Mooring system for mooring floating objects

Abstract

A mooring system for mooring a floating object, comprising a mooring line configured to extend between the object floating in water and an anchoring device, characterized in that the mooring line is made of polyoxymethylene and the ratio of the static stiffness to the dynamic stiffness of the mooring line is in the range of 0.8 to 1.

Description

Technical Field

[0001] The present invention relates to a mooring system for mooring a floating object, and in particular to a mooring line for the mooring system. Background Art

[0002] Floating facilities have proven to be an increasingly important solution for offshore oil and gas and offshore wind turbines (FWT) or in areas with limited infrastructure. As a result, floating production systems are used geographically and widely, operating in different environmental conditions. The permanent mooring of production systems is inherently required to provide long-term performance, in some cases more than 30 years, which increases the possibility of extreme or unknown operating conditions.

[0003] Mooring systems that maintain compliance (e.g., systems used to hold a FWT in place) can be configured as catenary, taut, or a combination of the two (often referred to as "hybrid"). Catenary systems use the deadweight of the mooring line (usually a steel chain) to create the restoring force. For shallow depths, chains or wire ropes may be the best solution in a catenary arrangement. However, for greater depths, they are expensive, corrosive, and cumbersome.

[0004] The taut mooring system uses the strain of the mooring line to generate a restoring force. Synthetic ropes are mainly used in deep and ultra-deep waters because of their light weight and high elasticity. Among synthetic ropes, polyester is the most common. Polyester ropes are used in semi-taut and taut mooring systems, which allows for efficient mooring systems at a lower cost. However, other synthetic ropes such as nylon, high modulus polyethylene (HMPE) or aramid can also be used. At medium depths (~100m), nylon ropes have shown some advantages. Due to its relatively high elasticity, if a nylon rope is stretched and released, it will not be permanently damaged quickly. If there are large waves during a storm at medium depths, the elongation of the mooring line may exceed the maximum allowable elongation if the mooring line is not too long, but no permanent damage will occur.

[0005] Catenary mooring has been around for a long time but has many disadvantages such as corrosion, weight, poor fatigue resistance of the chain, complex and dangerous operating characteristics, and the tendency to produce unacceptable tension in extreme conditions. Taut mooring overcomes the challenges associated with chain catenary but also brings additional challenges related to the length stability of the fiber rope. Nylon can allow 18% elongation without permanent damage, while polyester (14%), Dyneema (3%) or Aramid (3%) have much less elongation.

[0006] The demands on position-keeping devices are constantly evolving, requiring new and innovative approaches to the equipment and materials used. Summary of the invention

[0007] The inventors have discovered that the use of materials with properties not normally considered in taut moorings can significantly improve the performance of taut mooring systems by reducing deflection without increasing peak tension.

[0008] The design of a taut mooring system must balance two often conflicting requirements:

[0009] Limiting the lateral movement ("deflection") of the float when force loads (such as wind loads, water currents or turbine thrust) are applied,

[0010] Minimize tension changes caused by buoy movement (e.g. due to wave motion).

[0011] Offset is critical when the floater is connected to other subsea equipment with limited bending / tension fatigue or elongation capabilities, such as steel risers or power export cables.

[0012] Currently, the industry relies on a few simplified models that capture the most important characteristics while making conservative predictions of line tension and vessel excursions.

[0013] The improvements provided by the materials used in this application can best be understood using the "Static-Dynamic Model" described in ABS's "Guidelines for Application of Fiber Ropes for Offshore Mooring Systems". Figure 1 As shown in Figure 1, the model uses a bilinear slope with the inflection point at the mean tension. Below the mean tension, the slope is defined using the associated static stiffness. Above the mean tension, the slope is defined using the associated dynamic stiffness until the peak tension is reached. The effects of structural elongation and creep can be added to the model by applying an offset to the strain axis.

[0014] A static-dynamic model was developed to explain the elongation behavior of the fiber rope. In this model, the static stiffness is used 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] like Figure 1 As shown in curve I in Figure 1, the total deflection of the mooring line consists of four parts: structural elongation (A), static elongation (B), dynamic elongation (C) and elongation due to polymer creep effect (D). Static elongation (B), dynamic elongation (C) and elongation due to polymer creep (D) depend largely on the material used for the mooring line.

[0016] While taut mooring offers many operational and performance advantages, overcoming the challenges posed by the length instability of conventional fiber ropes is complex and expensive.

[0017] The model more accurately simulates the real conditions faced by fiber ropes in offshore moorings. Mooring lines in harsh environments typically experience a steady mean load and dynamic loads that oscillate around the mean load.

[0018] To minimize deflection, it is preferred to use mooring lines with high stiffness, whereas to minimize tension caused by float motion, it is preferred to use low stiffness. In modeling 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, while dynamic stiffness is used to represent the response to motion-based loads. Both static and dynamic stiffness depend on the tension at the time of measurement and are therefore represented by a range of values.

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

[0020] The axial stiffness of a rope can be defined as the secant stiffness or the slope of the line connecting the extreme values ​​during a tensile load-unload cycle. The load range in the load-unload cycle can be between 5% and 50% of the rope breaking strength. Dynamic stiffness is defined for a cycle time of 10-30 seconds. Static stiffness is defined for a cycle time of 30-120 minutes. The measurement of static and dynamic stiffness can refer to ISO 18692-1:2018.

[0021] A "wire" of the present invention may be any device providing compliance, such as a load relief device or a spring. Preferably, the wire is a rope.

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

[0023]

[0024] Where ΔF is the change in load, Δε is the change in strain, and EA is the stiffness or modulus of the rope multiplied by the cross-sectional area. Stiffness is usually expressed in units of force, such as kN or kips. Similarly, the dimensionless stiffness Kr can also be expressed as:

[0025]

[0026] Where MBS is the minimum breaking strength.

[0027] The above definition can be applied to structures of a single material and different sizes.

[0028] Conventional materials, such as polyester ropes and other fiber ropes, are made of materials with viscoelastic properties, so their stiffness characteristics are not constant, but vary with load duration and magnitude, number and frequency of load cycles, and loading history.

[0029] For conventional materials used in taut mooring lines (e.g. polyester and nylon), the dynamic stiffness values ​​range (as shown in Table 1 below) from approximately 1.5–2 times the static stiffness values. This is contrary to the desired optimization. Failure to take this behavior into account will inevitably lead to inaccurate predictions of line tension and moored object deflection, and the inaccuracy of moored object deflection can be particularly large.

[0030] Table 1 Typical static stiffness range and dynamic stiffness range

[0031] Material Static stiffness range (x MBS) Dynamic stiffness range (x MBS) Nylon fiber 2-12 5-25 Polyester 8-16 16-32

[0032] Typically, polyester mooring lines become stiffer with extended use. Historical loading above a certain level may result in a permanent increase in rope length, which can lead to a softening of the mooring system if re-tensioning is not performed. Due to the complex nature of ropes, it is not possible to develop a model that represents the exact stiffness characteristics of a rope.

[0033] Since increased length often results in unacceptable deflection, additional components (e.g., line chain tensioners) must be introduced into the taut mooring system in order to adjust the line length and thus reduce the total deflection. These components are expensive to purchase, require complex and expensive intervention procedures at a later stage, and hinder the optimization of the mooring system design due to the additional drag and weight of the components. Furthermore, these components require steel chains to operate, with all the attendant limitations.

[0034] Total deflection is the sum of the horizontal components of structural elongation, static elongation (strain due to static stiffness), dynamic elongation (strain due to dynamic stiffness) and creep. To reduce total deflection, the prior art is to "pre-stretch" the mooring lines to eliminate structural elongation at the beginning of their service life. However, since deflection is the sum of the four components, it is understood that total deflection can theoretically be reduced by reducing any of the four effective components.

[0035] The present invention defines a new taut mooring method, wherein the mooring rope is made of polyoxymethylene, and the ratio of the static stiffness to the dynamic stiffness of the polymer rope is close to one. Figure 1 As shown, curve II represents the linear stiffness of a rope as a mooring line according to the invention.

[0036] In the mooring system of the present invention, no length adjustment or tensioner is required during the operation or service life of the mooring system. The mooring line of the present invention requires no intervention throughout its life.

[0037] The linear stiffness of the mooring line can be achieved by a specific polymer material, polyoxymethylene (POM), which has properties that differ from nylon and polyester in that its stress-strain behavior is largely regressive. Figure 2 Shown is the stress-strain behavior of POM compared to nylon and polyester.

[0038] The mooring line is made of polyoxymethylene (POM). The mooring line may comprise 100 wt% of POM material. However, the mooring line may comprise, for example, about 90 wt% or more than 80 wt% of POM as a core and have a polymer sheath which is not POM, wherein the core is surrounded by the polymer sheath.

[0039] The regressive stress-strain behavior results in a relative increase in static stiffness and a relative decrease in dynamic stiffness values. This relative change results in similar static and dynamic values, rather than the 1.5-2 times increase in dynamic stiffness typical of nylon and polyester. This in turn reduces the unwanted deflections caused by force-based loads without significantly affecting compliance to motion-based loads (necessary deflections caused by dynamic stiffness). Therefore, for the same peak tension and rope size, significantly reduced deflections can be achieved using the proposed material. Such deflections are not possible with conventional materials such as nylon or polyester.

[0040] Compare the typical nylon static-dynamic curve I with Figure 1 Compared with the regression material curve II shown, it can be observed that for the same peak tension, the total deflection is significantly reduced. In a preferred example, the deflection can be reduced by up to 35%.

[0041] According to the invention, the static and dynamic stiffness may be in the range of 10 to 20 MBS (minimum breaking strength). In other words, the static and dynamic stiffness may be in the range of 7 to 15 GPa. Furthermore, the mooring line may have a tensile strength higher than 500 MPa, for example in the range of 500 to 1400 MPa.

[0042] Regarding the construction of the rope as a mooring line, it can have many forms. For example, the rope can be made of multiple polymer monofilaments twisted in a spiral shape. The multiple polymer monofilaments can have a diameter between 0.5 mm and 5 mm. Alternatively, the rope can be a strand rope, for example with 6 or 8 strands. The rope can be made of POM monofilament or multifilament fibers. The fibers are twisted into yarns, which are then twisted into strands. These strands are then used to produce a variety of possible rope structures. For example, some possible POM rope constructions are:

[0043] 1) Single braided rope consisting of an equal number of left-handed and right-handed strands. Possible numbers of strands are 6, 8, 12 and 16. So-called "single braided ropes" are made of an even number of strands woven in a circular pattern, half of which are clockwise and the other half counterclockwise. This type of rope has a slightly higher breaking force than twisted ropes and therefore costs a little more.

[0044] 2) A single braided rope as described in 1) above, but with the strands themselves also being separate rope structures:

[0045] a. 3-strand twisted rope.

[0046] b. 12-strand braided rope.

[0047] 3) Twisted rope (or twisted rope), consisting of three or four strands of wire twisted in left or right direction.

[0048] 4) Multi-layer twisted rope, in which the strands are twisted in a spiral manner into alternating left-handed and right-handed layers.

[0049] 5) Sheathed ropes, comprising a core of any of the above constructions but covered with a braided or extruded sheath:

[0050] a specific embodiment includes a single braided core, a plurality of parallel braided cores or a plurality of parallel twisted cores;

[0051] b. The number of strands in the cover layer can be 16, 20, 24, 32, 48, where each "strand" can consist of 1, 2 or 3 parallel strand groups.

[0052] The rope of the present invention is preferably wrapped by a polymer sheath. The polymer sheath can be selected from the group consisting of epoxy resin, polyamide, polyurethane, polyester (PES), polyethylene terephthalate (PET), nylon (PA), polypropylene (PP), polyethylene (PE), polypropylene-polyethylene (PP-PE) copolymer, polyethylene naphthalate (PEN), high modulus polyethylene (HMPE), polyester polyacrylate (LCP), para-aramid, aramid copolymer, or any combination or blend of the above.

[0053] The rope may have an elastic elongation higher than 10%. The diameter of the rope is not limited. For example, the rope may have a diameter between 100 mm and 300 mm.

[0054] In order to obtain a more ideal stress-strain response and Figure 1The polymer material of the polymer rope mooring line of the present invention preferably has a crystallinity of at least 60%, more preferably at least 80%, based on the overall properties shown in curve II. In addition, preferably at least 50% of the crystalline polymer is oriented, more preferably at least 70% of the crystalline polymer is oriented. This can be achieved by maintaining a semi-molten polymer to grow crystallinity and then drawing a polymer rod at a high drawing ratio to orient the crystals. When the polymer is stretched, the molecules can be preferentially aligned in the stretching direction.

[0055] The expected and accurate prediction of the deflection of a mooring line depends largely on the properties of the polymer material, especially the elongation properties. This is related to the macromolecular structure (i.e. "morphology") of the material. The morphology of a polymer material generally shows a crystalline part and a non-crystalline part (i.e., an amorphous part). Static stiffness is the stiffness of the tension member when it is slowly loaded, leaving time for the amorphous and crystalline parts to react to the load. The resulting fiber stiffness is the average of the stiffness of the two parts. Dynamic stiffness is the stiffness response of the tension member when it is cyclically loaded. Since the amorphous part does not react quickly enough to the rapidly changing loading mode, the more rigid crystalline part bears the load, causing the entire polymer to produce a more ridge-like response. For polyester ropes, this property causes the dynamic stiffness to be about twice the static stiffness. Failure to take into account this property will inevitably lead to inaccurate predictions of line tension and ship deflection, and the inaccuracy of ship deflection may be particularly high. According to the present invention, the desired material with a directional crystal structure exhibits linear stiffness over the entire strain variation range. Therefore, the deflection is significantly reduced and can be accurately predicted.

[0056] The floating object anchored by the mooring system of the present invention may be a floating ocean energy conversion platform, an offshore wind farm, a single point mooring buoy or a floating offshore oil or gas platform. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The following detailed description refers to the accompanying drawings, in which:

[0058] Figure 1 The static-dynamic stiffness model for two different types of materials is shown.

[0059] Figure 2 The stress-strain behavior of POM compared with nylon and PET is shown.

[0060] Figure 3 A cross section of a helical strand rope for use in a mooring system according to the invention is shown. DETAILED DESCRIPTION

[0061] Polymer ropes are manufactured according to the present invention. The production of the wire begins with melt extrusion of polymer particles to produce rods, which are then drawn into polymer wires having improved properties over the raw materials. Monofilament POM wires are made by conventional wire drawing processes. Similar to the processing of high tensile strength steel wires, the production of polymer wires (including the quality of the extruded rods and process control during wire drawing) is comparable to the processing of steel wires.

[0062] Larger wires can be twisted layer by layer in alternating directions to form a more rigid structural type of rope, known as a helical strand rope. The cross-section of this helical strand rope 30 is Figure 3 This torsionally balanced structure does not impose rotational loads on the attached mooring components. This closed structure is inherently more resistant to water ingress and any local corrosion. In addition, it is possible to apply Figure 3 The extruded jacket 32 ​​is shown to block water, thereby extending the design life.

[0063] The polymer wire produced has been used to make, for example, wrapped helical strand ropes with a diameter of 84 mm. The polymer wire undergoes a certain degree of plastic deformation without damaging the product; therefore, the wire can be pre-formed to facilitate consistent and accurate rope manufacturing. Due to the long helical geometry of the helical strand rope structure, the degree of deformation of the wire at the forming point of the rope is relatively low (each layer of wire is added in a separate pass through the machine). However, it is worth noting that this does not adversely affect the wire, and it is expected that more complex structures with greater degrees of relative deformation may be effective.

[0064] The wire components have suitable diameters and material properties so they are strong and inherently wear resistant. However, the outer wrapping sheath 32 serves as an extra precaution against installation damage and its presence does not impair performance since its weight is not a concern. The wrapping sheath 32 also serves to increase the rigidity of the product. Beneficial features such as the longitudinal marking line 34 will be retained to identify any twisting during deployment.

[0065] The application of the wrapped medium density polyethylene sheath is through a tube application wrapping process. During rope manufacturing it is also determined that the processing temperature of the sheath material does not affect the properties of the wire.

[0066] exist Figure 2 In the present invention, the stress-strain characteristics of the rope (curve C) are compared with those of high-strength fiber nylon (curve B) and high-strength PE (curve A). Figure 2It can be seen that the breaking strength of the rope of the present invention continues to increase until the strain reaches 35%. Compared with nylon and PE, the stress-strain characteristics of the rope of the present invention show regression. This makes the static stiffness relatively increase and the dynamic stiffness value relatively decrease. Therefore, the static stiffness value and dynamic stiffness value of the rope of the present invention are similar, and the rope of the present invention presents the following Figure 1 The linear stiffness is shown by curve II.

Claims

1. A mooring system for mooring a floating object, comprising a mooring line extending between the object configured to float in water and an anchoring device, characterized in that The mooring line is made of polyoxymethylene and has a ratio of static stiffness to dynamic stiffness in the range of 0.8 to 1.

2. The mooring system according to claim 1, wherein: The static and dynamic stiffnesses are in the range of 7 to 15 GPa.

3. A mooring system according to claim 1 or 2, wherein: The mooring line is a polymer rope made of a plurality of polymer monofilaments twisted in a spiral shape.

4. The mooring system according to claim 3, wherein: The plurality of polymer monofilaments have a diameter between 0.5 mm and 5 mm.

5. The mooring system according to claim 1 or 2, wherein: The mooring line is a single braided rope consisting of an equal number of left-hand and right-hand strands made of multifilament fiber yarns.

6. The mooring system according to claim 1 or 2, wherein: The mooring line is a twisted rope comprising strands twisted in a spiral manner and made of multifilament fiber yarns.

7. A mooring system according to any one of the preceding claims, wherein: The mooring line has a tensile strength higher than 500 MPa.

8. A mooring system according to any one of the preceding claims, wherein: The mooring line is a polymer rope surrounded by a sheath.

9. A mooring system according to claim 8, wherein: The polymer of the polymer sheath is selected from the group consisting of epoxy resin, polyamide, polyurethane, polyester (PES), polyethylene terephthalate (PET), nylon (PA), polypropylene (PP), polyethylene (PE), polypropylene-polyethylene (PP-PE) copolymer, polyethylene naphthalate (PEN), high modulus polyethylene (HMPE), polyester polyacrylate (LCP), para-aramid, aramid copolymer, or any combination or blend thereof.

10. A mooring system according to any one of the preceding claims, wherein: The mooring line has an elastic elongation greater than 10%.

11. A mooring system according to any one of the preceding claims, wherein: The mooring line is a polymer rope with a diameter between 100mm and 300mm.

12. A mooring system according to any one of the preceding claims, wherein: The polymer of the polymer rope has a crystallinity of at least 60%.

13. A mooring system according to claim 12, wherein: At least 50% of the crystalline polymer is oriented.

14. A mooring system according to any one of the preceding claims, wherein: The floating object is a floating ocean energy conversion platform, an offshore wind farm, a single point mooring buoy or a floating offshore oil and gas platform.

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