WO2022071793A1

WO2022071793A1 - An improved floating offshore structure

Info

Publication number: WO2022071793A1
Application number: PCT/MY2021/050082
Authority: WO
Inventors: Izleena Bt M IQBAR; Fauzy Omar BASHEER B OSMAN; Christian Hariady GIRSANG; Zahari A RAZAK; Hsiu Eik LEE; Frank Adam; Frank Dahlhaus; Habil Jochen GROSSMANN; Carsten HENNHÖFER; Michael Beyer
Original assignee: Petroliam Nasional Bhd Petronas
Current assignee: Petroliam Nasional Bhd Petronas
Priority date: 2020-10-01
Filing date: 2021-10-01
Publication date: 2022-04-07
Anticipated expiration: 2023-04-01
Also published as: EP4222047A1; EP4222047A4; CN116368062A; JP2023545026A; KR20230104144A; US20230415855A1

Abstract

A mooring connection including: a linkage comprising a plurality of articulated parts, adjacent articulated parts of said linkage in pin joint engagement and arranged to pivot at the pin joint about a first axis; wherein at least one pin joint within the linkage arranged to pivot about a second axis, said second axis orthogonal to the first axis.

Description

AN IMPROVED FLOATING OFFSHORE STRUCTURE

Field of the Invention

The invention relates to floating offshore structures to support above sea facilities including, for instance, wind turbines and offshore hydrocarbon exploration & production.

Background

The installation of offshore structures, particularly for tension leg platforms, are highly capital intensive structures and also involve high operational costs.

It is therefore beneficial to increase the efficacy of current design of supporting systems for said structures.

Summary of Invention

In a first aspect, the invention provides a mooring connection including: a linkage comprising a plurality of articulated parts, adjacent articulated parts of said linkage in pin joint engagement and arranged to pivot at the pin joint about a first axis; wherein at least one pin joint within the linkage arranged to pivot about a second axis, said second axis orthogonal to the first axis.

In a second aspect, the invention provides a gravity anchor for an offshore structure, said gravity anchor comprising; a base for contacting a seabed; interference members arranged to project from the base and arranged to embed into the seabed. In a third aspect, the invention provides a sub-structure for an offshore structure, the sub-structure comprising: an inlet and an outlet, both positioned above water level; said inlet and outlet in fluid communication through an air flow path; said air flow path having at least a portion below the water level; wherein the air flow path is in heat transfer communication with the water, and arranged to transfer heat from said air flow to said water.

It will be appreciated that, whilst each of the aspect may be used together for use with a floating platform, equally, each aspect may be used individually. The designer of a floating platform may therefore improve the construction and/or operation of a floating platform by using any one, or a combination, of the various aforementioned aspects and achieve a superior result to that of platforms according to the prior art. Thus, the use of any of these aspects of the invention is not contingent on using any of the other aspects.

Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figures 1A and IB are isometric views of connections according to one embodiment of the present invention;

Figures 2A and 2B are isometric views of adapted plates according to various embodiments of the present invention; Figures 3A to 3C are isometric views of a tension leg platform connector according to a further embodiment of the present invention;

Figure 4A and 4B are various views of a gravity anchor for an offshore structure according to one embodiment of the present invention;

Figure 5A and 5B are various views of a gravity anchor for an offshore structure according to a further embodiment of the present invention;

Figure 6 is an isometric view of a gravity anchor for an offshore structure according to a further embodiment of the present invention;

Figure 7 is a schematic view of a tower base for an offshore wind turbine system according to one embodiment of the present invention;

Detailed Description

For a floating offshore structure, a typical mooring line fixed connection to the hull is via padeye or equivalent. For such connections, the padeye can represent a weak link where cracks may propagate. A padeye type connection may also be weaker in out of plane moments. Hence for long term high intensity load cycle applications with stochastic loading (hence cyclic loading also on its weaker axes), padeye type connections may not be suitable.

Figures 1A and IB show an alternative arrangement, having a mooring line fixed connection 5, 65 used to connect to a tension leg platform 10 (TLP). The connection 5 includes a linkage having a number of articulated parts, each articulated connected by a pin joint to the adjacent part. The articulated parts of the connection 5 of Figure 1A include a pair of adapter plates 20, each connected to a mooring line 15 at a first end 35 and a TLP interface 25 at an opposed second end. The adapter plates 20 may be short 105 or long 110 and can be fabricated to any prescribed length, as shown in Figures 2A and 2B. Each adaptor plate comprises parallel plates having a pin joint at each end, to connect with adjacent articulated parts within the linkage.

The TLP interface 25 engages with a twisted Y-link 30 according to the present invention. The Y-link comprises a bifurcated flange at a first end for receiving a single flange therebetween. At the opposed end, the Y-link has a single flange. Where the Y- link differs from the prior art is that each end pivots about orthogonal axes 50, 55, 60. The Y-link then connects 40 to the adaptor plate 20 to transfer tensile load from the interface 25 to the mooring line 15. Thus, regardless of the direction in which the structure moves, there is free rotation about the two principal axes 50, 55, 60 within the horizontal plane and so redistributing load away from a weaker axis and consequently providing articulation about each horizontal axis.

Figure IB shows a further embodiment according to the present invention. Here the connection 65 connects 80 to a padeye 70 for a foundation system for engaging the seabed. The present invention is foundation agnostic and can be incorporated with arbitrary foundation system, for instance, piles, suction cans and gravity anchor. The connection 65 of Figure IB differs from the connection 5 of Figure 1A in that whilst both share a twisted Y-link 30, 75 according to one embodiment of the present invention, the connection of Figure IB includes a twisted H-link 85 which replaces the adaptor plate of Figure 1A. Similar to the Y-link, the H-link has a bifurcated flange at each opposed end. Similar to the Y-link, according to this embodiment however is that the pin joint connection at each end pivots about orthogonal axes. It will be noted that a twisted H-link 85 used in Figure IB could equally be used for the connection in Figure 1A. The combination of a twisted H-link and twisted Y-link therefore provides additional point of articulation about the horizontal principal axes 90, 95, 100 and thus is a useful adaptation of the connection of Figure 1A. When considering conventional TLP construction, the use of steel tendons for mooring lines is uniformly adopted. Not only is the initial capital expense significant, corrosion problems and general maintenance represent substantial life cycle costs also.

To this end, in one embodiment, the TLP includes the application of composite and / or polymer type rope / line as tendons for the TLP. Composites fibres may be Kevlar, Aramid, Polyester, HDPE, PP, nylon, UHMWPE or carbon.

Apart from the superior corrosion performance, composite and polymer ropes, having a specific gravity close to 1, are also buoyant or only slightly negatively buoyant. With the self-weight of a steel tendon contributing to the over design and extensive installation regime of offshore structures, the buoyancy benefits of polymer rope represent a reduction in sizing of other components, leading to a holistic reduction on capital expenditure and offshore operations risks.

As shown in Figures 3A to 3C, the connection 120 to the TLP 115 also allows for complete movement 122, 125, 130 of the adaptor plate and enables horizontal stowing 122 to reduce draft requirements and enable passage through shallow water areas.

Gravity based anchor design is unsuitable for a sea floor having soft clay and weak soils. The main issues faced is the settlement and sliding of the gravity anchor which will cause the floating structure to be non-functional, lose station-keeping or become unstable and is a risk to assets in its vicinity such as in Oil and Gas development areas where there are many pipelines and marine cables around its’ vicinity. Major sliding of gravity anchor may cause catastrophic disasters leading to pipeline burst or cable breakage. On the other hand, if fully enclosed skirts are utilized to enhance lateral stability (and vertical capacity), there may be challenges related to proper soil consolidation, egress of expulsed soil from underneath the gravity based anchor due to immediate settlement, coupled-suction effects which may impede proper ‘sitting’ of the gravity anchor and pose major long-term behaviour uncertainties. This is especially true for thick layer of soft topsoil, having a high proportion of mud, clay and/or peat without site preparation for shallow foundations. Figures 4 to 6 show various embodiments of a gravity anchor according to the present invention which are directed to solving these issues.

In overcoming the problems with lateral movements whilst engaged with soft soil on the seabed, the present invention includes several interference members arranged to increase the lateral resistance of the gravity anchor and so hinder sliding. The interference members are arranged to embed within the soft soil of the seabed and so create greater interference with the soil of the seabed through a greater bearing surface as compared to only the friction of the base of the gravity anchor.

In Figures 4A and 4B, the interference member is provided as a plurality of planar steel / Reinforced concrete sheets, or skirts 145 projecting from the base of the gravity anchor core 142, and placed peripherally around the gravity anchor core 142. Said skirts arranged to embed into the seabed 160 as the base of the gravity anchor contacts the seabed. Sliding of the gravity anchor 140 is therefore resisted 165 by the bearing surface of the skirt member 145, 155.

In an alternate embodiment, the skirts may be a single member on each side of the gravity anchor, however, in this embodiment each skirt member includes a slit 150 separating the skirt members 145, 155 through which the soft soil of the seabed can flow. These slits 150 allow the egress or expulsion of soil particles following immediate settlement and consolidation in order for the gravity anchor 140 to find an equilibrium position relative to the seabed. This feature is especially crucial in the case of soft soil such as clay, mud and/or peat. Having the gravity anchor according to the present invention to achieve an equilibrium position permits better and more predictable embedment in soft soil which reduces the risk of excessive differential settlement and unpredictable long-term consolidation. In a further embodiment, additional skirt members 185 may be mounted to the gravity anchor base 170 and not merely about the periphery, in order to provide greater resistance 180 against lateral sliding and on-bottom stability. It is the premise of this feature that the additional skirt members possess a depth that is less than or equal to the periphery skirts. The additional skirt members 185 may also be included for purposes of structural stiffening or strengthening of the gravity anchor base 170 in addition to providing greater resistance 180 against sliding and on-bottom stability. To this end, whilst additional skirts members may be positioned to prevent or limit lateral movement, reinforcing skirt members may be used on the underside of the core which are positioned to add reinforcement, and so may not be aligned to prevent or limit lateral movement, but instead to provide a structural function.

Further, the gravity anchor 140 may benefit from buoyancy bag assistance for load out and/or transportation of the gravity anchor to open water via wet tow in order to meet draft or Under Keel Clearance (UKC) requirements of yards / ports / quaysides/ channels / shallow water regions.

Figures 5A to 5C show a further embodiment using interference members. In this embodiment, the interference members are projections 190 projecting downwards from the base 170 of the gravity anchor core 142. The projections 190 may be relatively narrow so as to more easily penetrate the seabed and of the gravity anchor. The projections may be tubular / square / rectangular piles, rods, reinforcement bars and other rigid structural members having a sufficient diameter so as to maintain integrity and enable smooth embedment of the projection into the seabed 160.

It will be appreciated that the projections may be more easily embeddable into the seabed as compared to the skirt members. The number of projections used will therefore depend upon a balance between lateral resistance 200 and the ability to embed under the weight of the gravity anchor 140. In a further embodiment, the projections 194 maybe in sliding engagement with the gravity anchor core 142, with the core 142 having apertures 192, which may include ducts, that allow sliding of the projections 192 relative to the core 142. Sliding may then be controlled by the use of a shear grips 195 mounted to each projection 194. It will be appreciated that the consistency of the seabed may vary about the gravity anchor 140 and therefore some projections may meet greater resistance to penetration than others. If the soil of the seabed in one area is considerably stiffer than elsewhere, the projection 194 may buckle and potentially damage the gravity anchor 140. Further, the gravity anchor may not sit flush on the seabed due to the lack of penetration of one or more projections that are not fully embedded. Thus, in this further embodiment the a shear grip 195 may berated to resist an applied axial force to each projection. For instance, each shear grip 195 may be rated for a release force equal to the gravity anchor weight divided by the number of projections. A safety factor may or may not apply. Such a safety factor may include preventing the shear grip from releasing on impact, and thus a creep factor may also be adopted to prevent release on impact of the gravity anchor contacting the sea floor.

If one projection 205 A exceeds the rated force due to stiffer soil resisting penetration, the shear grip will release and push the unpenetrated length 205B of the projection upwards thus allowing the gravity anchor to settle flush on the seabed. It will be noted that even a partially penetrated projection 205 A may still provide sliding resistance 210. In a still further embodiment of the interference member invention, Figure 6 shows a gravity anchor 140 having a plurality of projections 215 projecting upwards from a top 220 of the gravity anchor core 142. In this embodiment, the gravity anchor is intended to first be placed on the seabed and then a mechanical application force to each projection to force the penetration of each projection into the seabed. The mechanical application of force may be as simple as a collar of steel or concrete that is arranged to sit on the projections and, through self-weight, mechanical impact or vibration, embed the projections. Alternatively, each projection may be separately hammered or otherwise driven/forced into the seabed. In the equatorial regions / tropical countries, the above water portions of offshore structures, including drilling platforms and wind turbine towers, may experience an unwanted heat build-up due to high ambient temperature, sun radiation, etc. Aside from being a heat related safety hazard, this is not favourable for temperature sensitive equipment such as converters / batteries, to name a few. To date the conventional solution involves energy intensive options such as air conditioning systems / HVAC.

Figure 7 shows one embodiment whereby the sub-structure for the offshore platform can be used as a heat exchanger. The sub-structure in this case involves support stiffening members 230, pylons 235 and anchors 240. It is known, for cost efficient construction, to include cavities within the structural members. By providing fluid path through the cavities of the various structural components 230, 235, 240, an air flow can be captured 250 and arranged to flow 255 into an inlet 262 from above sea level to below sea level 245. As the air flow passes through the cavities, the sub-structure acts as a heat exchanger against the cooler water interfaced areas, dissipating heat to vent cooler air 260 from an outlet 264. The air flow of cooled air can then be used to cool the overall structure, as well as to cool sensitive components within the offshore structure.

Capturing the air may be through naturally or forcefully directing wind into the substructure. For instance, movable louvres arranged to rotate into the direction of the wind may be used and to direct the air flow downward into the sub-structure.

Alternatively, fans along the fluid path may draw an air flow into the sub-structure.

Further, the sub-structure may be adapted, or purposely built, to provide fins and other heat exchanging devices to optimise the heat dissipation from the heated air to the cooler sub-structure. This can be in the form of dual purpose structural members such as in the case of a floating structure, whereby the hull stiffeners are used in dual purpose as both structural reinforcements as well as cooling fins / protrusions.

Claims

1. A mooring connection including: a linkage comprising a plurality of articulated parts, adjacent articulated parts of said linkage in pin joint engagement and arranged to pivot at the pin joint about a first axis; wherein at least one pin joint within the linkage arranged to pivot about a second axis, said second axis orthogonal to the first axis.

2. The mooring connection according to claim 1, wherein one of said linkage parts is a Y-link comprising a bifurcated double flange at a first end and a single flange at an opposed end, each end forming a pin joint arranged to pivot about orthogonal axes to each other.

3. The mooring connection according to claim 1 or 2, wherein one of said linkage parts is an H-link comprising a bifurcated double flange at each opposed end, each end forming a pin joint arranged to pivot about orthogonal axes to each other.

4. The mooring connection according to any one of claims 1 to 3, wherein one of said linkage parts is an adaptor plate, comprising a pair of parallel plates, and having opposed ends of the adaptor plate forming a pin joint.

5. The mooring connection according to any one of claims 1 to 4, wherein the mooring connection is connected at a first end to a tension leg platform, said tension leg platform including composite tendons. A gravity anchor for an offshore structure, said gravity anchor comprising; a base for contacting a seabed; interference members arranged to project from the base and arranged to embed into the seabed. The gravity anchor according to claim 6, wherein said interference members include skirts. The gravity anchor according to claim 6, wherein said adjacent skirts are separated by a slit, said slit arranged to permit soil from the seabed to move / flow between said skirts. The gravity anchor according to claim 6, wherein said interference members include projections, said projections including any one or a combination of tubular / square / rectangular piles, rods, tubes or reinforcement bars. The gravity anchor according to claim 9, wherein each projection is in sliding engagement with a core of the gravity anchor. The gravity anchor according to claim 9 or 10, wherein each projection is mounted to the core using shear grip, the shear grip arranged to resist axial loads applied to the projection up to a rated load, whereupon said shear grip is arranged to release the projection.

12. A sub-structure for an offshore structure, the sub-structure comprising: an inlet and an outlet, both positioned above water level; said inlet and outlet in fluid communication through an air flow path; said air flow path having at least a portion below the water level; wherein the air flow path is in heat transfer communication with the water, and arranged to transfer heat from said air flow to said water.

13. The sub-structure according to claim 12, wherein the air flow path includes heat exchange devices along the portion below the water level, said heat exchange devices arranged to increase heat transfer from the water to the air flow path.

14. The sub-structure according to claim 12 or 13, wherein the inlet includes wind directional devices arranged to guide wind into the air flow path. 15. The sub-structure according to claim 12 or 13, further including fans arranged to draw air into the inlet and drive said air flow through the air flow path.