Mooring line and system

Abstract

A mooring line 200 comprises a first section (210, Fig 1) for being in an inverse catenary shape under the seabed 20 when installed is disclosed. The mooring line comprises one or more plates 300 conn

Description

Technical Field The present disclosure relates generally to a mooring line, a mooring system for an offshore structure, and a method of installing an anchor member with such a mooring line. The mooring line is suitable for anchoring floating offshore structures. Background A floating offshore structure is anchored to the seabed to prevent the structure to drift away or out of its position. A mooring systems for this purpose may be taken, for example, from US5915326A. Here a chain, from an anchor embedded in the seabed, is in an inverse catenary shape. It is a problem how to maintain the inverse chain catenary connected to an anchor below the seabed. When tension is applied to the mooring line it reduces the inverse catenary. The resulting effect being the mooring line is effectively increased in length. This reduces pre-tension, weakens the performance of the mooring system, and increasing floater offsets. The section of the chain being in the inverse catenary shape may be a long part of the mooring line. A straightening of the inverse catenary shape causes therefore a large offset to the floating offshore structure. A further problem is that any solution should be practical in real life, inexpensive, and installable preferably with existing equipment. The present disclosure is directed to overcoming one or more of the problems as set forth above. SUMMARY It is an objective of the present invention to provide a mooring line, a mooring system for an offshore structure, and a method of installing an anchor member with such a mooring line. This objective can be achieved by the features as defined in the independent claims. Further characteristics are defined by the dependent claims. According to one embodiment, there is provided a mooring line 200 comprising a first section 210 for being in an inverse catenary shape under the seabed 20 when installed, the mooring line 200 comprising one or more plates 300 connected to the first section 210 of the mooring line 200, the one or more plates 300 comprise a surface area 310 configured for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. The one or more plates 300 may be shaped and attached as described herein to provide the resisting force, the drag. According to one embodiment, there is provided a mooring system for an offshore structure 10. The system comprises an anchor member 100 at least partly embedded in the seabed 20; and the mooring line 200 according to any one of the embodiments described herein. The mooring line 200 further comprises a second section 220 for being subsea, the mooring line 200 extending from the offshore structure 10 to a part of the anchor member 100 embedded in the seabed 20. The one or more plates 300 are connected under the seabed to the first section 210 of the mooring line 200 and are configured to keep the mooring line 200 in an inverse catenary shape. The mooring system may further comprise means for tensioning the mooring line. According to one embodiment, there is provided a method of installing an anchor member 100 with a mooring line according to any one of the embodiments described herein. The method comprises positioning the anchor member 100 at least partly embedded in the seabed with the one or more plates 300 on the mooring line 200 and the mooring line 200 connected to the anchor member 100. The method may further comprising one or more of: using a separate installation line for guiding the one or more plates 300; attaching one or more of the one or more plates 300 to the anchor member during the installation; providing buoyancy to the one or more plates 300 during the installation; using a remote operated vehicle, ROV, for the one or more plates 300 during the installation; and tensioning the mooring line 200 to a predetermined tension. At least one of the aspects and embodiments defined in the present application provides one or more solutions to the problems and disadvantages with the background art. Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following description and claims. Various embodiments of the present application obtain only a subset of the advantages set forth. No one advantage is critical to the embodiments. Any claimed embodiment may be technically combined with any other claimed embodiment or embodiments. Brief Description of the Drawings The disclosure will be further described with reference to examples depicted as schematic illustrations in the accompanying figures in which: FIG 1 is a schematic illustration of a mooring system according to an embodiment of the invention; FIG 2 is a schematic illustration used to explain straightening of the inverse catenary shape; FIG 3 is a schematic illustration of various embodiments of the invention; FIG 4 is a schematic illustration of various embodiments of the invention; and FIG 5 is a schematic illustration of a part of a mooring line towards a plate according to embodiments of the present invention. Detailed Description Turning first to figure 2, explaining what happens when the inverse catenary shape is straightened. A first section 210 of the mooring line 200 may be attached to an anchor member 100. The anchor member 100 may be embedded in the seabed 20. The dotted line in figure 2 illustrates a mooring line in the seabed having an inverse catenary shape. When tension is applied to the dotted mooring line it reduces the inverse catenary shape. This tension may occur for example when there is a storm and the axial pulling force in the mooring line is increased. This reduced inverse catenary shape of the mooring line 200 is illustrated by the solid line 200 in figure 2. The resulting effect being the mooring line is effectively increased in length. As an example, a taut mooring line may be 600 meters in length of which up to 100 meters may be below the seabed 20 connected to the anchor member 100 in an arced inverse catenary shape. A storm may increase tension in the mooring line, pulling the inverse catenary section of the chain through the soil, see the arrow in figure 2, into a straighter profile, from the dotted line to the solid line as illustrated in figure 2. This effectively lengthening the mooring line and reducing the tension in the system. This would then require an operation to increase tension and remove length from the mooring line, which is difficult, costly, and the mooring system would have had to have been designed to withstand this. By maintaining the mooring line's inverse catenary shape, the need for retensioning and higher allowable offsets is reduced or removed. This significantly reduces operational time for tensioning and system design for offsets. Turning now to all figures, the mooring line 200 is provided with one or more plates 300, for example flukes or restrictors, providing drag to oppose the straightening force applied by the tensile load on the mooring line 200. Therefore, the inverse catenary shape is maintained. The plates 300, connected to the mooring line 200, provide a restraining force which stops the mooring line's inverse catenary shape from cutting through the soil of the seabed when tension increases. This maintains pre-tension, strengthens the performance of the mooring system, and reduces floater offsets, as well as reducing the design requirements and operational intervention for a mooring system. According to one embodiment, a mooring line 200 comprises a first section 210 for being in an inverse catenary shape under the seabed 20 when installed, the mooring line 200 comprising one or more plates 300 connected to the first section 210 of the mooring line 200, the one or more plates 300 comprise a surface area 310 configured for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. The surface area 310 may preferably face towards the open part of the inverse catenary shape. The plates 300 may preferably be on the convex side of the inverse catenary shape. The plates 300 may be configured to provide drag, especially drag under the seabed. The plates 300 may for example be fluke shaped, or for example be shaped as a vertically loaded anchor, VLA, or for example in the shaped of a flat plate. Preferably, when installed, the plates are under the seabed 20. The mooring line 200 may, for example, be a wire or a chain, or a combination of the two. The mooring line 200, when installed, may have a first section 210 that is the section under the seabed, and a second section 220 that is the rest of the mooring line 200 from the seabed to a floating structure. The two sections 210 and 220 may be substantially of different material and / or properties. For example, the first section 210 may be a chain, and the second section 220 may be a wire or a cable or a rope, such as a fibre rope. According to one embodiment, the surface area 310 may be wider than a diameter D of the mooring line 200. As best illustrated in figure 5, the mooring line 200 may have a diameter D. The surface area 310 of the plate 300 may be wider than the diameter D. That is, the width L of the surface area 310 of the plate 300 stretches beyond the diameter D of the mooring line 200. Preferably the surface area 310 is several times wider than the diameter D, for example 2, 5, 10, 50, 100, 200 times wider than the diameter D. This may be one way of configuring the plate 300 to provide drag for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. While the surface area 310 is illustrated as a square in figure 5, the shape of the plate 300 may be any other suitable shape providing drag, for example round, triangle, oval, pentagon, hexagon, heptagon, octagon, nonagon, or decagon shape. According to one embodiment, the one or more plates 300 may be buoyant. Buoyancy may be added or included within the design of the plates 300 to aid orientation and alignment prior to being pulled to form the inverse catenary shape to maximise efficiency. A buoyant plate 300 may take its place as illustrates in figures 1, 3, and 4 during installation, or even thereafter, depending on the soil of the seabed. Installation of the mooring line 200 with buoyant plates 300 may place the first section 210 of the mooring line 200 in the inverse catenary shape. According to one embodiment, the one or more plates 300 may comprise guide means 330 for guiding the plates 300 when the mooring line 200 is installed. During installation the mooring line 200 may be pulled down into the seabed. The one or more plates 300 may be pulled down into the seabed with the installation of the mooring line 200. The guide means 330 may, for example, comprise one, or a combination, of arrow shaped, fluke shaped, and pointed part and / or edge. The guide means 330 may guide, direct, the plates into their position as illustrated in figures 1, 3, and 4. In addition, when axial pulling force is applied to the mooring line 200, then the guide means 330 may force the plates 300 to maintain their position and dig them self into a better position to provide drag in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape as illustrated in figures 1, 3, and 4. According to one embodiment, the one or more plates 300 may be connected to the inverse catenary section 210 of the mooring line 200 by attachment means 320. There may be attachment means 320 from the inverse catenary section 210 of the mooring line 200 to the plates 300, as best illustrated in figures 1, 3, and 4. The attachment means 320 may, for example, be attached to the centre of the plate 300, or to each corner, or to several point balancing the resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. The attachment means 320 are further disclosed herein after. According to one embodiment, each of the one or more plates 300 may be connected to the inverse catenary section 210 of the mooring line 200 by one or more attachment means 320. For example, with regard to the examples illustrated in figure 3, the highest, to the right hand side, plate 300 is connected to the mooring line 200 with one attachment means 320. The lowest, to the left hand side, plate 300 is connected to the mooring line 200 with two attachment means 320. According to one embodiment, the attachment means 320 may be movable attached to the inverse catenary section 210 of the mooring line 200. For example, with regard to the examples illustrated in figure 3, the middle plate 300 is connected to the mooring line 200 with an attachment means 320 that is looped around the mooring line 200. This allows the plate 300 to move along the inverse catenary section 210 of the mooring line 200. The plate 300 may also more easily rotate around the axis of the mooring line 200. This may be advantageous when the pulling force on the mooring line 200 also pulls to one side. According to one embodiment, the one or more plates 300 may be attached in parallel, or in series, to the inverse catenary section 210 of the mooring line 200. For example, with regard to the examples illustrated in figure 4, the highest, to the right hand side, three plates 300 are connected to the mooring line 200 in series with attachment means 320 between the plates 300 and the mooring line 200. According to one embodiment, the one or more plates 300 may be in line with the inverse catenary section 210 of the mooring line 200. For example, with regard to the examples illustrated in figure 4, the lowest, to the left hand side, plate 300 is connected directly to the mooring line 200 such that the plate 300 is in line with the mooring line 200. The plate 300 may be positioned along the same line as the mooring line 200. The plate 300 may be directly clamped onto the mooring line 200. According to one embodiment, a plurality of plates 300, each comprising a different shape, may connected to the inverse catenary section 210 of the mooring line 200. The plates 300 may differ from each other. Some may be smaller and some may be larger. For example, the surface area 310 may differ among the plurality of plates 300. For example, the surface area 310 of the plates 300 may be successively larger, or smaller, the closer the plates 300 are to an anchor member, or the end of the mooring line 200. According to one embodiment, the attachment means 320 may one or more of the following group: chain, wire, and rod. The attachment means 320 connects the one or more plates 300 to the first section 210 in the inverse catenary shape of the mooring line 200. The attachment means 320 may be one or more of chain segments, wire ropes, or solid rods connected to one or more of the plates, for example by a shackle. The attachment means 320 may be connected to the mooring line 200, for example like a tri plate connection, or may be a looped or sliding connection, for example a ring around the main mooring line. The different embodiments described herein describing the plates 300, the shape of the plates 300, the surface area 310, and the attachment means 320 may be combined in any way. According to one embodiment, a mooring system for an offshore structure 10 is provided. The system comprises an anchor member 100 at least partly embedded in the seabed 20; and the mooring line 200 according to any one of the embodiments described herein. The mooring line 200 further comprises a second section 220 for being subsea. The mooring line 200 extends from the offshore structure 10 to a part of the anchor member 100 embedded in the seabed 20. The one or more plates 300 are connected under the seabed to the first section 210 of the mooring line 200 and are configured to keep the mooring line 200 in an inverse catenary shape. This may best be taken from figures 1, 3, and 4. The system may comprise several mooring lines 200 each comprising one or more plates 300 connected to the first section 210 of the mooring line 200. The mooring system may further comprise means for tensioning the mooring line 200. The anchor member 100 may be any type of anchor member, for example a gravity anchor, suction anchor, plate anchor, fluke anchor, etc. The anchor member 100 may be a pilar forced into the seabed, as best illustrated in figure 3. The anchor member 100 may be a body 100, for example a plate shaped body 100, below the seabed, as best illustrated in figure 4. According to one embodiment, a method of installing an anchor member 100 with a mooring line according to any one of the embodiments described herein is provided. The method comprises positioning the anchor member 100 at least partly embedded in the seabed with the one or more plates 300 on the mooring line 200 and the mooring line 200 connected to the anchor member 100. The anchor member 100 with the mooring line 200 is connected to the anchor member 100, and the one or more plates 300 are connected to the first section 210 of the mooring line 200, during the installation of the anchor member 100. According to one embodiment, the method may further comprise using a separate installation line for guiding the one or more plates 300. The separate installation line may be connected to each of the plates 300 in parallel or in series. During the installation the separate installation line may guide the plates 300 into their correct position for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. According to one embodiment, the method may further comprise attaching one or more of the one or more plates 300 to the anchor member during the installation. The plates 300 may be attached to the anchor member 100 during the installation and then during the installation, or thereafter, released from the anchor member 100. In this way the one or more plates 300 may take their position for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. According to one embodiment, the method may further comprise providing buoyancy to the one or more plates 300 during the installation. During the installation the one or more plates 300 may be buoyant and pulled into the seabed after, following, the mooring line 200 being pulled in. In this way the one or more plates 300 are installed in their position for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. According to one embodiment, the method may further comprise using a remote operated vehicle, ROV, for the one or more plates 300 during the installation. The ROV may guide the one or more plates 300 during the installation of the anchor member 100. In this way the one or more plates 300 are installed in their position for providing a resisting force in a direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. According to one embodiment, the method may further comprise tensioning the mooring line 200 to a predetermined tension. After, or during, the installation, one may tension the mooring line 200 to a predetermined tension. Such tensioning may move the one or more plates 300 into their positions for provide a resisting force in a 5 direction perpendicular to the mooring line 200 for keeping the first section 210 in the inverse catenary shape when axial pulling force is applied to the mooring line. The installation method of the anchor member 100 and / or the one or more plates 300 may be any installation method, such as for example self-weight, suction assisted, drag embedded, torque, etc. 10 It will be apparent to those skilled in the art that various modifications and variations can be made to the mooring line, the mooring system for an offshore structure, and the method of installing an anchor member with such a mooring line. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed mooring line, mooring system, 15 and method. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.

Claims

1 A mooring line (200) comprising a first section (210) for being in an inverse catenary shape under the seabed (20) when installed, the mooring line (200) comprising one or more plates (300) connected to the first section (210) of the mooring line (200), the one or more plates (300) comprise a surface area (310) configured for providing a resisting force in a direction perpendicular to the mooring line (200) for keeping the first section (210) in the inverse catenary shape when axial pulling force is applied to the mooring line.2 The mooring line according to claim 1, wherein the surface area (310) is wider than a diameter (D) of the mooring line (200).3 The mooring line according to claim 1 or 2, wherein the one or more plates (300) are buoyant.4 The mooring line according to anyone of the preceding claims, wherein the one or more plates (300) comprises guide means (330) for guiding the plates (300) when the mooring line (200) is installed.5 The mooring line according to anyone of the preceding claims, wherein the one or more plates (300) are connected to the inverse catenary section (210) of the mooring line (200) by attachment means (320).6 The mooring line according to claim 5, wherein each of the one or more plates (300) are connected to the inverse catenary section (210) of the mooring line (200) by one or more attachment means (320).7 The mooring line according to claim 5 or 6, wherein the attachment means (320) is movable attached to the inverse catenary section (210) of the mooring line (200).8 The mooring line according to anyone of the preceding claims, wherein the one or more plates (300) are attached in parallel, or in series, to the inverse catenary section (210) of the mooring line (200).9 The mooring line according to anyone of the preceding claims, wherein the one or more plates (300) are in line with the inverse catenary section (210) of the mooring line (200).10 The mooring line according to anyone of the preceding claims, wherein a plurality of plates (300), each comprising a different shape, are connected to the inverse catenary section (210) of the mooring line (200).11 The mooring line according to claim 5, wherein the attachment means (320) is one or more of the following group: chain, wire, and rod.12 A mooring system for an offshore structure (10), the system comprising an anchor member (100) at least partly embedded in the seabed (20); and the mooring line (200) according to any one of the preceding claims, the mooring line (200) further comprising a second section (220) for being subsea, the mooring line (200) extending from the offshore structure (10) to a part of the anchor member (100) embedded in the seabed (20), wherein the one or more plates (300) are connected under the seabed to the first section (210) of the mooring line (200) and are configured to keep the mooring line (200) in an inverse catenary shape.13 The mooring system for an offshore structure (10), further comprising means for tensioning the mooring line.14 A method of installing an anchor member (100) with a mooring line according to any one of the preceding claims 1 to 9, the method comprising:positioning the anchor member (100) at least partly embedded in the seabed with the one or more plates (300) on the mooring line (200) and the mooring line (200) connected to the anchor member (100).15 The method according to claim 14, further comprising one or more of: using a separate installation line for guiding the one or more plates (300); attaching one or more of the one or more plates (300) to the anchor member during the installation;providing buoyancy to the one or more plates (300) during the installation;using a remote operated vehicle, ROV, for the one or more plates (300) during the installation; andtensioning the mooring line (200) to a predetermined tension.

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