Methods and systems for offshore mooring operations

Abstract

A method of preparing a mooring rope for use in a mooring system for a floating offshore structure. The method comprises laying the mooring rope between a buoy and a seabed anchor; and tensioning the mooring rope between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition. In another aspect, there is provided a method of preparing a mooring system for a floating offshore structure. The mooring system has at least one mooring line between a buoy and a seabed anchor, and the mooring line comprises a mooring rope. The method comprises tensioning the at least one mooring line between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition. Preferred embodiments include tensioning the at least one mooring rope or mooring line between a buoyancy force from the buoy and a tensioning device acting against the buoyancy force. The tensioning device may be a part of the mooring system.

Description

[0001] METHODS AND SYSTEMS FOR OFFSHORE MOORING OPERATIONS

[0002] The present invention relates to methods and systems for offshore mooring operations, and in particular for mooring and / or riser connection and disconnection operations associated with floating offshore assets. An aspect of the invention relates to a method of preparing a mooring line for use in offshore mooring operations, and another aspect of the invention relates to a method of managing mooring lines in offshore mooring operations. A further aspect relates to a method of connecting and / or disconnecting a mooring. The invention has particular, but not exclusive, application to the mooring of floating offshore wind installations, and the connection and disconnection of associated risers.

[0003] Background to the invention

[0004] Wind power is an increasingly important renewable energy source, and utilising offshore locations for wind turbines has benefits including higher wind energy yields and fewer spatial and planning constraints compared to those located onshore. Floating offshore wind turbines (FOWTs) are now in use to access locations in deeper water and reduce construction costs compared with the jacket or monopile construction systems used in fixed offshore wind generation.

[0005] It can be expected that FOWTs will have to be disconnected at various points in their service life to enable them to be removed from their operating locations for replacement, servicing and / or upgrade. The motions of a FOWT are generally greater than turbines attached to a jacket that is anchored and fixed to the seabed, which complicates in situ repair operations. The increasing size and height of the turbines makes motion at the nacelle level greater than with smaller turbines. Often the water will be too deep for jackup crane vessels, necessitating the use of floating crane vessels, with the disadvantage of increased motion at the crane tip compared with a jack-up crane and potential clash with mooring lines.

[0006] A typical FOWT arrangement is on a floating hull with spread moorings. Such spread moorings typically have multiple mooring lines which connect to anchors on the seabed, with each mooring line individually connected to a corner of the asset, which remains on a fixed heading. A dynamic power cable riser is connected to the FOWT with a suitable profile supported by buoyancies. A typical arrangement for pull-in of a mooring line uses a winch wire connected to the end of the hull mooring connector, the wire being disconnected or left slack when the mooring connector is fully engaged. This arrangement is applicable to semi-submersible floating structures and a range of other moored floating structures and vessels. Several types of mooring connectors are available to the industry.

[0007] A disadvantage of these arrangements is that to disconnect and reconnect a FOWT offshore is a long and expensive operation since they typically have between three and nine mooring lines, plus typically two dynamic power cables. The connection of the mooring lines and power cable system involves locating, surveying, lifting, and connecting the end of each line individually to the FOWT. During this time the FOWT will have to be held in position by towing vessels. The hook-up can be a long and difficult process, especially in deep water or if the area is congested with platforms, vessels or other floating structures (such as the other FOWTs in the field) and their moorings, plus associated cables. The pull-in of the dynamic power cable is performed after the completion of the mooring hook-up, and the cable hook-up itself can also be of a long duration with attendant risks of damage and limited sea state.

[0008] Other floating vessels such as ships use turret mooring systems that include a single point mooring (SPM) at which the mooring lines come to a central point around which the ship can rotate via a swivel. Turret mooring systems are normally used in harsh environments where the loads on the mooring system can be reduced by allowing the ship to vane or head into the weather.

[0009] WO2016069636 discloses a disconnectable buoy system for FOWTs. In this system, the power cables are supported by a buoy while the mooring lines are connected in the standard spread mooring arrangement, separate from the buoy and individually to each column of a multi-column hull. A disadvantage of this arrangement is that the power cable(s) are required to hold the disconnectable buoy in position when it is disconnected but must resist damage from the buoy movement. In addition, the buoy may become very large in deep water due to the weight of cables and may present a restriction to the marine operations.

[0010] The applicant’s WO2021 / 234148 describes a disconnectable mooring system for offshore floating structures which offers a number of benefits and advantages over previously proposed systems. In the configuration of WO2021 / 234148, mooring chains can be arranged from mooring points around the structure in the same pattern as for a spread mooring, with the advantage that each is also attached to the disconnectable buoy and thus need only be pulled in from a shallow water depth. A principal advantage of the WO2021 / 234148 system is that it provides a disconnectable mooring system for offshore semi-submersible structures which combines the convenience of a ‘turret’ style buoy with a desired fixed heading spread mooring configuration.

[0011] Many offshore mooring systems use fibre rope mooring lines for long term applications. When manufactured, the yarns and filaments that make up the fibre rope are loosely bound and require a period of bedding-in to remove the voids and spaces between the yarns and filaments and stretch into their working condition. This bedding-in can be at least partially achieved in a post-manufacturing process before operation, in which the rope is tensioned or ‘pre-stretched’ to the working condition and design length. Typically, the length of rope is measured with a small tension load applied, before and after the prestretching. Pre-stretching results in an increase in length of around 2% to 4% of the original length. This increase in additional length is normally termed a ‘construction stretch’ and is permanent, with a small amount of further ‘creep’ occurring during the inservice lifespan, depending on the material used.

[0012] The higher the tension applied during the pre-stretching, the quicker the elongation to a fully bedded-in state, or a ‘worked’ length which does not change significantly in service. The permanent elongation associated with construction stretch will also typically occur with a lower tension applied for a greater time.

[0013] Onshore pre-stretching of fibre ropes requires very high forces, together with long lengths requiring very large test equipment. When the rope is spooled back on a reel, the yarns can loosen, and some of the benefits of the pre-stretch are lost.

[0014] Offshore pre-stretching can also be performed. For large offshore moorings a very high load in the order of hundreds of tonnes may be applied to pre-stretch the ropes by application of vessel thrust. If insufficient pre-stretching is performed, the mooring lines may elongate or creep further in service and hence applying as much load for as long as possible is desirable. Such a process is very expensive, can require multiple vessels and uses a large amount of fuel with resultant CO2e emissions. There is also a risk that when the mooring line is laid down on the seabed, the tension is released and some of the construction stretch may then be lost, leading to a shorter rope and higher hook-up tensions. This is an uncertainty which can have significant impacts on the specification of the equipment used and costs.

[0015] Furthermore, there are integrity concerns associated with leaving fibre rope on the seabed, especially after pre-stretching, as the rope can be damaged by abrasion and particle ingress. In addition, the rope tends to be stiffer making it difficult to bend and lay down without kinking.

[0016] If the ropes are not fully pre-stretched, the rope lengths will increase further during the operational phase. To maintain the tension in the connected mooring system, adjustment of the mooring line length will be required by pulling in chain at either end or at a point along the mooring line. Normally in oil and gas facilities, the floating structure is the main single asset and chain tensioners are located on the floating structure, facilitating adjustment. However, for the floating offshore wind industry, the assets are multiple and unmanned, and locating the tensioning device on the floater is less likely. The option of tensioning at the seabed is more attractive both commercially and practically. Typical seabed tensioning device is shown in Figure 1, which can be located on the anchor, part way along the seabed or in the suspended portion of the mooring line. The upper end of the mooring line is typically a fixed connection, for example a male / female socket connector arrangement known in the art. The length of fibre rope can typically be 500m to 1500m in a mooring line for floating wind, therefore the additional permanent length which can be introduced by pre-stretching is significant.

[0017] As an alternative to pre-stretching, a mooring can be connected at a higher tension than its required operating tension during the hook-up, so that the when the construction stretch works its way out, the mooring is then at the design tension. The hook-up tensions for this method can be in excess of 150 tonnes, and thus the cost, required equipment size and safety issues can be problematic or prohibitive.

[0018] There have also been proposed devices for embedding anchors and stretching chain or wire mooring lines using cross-tensioning methods. These require mooring lines to be diametrically opposite one another. WO2023 / 239244 describes a method of tensioning moorings in three different directions. Other cross-tensioning methods include using a crane to lift mooring lines which are connected together at a central point. All of these methods use the ‘bow-string’ effect to generate high tensions by pulling on the central part of a stiff mooring line with little slack in it. Due to the stretch of fibre ropes these approaches risk the tensioning device reaching the surface without achieving the tension required, and have potential to generate uplift at the anchor. Therefore, cross-tensioning methods are difficult to achieve for taut or semi-taut moorings.

[0019] Maintaining the ropes under tension after pre-stretching, prior to and following the floating structure arrival is also a particular challenge. In operation during a storm the mooring lines which are not facing the weather (leeward lines) can also become slack and snatch, which raises concerns regards the integrity of the mooring system. This is due to the connection point on the floating structure potentially moving vertical downwards and / or backwards quicker than the rope can fall in water vertically.

[0020] The use of clump weights mid-way along wire rope or chain moorings is described in CA877709. The use of a clump weight with fibre rope moorings has been employed in an offloading system termed SAL (Single Anchor Leg) mooring.

[0021] Summary of the invention

[0022] It is amongst the aims and objects of the invention to provide a method and / or apparatus which obviates or mitigates one or more drawbacks or disadvantages of the prior art.

[0023] It is amongst the aims and objects of the invention to provide a method of preparing a mooring rope offshore, without reliance on large handling vessels or tugs.

[0024] Further aims and objects of the invention will become apparent from the following description.

[0025] According to a first aspect of the invention, there is provided a method of preparing a mooring rope for use in a mooring system for a floating offshore structure, the method comprising: laying the mooring rope between a buoy and a seabed anchor; and tensioning the mooring rope between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition.

[0026] The invention has the advantage that it enables pre-stretching of a mooring rope offshore, utilising the effect of the buoyancy to stretch the mooring rope, without reliance on the presence of vessels.

[0027] The mooring rope may be formed from a fibre rope material, which when manufactured, may comprise yarns or filaments that are loosely bound and require a period of bedding-in to remove the voids and spaces between the yarns or filaments and stretch into their working condition.

[0028] The mooring rope may be a part of a mooring line, and the method may comprise laying the mooring line comprising the mooring rope between a buoy and seabed anchor. The mooring line may comprise a mooring rope formed from a fibre rope material, and may comprise one or more additional components, such as lengths of wire, chain, and suitable connectors etc.

[0029] The method may be used on a single mooring rope. Alternatively, or in addition, the method may comprise laying two or more mooring lines, at least one of which comprises a mooring rope, between the buoy and respective seabed anchors. Each mooring line may comprise a mooring rope, or a subset of the mooring lines may comprise a mooring rope.

[0030] The method may comprise tensioning the at least one mooring line between a buoyancy force from the buoy and a tensioning device acting against the buoyancy force. The tensioning device may be a part of the mooring system.

[0031] The tensioning device may comprise a seabed tensioner apparatus. Alternatively, or in addition, the tensioning device may comprise an in-line tensioner apparatus. The mooring line may further comprise one or more lengths of chain, and typically comprises a length of chain disposed between the mooring rope and the seabed anchor. The tensioning device may be configured to pull in and / or pay out a part of the length of chain of the at least one mooring line. Where the mooring system comprises more than one mooring line, it may comprise a tensioning device associated with each mooring line. Alternatively, a tensioning device may be associated with a single or a subset of mooring line(s) in a group of two or more mooring lines. Mid-line buoyancy elements and / or clump weights may be connected to the mooring line, or some of each of the mooring lines.

[0032] Alternatively, or in addition, the method may comprise tensioning the at least one mooring line by applying an additional vertical force to the mooring line.

[0033] The additional vertical force may be applied by increasing the positive buoyancy of a buoy assembly comprising the buoy. In one embodiment, the buoy assembly has a first condition in which it is negatively buoyant, and a second condition in which it is positively buoyant. In the second condition, the buoy assembly may impart the buoyancy force on the mooring line to tension the mooring line. The buoy assembly may change from its first condition to its second condition by reducing the mass of the buoy assembly. The mass may be reduced by releasing ballast from the buoy assembly. For example, a clump weight may be released from the buoy assembly. Alternatively, or in addition, water may be discharged from the buoy assembly by pumping, or by displacement (purging) with gas or air.

[0034] Alternatively, or in addition, additional vertical force may be applied by adding additional buoyancy to the buoy assembly.

[0035] Alternatively, or in addition, additional vertical force may be applied by adding applying a force from a surface winch or crane, which may be located on a floating structure to be moored or a vessel.

[0036] The method may comprise monitoring an elongation of the mooring rope. Monitoring of an elongation of the mooring rope may comprise monitoring the water depth position of the buoy. Alternatively, or in addition, monitoring of an elongation of the mooring rope may comprise monitoring a tension in the at least one mooring line. Alternatively, or in addition, monitoring of an elongation of the mooring rope may comprise monitoring a position of the buoy, which may include a horizontal location of the buoy in relation to the seabed, a rotational or azimuthal position of the buoy, and / or an inclination of the buoy with respect to a vertical axis. The method may comprise monitoring an elongation rate of the mooring rope. The method may also comprise monitoring of the position and / or depth of a point on at least one mooring line. The buoy may comprise monitoring equipment. For example, the buoy may comprise equipment for monitoring water depth position, a tension in the at least one mooring line, a horizontal location of the buoy in relation to the seabed, a rotational or azimuthal position of the buoy, and / or an inclination of the buoy with respect to a vertical axis. The equipment may for example comprise one or more load cells, acoustic sensors, pressure sensors, and / or inclinometers. The monitoring system may record the data and / or relay data, for example via a surface transmitter. The method may comprise storage and / or real-time monitoring of measured data.

[0037] The method may comprise tensioning the at least one mooring line to a pre-determined tension selected for the mooring rope preparation operation. The pre-determined tension may be different from a typical operating tension for the mooring line in use in the mooring system.

[0038] The method may comprise maintaining a target tension, or a target tension range, during the mooring rope preparation operation. The method may comprise compensating for a reduction in tension due to elongation of the mooring rope during the mooring rope preparation operation. The method may comprise pulling on the at least one mooring line during the mooring rope preparation operation, optionally using a seabed or in-line tensioner apparatus, to increase the tension to or towards a target tension, or a target tension range. Alternatively, or in addition, the method may comprise increasing the buoyancy force of the buoy during the mooring rope preparation operation to increase the tension to or towards a target tension, or a target tension range. Increasing the buoyancy force may be achieved by increasing the net positive buoyancy of the buoy, for example by reducing a ballast weight of the buoy.

[0039] The method may comprise tensioning the at least one mooring line at a target tension, or a target tension range for a predetermined period of time. Alternatively, or in addition, the method may comprise tensioning the at least one mooring line at a target tension, or a target tension range, until an elongation length is obtained.

[0040] The buoy may comprise a connector which enables connection and disconnection of the buoy from a floating structure. The mooring system may have a disconnected configuration in which the buoy is not connected to the floating structure, and the mooring lines determine the position of the buoy for subsequent connection and mooring of the floating structure. The mooring system may have a connected configuration in which the buoy is connected to the structure, and in which a mooring connector of the mooring line may be pulled in to connect to a mooring point on the structure. Such a system is described in the applicant’s WO2021 / 234148.

[0041] The method may comprise tensioning, or adding tension to the at least one mooring line, by pulling on the at least one mooring line or buoy using a surface winch, and / or adding buoyancy to the buoy assembly. The surface winch may be on a floating structure.

[0042] The method may comprise adjusting a position of the buoy to a desired position for connection of the floating structure. Adjusting the position of the buoy may comprise pulling in or paying out at least one mooring line from a seabed tensioner apparatus. Alternatively, or in addition, adjusting the position of the buoy may comprise adjusting at least one mooring line at a position between the buoy and a respective seabed anchor.

[0043] The method may comprise maintaining and / or controlling the depth of the buoy using one or more clump weights on the mooring lines. The one or more clump weights may comprise one or more vertical lengths of chain.

[0044] The invention according to the first aspect extends to preparing a mooring rope or a mooring line comprising a mooring rope for subsequent use in a mooring system. The invention also relates to a method of preparing a mooring system which incorporates a mooring rope. Therefore, according to a second aspect of the invention, there is provided a method of preparing a mooring system for a floating offshore structure, the mooring system comprising at least one mooring line between a buoy and a seabed anchor, the mooring line comprising a mooring rope, and the method comprising: tensioning the at least one mooring line between a buoyancy force from the buoy and the subsea anchor, thereby elongating the mooring rope towards a worked length condition.

[0045] The mooring system may comprise at least two mooring lines between the buoy and respective seabed anchors, at least one of said mooring lines comprising a mooring rope. Preferably, the mooring system comprises first and second mooring lines, comprising first and second mooring ropes respectively. The mooring system may comprise three or more mooring lines between the buoy and respective seabed anchors. The method may comprise maintaining and / or controlling the depth of the buoy using one or more clump weights on the mooring lines. The one or more clump weights may comprise one or more lengths of chain, which may be vertical lengths of chain.

[0046] Alternatively, or in addition, the one or more clump weights may comprise a length of chain or a set of parallel chains of different lengths.

[0047] Alternatively, or in addition, the one or more clump weights may comprise a solid point mass.

[0048] The one or more clump weights may be positioned to prevent the buoy rising above a prescribed depth, when the rope approaches its worked length. The clump weight(s) may be positioned at a point along the mooring line between the buoy connection and the seabed touchdown point.

[0049] Where the mooring system has a connected configuration in which the buoy is connected to the structure, the method may comprise maintaining tension in the one or more mooring lines, using one or more clump weights on the mooring lines. The method may comprise maintaining tension in one or more leeward mooring lines, using one or more clump weights on the mooring lines. Thus, the method may mitigate slack conditions in leeward lines and reduce the risk of loss of construction stretch.

[0050] The one or more clump weights may allow a pre-stretch operation for a mooring rope to be performed in stages to suit the tension planned to be applied and frequency of retensioning. The clump weights may incorporate a mooring tensioning device within the clump weight assembly.

[0051] The one or more clump weights may be positioned between a mooring connector for the floating structure and a seabed touchdown point, and may be above the seabed under all operational conditions while having sufficient weight to maintain a positive tension in the mooring ropes in operation. The method may comprise lifting one or more clump weights by a surface winch or vessel, thereby reducing the tension in the mooring line during a connection and / or disconnection from the floating structure.

[0052] The position of the one or more clump weights may be such that it introduces a ‘bowstring’ effect to a mooring line creating additional compliance, and / or may maintain the mooring line under tension, especially in the leeward line case, reducing the risk of compression or snatching damage to mooring components.

[0053] Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa.

[0054] Brief description of the drawings

[0055] There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

[0056] Figure 1 is a schematic representation of a seabed anchor and tensioner system according to the prior art;

[0057] Figure 2 is a schematic representation of an arrangement for preparation of a mooring system according to an embodiment of the invention;

[0058] Figure 3A is a flow diagram illustrating steps of a mooring system preparation operation according to an embodiment of the invention, and Figures 3B to 3D are schematic representations of selected steps of Figure 3A;

[0059] Figures 4A to 4C are schematic representations of a mooring system preparation operation applied to a disconnectable mooring system for a floating structure, according to an embodiment of the invention;

[0060] Figures 5A to 5D are schematic representations of a mooring system preparation operation applied to a mooring system for a floating structure according to an alternative embodiment of the invention;

[0061] Figure 6A to 6C are schematic representations of a mooring system preparation operation applied to a mooring system for a floating structure according to an alternative embodiment of the invention to manage the depth of the tensioning buoy;

[0062] Figure 7A to 7D are schematic representations of a mooring system preparation operation applied to a mooring system for a floating structure according to an alternative embodiment of the invention to manage the depth of the tensioning buoy and improve operational performance.

[0063] Figure 8A to 8H are schematic representations of clump weight assembly options, methods of connection to the ropes and incorporating a chain tensioning device.

[0064] Detailed description of preferred embodiments

[0065] Referring firstly to Figure 1 , there is shown schematically a seabed anchor and tensioner system 10 according to the prior art. Normally in the oil and gas industry a floating structure is the main single asset in use, and chain tensioners are located on the floating structure. However, for the floating offshore wind industry, the assets are multiple and unmanned, and locating the tensioning device on the floating structures is less attractive, both commercially and practically. System 10 is a typical arrangement, and comprises a seabed anchor 11, which may be suction anchor in the seabed 14, and a tensioning device 12. The mooring line 20 may be formed from any suitable line, wire, rope or chain, but has at its lower end a length of chain 21. The tensioning device 12 is configured to pull-in and pay out the length of chain 21 to increase or decrease the tension in the mooring line 20. In a variation to the system 10, the tensioning device 12 is located part way along the mooring line and is connected to an anchor 11 by a length of chain. In a further variation to the system 10, the tensioning device 12 is located in a suspended part of the mooring line. Seabed or in-line tensioning devices are applied in the methods and systems of the present invention.

[0066] Referring now to Figure 2, there is shown schematically an arrangement for preparation of a mooring system according to an embodiment of the invention. The arrangement comprises a pair of mooring lines 20 arranged between a buoy 30 and respective seabed tensioning apparatus 12. In this embodiment, the tensioning apparatuses 12 are located at respective seabed anchors 10 in the manner of Figure 1 , but in alternative arrangements the tensioning apparatuses 12 may be part way along the mooring line or on the seabed 14. Each mooring line 20 comprises a lower length of chain 21 , which is designed to be pulled in and paid out from the tensioning apparatus, and a main mooring line 22. While the principles of the invention are applicable to any suitable mooring line material, including line, wire, rope or chain, the invention realises benefits when applied to fibre rope mooring line materials, which when manufactured, comprise yarns or filaments that are loosely bound and require a period of bedding-in to remove the voids and spaces between the yarns or filaments and stretch into their working condition. Example materials include polyester or nylon mooring ropes, which are known in the art and HMPE fibre based mooring lines such DYNEEMA®. Other examples of fibre materials and rope constructions include but are not limited to Aramid, Twaron®, Superline, and Steelite. In this embodiment, the main mooring line 22 is formed from a polyester fibre rope. An upper end of each mooring line 20 is connected to the buoy via a fixed connection, for example a male-female socket connection as is known in the art. The length of fibre rope can typically be 500m to 1500m in a mooring line for floating wind, although exceptions fall outside of this range. Any stretch that can be introduced to the mooring lines can therefore be significant. The lower length of chain may be around 50m in length or more to allow for pulling in of the mooring lines and may optionally include a temporary length of excess chain 24 to assist the installation by allowing engagement in the tensioner prior to stretching but removed once the working length is achieved.

[0067] The buoy 30 comprises connection points for each of the mooring lines it is required to connect, and monitoring equipment for monitoring any or all of a range of parameters during operations. For example, the buoy may optionally include equipment 38 for monitoring water depth position, a tension in the at least one mooring line, a horizontal location of the buoy in relation to the seabed, a rotational or azimuthal position of the buoy, and / or an inclination of the buoy with respect to a vertical axis. The equipment may for example comprise one or more load cells, acoustic sensors, pressure sensors, and / or inclinometers, which may be arranged in a sensor pack. In embodiments of the invention, buoy also comprises a communications and control module, which is capable of data storage, data transmission and reception to a remote processing or monitoring location, and / or a data storage device.

[0068] Steps of the mooring system preparation operation will now be described with reference to Figure 3.

[0069] The mooring lines 22 are installed (31) by laying the lines from the buoy 30 to their respective anchors, or laying the lines from the anchors to the buoy 30 under tension.

[0070] With the mooring lines connected at both ends, slack is removed (32) from the mooring lines by either pulling in one or more of the mooring lines at the tensioning device 12, until a predetermined initial tension exists in the mooring line between a buoyancy force from the buoy 30 and the anchor 11 or by letting the buoy reach its equilibrium position in the water column. Further tension is then applied to the mooring lines (33) by pulling on at least one mooring line using the seabed tensioning apparatus 12, until a target tension (or tension range) is reached. Tensioning causes elongation of the mooring lines as the fibres of the mooring rope bed-in, and voids and spaces between the fibre yarns are closed.

[0071] Elongation of the mooring line, and optionally the rate of elongation, is monitored (34) by equipment on the buoy and or the tensioning apparatus 12. Various techniques can be used to monitor the elongation. As the mooring line tension increases, the buoy is pulled deeper down into the water. As the mooring line elongates the buoy rises and the tension reduces. Thus, the elongation and rate of elongation can be monitored by monitoring the water depth position of the buoy, for example by pressure sensors or acoustic depth sensors. Other positional information relating to the buoy can be monitored, such as horizontal location of the buoy in relation to the seabed, a rotational or azimuthal position of the buoy, and / or an inclination of the buoy with respect to a vertical axis. Alternatively, or in addition, monitoring of an elongation of the mooring rope may be achieved by monitoring a tension in at least of the mooring lines. Monitoring equipment on the buoy may include any of a number of sensors for this purpose, including but not limited to load cells, acoustic sensors, pressure sensors, and / or inclinometers. Monitoring of an individual mooring line may be performed to check for any differential tension.

[0072] The target tension, or a target tension range, is maintained (35) by compensating for a reduction in tension due to elongation by pulling on at least one mooring line until the tension returns to a target tension, or a target tension range. Alternatively, the arrangement may be planned such that the fully pre-stretched condition is achieved without adjustment of the mooring lines.

[0073] The mooring line may be determined to have reached its working length (36) after a predetermined period of time at the target tension, or alternatively by monitoring of the elongation (or a combination). Although tension may be reduced in the mooring lines if desired, the buoy system enables the mooring ropes to be kept under tension. Laying down the mooring lines on the seabed under low or no tension carries a risk that the fibres will relax and the length of the ropes will reduce. Therefore, the system may be left to further elongate under the pre-tension, with varying tension in the mooring lines from wave and current action helping further bedding-in of the fibres within the rope, and maintaining the ropes under load prior to and during the hook-up operation.

[0074] With the mooring rope in its working condition, the mooring system can be connected (37) and based on the monitoring results final adjustment of the mooring may be performed once the FOWT is connected or prior to the hook-up.

[0075] Figure 3B to 3D are schematic representations of the key stages referred to in Figure 3A flowchart. In Figure 3B the buoy 30 has been installed and the tension increased in the mooring system by pulling the buoy down by pulling in chain at the tensioner 12 using the vessel 78, as described with reference to steps 32 and 33 in the Figure 3A. The ropes will begin to elongate as if it were be pulled by a ship’s thrust and the buoy 30 will rise higher in the water column. Figure 3C illustrates an optional method to further increase the tension in the mooring lines by applying a vertical upward force, for example by using the vessel crane or a winch onboard. In this case a vessel crane is used to pull up on the buoy 30 to generate additional tension, as described in step 33 in Figure 3A. In Figure 3D the buoy is shown higher in the water column indicating the ropes are approaching or have reached their worked length, as described in step 36 in the flowchart. The steps of adjusting the tension in the mooring line(s) as illustrated in Figure 3B may be repeated as required, and / or the optional step of Figure 3C, may be carried out to achieve the configuration in Figure 3D.

[0076] Referring now to Figures 4A to 4C, there are shown schematic representations of a mooring system preparation operation according to an embodiment of the invention. The operation is applied to a disconnectable mooring system 40 for a floating structure, of the type described in the applicant’s WO2021 / 234148. Figure 4A shows the system in a preparation operation; Figure 4B shows the system 40 in a disconnected configuration and Figure 4C shows the system 40 in a connected configuration. The mooring system 40 comprises a plurality of mooring lines, of which two 20a and 20b are shown, and a buoy 41. Each mooring line has a first end connected to a subsea anchor 51 with a subsea tensioning apparatus 52, and a second end connected to the buoy 41. Mooring line 20b has a mooring connector 58 for the floating structure a part way along the mooring line between the first and second ends. The mooring connector 58 is a three-way connector which connects two parts of the mooring line - the main mooring line and an upper line portion 23b - and also has a connection for pull-in of the mooring line to a mooring point on the floating structure 60 via a winch and winch line (not shown). Additional mooring lines similar to 20b are provided with respective seabed anchors and mooring connection points on the floating structure. A buoy pull-in winch and winch line are provided on the floating structure 60. The buoy 41 supports a riser 70, which in this case is a dynamic power cable riser 70 illustrated in a lazy-wave profile with buoyancies 72. In this embodiment, the mooring lines comprise a mooring rope, which is typically composed of either polyester or nylon.

[0077] Figure 4A shows the system 40 in a preparation operation, as described with reference to Figures 2 and 3. The mooring ropes of the lines 20 are pre-stretched to their working condition according to the methods of the foregoing description. With the pre-stretching complete and the mooring lines fully elongated, the mooring system can be used in the manner described below.

[0078] As shown in Figure 4B, the buoy 41 is lowered beneath the surface of the water to avoid offshore traffic. The mooring line 20b is in a catenary and the arrangement is stable in the water, without having to lower the mooring lines or riser to the seabed.

[0079] Figure 4C shows the system 40 in a connected configuration, in which the buoy 41 , is connected into the hull of the semi-submersible 60 at the base of one column, after pull-in by the winch and winch line. The mooring line 20b is connected onto the semisubmersible at a mooring point on the hull, and the upper portion 23b of the mooring line 20b runs from the buoy 41 to the connection point and mooring point. The lower part of the mooring chain 20b will take the environmental loads on the semi-submersible through the connector arrangement 58 and the upper portion 23b will hang in a slack catenary configuration to the buoy 41.

[0080] The buoy 41 is easily connected / disconnected from the semi-submersible hull and supports the moorings 20 and power cables 70 deeper in the water and avoids the requirement for swivels as the mooring arrangement is a spread mooring. When the buoy is pulled into the hull of the structure, if the elevation of the top of the buoy is above the water level, or the internal compartment can be made water-tight, the power cables can be connected to the electrical system without any handling of the dynamic power cable. The system 40 and systems based thereon offer numerous operational advantages over conventional mooring systems. Notably, the present invention is able to utilise the features of the novel buoy mooring system of WO2021 / 234148 in the mooring system preparation methods described herein. Therefore, the mooring systems and methods of WO2021 / 234148 can be preceded by the offshore pre-stretching of the mooring lines, reducing or eliminating the use of onshore pre-stretching methods and the uneconomical offshore stretching practices of the prior art.

[0081] A further advantage is that the buoy marks the position at which the floating structure will be located when connected, as it is at the equilibrium position of the forces of each mooring line. Any offset in one direction can be compensated prior to connection, by adjusting the mooring lines at the anchor or a mid-point to bring the buoy to the target location. By placing the buoy in the desired position, less adjustment will be required when the large floating structure is on station. In addition, there will be greater confidence that the final pull-in tensions to hook-up the mooring lines to the structure are as required.

[0082] Figures 5A and 5B are schematic representations of a mooring system preparation operation according to an alternative embodiment of the invention. The operation is applied to a disconnectable mooring system 80 for a floating structure, similar to the system 40 of Figures 4A to 4C, but with a number of differences as described below.

[0083] Figure 5A shows the system in an initial configuration, and Figure 5B shows the system 80 in a preparation operation, prior to connection to a FOWT structure 90. The mooring system 80 comprises a plurality of mooring lines, of which two 81a and 81b are shown, and a buoy 82. Both mooring lines comprise a main mooring line portion formed from a fibre rope, as is known in the art. To illustrate variations within the scope of the invention, lines 81a and 81b are shown as different mooring line types. Mooring line 81a comprises a polyester fibre rope mooring, which is denser than seawater, and which includes midline buoyancy 83 to raise the line 81a to a midwater position. Mooring line 81b comprises a floating fibre rope mooring with integral buoyancy along parts of its length, as is known in the art, which does not require supplementary buoyancy to suspend the line in midwater. Either approach is desirable to keep the fibre rope off the seabed. Either or both types can be used in embodiments of the invention, and indeed any suitable mooring line type is applicable to the illustrated configuration and method. Each mooring line has a first end connected to a subsea anchor 91 with a subsea tensioning apparatus 92, and a second end connected to the buoy 82. As shown in Figure 5A, the buoy 82 is initially negatively buoyant, by virtue of ballast in the buoy assembly, and operates as a clump weight resting on the seabed.

[0084] Mooring lines 81a, 81b have mooring connectors 88 for the floating structure a part way along the mooring line between the first and second ends. Additional mooring lines similar to 81a, 81b are provided with respective seabed anchors and mooring connection points on the floating structure. A buoy pull-in winch and winch line are provided on the floating structure 90.

[0085] In the initial configuration, shown in Figure 5A, the buoy rests on the seabed. Prior to hook-up of the mooring system to the floating structure, the buoy is made positively buoyant by purging water from ballast chambers by gas injection. With the buoy now buoyant, it rises in the water column to the position shown in Figure 5B, at which the mooring lines are in tension between the buoyancy force and the force from the seabed tensioning apparatus. In this condition, the fibre ropes are undergoing stretching, as described in relation to previous embodiments. When elongation is complete and the mooring lines are in their working condition, the mooring system can be hooked up by connecting the buoy to the floating structure 90 in the manner described with respect to system 40. As with the system 40 of Figures 4A to 4C, the buoy 82 may support a riser (not shown) such as a dynamic power cable riser and facilitate its pull-in to the floating structure.

[0086] Figures 6A to 6C are schematic representations of a mooring system preparation operation applied to a mooring system for a floating structure according to an alternative embodiment of the invention to manage the depth of the tensioning buoy. Figures 6A to 6C represent alternative configurations of the system to enable depth management of the buoy 82, as the ropes adjust in length and tension varies within the system. The arrangement of Figure 6A includes supplemental buoyancy in the form of a surface buoy 85, and may be used where the buoy 82 has a lower amount of buoyancy or where additional tension is to be impacted into the system to tension the ropes 81a and 81b. This additional buoyancy 85 is affixed to the buoy 82 by a pennant 84.

[0087] Figure 6B illustrates an arrangement to counteract larger buoyancy where the buoy is held at a maximum height above the seabed by a clump weight 96 connected to the buoy 82 via a pennant 94. This arrangement will allow the buoy 82 to move translationally in the horizontal axis to react to any imbalance in mooring line tensions.

[0088] Figure 6C illustrates a means to counteract larger buoyancy where the buoy is restricted to a maximum height range above the seabed by clump weights 100 connected to the buoy 82 via respective pennants 98. This arrangement will allow the buoy 82 to move in response to passing waves and minimises the transfer of dynamic tensions in the clump weights. Another advantage is that by positioning the clump weights on the mooring connectors, they are a distance away from power cables which may be pulled into the buoy (see Figure 4C). Additionally, when the mooring system is hooked up to the floating structure the clump weights can be easily accessed to remove or replace at a later date.

[0089] Figure 7A to 7D are schematic representations of a mooring system preparation operation applied to a mooring system for a floating structure according to an alternative embodiment of the invention to manage the depth of the tensioning buoy and improve operational performance. Figure 7A illustrates the use of clump weights 86 on the mooring lines in lieu of the clump weight on pennants as illustrated in Figure 6C. These clump weights may be a permanent part of the mooring system or removable, but performs the same function of controlling the maximum height above the seabed of the buoy 82 as the clump weights in Figure 6B or 6C. The clump weights also manage the height of the mooring lines 81b and buoyancy 83 above the seabed.

[0090] Figure 7B illustrates the clump weights in the operational condition after hook-up of the mooring system to the floating structure, in which the clump weights provide tension to the mooring system. In operation the clump weights should preferably be clear of the seabed at all times. The geometric advantage of the clump weights is that if any further rope elongation occurs the impact on the pre-tension within the mooring system is reduced as compared to a taut system. A further advantage is that the clump weight can be lifted during the hook-up phase by a crane or winch 104 to reduce the hook-up tension and avoiding the requirement for the support vessel 102 to thrust horizontally. The operational performance is also improved by the leeward lines being maintained under tension during a storm.

[0091] Figure 7C illustrates the clump weight arrangement in a deep water location, where the mooring lines 81c are substantially off the seabed at all times. In the disconnected arrangement the clump weights 86 ropes maintain the buoy 82 a safe distance from the sea surface during the tensioning phase. In the connected state in Figure 7D the clump weights 86 are off the seabed. This arrangement also reduces the amount of chain required at the anchor location and more importantly minimises the risk of the ground chain at the anchor degrading the soils in front of the anchor.

[0092] Figure 8A to 8H illustrate examples of clump weight configurations that may be used in embodiments of the invention. Figures 8A and 8B illustrates two examples of a junction in the mooring line consisting of mooring rope 118 with a joining plate 122 and chain ballast weight 120 which may touch down on the seabed gradually reducing the vertical load on the joining plate. In Figure 8A, a pair of chain ballast weights are provided. In Figure 8B, a single chain ballast weight is provided.

[0093] Figures 8C and 8D illustrate a mooring chain 124 between the joining plates principally for the purposes of easier handling offshore during installation. Additional weight can be incorporated by either hanging chain 125 vertically from each joining plate (Figure 8C) or providing a slack chain 126 between each joining plate (Figure 8D).

[0094] Figures 8E and 8F illustrate two arrangements of a solid clump weight 132 attached either directly to the rope via a padeye 134 (Figure 8F) or to a top connection 130 with inserts of chain 128 either side to aid handling (Figure 8E).

[0095] Figures 8G and 8H illustrate how a chain tensioner may be incorporated into a clump weight arrangement. Additional weight may be added to the chain tensioner 140 in the form of chain 142 hanging vertically downward to achieve the required weight to match the other mooring lines which may not have a chain tensioning fitted (Figure 8G).

[0096] Alternatively, a chain locker 146 may be incorporated underneath the chain tensioner 140 to store the chain and prevent the chain hanging underneath the tensioner in a variable length (Figure 8H).

[0097] The present invention is a means of applying the pre-stretch to all the mooring ropes simultaneously without the need for large anchor handling vessels or tugs, using a centrally located buoy at the confluence of the mooring lines. In embodiments of the invention, the buoy is also a connection buoy for a floating structure, and positions mooring lines where the floating structure will be finally located. Embodiments of the invention enable monitoring of the elongation of mooring lines from a buoy with monitoring equipment on board. Where there are multiple installations with similar or identical mooring systems, re-tensioning and monitoring can be performed as required, using data obtained about the rate of elongation to better understand the rope behaviour for subsequent installation within the field.

[0098] Variations to the above-described riser installation method are within the scope of the invention. In particular, although the system 80 is described in the context of a mooring line preparation method and subsequent mooring of a floating structure, the principles of operation can be used in a mooring operation itself, without necessarily including the prestretching operation. An aspect of the invention therefore relates to a disconnectable mooring system for a floating offshore structure, the system comprising: a buoy assembly comprising a connector which enables connection and disconnection of the buoy assembly from the floating structure; and a plurality of mooring lines connected between the buoy assembly and respective seabed anchors; wherein the buoy assembly comprises a first negatively buoyant configuration, and a second positively buoyant configuration, in which the mooring lines are at least partially supported by the buoy assembly. The buoy may be connected to the floating structure, and the mooring lines may be connected to the floating structure by a mooring connector disposed part way along the mooring line between the buoy and the anchor. In another aspect of the invention, a method of connecting and / or disconnecting the mooring system is provided.

[0099] The invention provides a method of preparing a mooring rope for use in a mooring system for a floating offshore structure. The method comprises laying the mooring rope between a buoy and a seabed anchor; and tensioning the mooring rope between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition. In another aspect, there is provided a method of preparing a mooring system for a floating offshore structure. The mooring system has at least one mooring line between a buoy and a seabed anchor, and the mooring line comprises a mooring rope. The method comprises tensioning the at least one mooring line between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition. Preferred embodiments include tensioning the at least one mooring rope or mooring line between a buoyancy force from the buoy and a tensioning device acting against the buoyancy force. The tensioning device may be a part of the mooring system. Various modifications to the above-described embodiments may be made within the scope of the invention, and the invention extends to combinations of features other than those expressly claimed herein. In particular, although the embodiments are described with reference to FOWT structures, the principles of the invention and its embodiments and aspects are applicable to other floating structures.

Claims

Claims1 . A method of preparing a mooring rope for use in a mooring system for a floating offshore structure, the method comprising: laying the mooring rope between a buoy and a seabed anchor; and tensioning the mooring rope between a buoyancy force from the buoy and the seabed anchor, thereby elongating the mooring rope towards a worked length condition.

2. The method according to claim 1 , wherein the mooring rope is a part of a mooring line, and the method comprises laying the mooring line comprising the mooring rope between a buoy and seabed anchor.

3. The method according to claim 1 or claim 2, wherein the mooring rope is formed from a fibre rope material.

4. The method according to claim 2 or claim 3, wherein the method comprises laying two or more mooring lines, at least one of which comprises the mooring rope, between the buoy and respective seabed anchors.

5. The method according to any preceding claim, wherein the method comprises tensioning the mooring rope between a buoyancy force from the buoy and a tensioning device acting against the buoyancy force.

6. The method according to claim 5, wherein the tensioning device is a part of the mooring system.

7. The method according to claim 5 or claim 6, wherein the tensioning device comprises a seabed tensioner apparatus or an in-line tensioner apparatus.

8. The method according to any of claims 2 to 7, wherein the mooring line further comprises one or more lengths of chain, for example a length of chain disposed between the mooring rope and the seabed anchor.

9. The method according to claim 8, wherein the tensioning device is configured to pull in and / or pay out a part of the length of chain of the at least one mooring line.

10. The method according to claim 4 and any of claims 5 to 9, comprising a tensioning device associated with each mooring line.

11. The method according to claim 4 and any of claims 5 to 9, comprising a tensioning device associated with a single mooring line or a subset of mooring lines in a group of two or more mooring lines.

12. The method according to any of claims 2 to 11 , wherein mid-line buoyancy elements and / or clump weights are present on the mooring line, or some of each of the mooring lines.

13. The method according to any preceding claim, comprising tensioning the at least one mooring rope by applying an additional vertical force to the mooring rope.

14. The method according to claim 13, wherein the additional vertical force is applied by increasing the positive buoyancy of a buoy assembly comprising the buoy.

15. The method according to claim 14 wherein the buoy assembly has a first condition in which it is negatively buoyant, and a second condition in which it is positively buoyant, and wherein in the second condition, the buoy assembly imparts the buoyancy force on the mooring line to tension the mooring rope.

16. The method according to claim 15, comprising changing the buoy assembly from its first condition to its second condition by reducing the mass of the buoy assembly, for example by one or more of: releasing ballast from the buoy assembly, relasing a clump weight from the buoy assembly, discharging water from the buoy assembly by pumping or by displacement (purging) with gas or air.

17. The method according to claim 13, wherein the additional vertical force is applied by adding buoyancy elements to the buoy assembly.

18. The method according to claim 3, wherein the additional vertical force is applied by adding applying a force from a surface winch or crane, for example located on a floating structure to be moored, or a vessel.

19. The method according to any preceding claim, comprising monitoring an elongation of the mooring rope.

20. The method according to claim 19, comprising monitoring of an elongation of the mooring rope by one or more of: monitoring the water depth position of the buoy; monitoring of an elongation of the mooring rope; monitoring a tension in the at least one mooring rope; monitoring a position of the buoy including one or more of a horizontal location of the buoy in relation to the seabed, a rotational or azimuthal position of the buoy, and / or an inclination of the buoy with respect to a vertical axis; or monitoring the position and / or depth of a point on at least one mooring rope.

21. The method according to any preceding claim, comprising monitoring an elongation rate of the mooring rope.

22. The method according to any preceding claim, wherein the buoy comprises monitoring equipment selected from one or more of: equipment for monitoring water depth position; equipment for monitoring a tension in the at least one mooring rope; equipment for monitoring a horizontal location of the buoy in relation to the seabed; equipment for monitoring a rotational or azimuthal position of the buoy; equipment for monitoring, and / or an inclination of the buoy with respect to a vertical axis.

23. The method according to any preceding claim, comprising tensioning the mooring rope to a pre-determined tension selected for the mooring rope preparation operation.

24. The method according to any preceding claim, comprising maintaining a target tension, or a target tension range, during the mooring rope preparation operation.

25. The method according to any preceding claim, comprising compensating for a reduction in tension due to elongation of the mooring rope during the mooring rope preparation operation.

26. The method according to claim 25, comprising pulling on the mooring rope during the mooring rope preparation operation, optionally using a seabed or in-line tensioner apparatus, to increase the tension to or towards a target tension, or a target tension range.

27. The method according to claim 25 or claim 26, comprising increasing the buoyancy force of the buoy during the mooring rope preparation operation to increase the tension to or towards a target tension, or a target tension range.

28. The method according to any preceding claim, comprising tensioning the at least one mooring line at a target tension, or a target tension range, for a predetermined period of time.

29. The method according to any preceding claim, comprising tensioning the at least one mooring line at a target tension, or a target tension range, until an elongation length is obtained.

30. The method according to any preceding claim, wherein the buoy comprises a connector which enables connection and disconnection of the buoy from a floating structure, wherein the mooring system has a disconnected configuration in which the buoy is not connected to the floating structure, and the mooring lines determine the position of the buoy for subsequent connection and mooring of the floating structure.

31. The method according to claim 30, wherein the mooring system has a connected configuration in which the buoy is connected to the structure, and in which a mooring connector of the mooring line may be pulled in to connect to a mooring point on the structure.

32. The method according to any preceding claim, wherein the method comprises tensioning, or adding tension to the mooring rope, by pulling on the mooring rope or buoy using a surface winch, and / or by adding buoyancy to a buoy assembly comprising the buoy.

33. The method according to any of claims 2 to 32, wherein the method comprises adjusting a position of the buoy to a desired position for connection of the floating structure by pulling in or paying out at least one mooring line from a seabed tensioner apparatus, and / or by adjusting at least one mooring line at a position between the buoy and a respective seabed anchor.

34. The method according to any preceding claim, comprising maintaining and / or controlling the depth of the buoy using one or more clump weights on the mooring lines.

35. A method of preparing a mooring system for a floating offshore structure, the mooring system comprising at least one mooring line between a buoy and a seabed anchor, the mooring line comprising a mooring rope, and the method comprising: tensioning the at least one mooring line between a buoyancy force from the buoy and the subsea anchor, thereby elongating the mooring rope towards a worked length condition.

36. The method according to claim 35, wherein the mooring system comprises at least two mooring lines between the buoy and respective seabed anchors, at least one of said mooring lines comprising a mooring rope.

37. The method according to claim 36, wherein the mooring system comprises first and second mooring lines, comprising first and second mooring ropes respectively.

38. The method according to claim 37, wherein the mooring system comprises three or more mooring lines between the buoy and respective seabed anchors.

39. The method according to any of claims 35 to 38, comprising maintaining and / or controlling the depth of the buoy using one or more clump weights on the mooring lines.

40. The method according to claim 39, wherein the one or more clump weights is positioned to prevent the buoy rising above a prescribed depth, when the rope approaches its worked length.

41. The method according to claim 39 or claim 40, wherein the one or more clump weights is positioned at a point along the mooring line between the buoy connection and the seabed touchdown point.

42. The method according to any of claims 35 to 41 , wherein the buoy comprises a connector which enables connection and disconnection of the buoy from a floating structure, wherein the mooring system has a disconnected configuration in which the buoy is not connected to the floating structure, and the mooring lines determine the position of the buoy for subsequent connection and mooring of the floating structure.

43. The method according to claim 42, wherein the mooring system has a connected configuration in which the buoy is connected to the structure, and in which a mooring connector of the mooring line may be pulled in to connect to a mooring point on the structure.

44. The method according to any of claims 35 to 43, comprising maintaining tension in the one or more mooring lines, using one or more clump weights on the mooring lines.

45. The method according to claim 44, comprising maintaining tension in one or more leeward mooring lines, using one or more clump weights on the mooring lines.

46. The method according to claim 44 or claim 45, wherein the one or more clump weights are positioned between a mooring connector for the floating structure and a seabed touchdown point, and are above the seabed under operational conditions while having sufficient weight to maintain a positive tension in the mooring ropes in operation.

47. The method according any of claims 44 to 46, comprising lifting one or more clump weights by a surface winch or vessel, thereby reducing the tension in the mooring line during a connection and / or disconnection from the floating structure.

48. A mooring system prepared according to the method of any of claims 35 to 47.

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