing the progressive cutting action of the tether, over time, through the geological formation, defining a curve with increased curvature and resulting in reduced lateral displacement, over time, of the platform from its original location relative to the offshore foundation (when compared to a conventional tether). The presence of the radially extending member(s) on the tether of the tether assembly therefore reduce the length of tether required to maintain the platform in a predetermined position and reduce the need to shorten the tether over time to maintain the platform in a predetermined position relative to the foundation. The tether assembly of the present invention has associated cost savings compared to conventional mooring lines. Installation and maintenance of the tether assembly is more efficient than conventional mooring lines giving rise to further cost savings.

[0014] The tether assembly of the present invention (and in particular the radially extending member(s)) is preferably configured to shed normal loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole. The tether assembly of the present invention (and in particular the radially extending member(s)) is preferably configured to shed normal and axial loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole.

[0015] The tether assembly of the present invention is preferably configured to reduce normal loading (by for example normal load shedding) on an offshore foundation, for example anchor pile, located within a bore hole. The term “normal loading” is used herein to refer to loading of forces extending in a direction normal to the elongate axis of the foundation, or anchor pile, extending between opposed ends thereof. As a result of the normal load shedding, the foundation (for example anchor pile) has improved stability with reduced risk of lateral movement of the platform over time from a predetermined position which could lead to failure of the tether.

[0016] The tether assembly of the present invention is preferably configured to reduce axial loading (by for example axial load shedding) on an offshore foundation, for example anchor pile, located within a bore hole. The term “axial loading” is used herein to refer to loading of forces extending in a direction parallel to the elongate axis of the foundation, or anchor pile, extending between opposed ends thereof. As a result of the axial load shedding, the foundation (for example anchor pile) has improved stability with reduced risk of failure as a result of axial movement of the foundation within the borehole.

[0017] The presence of radially extending members has been found to improve the longevity of the tether and reliability of the engagement of the anchor pile within the bore hole, with reduced risk of normal movement.

[0018] A protective portion (for example a sheath) may be mounted between the tether and the at least one radially extending member. The protective portion may be configured in use to reduce, for example prevent, exposure of the tether to mechanical damage (for example frictional forces from the geological formations). For example, the protective portion may substantially surround the peripheral region of a portion of the tether located adjacent the or each radially extending member. The protective portion may extend axially beyond (at at least one end thereof) the or each radially extending member. A first end of a protective portion (which is located adjacent a first radially extending member) may be configured in use to engage with a second radially extending member. For example, a first end of a protective portion (which is located adjacent a first radially extending member) may be configured in use to be retained within a spacing provided between the tether and a second radially extending member. The protective portion may be configured to provide a continuous surface extending between adjacent pairs of radially extending members to reduce the risk of mechanical damage to the underlying tether. The protective portion together with the adjacent radially extending member may provide a female connector at a first end of the radially extending member and a male connector at an opposed second end of the radially extending member. The male and female connectors provided by the protective portion and radially extending member being configured to cooperatively engage respective female and male connectors provided by a further protective portion. The substantially continuous surface preferably provides a seal extending across the join formed between adjacent ends of an adjacent pair of radially extending members, thereby preventing dirt ingress and protecting the underlying tether.

[0019] Preferably, the tether assembly is configured in use to reduce the normal load exerted on a foundation (for example anchor pile) in a borehole by at least 5%, preferably by at least 10%, for example by at least 15% compared to a tether assembly which is free of radially extending member(s).

[0020] Preferably, the tether assembly is configured in use to reduce the normal load exerted on a foundation (for example anchor pile) in a borehole by up to at least 99%, preferably by at least 90%, for example by at least 80% compared to a tether assembly which is free of radially extending member(s).

[0021] Preferably, the tether assembly is configured in use to reduce the axial load exerted on a foundation (for example anchor pile) in a borehole by at least 5%, preferably by at least 10%, for example by at least 15% compared to a tether assembly which is free of radially extending member(s).

[0022] Preferably, the tether assembly is configured in use to reduce the axial load exerted on a foundation (for example anchor pile) in a borehole by up to at least 99%, preferably by at least 90%, for example by at least 80% compared to a tether assembly which is free of radially extending member(s).

[0023] The at least one radially extending member is preferably configured in use to be spaced away from the foundation.

[0024] The at least one radially extending member is preferably configured in use to engage the geological foundation at the borehole and to extend through at least a portion of the adjacent geological foundation.

[0025] A protective portion is preferably configured in use to extend along a portion of the tether which contacts the geological formation during use. The at least one radially extending member is preferably mounted on and extends outwardly from a portion of the tether or protective portion (for example sheath).

[0026] The at least one radially extending member may be slideably mounted on and extends outwardly from a portion of the tether or protective portion (for example sheath).

[0027] The at least one radially extending member is preferably secured to, for example fixedly secured, to at least a portion of the tether or sheath.

[0028] The at least one radially extending member preferably extends outwardly from and surrounds at least a portion of the periphery of at least a portion of the tether (and optional underlying protective portion). In one embodiment, the at least one radially extending member preferably extends outwardly from and substantially entirely surrounds the periphery of at least a portion of the tether (and optional underlying protective portion).

[0029] The outer diameter (as measured between opposing side thereof) of the at least one radially extending member is preferably greater than the diameter of the tether (optionally with a protective portion (for example sheath) mounted thereon).

[0030] The outer diameter (as measured between opposing sides thereof) of the at least one radially extending member may be constant along the length of the radially extending member.

[0031] The outer diameter (as measured between opposing sides thereof) of the at least one radially extending member may vary along the length of the radially extending member.

[0032] The variations of the outer diameter of the at least one radially extending member may be uniform along the length of the at least one radially extending member.

[0033] In one embodiment, the outer diameter (as measured between opposing sides thereof) of the at least one radially extending member may increase along the length of the radially extending member. For example, the radially extending member may taper outwardly in a direction from one end to the opposed end thereof. The radially extending member may comprise undulating portions along the length of the member.

[0034] The tether assembly may comprise a plurality of radially extending members, In one embodiment, the at least one radially extending member has a first end and an opposed second end and defines an open ended channel extending therebetween. The open ended channel is preferably configured in use to receive a portion of the tether (and optional underlying protective portion) therein.

[0035] The at least one radially extending member may be formed integrally with the tether. Alternatively, the at least one radially extending member may be configured to be mounted on to the tether by for example retrofitting of the at least one radially extending member to the tether.

[0036] The at least one radially extending member is preferably configured to form a tight fit with the adjacent portion of the tether (and optional underlying protective portion) to reduce, preferably prevent, axial and / or normal movement of the tether relative to the at least one radially extending member.

[0037] The at least one radially extending member may be configured to be permanently mounted on the tether (and optional underlying protective portion). In one embodiment, the at least one radially extending member may be configured to be releasably mounted on the tether (and optional underlying protective portion).

[0038] In one embodiment, the at least one radially extending member is preferably moveable between: a first open position for mounting on at least a portion of the tether (and optional underlying protective portion); and a second closed position in which the at least one radially extending member is mounted on at least a portion of the tether (and optional underlying protective portion).

[0039] The at least one radially extending member preferably comprises a pair of radially extending member portions configured in use to be engageable (for example releasably engageable) together to define the radially extending member.

[0040] The at least one radially extending member may be configured in use to be mounted on the tether (or protective portion for example sheath) by clamping.

[0041] For example, a first radially extending member portion may be moveable relative to the second radially extending member portion to move the at least one radially extending member between the first open position and the second closed position. In one embodiment, the pair of radially extending member portions are hingeably connected together.

[0042] In the second closed position, the pair of radially extending member portions preferably define an open ended channel extending therebetween for receiving a portion of the tether (and optional underlying protective portion) to enable the tether to extend through the radially extending member.

[0043] The tether may be composed of any suitable material, such as for example wire. The wire may have any suitable dimensions, such as for example a diameter of about 130 mm.

[0044] The protective portion (for example sheath) may be composed of any suitable material, such as for example plastic coating. The protective portion (for example sheath) may have any suitable dimensions, such as for example a thickness of at least 5 mm, preferably a thickness of no more than 10 mm.

[0045] The least one radially extending member may be composed of any suitable material, such as for example ***

[0046] The tether assembly preferably comprises a plurality of radially extending members configured in use to be arranged in series along at least a portion of the tether (and optional underlying protective portion).

[0047] The plurality of radially extending members are preferably arranged such that at least one radially extending member abuts an adjacent radially extending member. It is however to be understood that the plurality of radially extending members may be spaced apart from each other along at least a portion of the tether (and optional underlying protective portion).

[0048] The at least one radially extending member may have any suitable shape and dimensions.

[0049] The at least one radially extending member may be elongate and defining a longitudinal axis which is configured in use to extend substantially parallel to the adjacent portion of tether. The at least one radially extending member may comprise a tapered portion. The at least one radially extending member may taper (for example inwardly) between opposing ends thereof.

[0050] The at least one radially extending member preferably has a substantially circular cross-section. It is however to be understood that the radially extending member may have a polygonal cross-section.

[0051] The tether assembly may further comprise an abutment member configured in use to be located between adjacent radially extending members and to receive a portion of the tether therethrough.

[0052] In one embodiment, the radially extending member comprises an abutment member located at an end thereof. The abutment member is configured in use to abut an end (for example an opposed end) of an adjacent radially extending member. The abutment member may be an annular flange defining a channel extending therethrough.

[0053] The at least one radially extending member preferably further comprises one or more surface features, such as for example one or more of: groove(s), slot(s), protrusion(s), ridge(s), or any combination thereof located on an outer surface thereof. The presence of one or more surface features may aid frictional engagement with adjacent geological formations and / or the tether (or optional underlying protection portion) providing for improved load shedding from the tether assembly.

[0054] The one or more surface features may be provided in any suitable location and / or in any suitable configuration.

[0055] The outer diameter of the radially extending member(s) is at least 10% greater, preferably at least 20% greater, for example at least 30% greater than the diameter of the tether (optionally with an underlying protective portion).

[0056] The at least one radially extending member is preferably configured in use to reduce (and preferably remove) normal load along the length of the tether, and to direct the normal load into geological formations surrounding a borehole, prior to reaching the second end of the tether. According to a second aspect of the present invention, there is provided a method for anchoring a floating offshore platform to a foundation, for example an anchor pile, in a borehole, the method comprising: installing a tether assembly to secure a floating offshore platform or a mooring line extending from a floating offshore platform to a foundation (for example anchor pile) in a borehole, such that the tether assembly is configured in use to shed loads into geological formations located between the floating offshore platform and the borehole prior to reaching the foundation, for example anchor pile.

[0057] In use, the tether assembly is configured to maximise load shedding (preferably normal and / or axial load shedding) along the tether. As a result, the tether assembly is configured in use to significantly reduce the normal and / or axial load transmitted to the foundation, for example anchor pile.

[0058] The tether assembly is preferably configured to maximise normal and axial load shedding along the tether.

[0059] The tether assembly is preferably configured in use to shed normal loads (i.e. loads extending normal to the axis of the tether) into geological formations through frictional engagement between the radially extending member(s) and the geological formations.

[0060] The tether assembly is preferably configured in use to shed axial loads (i.e. loads extending substantially parallel to the axis of the tether) into geological formations through frictional engagement between the tether (and optional underlying protective portion) and the radially extending member(s).

[0061] In one embodiment, movement of the tether (and optional underlying protective portion) relative to the radially extending member(s) in the axial direction is restricted, preferably prevented. For example, the radially extending member(s) provides for frictional engagement with the tether (and optional underlying protective portion) restricting axial movement of the tether relative to the radially extending member(s). For example, the outer surface of the radially extending member(s) may comprise a high friction coating. In one embodiment, the radially extending member(s) is bonded to the tether (for example by adhesive or welding) to prevent axial movement of the tether relative to the radially extending member(s). During use, axial movement of the tether, as a result of axial loading, is restricted by the radial extending member(s) resulting in axial load shedding into the adjacent geological formations.

[0062] During installation, the second opposed end of the tether is preferably attached to the foundation, for example anchor pile, such that at least a portion of the tether adjacent the second opposed end extends substantially parallel to a longitudinal axis defined by the foundation, for example anchor pile (i.e. providing for a substantially in-line installation providing for improved axial loading of the foundation, for example anchor pile).

[0063] During installation, the first end of the tether is either attached to the floating offshore platform, or to a mooring line located between the floating offshore platform and the foundation, such that at least a portion of the tether adjacent the first end thereof extends at an angle of at least 5 degrees relative to a plane defined by the floating offshore platform.

[0064] The tether assembly is preferably configured to increase the effective bend radius (i.e. curvature) of the tether between the first and second ends thereof (when compared to a tether without radially extending member(s)) by engaging with the geological formations.

[0065] Furthermore, the tether assembly is preferably configured to increase the angle of the portion of the tether adjacent the second end thereof for connection to the anchor pile.

[0066] The angle of the portion of the tether adjacent the second end may extend substantially perpendicular to a plane defined by the floating offshore platform. By increasing the angle of the portion of the tether adjacent the second end thereof (i.e. by bringing the portion of the tether adjacent the second end thereof into for example alignment with the longitudinal axis of the foundation, for example anchor pile), it has been found that significantly less normal loading is transmitted to the foundation, for example the anchor pile, resulting in less risk of failure or movement of the foundation, for example the anchor pile, post installation.

[0067] The presence of radially extending members has been found to reduce the length of tether required to connect a foundation, for example an anchor pile, to a floating offshore platform leading to reduced associated costs and installation time periods. By increasing the effective bend radius of the tether and / or by increasing the angle of the portion of the tether adjacent the second end thereof, the length of tether required to secure a floating offshore platform may be reduced.

[0068] The tether is the portion of the assembly which is prone to damage due to interactions with the seabed sediments. Furthermore, the tether is at risk from damage due to ingress of sediment. The presence of radially extending members has also been found to reduce the risk of damage to the tether.

[0069] Embodiments of the present invention may be described in further detail in relation to the accompanying Figures:

[0070] BRIEF DESCRIPTION OF FIGURES

[0071] Figure 1 is a schematic illustration of a tether assembly according to one embodiment of the present invention in communication with an anchor pile within a borehole;

[0072] Figure 2 is a schematic illustration of a radially extending member of the tether assembly of Figure 1 ;

[0073] Figure 3 is a schematic illustration of a variety of different radially extending members of the tether assembly;

[0074] Figures 4A to 4C are schematic illustrations of a conventional tether connected to a floating platform and an offshore foundation extending through dense / strong soil under different loading conditions (0 tonnes (Figure 4A), 400 tonnes (Figure 4B) and 800 tonnes (Figure 4C));

[0075] Figures 5A to 5C are schematic illustrations of a conventional tether connected to a floating platform and an offshore foundation extending through weak / loose soil under different loading conditions (0 tonnes (Figure 5A), 400 tonnes (Figure 5B) and 800 tonnes (Figure 5C));

[0076] Figures 6A to 6C are schematic illustrations of a conventional tether connected to a floating platform and an offshore foundation extending through dense / strong soil under different loading conditions (0 tonnes (Figure 6A), 400 tonnes (Figure 6B) and 800 tonnes (Figure 6C)); and

[0077] Figures 7A to 7C are schematic illustrations of a conventional tether connected to a floating platform and an offshore foundation extending through weak / loose soil under different loading conditions (0 tonnes (Figure 7A), 400 tonnes (Figure 7B) and 800 tonnes (Figure 7C)).

[0078] DETAILED DESCRIPTION

[0079] With reference to the Figures, a first end (not shown) of a tether 1 of a tether assembly 2 is connected to a floating offshore platform (not shown), and a second opposed end 4 of the tether 1 is attached to an anchor pile 6 embedded within a borehole 8 of a seabed. It is to be understood that the tether assembly may be connected to a mooring line which is connected to a floating offshore platform. For example, the tether 1 may extend between the offshore foundation and a seabed crossing point, and configured in use to be connected to a mooring line.

[0080] The tether assembly 2 further comprises a plurality of radially extending members 10 mounted on the tether 1 . It is however to be understood that the tether assembly 2 may include a single radially extending member.

[0081] Each radially extending member(s) is secured to the tether by welding or adhesive. It is however to be understood that the radially extending member(s) may be secured to the tether by any suitable means, including by frictional engagement. The radially extending member(s) may comprise one of more attachment features for securing to the tether. The radially extending member(s) are configured to prevent movement of the tether 1 relative to the radially extending member(s).

[0082] In the illustrated embodiment, each radially extending member 10 extends outwardly from and substantially entirely surrounds the periphery of an adjacent portion of the tether. The radially extending members 10 each provide a continuous outer surface extending entirely around the periphery of an adjacent portion of the tether 1 . The radially extending members 10 are provided in a series and abut adjacent radially extending members. The plurality of radially extending members 10 are arranged along a portion of the tether 1 to provide a continuous outer surface extending along the plurality of radially extending members 10.

[0083] It is however to be understood that one or more of the radially extending members may be configured to only surround a portion of the periphery of the tether 1 . For example, one or more of the radially extending members may provide a discontinuous outer surface surrounding a portion of the periphery of an adjacent portion of the tether 1.

[0084] The outer diameter of the radially extending members 10 is greater than the diameter of the tether 1 . The diameter is measured between opposing surfaces of the member or tether.

[0085] In the illustrated embodiment, the radially extending members 10 each have a first end 12 and an opposed second end 14. An open ended channel 16 extends between the first 12 and second 14 ends of the radially extending member 10. The open ended channel 16 is configured in use to receive a portion of the tether 1 therein.

[0086] The radially extending members 10 are elongate in shape. The radially extending members 10 are substantially cylindrical in shape. Each of the first and second ends 12, 14 of the radially extending members 10 tapers inwardly in a direction extending away from the radially extending member. A first end 12 of the radially extending member 10 comprises an annular abutment surface provided by an annular flange. The annular flange is configured in use to abut a second end of an adjacent radially extending member 10.

[0087] It is to be understood that the radially extending members 10 may have any suitable shape and / or dimensions depending on the particular requirements, for example depending on the geological formations present. The radially extending members 10 are configured in use to increase frictional engagement with the surrounding geological formation in comparison to the tether alone. For example, one or more of the radially extending members may be disc-shaped, fan-shaped or sail-shaped.

[0088] In the illustrated embodiment, the outer surface of each radially extending member 10 is substantially smooth. It is however to be understood that the outer surface of one or more radially extending members 10 may have any suitable surface decoration or markings in order to increase the load shedding ability of the tether assembly. For example, the outer surface of the radially extending members 10 may comprise one or more of: openings, grooves, protrusions, ridges, or any combination thereof.

[0089] Figures 3A to 3F illustrate a number of different radially extending members 10A, 10B, 10C, 10D and 10E. As shown in Figure 3A, the radially extending member 10A defines an open ended channel 12A. A protective portion 14A is located within the open ended channel 12A. The protective portion 14A defines an open ended channel 16Afor receiving a portion of the tether 1 therein. A first end 18A of the protective portion 14A extends beyond the first end 20A of the radially extending member 10A. A second end 22A of the protective portion 14A is located within the open ended channel 12A of the radially extending member 10A. The first end 18A of the protective portion 14A together with the first end 20A of the radially extending member 10A provide a male connector. The second end 22A of the protective portion 14A together with the second end 24A of the radially extending member 10A provide a female connector. In use, the first end 18A of the protective portion 14A is configured to be received within and to engage with the open ended channel 12A at the second end 24A of a further radially extending member 10A to provide a substantially continuous outer surface extending between adjacent pairs of radially extending members 10A as shown in Figure 3B.

[0090] The radially extending member 10B of Figure 3C is similar in shape and configuration to the radially extending member 10A. The outer surface 26B of the radially extending member 10B has a high friction coating to optimise axial load shedding in use.

[0091] The radially extending member 10C of Figure 3D is also similar in shape and configuration to the radially extending member 10A. The outer surface 26C of the radially extending member 10C is covered with a protective layer 28C (in the form of a layer of sheet metal, for example stainless steel) to provide increased protection to the radially extending member 10C from for example glacial tills or similar. The radially extending member 10C prevents loads being transmitted / transferred from the protective layer 28C to the tether.

[0092] The radially extending member 10D of Figure 3E has an undulating cross-section along the length of the member 10D. Each undulation extends substantially around the periphery of the radially extending member. It is however to be understood that the undulations may only extend across a portion of the periphery of the radially extending member. Each undulation extends in a direction extending substantially perpendicular to the length of the radially extending member. It is to be understood that one or more undulations may undulate in a radial direction. In the illustrated embodiment, the undulations are uniformly spaced along the length of the member 10D and are uniform in dimensions. It is however to be understood that each individual undulation may have any suitable dimensions and may be located at any suitable position on the radially extending member. The undulations are configured in use to create a sheer / reaction surface to interact with surrounding soil and to increase axial load shedding.

[0093] The radially extending member 10E of Figure 3F is configured to receive drag inducing members 30E thereon. The drag inducing member 30E in the illustrated embodiment is a recycled vehicle tyre. It is to be understood that the radially extending member 10E may retain any suitable number of drag inducing members and is not limited to one. The drag inducing member(s) is preferably strong and resilient. The drag inducing member(s) in addition to the radially extending member will help to increase normal and axial load shedding.

[0094] The radially extending members 10 may be mounted on the tether 1 after production and prior to installation of the tether 1 , for example the radially extending members 10 may be retrofitted to the tether 1 . It is however to be noted that the radially extending members 10 may be mounted on the tether 1 during production of the tether system. The radially extending members 10 may be integrally formed with the tether 1 .

[0095] The plurality of radially extending members 10 are mounted in series on the tether 1 . As shown in Figure 1 , each radially extending member 10 is located to abut an adjacent radially extending member 10. It is however to be understood that the radially extending members may be spaced apart along the tether 1 , for example along a portion of the tether 1 .

[0096] The radially extending members are each configured in use to engage with and to shed normal and axial loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole.

[0097] In use, the tether assembly 2 is installed to secure a floating offshore platform (not shown) to the anchor pile 6 in a borehole 8, such that the tether assembly 2 is configured in use to shed normal and axial loads into geological formations located between the floating offshore platform and the borehole 8 prior to reaching the anchor pile 6. A first end 20 of the tether 1 is attached to a floating offshore platform (not shown). A second opposed end 18 of the tether 1 is attached to an anchor pile 6 within a bore hole. The tether assembly 2 is configured in use to shed normal loads into geological formations through frictional engagement of the radially extending members 10 with the geological formations. The tether assembly 2 is also configured in use to shed axial loads into geological formations by the radially extending members 10 preventing relative axial movement of the tether 1 .

[0098] In use, the tether assembly is configured to maximise load normal and / or axial load shedding along the tether 1 . As a result, the tether assembly 2 is configured in use to significantly reduce the normal and / or axial load transmitted to the anchor pile 6.

[0099] The tether assembly 2 is configured to increase the effective bend radius (i.e. curvature) of the tether 1 between the first and second ends thereof by engaging with the geological formations compared to the effective bend radius of conventional tethers.

[0100] It can be seen that during installation, the second opposed end 4 of the tether 1 is attached to the anchor pile 6 such that at least a portion of the tether 1 adjacent the second opposed end 4 extends substantially parallel to a longitudinal axis defined by the anchor pile 6.

[0101] It can also be seen that during installation, the first end of the tether 1 is attached to the floating offshore platform (not shown) such that at least a portion of the tether 1 adjacent the first end thereof extends at an angle of at least 5 degrees relative to a plane defined by the floating offshore platform (not shown).

[0102] Furthermore, the tether assembly 2 is configured to increase the angle of the portion of the tether 1 adjacent the second end 4 thereof for connection to the anchor pile 6. The portion of the tether 1 adjacent the second end 4 thereof may extend substantially parallel to a longitudinal axis of the anchor pile 6 (i.e. providing for a substantially inline installation providing for improved axial loading of the anchor pile). The angle of the portion of the tether 1 adjacent the second end 4 may extend substantially perpendicular to a plane defined by the floating offshore platform. By increasing the angle of the portion of the tether 1 adjacent the second end 4 thereof (i.e. by bringing the portion of the tether 1 adjacent the second end 4 thereof into for example alignment with the longitudinal axis of the anchor pile 6), it has been found that significantly less normal loading is transmitted to the anchor pile 6 resulting in less risk of failure or movement of the anchor pile post installation. The presence of radially extending members 10 has been found to reduce the length of tether 1 required to connect an anchor pile 6 to a floating offshore platform leading to reduced associated costs and installation time periods. By increasing the effective bend radius of the tether 1 and / or by increasing the angle of the portion of the tether 1 adjacent the second end 4 thereof, the length of tether required to secure a floating offshore platform may be reduced.

[0103] The tether 1 is the portion of the assembly which is prone to damage due to interactions with the seabed sediments. Furthermore, the tether 1 is at risk from damage due to ingress of sediment. The presence of radially extending members 10 surrounding portions of the tether 1 has also been found to reduce the risk of damage to the tether.

[0104] The presence of radially extending members 10 has been found to improve the longevity of the tether and reliability of the engagement of the anchor pile 6 within the bore hole, with reduced risk of normal movement.

[0105] Figures 4Ato 4C, 5Ato 5C, 6Ato 6C and 7Ato 7C demonstrate the angle of curvature of a conventional tether and tether assembly of the present invention under various conditions in dense / strong soil and weak / loose soil.

[0106] Comparative Example: Conventional tether within Dense / Strong soil Table 1

[0107] Comparative Example: Conventional tether within Weak / Loose soil

[0108] Table 2 Example 1 : Tether Assembly of the present invention within Dense / Strong soil

[0109] Table 3

[0110] Example 2: Tether Assembly of the present invention with Weak / Loose soil

[0111] Table 4

[0112] With reference to Figures 4A to 4C and Table 1 , a conventional tether (securing a floating offshore platform to a foundation) within dense / strong soil is loaded with 0 tonne (Figure 4A), 400 tonnes (Figure 4B) and 800 tonnes (Figure 4C). It can be seen that as the load increases the bend radius (angle of curvature) of the tether adjacent the second end of the tether reduces from 90 degrees (0 tonne; Figure 4A), to 75 degrees (400 tonnes; Figure 4B), and to 70 degrees (800 tonnes; Figure 4C). The bend radius is decreased by 20 degrees when the loading is increased from 0 tonnes to 800 tonnes. Furthermore, the lateral displacement of the floating platform relative to the location of the foundation increases from 30 metres (0 tonne; Figure 4A), to 36 metres (400 tonnes; Figure 4B), to 38 metres (800 tonnes; Figure 4C). This represents a 26.7% increase in the lateral displacement of the floating platform when loaded with 800 tonnes. The tension at the top of the foundation increases from 0 to 700 tonnes, when increasing the loading of the tether assembly from 0 tonnes to 800 tonnes.

[0113] In comparison, with reference to Figures 6A to 6C and Table 3, the tether assembly of the present invention (securing a floating offshore platform to a foundation) within dense / strong soil is loaded with 0 tonne (Figure 6A), 400 tonnes (Figure 6B) and 800 tonnes (Figure 6C). It can be seen that as the load increases the bend radius (angle of curvature) of the tether adjacent the second end of the tether reduces from 90 degrees (0 tonne; Figure 6A), to 80 degrees (400 tonnes; Figure 6B), and to 75 degrees (800 tonnes; Figure 6C). The bend radius is decreased by 15 degrees when the loading is increased from 0 tonnes to 800 tonnes. The bend radius is therefore significantly higher compared to the equivalent bend radius achieved by a tether under the same loading conditions. Furthermore, the lateral displacement of the floating platform relative to the location of the foundation increases from 30 metres (0 tonne; Figure 6A), to 33 metres (400 tonnes; Figure 6B), to 34 metres (800 tonnes; Figure 6C). This represents a 13% increase in the lateral displacement of the floating platform when loaded with 800 tonnes. The increase in the lateral displacement of the floating platform provided by the tether assembly of the present invention is less than half the increase in the lateral displacement provided by a conventional tether under the same loading conditions. The tension at the top of the foundation increases from 0 to 640 tonnes, when increasing the loading of the tether assembly from 0 tonnes to 800 tonnes. The tension at the top of the foundation in connection with the tether assembly of the present invention is reduced by 8.5% compared to the tension at the top of a foundation in connection with a conventional tether. As a result, the tether assembly has a lower associated risk of failure or movement of the foundation, for example the anchor pile, post installation.

[0114] With reference to Figures 5A to 5C and Table 2, a conventional tether (securing a floating offshore platform to a foundation) within weak / loose soil is loaded with 0 tonne (Figure 5A), 400 tonnes (Figure 5B) and 800 tonnes (Figure 5C). It can be seen that as the load increases the bend radius (angle of curvature) of the tether adjacent the second end of the tether reduces from 90 degrees (0 tonne; Figure 5A), to 70 degrees (400 tonnes; Figure 5B), and to 50 degrees (800 tonnes; Figure 5C). The bend radius is decreased by 40 degrees when the loading is increased from 0 tonnes to 800 tonnes. Furthermore, the lateral displacement of the floating platform relative to the location of the foundation increases from 30 metres (0 tonne; Figure 5A), to 40 metres (400 tonnes; Figure 5B), to 45 metres (800 tonnes; Figure 5C). This represents a 50% increase in the lateral displacement of the floating platform when loaded with 800 tonnes. The tension at the top of the foundation increases from 0 to 740 tonnes, when increasing the loading of the tether assembly from 0 tonnes to 800 tonnes.

[0115] In comparison, with reference to Figures 7A to 7C and Table 4, the tether assembly of the present invention (securing a floating offshore platform to a foundation) within dense / strong soil is loaded with 0 tonne (Figure 7A), 400 tonnes (Figure 7B) and 800 tonnes (Figure 7C). It can be seen that as the load increases the bend radius (angle of curvature) of the tether adjacent the second end of the tether reduces from 90 degrees (0 tonne; Figure 7A), to 75 degrees (400 tonnes; Figure 7B), and to 60 degrees (800 tonnes; Figure 7C). The bend radius is decreased by 30 degrees when the loading is increased from 0 tonnes to 800 tonnes. The bend radius is therefore significantly higher compared to the equivalent bend radius achieved by a tether under the same loading conditions. Furthermore, the lateral displacement of the floating platform relative to the location of the foundation increases from 30 metres (0 tonne; Figure 7 A), to 35 metres (400 tonnes; Figure 7B), to 37 metres (800 tonnes; Figure 6C). This represents a 23% increase in the lateral displacement of the floating platform when loaded with 800 tonnes. The increase in the lateral displacement of the floating platform provided by the tether assembly of the present invention is less than half the increase in the lateral displacement provided by a conventional tether under the same loading conditions. The tension at the top of the foundation increases from 0 to 580 tonnes, when increasing the loading of the tether assembly from 0 tonnes to 800 tonnes. The tension at the top of the foundation in connection with the tether assembly of the present invention is reduced by 22% compared to the tension at the top of a foundation in connection with a conventional tether. As a result, the tether assembly has a lower associated risk of failure or movement of the foundation, for example the anchor pile, post installation.

Claims

CLAIMS1 . A tether assembly for use in securing a floating offshore platform to an offshore foundation located within a borehole, the tether assembly comprising: a tether comprising: a first end configured in use to be attached to a floating offshore platform or to a mooring line extending from a floating platform; and a second opposed end configured in use to be attached to an offshore foundation, for example anchor pile, embedded within a borehole of a seabed; and at least one radially extending member mounted on the tether located between the first end and second ends thereof, in which the radially extending member(s) is configured in use to engage with and to shed loads to geological formations located adjacent the tether and between the floating offshore platform and the borehole.

2. A tether assembly as claimed in claim 1 , in which the tether is configured in use to extend between the offshore foundation and a mooring line extending from a floating platform.

3. A tether assembly as claimed in either of claims 1 and 2, in which the tether further comprises a protective portion mounted on and extending along at least a portion of the tether, and in which the at least one radially extending member is mounted on and extends outwardly from the protective portion.

4. A tether assembly as claimed in any one of claims 1 to 3, in which the at least one radially extending member extends outwardly from and surrounds at least a portion of the periphery of at least a portion of the tether.

5. A tether assembly as claimed in claim 4, in which the at least one radially extending member extends outwardly from and substantially entirely surrounds the periphery of at least a portion of the tether.

6. A tether assembly as claimed in any preceding claim, in which the outer diameter of the at least one radially extending member is greater than the diameter of the tether.

7. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member has a first end and an opposed second end, in which an open ended channel configured in use to receive a portion of the tether therein is defined between the first and second ends of the radially extending member(s).

8. A tether assembly as claimed in any preceding claim, in which at least one radially extending member is formed integrally with the tether.

9. A tether assembly as claimed in any one of claims 1 to 7, in which the at least one radially extending member is moveable between: a first open position for mounting on at least a portion of the tether; and a second closed position in which the at least one radially extending member is mounted on at least a portion of the tether.

10. A tether assembly as claimed in claim 9, in which the at least one radially extending member comprises a pair of radially extending member portions releasably engageable together to define the radially extending member, and in which a first radially extending member portion is moveable relative to the second radially extending member portion to move the at least one radially extending member between the first open position and the second closed position.11 . A tether assembly as claimed in claim 10, in which the pair of radially extending member portions are hingeably connected together.

12. A tether assembly as claimed in either of claims 10 and 11 , in which in the second closed position the pair of radially extending member portions define an open ended channel extending therebetween for receiving a portion of the tether.

13. A tether assembly as claimed in any preceding claim, comprising a plurality of radially extending members configured in use to be arranged in series along at least a portion of the tether.

14. A tether assembly as claimed in claim 13, in which the plurality of radially extending members are arranged such that at least one radially extending member abuts an adjacent radially extending member.

15. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member has a substantially circular cross-section.

16. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member is an elongate member.

17. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member further comprises one or more of: groove(s), slot(s), protrusion(s), ridge(s) located on an outer surface thereof.

18. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member is configured in use to reduce normal load along the length of the tether, and to direct the normal load into geological formations surrounding a borehole, prior to reaching the second end of the tether.

19. A tether assembly as claimed in any preceding claim, in which the at least one radially extending member is configured in use to reduce axial load along the length of the tether by preventing or reducing movement of the tether relative to the radially extending member(s), and to direct the axial load into geological formations surrounding a borehole, prior to reaching the second end of the tether.

20. A method for anchoring a floating offshore platform to foundation within a borehole, the method comprising: installing a tether assembly as claimed in any one of claims 1 to 19 to secure a floating offshore platform or mooring line extending from a floating offshore platform to a foundation in a borehole, such that the tether assembly is configured in use to shed loads into geological formations located between the floating offshore platform and the borehole prior to reaching the foundation.21 .A method as claimed in claim 20, in which the tether assembly is configured in use to shed normal loads into geological formations through frictional engagement.

22. A method as claimed in either of claims 20 and 21 , in which the at least one radially extending member is configured in use to reduce axial load along the length of the tether by preventing or reducing movement of the tether relative to the radially extending member(s), and to direct the axial load into geological formations surrounding a borehole, prior to reaching the second end of the tether.

23. A method as claimed in any one of claims 20 to 22, in which the tether assembly comprises a tether comprising: a first end configured in use to be attached to a floating offshore platform or a mooring line extending from a floating offshore platform; and a second opposed end configured in use to be attached to a foundation embedded within a borehole of a seabed; and in which during installation, the second opposed end of the tether is attached to the foundation such that at least a portion of the tether adjacent the second opposed end extends substantially parallel to a longitudinal axis defined by the anchor pile.

24. A method as claimed in any one of claims 20 to 23, in which the tether assembly comprises a tether comprising:a first end configured in use to be attached to a floating offshore platform or a mooring line extending from a floating offshore platform; and a second opposed end configured in use to be attached to a foundation embedded within a borehole of a seabed; and in which during installation, the first end of the tether is attached to the floating offshore platform such that at least a portion of the tether adjacent the first end thereof extends at an angle of at least 5 degrees relative to a plane defined by the floating offshore platform. 1

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