Mooring system and installation method for a floating structure

Abstract

The present invention relates to a mooring system (5) for a floating structure (3), preferably for a floating wind turbine platform, said mooring system (5) comprising at least one mooring line (51) made of synthetic material and without chain sections, said mooring line (51) comprising a wear-resistant section (52) connected to an anchor (53), said wear-resistant section (52) being in permanent or at least intermittent contact with the seabed (Sb), said anchor (53) protruding from the seabed (Sb) or buried below the seabed (Sb).

Description

[Technical Field]

[0001] The present invention relates to floating structures, for example floating structures for offshore wind turbines, and more precisely to the subsea mooring of floating structures to the seabed. [Background technology]

[0002] For the installation of offshore wind units, which comprise a turbine and a support connected to the seabed, it is well known to use a support structure, such as a jacket structure. Such a support structure rests on the seabed and is fixed to the ground using anchoring devices. The support structure extends above the sea surface to accommodate the wind turbine mast. Generally, this support structure is made in one piece, and the greater the depth, the higher the support structure must be. However, this solution is expensive.

[0003] Another solution for installing offshore wind units is to use floating structures moored to the seabed with mooring lines. This solution requires the use of floating technology. Floating offshore wind units are then: a floating structure designed to support a turbine, for example a rotor nacelle assembly (RNA) with blades, hub and nacelle, and tower, or to support some other element such as an electrical substation; a station keeping system corresponding to the mooring lines and anchors, allowing the system to remain in place; It can be subdivided as follows:

[0004] Mooring lines are typically made of steel chains and lengths of synthetic material, such as polyester. This type of mooring line requires additional effort during installation when standard procedures are followed. In the floating offshore wind farm domain, chains have proven susceptible to fatigue due to the large tension cycles caused by the floating structure's motion, which occur with high reproducibility, in addition to external loads (e.g., wind turbine thrust) that must be recovered by the mooring system. These external loads and constraints increase as the size of the floating structure and wind turbines increase and their rotational speeds increase. As a result, it has been observed that the diameter of mooring chains is continuously increasing with increasing wind turbine power output, up to the limits of supply chain capacity. This increase in chain diameter significantly increases costs, restricts the supply chain, and complicates mooring line installation. Furthermore, feedback from existing floating structures, for example in the oil and gas domain, indicates that chains are the main cause of mooring line breakages (approximately 50%).

[0005] Furthermore, it has often been found beneficial to insert more or less long lengths of synthetic material, such as polyester, into the mooring line assemblies. This material is used in many floating offshore wind farms due to its excellent properties: low line weight, high strength, good fatigue resistance, and compliant stiffness (relatively low EA compared to standard steel mooring elements). Moreover, from an economic point of view, synthetic materials are less expensive to manufacture and more cost-effective compared to steel materials, making them even more suitable in the floating offshore wind turbine domain.

[0006] However, synthetic materials such as polyester exhibit visco-elasto-plastic behavior, undergoing changes in stiffness and elongation properties over time depending on past loading. These changes may be more or less reversible depending on the time the rope is left unstressed (i.e., the time it takes for the recovery process to occur).

[0007] A known solution to address this issue with synthetic materials is to perform a pre-tensioning operation during mooring line installation, which consists in applying a load to the mooring line using an installation vessel in order to stretch the synthetic material of the mooring line assembly, so that the high loads observed, for example, during a storm, do not cause additional permanent stretching of the rope, at least reducing this stretch to an acceptable level.

[0008] However, to be beneficial, this pre-tensioning operation must be performed in such a way as to ensure that tension is maintained in the ropes, otherwise the ropes will retract and the benefit of the pre-tensioning operation will be lost. In practice, maintaining tension in the mooring lines after a pre-tensioning operation is not possible in the field of floating offshore wind turbines, as the operation is performed away from the floating structure before hooking the mooring lines to the floating structure.

[0009] Furthermore, a commercial wind farm typically consists of 20-30 floating structures, with three to six mooring lines per unit, requiring 60-180 pre-tensioning operations, which significantly increases the deployment time of the offshore spreads and impacts project costs. Summary of the Invention [Problem to be solved by the invention]

[0010] One object of the present invention is to provide an improved, inexpensive mooring system for offshore wind turbines with composite mooring lines. [Means for solving the problem]

[0011] To this end, the present invention relates to a mooring system for a floating structure, preferably a floating wind turbine platform, comprising at least one mooring line made of synthetic material and having no chain section, the mooring line comprising a wear-resistant section connected to an anchor, the wear-resistant section being in permanent or at least intermittent contact with the seabed.

[0012] The anchor may be buried beneath the seabed.

[0013] The anchor may protrude from the seabed.

[0014] The present invention also relates to a mooring system for a floating structure, preferably a floating wind turbine platform, comprising at least one mooring line made of synthetic material connected directly to an anchor, the mooring line comprising a protective section connected to the anchor, the protective section being in permanent or at least intermittent contact with the seabed.

[0015] The anchor may protrude from the seabed.

[0016] The anchor may be buried beneath the seabed.

[0017] The present invention also relates to a method for installing a floating structure, said method comprising the step of installing at least one mooring line made of synthetic material, the length of the synthetic mooring line being 1-4% shorter at equal tension than the calculated design length of the synthetic mooring line suitable for the floating structure at its implantation site.

[0018] According to the installation method, after a period of passive pre-tensioning, the length of the synthetic mooring line is extended to the calculated design length of the synthetic mooring line suitable for the floating structure at the landing site.

[0019] Depending on the installation method, after a period of passive pre-tensioning, the quasi-static stiffness of the synthetic material of the mooring line can increase between 10% and 100%.

[0020] According to the installation method, the step of installing at least one mooring line made of synthetic material includes: a first step of connecting the bottom of the mooring line to its anchor on the seabed; The second step is towing the floating structure on site; A third step is to hook the mooring lines to the floating structure; may include:

[0021] According to the installation method, the mooring line may comprise at least one buoyant segment or may be connected to at least one submerged buoy at least between the first and third steps. [Brief explanation of the drawings]

[0022] Further characteristics and advantages of the invention will become apparent from the following description, given by way of non-limiting example with reference to the accompanying drawings, in which:

[0023] [Figure 1] FIG. 2 shows a side view of a schematic illustration of an offshore wind farm of wind turbines with one mooring line according to a first embodiment in a first variant. [Figure 2] FIG. 1 shows a side view of a schematic illustration of an offshore wind farm of wind turbines with one mooring line according to the first embodiment in a second variant. [Figure 3] FIG. 3 is a side view of the schematic illustration of the offshore wind farm of wind turbines of FIG. 2 in a first state; [Figure 4] FIG. 3 is a side view of a schematic illustration of an offshore wind farm of wind turbines of FIG. 2 in a second state. [Figure 5] FIG. 1 shows a side view of a schematic illustration of an offshore wind farm of wind turbines with one mooring line according to a second embodiment of the first variant. [Figure 6] FIG. 6 is a side view of the schematic illustration of the offshore wind farm of wind turbines of FIG. 5 in a first state. [Figure 7] FIG. 6 is a side view of the schematic illustration of the offshore wind farm of wind turbines of FIG. 5 in a second state. [Figure 8] FIG. 10 shows a side view of a schematic illustration of an offshore wind farm of wind turbines with one mooring line according to a second embodiment in a second variant.

[0024] In these figures, like elements have like reference numerals. The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference pertains to the same embodiment or that a feature applies only to a single embodiment. Individual features of different embodiments can be combined or interchanged to provide other embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0025] 1 to 8 show a floating structure 3, for example a floating wind turbine platform, designed to receive a wind turbine 4 and float above the sea surface S1. The floating structure 3 is moored to the seabed Sb by a mooring system 5.

[0026] The mooring system 5 comprises at least one mooring line 51 made of synthetic material and without any chain section, by which is meant a segment of two or more chain links made of steel.

[0027] In the first embodiment shown in Figures 1 to 4, the mooring line 51 has a wear-resistant section 52 made of synthetic material and connected to an anchor 53. The wear-resistant section 52 is in permanent or at least intermittent contact with the seabed. This wear-resistant section 52 is connected to the anchor 53 which moor the floating structure 3 to the seabed Sb.

[0028] This wear-resistant section 52 may be, for example, up to the last 5% of the length of the mooring line 51. In this first embodiment, the mooring line 51 only comprises a section made of synthetic material and a wear-resistant section 52. This wear-resistant section 52 may be, for example, a metal cable or a rope section made of High Modulus Polyethylene.

[0029] As shown in Figure 1, the anchor 53 is, for example, buried beneath the seabed Sb. The wear-resistant section 52 is then in permanent contact with the seabed. The wear-resistant section 52 of the mooring line 51 allows the mooring line 51 to be connected to this buried anchor 53 without the synthetic material becoming embedded in the seabed Sb. Indeed, synthetic materials are sensitive to wear and can deteriorate due to friction with sediments and the seabed Sb.

[0030] 2 to 4, the anchor 53 protrudes from, for example, the seabed Sb, and the wear-resistant section 52 then intermittently contacts the seabed.

[0031] In fact, in calm weather as shown in Figure 3, the wear-resistant section 52 remains above the seabed Sb without touching it. To avoid any contact of the synthetic material of the mooring line 51, said mooring line 51 can be provided with at least one buoyant segment 54 or can be connected to at least one underwater buoy.

[0032] However, in bad weather, for example, the floating structure 3 may be pushed by wind or current, as shown in Figure 4. The mooring lines 51 are then tensioned on one side and relaxed on the other side. In these conditions, the wear-resistant sections 52 may come into intermittent contact with the seabed Sb, as shown in Figure 4.

[0033] In the second embodiment shown in Figures 5 to 7, the mooring line 51 is made of synthetic material and is directly connected to the anchor 53. Direct connection to the anchor 53 means that, unlike the first embodiment, the mooring line 51 does not have an abrasion-resistant section 52.

[0034] According to a first variant of the second embodiment shown in Figures 5 to 7, the anchor 53 protrudes from the seabed Sb.

[0035] The mooring line 51 may include at least one protective section 51 a for protecting the synthetic material from friction with the sediments and the seabed Sb. This protective section 51 a is in permanent or at least intermittent contact with the seabed. This at least one protective section 51 a may be any section of the mooring line 51 that may be in contact with or buried in the sediments and the seabed Sb. This at least one section of the mooring line 51 may be coated with a protective layer or inserted into a sleeve.

[0036] In fact, in calm weather as shown in Figure 6, the protected part 51a remains above the seabed Sb without touching it. To avoid any contact of the synthetic material of the mooring line 51, said mooring line 51 can be provided with at least one buoyant segment 54 or can be connected to at least one underwater buoy.

[0037] However, in bad weather, for example, the floating structure 3 may be pushed by wind or water currents, as shown in Figure 4. The mooring lines 51 are then tensioned on one side and relaxed on the other side. In these conditions, the protected section 51a may come into intermittent contact with the seabed Sb, as shown in Figure 7.

[0038] If the anchor 53 is buried under the seabed Sb, as shown in the second variant of Figure 8, the section of the mooring line 51 connected to the anchor 53 and partially buried can be the protected section. The protected section 51a is then partially buried and in permanent contact with the seabed.

[0039] The synthetic material of the mooring lines 51 may be selected from, for example, polyester or polyamides such as nylon.

[0040] In both embodiments, the end of the mooring line 51 that is hooked to the floating structure 3 may comprise a short metal section (not shown) made of metal cable or a rope section made of high modulus polyethylene.

[0041] This mooring line 51 without chain sections is a cheaper mooring line than a mooring line with chain sections. Indeed, as the size of the floating structure 3 and the wind turbine 4 increases, the size and cost of the chain links also increase. Since the installation of the floating structure 3 and the entire offshore wind farm is easier with the composite mooring line 51, which is lighter and easier to handle than chain, the requirements on installation vessel capacity are reduced. The ships and vessels required for installation are smaller and cheaper.

[0042] The present invention also relates to a method for installing a floating structure 3, the method comprising the step of installing at least one mooring line 51 made of synthetic material, wherein, upon installation, the length of the synthetic mooring line 51 is 1-4% shorter at equal tension than the calculated design length of the synthetic mooring line suitable for the floating structure 3 at its landing site.

[0043] The calculated design lengths correspond to mooring line lengths resulting from a standard mooring design, which adjusts the mooring line lengths and strengths to ensure that the maximum flotation offset remains within acceptable limits (typically less than 50% of the water depth in shallow waters, i.e., less than 100m depth) and that the maximum mooring line tension is typically less than 60% of the minimum breaking load under an extreme event (typically a 1 in 50 year storm).

[0044] As described above in the different embodiments, the mooring lines 51 are made of synthetic material and do not have chain sections.

[0045] Due to the shorter length of the mooring lines 51 compared to the calculated design length, the mooring lines 51 connected to the anchors 53 and the floating structure 3 are under higher tension during installation. Due to this increased tension in the mooring lines 51 during installation, active pre-tensioning of the mooring lines 51 can be avoided. This eliminates the need to use dedicated vessels for the pre-tensioning work and the need to equip the floating structure 3 with high-capacity tensioning means. The installation of the floating structure 3, and thus of the entire offshore wind farm, is cheaper and easier.

[0046] Pre-stretching of the mooring lines 51 occurs passively during the first few weeks or months of operation of the offshore wind turbine and exposure to environmental loads. Wind, swell, and annual storms create significant tension in the mooring lines 51, stretching them similarly to active pre-stretching. Unlike active pre-stretching, which uses high-tension marine spreads to over-tension the synthetic lines during the installation process (40% of minimum breaking load is common in the industry), passive pre-stretching uses environmental loads (waves, wind, currents) to apply high tension to the synthetic lines, resulting in a permanent stretch.

[0047] The invention consists in installing the lines with intentionally shortened lengths of the composite mooring ropes (1% to 4% as mentioned above) in order to predict the future elongation of the ropes caused by environmental loads shortly after installation and reach the "nominal" or "calculated design" rope length.

[0048] For example, for a floating structure 3 moored at a depth of 70-100 m with several polyester mooring lines 51, the tension during installation may be 12%-20% of the minimum break limit of the mooring lines 51, in order for the nominal tension to reach 3%-10% of the minimum break. After a passive pre-tensioning period, the tension in the mooring lines 51 reaches the aforementioned target nominal tension.

[0049] Pre-stretching also affects the quasi-static stiffness of the synthetic material of the mooring line 51. Quasi-static stiffness refers to the stiffness of the mooring line 51 when a load is applied slowly, allowing time for both the amorphous and crystalline portions of the synthetic material to react to the load. The resulting fiber stiffness is the average of the stiffness of both the amorphous and crystalline portions.

[0050] After the passive pre-tensioning period, the length of the synthetic mooring line 51 (initially 1% to 4% shorter than the calculated design length) is extended to the calculated design length of the synthetic mooring line suitable for the floating structure 3 at its landing location.

[0051] After the passive pre-tensioning period, the quasi-static stiffness of the synthetic material of the mooring line 51 may increase between 10% and 100%. For polyamide mooring lines 51, such as nylon, the increase in quasi-static stiffness may be between 10% and 50%. For polyester mooring lines 51, the increase in quasi-static stiffness may be between 30% and 100%.

[0052] The step of installing at least one mooring line 51 made of synthetic material comprises: a first step of connecting the bottom of the mooring line 51 to its anchor 53 on the seabed Sb; The second step is towing the floating structure on site; a third step of hooking the mooring lines 51 to the floating structure 3; may include:

[0053] During the first step, the mooring lines 51 are typically connected to anchors 53. These anchors 53 can be pre-installed on the seabed Sb or can be installed during this first step with the mooring lines 51 already connected. If the exact position of the anchors 53 is known, particularly in the case of towing anchors, the length of the mooring lines can be adjusted during installation so that the synthetic mooring lines 51 are, at equal tension, 1 to 4% shorter than the calculated design length of the synthetic mooring lines suitable for the floating structure 3 at its landing site.

[0054] During the second step, the floating structure 3 is towed to the site by known means, for example on a barge or towed to the site.

[0055] During the third step, the mooring lines 51 are typically lifted and pulled in order to be hooked to the floating structure 3. During this third step, the mooring lines 51 are subjected to higher tensions, as mentioned above, due to their shorter installed length compared to the calculated design length.

[0056] Between the first step of connecting the bottom of the mooring line 51 to its anchor 53 on the seabed Sb and the third step of hooking the mooring line 51 to the floating structure 3, at least one buoyant segment 54 can be installed on the mooring line 51 to prevent the synthetic material from coming into contact with the sediments and the seabed.

Claims

1. A mooring system (5) for a floating structure (3), preferably for a floating wind turbine platform, comprising: The mooring system (5) comprises at least one mooring line (51) made of synthetic material and having no chain section, the mooring line (51) comprising a wear-resistant section (52) connected to an anchor (53), the wear-resistant section (52) being in permanent or at least intermittent contact with the seabed.

2. 2. The mooring system (5) according to claim 1, wherein the anchor (53) is buried below the seabed (Sb).

3. 2. The mooring system (5) according to claim 1, wherein the anchor (53) protrudes from the seabed (Sb).

4. A mooring system (5) for a floating structure (3), preferably for a floating wind turbine platform, comprising: The mooring system (5) comprises at least one mooring line (51) made of synthetic material connected directly to an anchor (53), the mooring line (51) comprising a protective section (51 a) connected to the anchor (53), the protective section (51 a) being in permanent or at least intermittent contact with the seabed.

5. 5. A mooring system (5) according to claim 4, wherein the anchor (53) protrudes from the seabed (Sb).

6. 5. A mooring system (5) according to claim 4, wherein the anchor (53) is buried below the seabed (Sb).

7. 1. A method for installing a floating structure (3), the method comprising the step of installing at least one mooring line (51) made of synthetic material, the length of the mooring line (51) made of synthetic material being 1-4% shorter, at equal tension, than a calculated design length of the mooring line made of synthetic material suitable for the floating structure (3) at its landing site.

8. 8. The installation method according to claim 7, wherein after a passive pre-tensioning period, the length of the synthetic mooring line (51) extends to the calculated design length of the synthetic mooring line suitable for the floating structure (3) at its landing site.

9. 9. The installation method according to claim 7 or 8, wherein after a period of passive pre-tensioning the quasi-static stiffness of the synthetic material of the mooring line (51) increases by between 10% and 100%.

10. The step of installing at least one mooring line (51) made of synthetic material comprises: a first step of connecting the bottom of said mooring line (51) to its anchor (53) at the seabed (Sb); a second step of towing the floating structure (3) on site; a third step of hooking the mooring lines (51) to the floating structure (3); The installation method according to any one of claims 7 to 9, comprising:

11. 11. The installation method of claim 10, wherein the mooring line (51) comprises at least one buoyant segment (54) or is connected to at least one underwater buoy at least between the first step and the third step.

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