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EP-4741275-A1 - MOORING SYSTEM FOR TENSION LEG PLATFORMS

EP4741275A1EP 4741275 A1EP4741275 A1EP 4741275A1EP-4741275-A1

Abstract

The invention is directed to a reusable tension element for a tendon for tension leg platforms, said tension element comprising a lamella having an essentially rectangular cross-section with an aspect ratio that is defined as the ratio between the longest side and the shortest side of the rectangle of at least 10, preferably at least 15, wherein said lamella has thickness of at least 1 mm, and a length of at least 20 m, wherein said lamella comprises a material having an elastic modulus of more than 50 GPa. Further aspects of the invention are a tendon comprising said elements, a transportation system to transport said elements, and a tension leg platform that is tethered to a seabed with said tendons.

Inventors

  • Eblagon, Fernando Andres

Assignees

  • Lankhorst Euronete Portugal, S.A.

Dates

Publication Date
20260513
Application Date
20241108

Claims (16)

  1. A tension element for use in a tendon for tethering tension leg platforms, said tension element comprising a lamella having an essentially rectangular cross-section with an aspect ratio that is defined as the ratio between the longest side and the shortest side of the rectangle of at least 10, preferably at least 15, wherein said lamella has a thickness of at least 1 mm, and a length of at least 20 m, wherein said lamella comprises a material having an elastic modulus of more than 50 GPa, as determined according to the ISO 527 series of standards.
  2. The tension element according to claim 1, wherein the material comprises metal and/or fiber composite, preferably steel, stainless steel, silicon nitride, silicon carbide, glass fiber composite, carbon fiber composite and/or ceramic fiber composite, more preferably carbon fiber composite.
  3. The tension element according to any one of the previous claims, wherein the material has an elastic modulus of more than 100 GPa, preferably more than 130 GPa, more preferably in the range of 130 GPa to 160 GPa, as determined according to the ISO 527 series of standards.
  4. The tension element according to any one of the previous claims, wherein the material has a compressive strength of more than 1 GPa, preferably more than 1.5 GPa, more preferably in the range of 1.5 GPa to 2 GPa, as determined according to the ISO 14126 standard.
  5. The tension element according to any one of the previous claims, having a length of at least 50 m, more preferably in the range of 50 to 500 m and/or a width in the range of 0.1 m to 2 m.
  6. The tension element according to any one of the previous claims, having a thickness in the range of 1 to 8 mm, preferably 1 to 6 mm.
  7. The tension element according to any one of the previous claims, comprising one or more through holes at one end of the lamella and/or one or more through holes at the other end of the lamella.
  8. The tension element according to any one of the previous claims, comprising a first terminator at one end of the lamella and second terminator of the other end of the lamella.
  9. A tendon for deepwater tension leg platforms comprising a plurality of tension elements in accordance with any one of the previous claims.
  10. The tendon in accordance with claim 9, wherein the plurality of tension elements is joined together at both ends of each the lamellae of said tension elements.
  11. The tendon in accordance with any one of claims 9-10, further comprising a first mooring line connector at one end of the tension elements and a second mooring line connector at the other end of the tension elements.
  12. The tendon in accordance with any one of claims 9-11, the lamellae of the tension elements are oriented in the same direction and wherein preferably at least part of the tension elements are arranged in a stacked configuration and/or at least part of the tension elements are arranged in coplanar and parallel configuration.
  13. A tension leg platform such as an offshore wind turbine, comprising a floating platform and one or more tendons according to any one of claims 9-12 attached to the floating platform.
  14. The tension element according to any one of claims 1-8, which is wound onto a bobbin to a wound configuration with an outer diameter of less than 2.9 m, preferably less than 2.5 m.
  15. A transport system comprising a frame and a set of tension elements in accordance with claim 14, which set is placed on said frame and said frame preferably fits into a 20-foot container or a 40-foot container.
  16. Method for reusing the tension element or the tendon according to any one of claims 1-12 that is attached to a tension leg platform (TLP), said method comprising reversibly disconnecting the tension element from the TLP, for instance from the mooring line connector, and winding the tension element onto a bobbin, placing the wound tension element on a transport frames, and transporting the wound tension element to a different site or platform for further use, optionally followed by unwinding the tension element and constructing a new tendon with the unwound tension element.

Description

FIELD The invention is in the field of mooring systems for tension leg platforms (TLPs). The invention is particularly directed to a tendon and reusable elements therefor for tethering deepwater TLPs. BACKGROUND Mooring systems keep TLPs in position by tethering the TLP under tension to the bottom of the body of water (sea or ocean) it is floating on. Mooring systems for these purposes require extremely high rigidity in order to guarantee stability on the floating platform. As such, those as developed in the 1970s rely on steel pipes or steel wire mooring lines due to their high stiffness. Examples of steel tubes that have been used extensively for TLPs in the past are described in US4297965A, US3648638A and US3780685A. However, due to the extremely high stiffness of the tethers and their circular cross-section, the flexural stiffness is also high, leading to a large minimum bending radius, or no allowable bending at all as in the case of tubes. This in turn translates onto extremely long steel tubes that need to be transported on barges or wire ropes that have to be wound for transport and installation onto large diameter bobbins, up to 15 meters in diameter. Transportation and installation are difficult, expensive and cumbersome. These mooring systems require specialized transport and installation vessels which not only make them more expensive to install but also limit their scalability due to the limited availability of specialized vessels. As such, these systems can typically only be produced on a factory located next to a port facility due to the difficulties in transporting them. Moreover, the conventional steel-based systems have a limited lifetime in corrosive environments such as seawater which requires periodical inspection and replacement of the mooring tethers. Yet a further drawback of steel-based systems is their heavy weight. A paper by Jackson et al. in Offshore Technology Conference (2005), OTC 17535 discloses a pultruded carbon fiber reinforced plastic (CFRP) rod for ultra-deepwater mooring line application such as mobile offshore drilling units (MODUs) and reviews other technologies such as polyester, aramide and HMPE as the main synthetic fibers for deepwater and ultra-deepwater applications. A drawback of such fiber ropes, however, is that due to their circular cross-section, fiber ropes also have the same limitations as steel wire in terms of high flexural stiffness and minimum bending radius which leads to very large bobbins. Also, for the same stiffness, synthetic ropes require a bigger diameter due to the lower elastic modulus of synthetic fibers compared to steel wire. This effect punishes synthetic ropes even further leading to even bigger rope coils. A paper by Sparks et al. in Offshore Technology Conference (2003) OTC 15164 describes a rod tendon that is composed of a two-level hierarchy construction. The first level is composed of 19 rods (profiles) of 6 mm in diameter that are grouped together and clamped to a termination to form a sub-element having a circular cross-sectional shape. The second level is composed of 37 sub-elements to form a tendon, which also has a circular cross-sectional shape. Both the sub-elements and the tendon have to be assembled on site as even the 6 mm rods have a minimum winding diameter exceeding the road-transportable size when they reach longer lengths. There is a desire to provide a mooring system that can readily be transported via the road, is compatible with the high stiffness requirements of tension leg platforms and can easily and economically be installed using commonly available Anchor Handling Tug Supply Vessels (ARTS). As such, quayside or offshore assembly can be obviated. Time on vessels or on the quayside is extremely expensive and any operation that can be avoided or sped up reduces the cost of installation significantly. As a reference, typical cost of an ARTS vessel is in the order of tens of thousands of euros per day. SUMMARY The present inventor realized that the high tensile stiffness requirements of tension leg platforms and the low flexural stiffness requirements for road-transport can be coincided by providing a modular system, wherein a tendon is constructed from tension elements and which tension elements each have an appropriate geometry such that they can readily be joined onsite without requiring specialized transportation methods and/or construction facilities. The inventor realized that the circular cross-sectional shape of conventional tendons and elements therefor, limit their transportability. Accordingly, the present invention is directed to a tension element for a tendon for tension leg platforms, said tension element comprising a lamella having an essentially rectangular cross-section with an aspect ratio that is defined as the ratio between the longest side and the shortest side of the rectangle of at least 10, preferably at least 15, wherein said lamella has a thickness of at least 1 mm, a length of at least