Method for handling a flexible subsea power cable

Abstract

A method of handling a flexible subsea power cable (10) for an offshore installation, the cable comprising two end sections (11) and a fitting (12) on at least one end section (11), the method comprising at least the following steps in order; (i) wet-parking the entire cable (10) to a first wet-parking position flat on a seabed (17); (ii) recovering one end section (11) of the cable (10) onto a vessel deck (28); (iii) adding one or more elements (14) to one or more parts of the end section (11) of the cable (10); and (iv) returning the cable (10) to a second wet-parking position with at least a part of the one end section (11) including the one or more elements being above the seabed (17).

Description

[0001] METHOD FOR HANDLING A FLEXIBLE SUBSEA POWER CABLE

[0002] Technical Field

[0003] The present disclosure relates generally to handling of power cables in the field of offshore structures and more generally to the renewable energy industry. More specifically, the present invention relates to a method for handling a flexible subsea power cable, for example, as part of installation to a floating offshore structure.

[0004] Background

[0005] There is a need for developing ways of harvesting renewable energy, and a possible way is to provide wind farms positioned offshore. Such wind farms can be formed of a number of floating structures. There is a need to provide inter array cables to transport the energy harvested from each floating structure, and transport this energy to an offshore user or to an export line to shore.

[0006] Power cables are sometimes wet-parked subsea. Conventionally, a wet- parked power cable is ‘parked’ for a short period subsea with ancillaries, for example buoyancy modules, mounted on the power cable. Conventionally, such wet-parking is temporary as the cable remains able to move, i.e. is dynamic in the sea, so that the retriever must arrive soon.

[0007] Typically, an end of the power cable is retrieved from the sea-bed onto a vessel, so that ancillaries, for example buoys, clamps, protection, etc., can be installed on the power cable. Counting from the end of the power cable, it may be necessary to retrieve 200 meters, or more, or less, of the power cable. This length of the power cable must be presented on the vessel’s work area so that the ancillaries can be installed on the power cable.

[0008] Considerations also include where to park the end of the power cable on the vessel, and not to damage the power cable or any ancillaries thereon while handling the power cable. Any solution must provide a good area to work on for installing ancillaries. Further technical problems include that the power cable terminations are often long and do not go through a tensioner, the length that must be retrieved and the depth of the sea where the vessel operates.

[0009] It is desirable that any solution can use an existing vessel without too much of a change or need to rebuild the deck and / or the vessel. It is a problem to comply with the relevant regulations technically and in a cost effective manner. It is further a problem to avoid damage to the power cable, terminations, ancillaries, and any part of the vessel. The power cable is normally heavy, especially when pulling of the power cable up from the sea. Retrieving a long part of the power cable results in complicated handling of that length on the vessel. Any solution needs to be flexible and fit different types of power cables.

[0010] A further technical problem is that any part of handling a power cable on a vessel must be safe and function without a possibility to fail, fulfil technical and legal requirements, and be easy to use. It is desirable that any solution is simple, not expensive to produce, and is reliable. It is a further technical problem to avoid cumbersome arrangements that are expensive to realise.

[0011] The present invention seeks to improve the handling of such cables as part of their installation.

[0012] Summary

[0013] It is an objective of the present invention to provide a method and system for handling a flexible subsea power cable. This objective can be achieved by the features as defined in the independent claims. Further characteristics and embodiments are defined by the dependent claims.

[0014] According to one aspect, there is provided a method of handling a flexible subsea power cable for an offshore installation, the cable comprising two end sections and a fitting on at least one end section, the method comprising at least the following steps in order;

[0015] (i) wet-parking the entire cable to a first wet-parking position flat on a seabed;

[0016] (ii) recovering one end section of the cable onto a vessel deck;

[0017] (iii) adding one or more elements to one or more parts of the end section of the cable; and

[0018] (iv) returning the cable to a second wet-parking position with at least a part of the one end section including the one or more elements being above the seabed.

[0019] The first wet-parking position may provide a storage of the entire cable. The entire length of the cable, including any fittings, may be supported by the seabed, for a longer time period, in a static position not causing any damage or wear to the cable in the first wet-parking position. The cable may move and may be subject to damage and wear in the second wet-parking position and may only be in the second wetparking position a shorter period of time, for example shorter than the longer time period. The fitting may comprise a bend stiffener and / or a pigtail. At least a part of the cable may be additionally protected in the first wetparking position by protection means, for example a mattress supporting at least a fitting on the cable. At least a part of the cable may be further protected by stabilising means and / or protective means. At least a part of the cable may be buried and / or protected with a sufficient length of the end section of the cable unburied to allow recovering the end section to the vessel for steps (ii) and (iii). The remaining part of the cable may remain stabilised, for example buried and / or protected, on the seabed.

[0020] At least one of the above embodiments 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.

[0021] Brief Description of the Drawings

[0022] The disclosure will be further described with reference to examples depicted as schematic illustrations in the accompanying figures in which:

[0023] FIG 1 is a schematic illustration of a wet-parked cable in a first position on a sea-bed as part of one embodiment of the invention;

[0024] FIG 2 is a schematic illustration of recovering a part of one end section of the cable of Figure 1 onto a vessel deck as part of an embodiment of the invention;

[0025] FIGs 3 to 7 are schematic illustrations of onboarding the end section of the cable in Figure 2 onto a vessel deck as part of an embodiment of the invention.

[0026] FIG 8 is a schematic illustration of a wet-parked cable in a second position on a sea-bed as part of an embodiment of the invention;

[0027] FIG 9 is a schematic illustration of positioning an offshore installation in a proximity of one end section, and installing the one end section of the cable in FIG. 8 to the offshore installation for operation therewith, as part of an embodiment of the invention; and

[0028] FIG 10 is a schematic illustration of the method according to an embodiment.

[0029] Detailed Description

[0030] There is a need for developing ways of harvesting renewable energy, and a possible way is to provide wind farms positioned offshore. Such wind farms can be formed of a number of floating structures. There is a need to provide inter array cables to transport the energy harvested from each floating structure, and transport this energy to an offshore user or to an export line to shore.

[0031] Power cables may be temporary wet-parked subsea, i.e. still moving and therefore being dynamic. However, this requires timely retrieval by the user of the cable.

[0032] According to one aspect of the present invention, there is provided a method of handling a flexible subsea power cable for an offshore installation, the cable comprising two end sections and a fitting on at least one end section, the method comprising at least the following steps in order;

[0033] (i) wet-parking the entire cable to a first wet-parking position flat on a seabed;

[0034] (ii) recovering one end section of the cable onto a vessel deck;

[0035] (iii) adding one or more elements to one or more parts of the end section of the cable; and

[0036] (iv) returning the cable to a second wet-parking position with at least a part of the one end section including the one or more elements being above the seabed.

[0037] In this way, a first provider can provide step (i) at a first operation, and the same or a different provider can provide steps (ii) to (iv) at a different time, optionally a delayed time, and optionally a relatively open-ended time. The first provider may store the cable in the first wet- parking position instead of storing the cable onshore.

[0038] The present invention is also not limited by having the same provider for all of steps (i) to (iv), or by having steps (i) to (iv) carried out at the same time, or by both.

[0039] Thus, the present invention provides significant increased flexibility in providing a flexible subsea power cable for an offshore installation, which does not have to be limited to specific co-ordination of some steps of the provision to, and the subsequent installation of, the cable with the offshore installation. For example, the manufacturer of the cable can provide the cable for step (i) when convenient to the manufacturer and not the installer. Increased flexibility can lead to reduced operating costs for one or more of these steps.

[0040] The offshore installation may be a floating offshore installation or floating offshore structure.

[0041] Optionally, the offshore installation is a renewable power floating structure (e.g. a floating structure which supports a renewable power source or generator) arranged or to be arranged in a body of water. A renewable power source or generator could be a wind turbine, a wave harvesting solution or a tidal harvesting solution or a combination of these. The power cable may be used to export energy harvested on the offshore installation. An “inter array” power cable or a power cable in general, can extend from the floating structure into the body of water.

[0042] The flexible subsea power cable may be any suitable cable able to carry power. Optionally, the flexible subsea power cable is a Dynamic Inter Array Cable.

[0043] The flexible subsea power cable has two end sections, an end section may start from the very end of the power cable and extend along the cable. Each end section may be the same or different. Each end section may extend for a predetermined length along the cable. One or both end sections may include a fitting.

[0044] Optionally, one or both end sections include one or more components, such as a bend stiffener and / or a connector and / or a pigtail. The length of an end section may be a length sufficient for all the ancillaries, such as bend stiffener, pigtail, buoyancy elements, clamps, connectors, weights, protectors, etc. to be mounted or handled on the power cable while taking into consideration the depth where the power cable is to be recovered on to the vessel.

[0045] Optionally, the length of at least one of the end sections is related to or determined by the amount of cable required to be recovered in step (ii) to achieve step (iii). As the number and / or nature and / or relative positioning of the one or more elements to be added to one or more parts of the end section of the cable may vary according to the requirements of the later cable installation, the present invention is not limited by the length of at least the end section or the length of the part of one end section to be recovered in step (ii) onto a vessel deck.

[0046] Optionally, counting from the end of the power cable, an end section may be at least 50, 100, 150, 200, 250, 300, 400, 500, 600, or more metres long. Each end section may be the same or different length.

[0047] In the present invention, the wet-parking of the cable in step (i) can be carried out at any suitable time in advance of and independent from steps (ii) to (iv). Since the two wet-parking may be separately and independent of each other, a much shorter weather window is required for each phase, compared with installing the power cable from start to finish. Thus, the present invention provides flexibility in providing the flexible subsea power cable ready for subsequent offshore installation, which does not have to be limited to specific co-ordination of some steps of the provision to, and the subsequent installation of, the cable with the offshore installation. According to one embodiment, the cable (10) may be completely static in the first wet-parking position for the predetermined time period, and the cable (10) may be at least partly dynamic in the second wet-parking position for a time period shorter than the predetermined time period.

[0048] Optionally, a time period between steps (i) and (ii) is a predetermined time period, or more than a predetermined time period. The pre-determined time period could be a week, more than a week, several weeks, several months, or longer. For example, the pre-determined time period could be more than 4 weeks, or more than 8, 10, 20, 26, 30, 40, 52 weeks, or longer such as 1.5 or even 2 years. The time period of wet-parking the cable in the first wet-parking position may be several times longer than the time period of wet-parking the cable in the second wet-parking position, for example three, five, ten or more times longer. One benefit of the present invention is that the wet-parking in step (i) could be carried out at a time suitable to the manufacturer and / or layer of the cable, rather than the installer of the cable to the offshore structure. Step (i) may be done by using a tensioner and a chute.

[0049] Step (ii) of the method of the present invention can be carried out using any apparatus or process.

[0050] Optionally, step (ii) includes recovering one end section of the cable including the fitting.

[0051] Optionally, step (ii) includes recovering one end section of the cable including a length of cable to allow step (iii) to be achieved on the end section of the cable on the vessel deck, or on a part of the end section near to the vessel deck. That is, step (ii) achieves recovery of a sufficient portion of the cable to achieve step (iii).

[0052] According to one embodiment of the present invention, step (ii) includes recovering one end section of the cable using a lay tower on the vessel deck.

[0053] Optionally, the lay tower comprises a tower mountable on a deck of a vessel, the tower comprising a firing line for laying the cable; an arcuate guide for the cable, the arcuate guide being rotatable around an axis on the tower; a first handling arm, the first handling arm being rotatable around the axis, configured to hold the fitting or cable, and configured to move the fitting along a path outside of the arcuate guide; and a tensioner for the cable, the tensioner being movable in and out of the firing line. The tensioner may be moved away from the firing line to give space to the first handling arm or any other equipment. Optionally, the tower includes a second handling arm, the second handling arm being configured to position the cable to allow the tensioner to move into the firing line around the cable (i.e. not around the fitting) by guiding the cable end further away from the arcuate guide than the first handling arm.

[0054] Optionally, the lay tower is able to carry out at least step (i) of the present invention.

[0055] Optionally, the lay tower is able to carry out at least steps (ii) to (iv) of the present invention.

[0056] Optionally, the lay tower is able to carry out at least steps (i) to (iv) of the present invention. These options may use the one and the same lay tower for steps (ii) to (iv), or even steps (i) to (iv).

[0057] Alternatively or additionally, step (ii) includes recovering one end section of the cable using a winch wire.

[0058] Optionally, a portion of the one end section of the cable recovered in step (ii) is stored on the vessel deck, or at least across the vessel deck. Optionally, a portion of the one end section of the cable is stored on a reel on the vessel deck. Optionally, a portion of the one end section of the cable is stored over one side of the vessel. Optionally, a portion of the one end section of the cable is stored on a carousel on the vessel.

[0059] Where the one end section of the cable recovered in step (ii) includes a fitting, the fitting can be protected, during step (iii), and optionally during one or more of the other steps of the present invention. For example, the fitting may include a protection from during manufacture of the cable, and / or from wet-parking in step (i) or as part of step (ii) of the method of the present invention.

[0060] Thus, optionally, the method further comprises using a protective to protect the fitting during step (i).

[0061] Optionally, the method further comprises using a protective to protect the fitting during during at least one or more of the steps (ii) to (iv).

[0062] Optionally, the fitting is a unit, device or component that is damageable during handling on or around a vessel.

[0063] Optionally, the fitting is a bend stiffener, or an end connector, or a bend stiffener with a pig tail.

[0064] Optionally, the one or more elements to be added to the one or more parts of the end section of the cable include but are not limited to buoyancy elements, clump weights, or tether clamps. The buoyancy elements, clump weights, or tether clamps may be configured for power cables. The addition of one or more elements to be added to the one or more parts of the end section of the cable in step (iii) may be for preparation of the cable for subsequent installation of the cable with the offshore installation. For example, an inter-array cable in a wind farm array may require to have a certain buoyancy, and the addition of the or each element to the one or more parts of the end section of the cable can be to achieve such buoyancy in use. The number and / or nature and / or relative positioning of the one or more elements added to one or more parts of the end section of the cable may vary according to the requirements of the later cable installation, the present invention is not limited by the number and / or nature and / or relative positioning of the one or more elements on to one or more parts of the end section of the cable. The end section of the power cable with the addition of one or more elements may be the dynamic part of the power cable in the second wet- parked position. In the first wet-parked position this end section is, as explained herein, static, for example flat and still, on the seabed. In the second wet-parked position the end section may be configured by the one or more elements to a dynamic end section forming a wave, such as for example a lazy wave, pliant wave, and / or steep wave. This may apply to one or both the end sections.

[0065] In one embodiment of the present invention, the cable is re-positioned between the addition of one element and the addition of another element. Optionally, a first element is added to the cable, and the cable is part returned into the sea to allow a second (and any subsequent) element to be added in a sequential manner.

[0066] Optionally, the cable is held by one or more tensioners on the vessel deck, and positioning of the cable to achieve the addition of one or more elements is achieved by operation of the tensioner. Optionally, any such tensioner(s) could be part of a lay tower on the vessel deck.

[0067] In step (iv) of the present invention, the cable of step (iii) is returned to a second wet-parking position with at least a part of the one end section now being above the seabed. As such, the second wet-parking position is different to the first wet-parking position. The second wet-parking position may be a result of the addition of the one or more elements in step (iii) of the present invention. For example, the addition of one or more buoyancy modules on one end section of the power cable results in that that end section now floating above the seabed. In the second wet-parked position the end section may be configured by the one or more elements to a dynamic end section forming a wave subsea and above the seabed, such as for example a lazy wave, pliant wave, and / or steep wave.

[0068] In step (iv) of the present invention, it is possible that in the second wetparking position, one or more elements in step (iii) may still lie on the seabed. The present invention is not limited by the extent of the one end section now being above the seabed. Typically, the extent of the one end section now being above the seabed will be determined by the nature of the sea, such as the sea depth, the nature and number and position of the one or more elements along the end section, etc.

[0069] Optionally, the part of the one end section including the one or more elements being above the seabed in the second wet-parking position is dynamic.

[0070] Optionally, the second wet-parking position may be wholly or substantially a position for subsequent installation of the cable to the offshore installation. The end of the cable in the second wet-parking position, and optionally also the first wetparking position, may be in the proximity of the offshore installation, or where the offshore installation will be positioned, such that a subsequent installation of the cable to the offshore installation is possible.

[0071] The method may provide a cable wholly or substantially ready for installation to an offshore installation. Such installation may be carried out by a separate vessel or operator.

[0072] According to another embodiment of the present invention, the method further comprise:

[0073] (v) positioning an offshore installation in a proximity of one end section; and

[0074] (vi) installing the one end section to the offshore installation for operation therewith.

[0075] Thus, the present invention provides significant increased flexibility in not only providing a flexible subsea power cable for an offshore installation, but also to the subsequent installation of the cable with the offshore installation. Increased flexibility can lead to reduced operating costs for all these steps. This may be especially where one or more parts of the system to set up and install the offshore installation are being made and / or provided by separate providers. This may also be especially where the weather may influence, in particular delay, one or more of steps (i) to (iv) or (i) to (vi). A delay for bad weather or production issues can be accommodated by variance in the timing of one or more of steps (i) to (iv) or (i) to (vi), such as between step (i) and steps (ii) to (iv), and / or, if included, in each of steps (v) and (vi). Optionally, the cable is static along the entire length of the cable in the first wet-parking position for the predetermined time period, and the cable is at least partly dynamic in the second wet-parking position for a time period shorter than the predetermined time period. Optionally, the first wet-parking of the power cable in step (i) provides a wholly or substantially wholly static cable, that is conveniently parked, stored, away from other activities, such that the cable can be parked for longer periods than hitherto expected in the art, i.e. extending months and even years. Such a time period is effectively ‘indefinitely’ in terms of the overall offshore installation. The cable can lie flat or relatively flat on the seabed as suits the user. This first wet-parking position may be static, while the second wet-parking position may be dynamic. It is in the first wet-parking position that the cable can be stored for a longer time period without being subject to damage or wear. It is in the second wetparking position that the cable is subject to damage or wear and may therefore be parked only for a shorter time period.

[0076] Optionally, step (i) includes wet-parking the fitting on a support on the seabed. The support may be any suitable device or component, able to support the fitting away from disruptive features on the sea bed. The support may be means that support the fitting and at least a part of the cable. The support may protect the fitting on all sides. Where the cable may include a fitting at both end sections of the cable, optionally step (i) includes wet-parking each fitting on a support on the seabed. Where the cable has a number of fittings, step (i) may include wet-parking the, or each, fitting with a support on the seabed.

[0077] Optionally, the power cable may be connected (e.g. anchored) to the seabed by a clamp attached to the power cable at a tethered point. Optionally, the clamp is connected to a foundation structure or anchored to the seabed via a tether or similar. This can also be called a hold-down system and may be used in the second wetparking position. The tethered point of the power cable, anchored to the seabed, is limited in the distance it can move away from the seabed but can move towards to seabed. Put another way, the tethered point has a maximum distance from which is can vertically move away from the seabed. As the hold-down system is via a tether / sling, the Touchdown Point (TDP) of the power cable can change since the clamp can move up / down and also laterally in the water column. Because of this, the power cable at the TDP location could also move laterally on the seabed. Alternatively, the point anchored to the seabed may have limited to substantially no movement on the seabed (e.g. be a more fixed point where the power cable is fixed to a bottom structure and in this instance the TDP is more fixed).

[0078] According to one embodiment, a system for handling a flexible subsea power cable for an offshore installation may be provided. The cable may comprise two end sections and a fitting on at least one end section. The system may comprise means to wet-park the entire cable to a first wet-parking position flat on a seabed, means to recover at least a part of one end section of the cable onto a vessel deck, means to add one or more elements to one or more parts of the end section of the cable, and means to return the cable to a second wet-parking position with at least a part of the one end section including the one or more elements being above the seabed.

[0079] The means to wet-park the entire cable to a first wet-parking position on a seabed may include a first vessel and cable laying unit, optionally including a lay tower as described herein, in particular able to lay and wet-park a cable having a fitting on at least one end section of the cable. Where such a fitting is or includes a bend stiffener, pigtail, or both, the cable laying unit may be specifically adapted thereto.

[0080] The means to recover at least a part of one end section of the cable onto a vessel deck may include a second vessel, optionally being the same or different to the first vessel, able to partly, substantially or fully onboard the part of one end section of the cable onto a vessel deck. Optionally, the means includes a lay tower as described herein. Where the end section includes a fitting, the lay tower may be specifically adapted thereto.

[0081] The means to add one or more elements to one or more parts of the end section of the cable may include one or more clamps, arms and other handling equipment able to provide, line up and allow the addition of the element(s) to the cable.

[0082] The means to return the cable to a second wet-parking position with at least a part of the one end section above the seabed may be the same or different to the means to recover at least a part of one end section of the cable onto a vessel deck. Optionally, the means to return the cable to a second wet-parking position is the same as the means to recover at least a part of one end section of the cable onto a vessel deck. Optionally, the means includes a lay tower as described herein. Where the end section includes a fitting, the lay tower may be specifically adapted thereto.

[0083] At least one of the above embodiments 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.

[0084] Turning to the drawings, figure 1 shows a flexible subsea power cable 10 for an offshore installation. The cable 10 comprises two end sections 11, each having at least one fitting thereon. In the example shown in Figure 1, each end section 11 has a first fitting comprising a bend stiffener 12, and a second fitting being a pigtail 13. The end section 11 may extend further along the cable for the buoyancy elements, clump weights, or tether clamps to be subsequently attached. The one or more elements 14 that are added to the end section 11 may best be taken from figure 8.

[0085] Figure 1 shows the cable 10 wet-parked flat on a seabed 17, with each bend stiffener 12 resting on a suitable pre-laid support 16 so as to provide some protection against any damaging features on the seabed 17. The support 16 may be a support structure or equipment, and may include protective means for protecting the fitting, the bend stiffener 12. Figure 1 also shows one end of the cable 10 connected (e.g. being tied) to a clump weight 18. The entire cable 10 may lay flat on the seabed. The first wet-parking position illustrated by figure 1 is merely a schematic illustration and the cable 10 and the bend stiffener 12 are supported by their entire length on the seabed even if there may be a gap in the illustration in figure 1. The part of the cable 10 that is between the end sections, as described herein, may be fixed and laid flat on the seabed. This may be done by, for example, burying the part at least partly, as schematically illustrated in figure 1. This may be done, in addition or as an alternative by fixation means, such as for example sand sacks, weights, and / or clamps. The end sections 11 may also be static, fixed and laid flat on the seabed. This may be made with fixation means, for example weights and / or clamps, that are removably attached to the end section 11 and may be removed during step (ii).

[0086] Thus, figure 1 shows the outcome of step (i) of a method of handling a flexible subsea power cable 10 for a floating offshore installation, the cable comprising two end sections 11 and a fitting 12 on at least one end section 11 , the method comprising at least the following steps in order;

[0087] (i) wet-parking the entire cable 10 to a first wet-parking position flat on a seabed 17; (ii) recovering one end section of the cable 10 onto a vessel deck 28;

[0088] (iii) adding one or more elements 14 to one or more parts of the end section 11 of the cable 10; and

[0089] (iv) returning the cable 10 to a second wet-parking position with at least a part of the one end section 11 including the one or more elements being above the seabed 17.

[0090] The wet-parking of the cable 10 can be carried out by a suitable first vessel following its manufacture. As the cable 10 can be suitably robust or otherwise protected against the sea, the cable 10 can be wet-parked at a suitable time in advance of its installation with an offshore installation. This provides flexibility to the organiser and installer of the offshore installation.

[0091] Figure 2 shows a method for recovering one end section 11 of the cable 10 on to a vessel deck of a vessel 20. A suitable wire, such as a winch wire 22, can be extended from the vessel 20 to reach the wet-parked cable 10, and connected to the pigtail 13. In this way, one end of the cable 10 can be winched from the seabed (not shown in the Figure 2) towards the vessel 20.

[0092] Figure 3 shows a lay tower 30 on the vessel 20 generally comprising two support structures 32, an arcuate guide 34 for the cable, the arcuate guide 34 being rotatable around an axis on the tower 30, a first handling arm 36, the first handling arm being rotatable around the same axis, and configured to hold the cable 10 or at least the end of the cable 10, and configured to move the bend stiffener 12 along a path outside the arcuate guide 34 in a manner described herein after, at tensioner 38 for the cable 10, the tensioner being moveable in and out of the firing line of the tower 30. The tensioner 38 may be moved away from the firing line to give space to the first handling arm 36 or any other equipment.

[0093] Figure 3 also shows a hang-off work table 26, for example a balcony or moonpool table, extending from the side of the vessel 20, and a winch 24 able to operate the winch wire 22 using a suitable motor. The hang-off work table may have an opening in the fire line for the cable 10, and means for opening or closing the opening. The hang-off work table 26 may comprise a hang-off for the cable 10.

[0094] Figure 3 shows part of the recovery of one end section 11 of the cable 10 on to the vessel deck 28. The recovery is carried out by operation of the winch 24 so as to bring the winch wire 22 and connected cable 10 up to the firing line of the tower 30. Figure 3 shows the path of the winch wire 22 extending to the end of the handling arm 36, through the distal end of the handling arm 36 and along a route being the same as or similar to the axis on the tower mentioned above, and then extending outwardly from one support structure 32 of the tower and downwardly to the base of the tower towards the winch 24. In this way, the winch wire 22 can be automatically operated to bring the end of the cable into the firing line of the tower 30, and the winch wire 22 is substantially out of the way for operators on the vessel deck. The winch wire 22 may be routed internally of the handling arm 36 and / or internally of the support structure 32.

[0095] Figure 4 shows the movement, see the black arrow, of the tensioner 38 from a position not in line with the firing line of the tower 30, to a position wherein its tracks 39 can be positioned around the firing line of the tower 30. The tensioner 38 may be moved away from the firing line to give space to the first handling arm 36 or any other equipment. Figure 4 shows movement of two of the tracks 39 from an open position shown in figure 4 to a closed position figure 5 to symmetrically surround the cable 10, and to operate in a coordinated manner. In figure 5, the track 39 of the tensioner 38 can be positioned to now engage the cable 10, so as to support the weight of the cable 10 and achieve motion of the cable. By operating the tensioner 38 the cable 10 may be moved.

[0096] Figure 5 shows movement, see the black arrow, of the arcuate guide 34 and handling arm 36 along a travel path perpendicular to the firing line of the tower 30. The arcuate guide 34 and handling arm 36 may move along a suitable guide frame 40, from a first position shown in figures 3 and 4, to a second position shown in Figure 5 where the cable 10 now meets with the arcuate guide 34.

[0097] Figure 6 shows rotation of the arcuate guide 34 and the handling arm 36 so as to continue the recovery of one end section of the cable 10 on to the vessel deck 28. In the rotation of the arcuate guide 34 and handling arm 36, the bend stiffener 12 and the pigtail 13 can avoid the surface of the arcuate guide 34, and thus avoid damage of at least the bend stiffener 12 whilst being recovered to the vessel deck 28. Upon further rotation of the arcuate guide 34 and handling arm 36 the bend stiffener 12 and pigtail 13 are moved to a vertical position on the other side of the lay tower 30 to the firing line. This is best illustrated in figure 7, where the tensioner and cable coming up have been removed for clarity purposes. An operator may now either relocate the cable 10 on to a reel (not shown), or relocate part of the cable 10 to the other side of the vessel 20 than the lay tower 30, as long as a suitable length of the one end section of the cable is provided for access for step (3) of the method of the present invention. Optionally, the bend stiffener 12 is located in a protection provided on the lay tower 30, and the end of the pigtail 11 of the cable 10 is connected to a vessel wire 46.

[0098] An operator can now add one or more elements 14 to one or more parts of the end section of the cable 10 being held in a suitable operating position, and being moveable to each required position using the tensioner 38. In this way, the operator can add one or more elements to the cable 10 to complete the pre-installation configuration of the cable 10, optionally shortly before expected installation of the cable 10 to an offshore installation. For example, using suitable guidance and support means, an operator can position a buoyancy module 14, shown in figure 8, around a part of the end section 11 of the cable 10 at the hang-off work table 26 next to the lay tower 30. A buoyancy module 14 may be added to the cable 10. Opening of doors to an opening of the hang-off work table 26 allows movement of the cable 10 and now connected buoyancy module 14 downwardly into the sea, to then allow the operator access to another part of the end section of the cable 10 on the operating position of the hang-off work table 26. In this way, one or more further elements 14, such as a slinker clumpweight or tether clamp or more buoyancy elements, can be added at suitable positions along the length of the cable 10.

[0099] The steps (ii)-(iv) configures the end section to be a dynamic end section 11 of the power cable 10. The end section 11 being dynamic may be the section of the power cable between touch a TDP and the offshore installation, for example a floating wind turbine platform. The subsequent operation, for example step (vi), may then be limited, for example only pulling in and installing the cable end to the offshore installation directly. As mentioned herein, several ancillaries 14, for example buoyancy members, connectors, clump weights, tether clamp, etc., may be attached to the end section 11 of the power cable 10 thereby forming a predetermined dynamic end section 11 of the power cable. The end section 11 of the power cable 10 with the addition of one or more elements may be the dynamic part of the power cable 10 in the second wet-parked position. In the first wet-parked position this end section 11 may be, as explained herein, static, for example flat and still, on the seabed. In the second wet-parked position the end section 11 may be configured by the one or more elements 14 to a dynamic end section 11 forming a wave, such as for example a lazy wave, pliant wave, and / or steep wave. This may apply to one or both the end sections 11. Once the operator has added the or all elements 14 to the end section 11 of the cable 10, the cable 10 can be returned to the second wet-parking position as discussed herein after with at least a part of that one end section above the seabed 17. Returning the cable 10 can be carried out in a reverse manner to the steps described herein above. In particular, the cable 10 can be fed back into the sea until the bend stiffener 12 and pigtail 13 are relocated on one side of the tower 30 in a position shown in Figure 7. By reversal of the steps shown and described herein in relation to Figures 7, 6, 5, 4 and 3, rotation of the arcuate guide 34 and the handling arm 36 can reposition the bend stiffener 12 and the pigtail 13 back into the firing line of the tower 30 on the sea-side of the tower 30 without the bend stiffener 12 touching the arcuate guide 34, followed by movement of the arcuate guide 34 and the handling arm 36 from the second position to the first position as described above along the transfers support 40, and movement of the tensioner 38 from the firing line position shown in Figure 5 to a suitable out of line position shown in Figure 3, to allow the return of the end of the cable 10 back into the sea using the winch wire 22.

[0100] Figure 8 shows the cable 10 in a second wet-parking position with at least a part of one end section 11 above the seabed 17. In the second wet-parked position the end section 11 may be configured by the one or more elements 14 to a dynamic end section 11 forming a wave, such as for example a lazy wave, pliant wave, and / or steep wave, subsea and above the seabed. This may apply to one or both the end sections 11. For example, the entire end section 11 may be above the seabed 17. Optionally, the second wet-parking position conveniently positions at least a part of one end section 11 of the cable 10 for the installer of the cable 10 to retrieve and then connect and install with an offshore installation 60. The end section 11 may be in the proximity of the offshore installation 60 where it will subsequently be installed.

[0101] Figure 9 shows an example of an offshore installation 60 and an installation vessel 20. With a suitable winch wire 22, the installation vessel 20 is able to connect to the end of the cable 10 and install the one end section of the cable 10 to the offshore installation 60 for operation therewith.

[0102] Figure 10 is a schematic illustration of the method steps (i) to (vi). Figure 10 illustrates these steps as defined herein and optionally their further characterisation disclosed herein.

[0103] The example of the present invention shown in figures 1-10 illustrate the increased flexibility of providing and installing a flexible subsea power cable for or to an offshore installation, through the ability of parts of the installation to be provided separately, optionally provided by separate providers. In this way, a first provider of a flexible subsea power cable can wet-park the cable at any suitable time prior to the subsequent steps of the method of the present invention. The power cable can be a static cable able to remain, to be stored, on a suitable seabed for the convenience of the next operator. The next provider can provide steps (ii) to (iv) of the method of the present invention when best convenient, so as to position the cable in a second wet-parking position, ideally having one end of the cable in proximity to the offshore installation. The offshore installation may already be located, or may be located separately relative to the cable as step (v). The subsequent installation, step (vi), may be taken later and only needs a small weather window, because this final step (vi) is now well prepared for being done quickly. The present invention allows flexibility in any such positioning arrangement.

[0104] The method may allow the first wet-parking of the cable in the first wetparking position to be done well in advance and independently of the subsequent steps. The power cable may be laid in the first wet-parking position where a major part of the power cable, the part excluding the two end sections, is already in a permanently fixed position flat on the seabed, i.e. in the power cables' final installation position. However, the entire cable is static, fixed, and flat on the seabed and can therefore be stored in the first wet-parked position for a longer time until the subsequent steps are due. After performing steps (ii) and (iii) the power cable is returned in step (iv) to a second wet-parked position. In this position one, or both, of the end sections are dynamic, i.e. the end sections have may be configured by the one or more elements to a dynamic end section forming a wave, such as for example a lazy wave, pliant wave, and / or steep wave, subsea and above the seabed. At least a part of the end sections is therefore subsea and above the seabed. The end sections of the power cable are now ready for subsequent installation, which can be done swiftly and only needs a small weather window, because the power cable is ready for the final installation.

[0105] When ready, the next provider having positioned an offshore installation in proximity to one end section of the cable, can proceed with installing the one end section to the offshore installation for operation therewith.

[0106] It will be apparent to those skilled in the art that various modifications and variations can be made to the method and system for installing a flexible subsea power cable. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed system 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

CLAIMS1. A method of handling a flexible subsea power cable (10) for an offshore installation, the cable comprising two end sections (11) and a fitting (12) on at least one end section (11), the method comprising at least the following steps in order;(i) wet-parking the entire cable (10) to a first wet-parking position flat on a seabed (17);(ii) recovering one end section (11) of the cable (10) onto a vessel deck (28);(iii) adding one or more elements (14) to one or more parts of the end section (11) of the cable (10); and(iv) returning the cable (10) to a second wet-parking position with at least a part of the one end section (11) including the one or more elements being above the seabed (17).

2. The method according to claim 1 , further comprising:(v) positioning an offshore installation (60) in a proximity of one end section (11); and(vi) installing the one end section (11) to the offshore installation (60) for operation therewith.

3. The method according to any one of the preceding claims, wherein the cable (10) is a Dynamic Inter Array Cable, and the one or more elements (14) include at least one or more buoyancy elements, connectors, clump weights, or tether clamps.

4. The method according to any one of the preceding claims, wherein a protection (42) is used to protect the fitting (12) during step (i).

5. The method according to any one of the preceding claims, wherein a protection (42) is used to protect the fitting (12) during at least one or more of the steps (ii) to (iv).

6. The method according to any one of the preceding claims, wherein the fitting (12) is a bend stiffener.7 The method according to any of the preceding claims, wherein step (i) includes wet-parking the fitting (12) on a support (16) on the seabed (17).

8. The method according to any one of the preceding claims, wherein the part of the one end section (11) including the one or more elements above the seabed is dynamic.9 The method according to any one of the preceding claims, further comprising the step of using a lay tower (30) for step (i).

10. The method according to any one of the preceding claims, further comprising the step of using one lay tower (30) for steps (ii) to (iv).

11. The method according to any one of claims 9 or 10, wherein the lay tower (30) comprises an arcuate guide (34) mounted at an elevated position on the vessel deck (28) providing an arcuate guide surface in a vertical plane and a transfer mechanism (36) able to transfer the fitting (12) of the end section of the cable (10) along a path outside the arcuate guide surface.

12. The method according to any one of claims 9 to 11 , wherein the lay tower (30) comprises a winch wire (22) for handling an end of the power cable (10), the winch wire (22) being routed internally through at least one structure of the lay tower (30).

13. The method according to any one of the preceding claims, wherein over boarding a part of the end section (11) on the opposite side of the vessel (20), or reeling a part of the end section (11) on the vessel (20), or using a carousel for a part of the end section (11), during at least one or more of steps (ii) and (iii).

14. The method according to any one of the preceding claims, wherein a time period between steps (i) and (ii) is a predetermined time period.

15. The method according to claim 14, wherein the cable (10) is static along the entire length of the cable in the first wet-parking position for the predetermined time period, and the cable (10) is at least partly dynamic in the second wet-parking position for a time period shorter than the predetermined time period.

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