Description

Technical Field

[0001 ] The invention relates to a mooring system for mooring a floating object, in particular relates to mooring lines for a mooring system.

Background Art

[0002] Floating facilities have proved an increasingly important solution for offshore oil&gas and offshore wind turbines (FWTs), or where infrastructure is limited. Therefore, floating production systems usage is geographically widespread operating in varying environmental conditions. Permanent moorings for production systems, by their nature are required to offer long term performance, in some cases in excess of 30 years, increasing the potential for extreme or unknown operating conditions.

[0003] Compliant station keeping mooring systems (e.g., those used to keep FWTs in position) can be configured as either catenary, taut or a combination of the two (normally referred to as “hybrid”). Catenary systems use the selfweight of the mooring line, normally steel chains, to generate a restoring force. For low depths, chain or steel rope might be the best solution in a catenary set-up. For much higher depths though, they are expensive, corrosion sensitive and heavy.

[0004] Taut mooring systems use the strain of the mooring line to generate a restoring force. Synthetic ropes are mainly used in deep and ultradeep waters, thanks to their lightweight and high elasticity. Amongst synthetic ropes, polyester is most common. Polyester ropes are used in semi-taut and taut mooring systems, allowing for efficient mooring systems at lower costs. However, other synthetic ropes are also used such as nylon, high modulus polyethylene (HMPE) or aramid. At intermediate depths (~100 m), nylon rope has shown some advantages. Due to its higher relative elasticity, the nylon rope is less quickly permanently damaged if it is elongated and released. If at medium depths, there are high waves during a storm, the elongation on a mooring line could go beyond the maximum elongation allowed, without permanent deterioration, if the line is not very long.

[0005] Catenary mooring is long established but has many disadvantages e.g., corrosion, weight, poor fatigue resistance of chain, complex and dangerous handling characteristics and a tendency to generate unacceptable tensions in extreme conditions. Taut mooring overcomes the challenges associated with chain catenary but introduces additional challenges associated with the length stability of fiber ropes. Nylon might allow 18% elongation without permanent damage, whereas polyester (14%), dyneema (3%), or aramid (3%) is much less.

[0006] The demands on station keeping equipment are continuously evolving, requiring a novel and innovative approach to equipment and materials utilized.

Summary of Invention

[0007] The inventors have identified that the use of materials with characteristics not normally considered for taut mooring can significantly improve the performance of taut mooring systems by decreasing offset without increasing peak tensions.

[0008] Taut mooring system design must achieve a balance between two often contradictory requirements:

[0009] Limit the lateral movement of a floater (“offset”) when a force load is applied e.g., wind loads, currents or turbine thrust,

[0010] Minimize the tension variations caused by floater motions from e.g., wave motion.

[0011 ] Offset is of critical importance where the floater is connected to other subsea equipment with limited flex/tension fatigue or elongation capacity e.g., steel risers or electricity export cables.

[0012] Currently the industry relies on some simplified models that capture the most important characteristics and at the same time yield conservative prediction of line tensions and vessel offsets.

[0013] The improvement offered by the material used for the present application can best be understood using the “static-dynamic model” described in ABS’s Guidance Notes on the Application of Fiber Ropes for Offshore Mooring. As shown in Fig.1 , this model uses a bi-linear slope with an inflection point at the mean tension. Below mean-tension, the slope is defined using a relevant static stiffness. Above the mean tension the slope is defined by using a relevant dynamic stiffness up to the peak tension. The effects of construction elongation and creep can be added to this model by applying an offset to the strain axis.

[0014] The static-dynamic model was developed to account for the fiber rope elongation behavior. In this model, the static stiffness is utilized for the initial region of the loading curve up to the mean load. Afterwards, the dynamic stiffness is used to predict the cyclic part of the loading (Figure 1 ).

[0015] As shown by curve I in Fig.1 , the total offset of a mooring line includes four parts: construction elongation (A), static elongation (B), dynamic elongation (C) and elongation due to polymer creep effect (D). The static elongation (B), dynamic elongation (C) and elongation due to polymer creep (D) are very much dependent on the material used for mooring lines.

[0016] Whilst taut mooring offers many operational and performance advantages, the length instability of conventional fiber ropes creates a challenge which is complex and expensive to overcome.

[0017] This model more accurately simulates the actual conditions faced by a fiber rope mooring out at sea. A mooring line under a severe environment typically experiences a steady mean load and dynamic loads oscillating around the mean load.

[0018] To minimize offset, a mooring line with high stiffness is preferred, whilst to minimize tensions caused by floater motions a low stiffness is preferred. In modelling and simulation of mooring systems two categories of stiffness are considered: “static stiffness” and “dynamic stiffness.” Static stiffness is used to represent the response to force-based loads, whilst dynamic stiffness is used to represent the response to motion based loads. Both static and dynamic stiffnesses are dependent on the tensions at which they are measured and are therefore represented by a range of values.

[0019] According to the present invention, there is provided a mooring system used for mooring a floating object, comprising a mooring line configured for extending between said object floating in water and an anchoring device, wherein said mooring line is made from polyoxymethylene and the ratio of static stiffness to dynamic stiffness of said line is preferably close to one. The ratio of static stiffness to dynamic stiffness of said line can be in a range of 0.8 to 1 and preferably in a range of 0.9 to 1 .

[0020] Rope axial stiffness may be defined as the secant stiffness, or the slope of the line connecting the extrema during a tensile load-unload cycle. The range of the load in the load-unload cycle may be between 5 and 50% of the rope breaking strength. The dynamic stiffness is defined with a cycle time of 10-30 seconds. The static stiffness is defined at a cycle time of 30- 120 minutes. Measurement of the static stiffness and the dynamic stiffness can be referred to ISO 18692-1 :2018.

[0021 ] The "line" of the invention can be any devices giving compliance such as a load reduction device or spring. Preferably, the line is a rope.

[0022] Definition of static and dynamic stiffness is known is the art. The stiffness of a fiber or polymer rope is expressed as:

[0024] where AF is the change in load, As is change in strain, and EA is the stiffness or the modulus times the cross-sectional area of the rope. The stiffness is usually expressed in units of force such as kN or kips. Equivalently, a non- dimensional stiffness Kr can also be expressed as:

:° ⁰²⁵ : ^K ’ = -Si . (1-2)

[0026] where MBS is the Minimum Breaking Strength.

[0027] The definition above can be applied for a single material and for construction but different sizing.

[0028] For conventional materials, like polyester ropes as well as other fiber ropes are made of materials with visco-elastic properties, so their stiffness characteristics are not constant and vary with the load duration and magnitude, the number and frequency of load cycles, and the loading history.

[0029] For conventional materials used in taut mooring lines (e.g., polyester and nylon) the range of dynamic stiffness values, as shown in table 1 below, are approximately 1.5 - 2 times the static stiffness values. This is counter to the required optimization. Failure to account for this behavior will inevitably yield inaccurate line tension and mooring object offset predictions, and the inaccuracy for mooring object offset can be particularly large.

[0030] Table 1 Typical static and dynamic stiffness ranges

[0031 ] In general, polyester mooring lines become stiffer after a long time in service. Historical loading above a certain level may lead to a permanent increase of the rope length and results in a softer mooring system if no retensioning is performed. Because of this complex rope behavior, it is not possible to develop models that represent the precise stiffness characteristics of the rope.

[0032] Because an increase in length often leads to unacceptable offsets, additional components must be introduced to taut mooring systems (such as in line chain tensioners) to allow line length to be adjusted and hence reduce total offset. These components are expensive to purchase, require complex and expensive intervention procedures later in life and inhibit optimized mooring system design due to the added drag and weight of the components. Additionally, these components require steel chain for operation with all its attendant limitations.

[0033] Total offset is the sum of the horizontal components of construction elongation, static elongation (strain due to static stiffness), dynamic elongation (strain due to dynamic stiffness) and creep. To reduce total offset, the state of the art is to “pre-stretch” mooring lines to remove the construction elongation at the start of life. However, as offset is the sum of four components it can be understood that the total offset can be reduced in theory by reducing any of the four contributing components.

[0034] A new approach to taut mooring has been identified by the present invention, wherein the mooring rope is made from polyoxymethylene and the ratio of static stiffness to dynamic stiffness of the polymer rope is close to one. As shown Fig. 1 , curve II represents a linear stiffness of the rope as mooring lines according to the present invention. [0035] In the mooring system of the present invention, there is no length adjustment requirement or tensioner during operation or service life of said mooring system. There is no lifetime intervention for the mooring lines of the present invention.

[0036] The linear stiffness of mooring lines can be achieved by a specific polymer material polyoxymethylene (POM), which has different characteristics to those of nylon and polyester in that its stress strain behavior is largely regressive. The stress strain behavior of POM compared with nylon and polyester is shown in Fig. 2.

[0037] The mooring line is made from polyoxymethylene (POM). The mooring line can comprise 100 wt% of POM material. However, the mooring line may comprise e.g., about 90 wt% or more than 80 wt% of POM as core and have a polymer jacket which is not POM, wherein the core is sheathed by the polymer jacket.

[0038] The regressive stress strain characteristics lead to a relative increase in static stiffness and a relative decrease in dynamic stiffness values. This relative change results in that static and dynamic values which are similar, rather than the 1.5-2 times increase in dynamic stiffness typical of nylon and polyester. This in turn reduces the unwanted offsets caused by forcebased loads, without significantly impacting the compliance to motionbased loads (necessary offset caused by dynamic stiffness). Consequently, for the same peak tension and hence rope size, a significantly reduced offset can be achieved using the proposed materials. This offset would be impossible with conventional materials such as nylon or polyester.

[0039] Comparing with a typical nylon static-dynamic curve I with that of a regressive material curve II as shown in Fig.1 , it can be observed that for the same peak tension, the total offset is significantly reduced. In a preferred example, the offset can be reduced up to 35%.

[0040] According to the present invention, said static stiffness and dynamic stiffness can be in the range of 10 to 20 MBS (Minimum Breaking Strength). In other word, said static stiffness and dynamic stiffness can be in a range of 7 to 15 GPa. In addition, the mooring line can have a tensile strength above 500 MPa, e.g., in a range of 500 to1400 MPa. [0041 ] With respect to the construction of a rope as a mooring line, it can be in varied formats. For instance, the rope can be made from a plurality of polymer monofilaments laid in a spiral shape. The plurality of polymer monofilaments can have a diameter between 0.5 mm and 5 mm. Alternatively, the rope can be a strand wire rope, e.g., having 6 or 8 strands. The ropes can be produced from POM monofilament or multifilament fiber. Fibers are twisted into yarns and the yarns may be twisted into strands. These strands are then used to produce a number of possible rope structures. For instance, some possible POM rope constructions are:

1 ) Single braided ropes consisting of equal number of left and right- handed strands. Possible number of strands are 6, 8, 12, and 16. So-called "single braided ropes" consist of an even number of strands braided according to a circular pattern, half of them clockwise, the other half counterclockwise. This type of rope offers a somewhat higher breaking force than a twisted rope, which makes it a bit more expensive.

2) Single braided ropes as in above item 1 ) but where the strands themselves are individual rope structures either: a. 3 strand laid ropes b. 12 strand braided ropes.

3) Laid (or twisted) ropes consisting of 3 or 4 strands laid in either the left or right hand direction.

4) Multilayer laid ropes where strands are laid in a helical manner in alternating left and right hand layers.

5) Jacketed ropes comprising a core of any of the above structures but with a braided or extruded jacket covering the core: a. particular embodiments include a single braided core, multiple parallel braided cores or multiple parallel laid cores b. number of strand in the cover can be 16,20,24,32,48 where each “strand” can consist of 1 , 2 or 3 parallel strand groups.

[0042] The rope of the present invention is preferably sheathed by a polymer jacket. The polymer jacket can be chosen from the group consisting of: epoxy, polyamide, polyurethane, polyester (PES), polyethylene terephthalate (PET), nylon (PA), polypropylene (PP), polyethylene (PE), polypropylene-polyethylene (PP-PE) copolymer, polyethylene napthalate (PEN), high modulus polyethylene (HMPE), polyester polyacrylate (LCP), para-aramid, aramid copolymer, or any combination or blend of the above. [0043] The rope can have an elastic elongation above 10%. There is no limitation on the diameter of the rope. For instance, the rope can have a diameter between 100 mm to 300 mm.

[0044] In order to get a more desired stress strain response and the overall characteristics as shown by curve II in Fig.1 , the polymer material of the invention polymer rope mooring lines preferably have at least 60% crystallinity, more preferably at least 80% crystallinity. In addition, preferably at least 50% of the crystalline polymer is orientated and more preferably at least 70% of the crystalline polymer is orientated. This can be accomplished by keeping the semi-molten polymer to grow crystallinity and then draw the polymer rod with a high drawing ration to orient the crystals. When the polymer is stretched the molecules may be preferentially aligned along the stretching direction.

[0045] The desired and accurate offset predication of the mooring lines is very much dependent on the behavior of polymer material, especially the elongation behavior. This is linked with the macro-molecular structure of the material, i.e., “morphology”. The morphology of polymer materials typically shows crystalline parts and non-crystalline, i.e., amorphous parts. Static stiffness is the stiffness of a tension member when it is loaded slowly, leaving time for both the amorphous and crystalline part to react the load. The resulting fiber stiffness is an average of the stiffness of both parts. Dynamic stiffness is the stiffness response of a tension member when it is under cyclic loading. As the amorphous part does not react fast enough to the quickly changing loading regime, it is the stiffer crystalline part that takes on the load, resulting in a more ridged response of the whole polymer. For polyester rope, this behavior results in dynamic stiffness being about 2 times the static stiffness. Failure to account for this behavior will inevitably yield inaccurate line tension and vessel offset predictions, and the inaccuracy for vessel offset can be particularly large. According to the invention, the desired material with oriented crystalline structure has presented a linear stiffness over the whole strain change range. Thus, the offset is significantly reduced and can be accurately predicted.

[0046] The floating object anchored by the mooring system of the invention can be a floating marine energy converter platform, an offshore wind farm, a single point mooring buoy or a floating offshore oil or gas platform.

Brief Description of Figures in the Drawings

[0047] The following detailed description makes reference to the accompanying drawings, in which:

[0048] Fig. 1 illustrates static-dynamic stiffness model of two different type of material.

[0049] Fig. 2 illustrates the stress strain behaviour of POM compared with nylon and PET.

[0050] Fig. 3 illustrates a cross-section of a spiral strand rope used for the mooring system according to the invention.

Mode(s) for Carrying Out the Invention

[0051 ] A polymer rope was made according to the present invention. Wire production starts with melt extrusion of polymer pellets to product a rod, which is then drawn into a polymer wire with improved properties over the raw material. Monofilament POM wire is made by conventional wire drawing process. Similar to the work on the high tensile steel wire, the production of the polymer wires including quality of the extruded rod and process control during wire drawing is comparable to the processing of steel wire.

[0052] Larger wires may be spun layer by layer in alternate direction creating a stiffer structural type of rope, referred to as a spiral strand rope. The crosssection of such a spiral strand rope 30 is illustrated in Fig. 3. This torsional balanced construction imparts no rotational load to attached mooring components. This closed structure is inherently more resistant to water ingress and any subset corrosion. Furthermore, an extruded jacket 32 as shown in Fig. 3 can be applied to exclude water, enhancing the design life. [0053] The polymer wires produced have been utilized to manufacture e.g., an 84 mm diameter sheathed spiral strand rope. The polymer wires take a level of plastic deformation without damage to the product; hence it is possible to impart a preformation on the wires to aid with consistent and precise rope manufacture. The level of deformation in the wires at formation point of the rope is relatively low due to the long helix geometry of the spiral strand rope construction - each layer of the wires being added in an individual pass through the machine. However, it was noted that there were no detrimental effects on the wires, and it is anticipated that more complex constructions with a greater degree of relative deformation could be effective.

[0054] The component wires are of suitable diameter and material properties such that they are robust and inherently abrasion resistant. However, the external sheathed jacket 32 acts as an extra precautionary aid against installation damage and as its weight is not a concern, its presence is not detrimental to performance. The sheathed jacket 32 also serves to increase the stiffness of the product. Beneficial elements such as the longitudinal marker line 34 to identify any twist during deployment would remain present.

[0055] Application of the sheathed medium density polyethylene jacket is applied via tube on sheathing process. During the rope manufacture, it was also established that the processing temperature of the jacket material does not influence the properties of the wire.

[0056] The stress-strain characteristics of the invention rope (curve C) is compared with high strength fiber nylon (curve B) and high strength PE (curve A) in Fig. 2. As can be seen from Fig. 2, the breaking tenacity of the invention rope continues to increase up to 35% strain. The stressstrain characteristics of the invention rope is regressive in comparison with nylon and PE. This results in a relative increase in static stiffness and a relative decrease in dynamic stiffness values. Thus, the static and dynamic stiffness value of the invention rope are similar, and the invention rope presents a linear stiffness as shown by cure II in Fig.1 .