System and method for increasing service life of a power cable

Abstract

A system and corresponding method for increasing a service life of a power cable (10) attached to an offshore structure (100). The system comprises a guide unit (102) for guiding a power cable (10) from a body of water to the offshore structure (100) such that the power cable (10) is in a position to be electrically connected to the offshore structure (100), the power cable having a first portion of length arranged mainly vertically along the offshore structure (100), and a second portion of length deviating out from the offshore structure (100) to extend through the body of water. A transition area between the first and second portions of length defines a main fatigue region (104); and the guide unit (102) further comprises a part engageable with the transition area of the power cable (10).

Description

[0001] SYSTEM AND METHOD FOR INCREASING SERVICE LIFE OF A POWER CABLE

[0002] Technical Field

[0003] The present disclosure relates generally to the field of offshore power generating structures and power cables. More specifically, the present invention describes a system and method for increasing the service life of a power cable attached to an offshore structure.

[0004] Background

[0005] Power cables, such as inter array power cables, are used to transport the energy generated at offshore structures. For example a wind mill, or wind turbine, on an offshore structure uses power cables to transfer electric power to shore. Similar power cables can also be used for other renewable energy productions sites or even for oil and gas structure offshore. The power cables extend from the offshore structure into the sea. Waves, tide, wind, movement of the offshore structure causes a relative movement between the power cable and the offshore structure. This relative movement wears the power cable. l-tubes and J-tubes are fixed onto the offshore structure and used for guiding and protecting the power cables as they extend from the offshore structure and down into the body of water, the sea. In operation, both the offshore structure with the tubes and cables are subjected to interference from wind, waves, and ocean currents while deployed at sea. These place periodic mechanical stresses on the power cables, which lead to mechanical failure of the power cables. The weakest point of each power cable is the region adjacent to the connection at a floating vessel. This main fatigue region is the region of the cable subjected to the highest mechanical stress, and therefore the most likely to fail.

[0006] A design challenge when providing cables with tubes is how to handle fatigue caused by interactions between the cable and the tube. Existing systems possess shortcomings relating to the service life of such cables. Accordingly, there remains a need for further contributions in this area of technology.

[0007] SUMMARY

[0008] It is an objective of the present invention to provide a system for increasing a service life of a power cable, and a method for the same. This objective can be achieved by the features as defined by the independent claims. Further enhancements are characterized by the dependent claims. The invention is defined by the claims.

[0009] According to a first embodiment, there is provided a system for increasing the service life of a power cable attached to an offshore structure. The system comprises a guide unit for guiding a power cable from a body of water to the offshore structure such that the power cable is in a position to be electrically connected to the offshore structure. The power cable has a first portion of length arranged mainly vertically along the offshore structure, and a second portion of length deviating out from the offshore structure to extend through the body of water. A transition area between the first and second portions of length defines a main fatigue region. The guide unit further comprises a part engageable with the transition area of the power cable. The system further comprises a means for providing a relative movement between the guide unit and the power cable, and a locking means for locking the relative movement between the guide unit and the power cable, such that the part of the guide unit engageable with the transition area of the power cable can have a variable position along a longitudinal length of the power cable. The system may be configured such that the main fatigue region is variable along the length of the power cable. This increases the service life of the power cable since the fatigue region of the power cable can be moved in relation to the guide unit of the offshore structure.

[0010] The relative movement provided and prevented by the system may be a substantially vertical relative movement between the guide unit and the power cable. Alternatively, or in addition, the said relative movement may in a direction along the longitudinal length of the guide unit.

[0011] The system may further comprises a plurality of locking positions distributed in a longitudinal direction of the guide unit. Each locking position may be engageable with the locking means to prevent the relative movement. The plurality of locking positions may be predefined in the longitudinal direction of the guide unit.

[0012] The means to provide the relative movement may comprise an actuator and / or a hoisting device. The means may be further configured to move the power cable from a first locking position to a second locking position of the plurality of predetermined locking positions. The means may provide the relative movement between the guide unit and the power cable. The hoisting device may be arranged on the offshore structure, and further comprise a pulling element attachable to the power cable. The hoisting device may be operable to raise or lower the power cable accordingly.

[0013] The system may further comprises a controller communicatively coupled to the locking means and the means for providing the relative movement. The controller may be configured to monitor a condition of the main fatigue region, and generate instructions to provide the relative movement, and / or lock the relative movement via the locking means, based on the condition of the main fatigue region. Optionally, or in addition, the controller is communicatively coupled to a sensor. The sensor may be configured to monitor the condition of the main fatigue region, and communicate the condition to the controller. The controller may be further configured to generate instructions to activate the locking means upon the power cable being moved from the first locking position to the second locking position. The controller may be further configured to generate instructions to deactivate the locking means and provide the relative movement upon the condition exceeding a predetermined threshold. Optionally, the condition comprises a duration of time relative to when the relative movement was last provided. The controller may be configured to generate instructions to activate the locking means upon the main fatigue region being displaced by a predetermined length. The predetermined length may be a predetermined clearance length.

[0014] The guide unit may be selected from a group consisting of: an l-tube, a J- tube, a telescopic tube, a short tube element, a bell mouth, and a connector unit. Optionally, a bend stiffener is attachable to the lower end of the guide unit. The offshore structure may be a floating offshore structure. Preferably, the floating offshore structure comprises a wind turbine and / or a wave energy converter and / or a tidal turbine.

[0015] According to a second embodiment, there is provided a method for increasing a service life of a power cable attached to an offshore structure. The method comprises: providing a system according to the first aspect of the present invention; deactivating the locking means and providing the relative movement to move the power cable relative to the guide unit; and activating the locking means to lock the relative movement between the power cable and the guide unit. The step of providing the relative movement between the power cable relative to the guide unit may further comprise pulling the power cable from the first locking position to the second locking position. The method may further comprise detecting, via the controller, the condition of the main fatigue region. The step of deactivating the locking means and providing the relative movement may be executed upon the condition exceeding a predetermined threshold. In addition, or as an alternative, the step of activating the locking means may be executed upon the main fatigue region being displaced by a predetermined clearance length.

[0016] 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.

[0017] Brief Description of the Drawings

[0018] The disclosure will be further described with reference to examples depicted in the accompanying figures in which:

[0019] FIG. 1 is a schematic illustration of an offshore structure, and a transfer apparatus;

[0020] FIG. 2A and 2B are schematic views of a guide unit held in a first and a second locking positions, respectively;

[0021] FIG. 3 is a detailed view of a hoisting device arranged on the offshore structure; and

[0022] FIG. 4 is a detailed view of an actuator arranged on the offshore structure.

[0023] Detailed Description

[0024] Embodiments of the present invention provide a system and method for increasing the service life of a power cable attached to an offshore structure. This detailed description describes in detail at least one way of carrying out the claimed invention by referring to the drawings.

[0025] As shown in FIG. 1 , the system comprises a guide unit 102 and a means 300, 400 for providing a relative movement between the guide unit 102 and a power cable 10. An offshore structure 100 may have a guide unit 102 and the guide unit 102 may be moveably attached to the offshore structure 100. As illustrated in FIG. 1, the guide unit 102 is suitable for a power cable 10 attachable to the offshore structure 100. The power cable 10 having a first portion of length arranged vertically along the offshore structure 10, and a second portion of length deviating out of the offshore structure into a body of water. The power cable 10 may eventually be laying on a sea bed 50. In other words, the power cable 10 may have a first portion of length extending between an upper end of the guide unit 102 to a lower end of the guide unit 102, and a second portion of length extending between the lower end of the guide unit 102 to below a sea level 40.

[0026] The guide unit 102 serves to guide the power cable 10 from the body of water to the offshore structure 100, such that the power cable 10 is in position to be electrically connected to the offshore structure 100. Electrically connecting to the offshore structure 100 may also include connections to one or more electrical components on the offshore structure. The offshore structure 100 may further comprise a windmill 30, such that the offshore structure 100 forms part of a power distribution system that facilitates the transmission of electricity from a windmill 30 to various components. While the offshore structure 100 comprises a windmill 30 in FIG.1 , the offshore structure 100 may comprise any suitable structure for generating electricity.

[0027] The guide unit 102 may be moveably attached to a supporting post 101 , with the guide unit 102 and supporting post 101 each extending parallel to one another in a direction substantially perpendicular to a sea level 40. As may best be taken from figures 2A and 2B, the guide unit may be slidably arranged on the supporting post

[0028] 101, for example with wheels 108. This may allow the relative movement between the part of the guide unit 102 engagable with the transition area of the power cable 10. There may be provided a transfer apparatus to deliver the power cable 10 from a floating vessel, such as a ship, to the offshore structure 100.

[0029] The guide unit 102 may be a generally hollow body structured to guide the power cable 10. The guide unit 102 may interact with the main fatigue region 104 of a power cable 10. The main fatigue region 104 of the power cable 10 is the region adjacent to where the power cable 10 connects via the guide unit 102 to the offshore structure 100. The main fatigue region 104 may be further defined as being in an area of transition between the first and second portions of length of the power cable 10. A part of the guide unit 102 may further interact with the power cable 10 within the transition due to wind and tidal forces. The main fatigue region 104 may be the region of the power cable 10 most prone to failure due to external forces. As may be taken from FIG 1, the lower end of the guide unit 102 interact with the power cable 10 as the two move relative to each other. The lower end of the guide unit 102 may be the part engageable, interacting, with the transition area of the power cable 10. The interaction may cause wear to the main fatigue region 104 of the power cable 10.

[0030] To protect the power cable 10, a bend stiffener 20 may be provided towards the lower end of the guide unit 102 for reducing the bending movement of the power cable 10. The bend stiffener 20 may be formed with an elongate sleeve defining a substantially cylindrical passage which can receive at least a portion of the power cable 10. The bend stiffener 20 may be provided with a bend stiffener connector removably connectable to the guide unit 102. In FIG 1 , the guide unit 102 comprises a J-tube having a flange attachable with the bend stiffener connector. However, different types of guide units 102, such as an l-tube, a J-tube, a telescopic tube, a short tube element, a bell mouth, and a connector unit, may be used with or without the bend stiffener 20.

[0031] The system comprises a locking means 110 for locking a relative movement between the guide unit 102 and the power cable 10, and a means 300, 400 for providing the relative movement between the guide unit 102 and the power cable 10. The locking means 110 locks the relative movement between the guide unit 102 and the power cable 10. The relative movements provided by the means 300, 400 are in reference to a substantially vertical relative movement between the guide unit 102 and power cable 10. The relative movement may be in a direction along the longitudinal length of the guide unit 102. There may be an additional horizontal relative movement between the power cable 10 and the guide unit 102 due to wind and / or tidal forces. While the present invention can provide and / or prevent a relative movement vertically or in a direction of the guide unit 102, the power cable 10 may still be capable of being displaced in a horizontal direction inside the guide unit 102 such that the main fatigue region 104 of the power cable 10 is engageable with the guide unit 102.

[0032] The locking means 110 may comprise any known apparatus suitable to selectively restrict relative movement between the between the guide unit 102 and the power cable 10. For example, the power cable 10 may be held inside the guide unit 102 by the locking means. For example, the power cable 10 may be held by the offshore structure 100 and the guide unit 10 may also be held by the offshore structure 100. The means 300, 400 and the locking means 110 are operable to move the main fatigue 104 region along a longitudinal length of the power cable 10, and lock the power cable 10 in place upon the main fatigue region 104 being moved. The main fatigue region 104 may be moved by moving the power cable 10 while the guide unit 102 is not moved; or moving the guide unit 102 while the power cable 10 is not moved; or a combination of the two. Moving the guide unit 102 is illustrated in FIGs 2A and 2B, where the guide unit 102 is raised from a lower position, elevation, in FIG 2A to a higher position, elevation, in FIG 2B. In FIG 2A the guide unit 102 is thus closer to the sea bed 50 than in FIG 2B. The main fatigue region 104 may in this way be moved along the power cable 10. This provides the technical benefit of extending the service life of the power cable 10. The distance between the position in FIG 2A and the position in FIG 2B may be a predetermined length. The lower position may be a first locking position and the higher position may be a second locking position. While figures 2A and 2B show two locking positions, there may be more than two locking positions. The locking positions may be predetermined. The means to provide the relative movement may comprise an actuator 300 or a hoisting device 400 and may be configured to move the power cable 10 from the first locking position to the second locking position of a plurality of locking positions.

[0033] The system illustrated in the figures allows the part of the guide unit 102 that interacts with the transition area in the main fatigue region of the power cable 10 to be variable over time. This extends the service life of the power cable 10. The system may further comprise a storage space for storing excess lengths of cable that have been pulled towards the offshore structure 100, and a means for guiding excess folded lengths of the power cable 10 into the storage space.

[0034] A controller 120 may be communicatively coupled to a sensor that is configured to monitor a condition of the main fatigue region 104, and further communicatively coupled to the means to provide the relative movement and locking means. The controller 120 may operable to generate instructions, based on the condition received from the sensor, to activate or deactivate the locking and / or the means to provide the relative movement. The condition of the guide unit 102 and / or the condition of the power cable 10 may provide appropriate data for determining whether the power cable 10 in the main fatigue region 104 has been damaged, such as, for example, a thickness of a power cable wall, or the duration of time since the portion of the power cable in the main fatigue region has last been moved. The controller 120 may also utilize empirical data and / or statistical analysis to decide an appropriate length of time between activations of the means to provide the relative movement. The length of time may be for example 6 months, 1 year, 3 years, or 5 years. The controller 120 may also be manually operated to move the main fatigue region by a relative movement between the guide unit 102 and the power cable 10.

[0035] The end of the power cable 10 closest to the offshore structure 100 may comprise a plurality of predetermined locking positions, distributed in a longitudinal direction of the guide unit 102. Each locking position may be engageable with the locking means 110 to prevent the relative movement between the power cable 10 and guide unit 102. The means to provide the relative movement may be further configured to move the end of the power cable 10 from a first locking position to a second locking position of the plurality of predetermined locking positions.

[0036] A predetermined threshold associated with a minimum or maximum amount of a numerical quantity may be programmed into the controller 120, such that the controller 120 is further configured to activate or deactivate the locking means 110 and means to provide the relative movement upon the condition meeting the predetermined threshold. The predetermined threshold may be associated with a minimum amount of thickness of a layer of the power cable 10 in the main fatigue region 104. The sensor may be further operable to monitor a difference in time relative to when the means to provide the relative movement was last activated. In such a configuration, the predetermined threshold may be a maximum duration in time. Optionally, the controller 120 is further configured to generate instructions to deactivate the means to provide the relative movement and activate the locking means 110 upon the main fatigue region 104 being displaced by the predetermined length. The controller 120 may be configured to generate instructions to deactivate the means to provide the relative movement and activate the locking means 110 upon the power cable 10 being moved from the first locking position to the second locking position.

[0037] Turning to FIG 3, the means to provide the relative movement may comprise a hoisting device 400, for example arranged on the offshore structure 100. Sections of the guide unit 102 have been removed from the illustration for clarity purposes. The hoisting device 400 may comprise a frame 410 removably mounted to the offshore structure 100. A rotatable drum 420 having a wire 432 wound thereon may be positioned on an upper end 412 of the frame 410. The wire 432 hangs downwardly due to gravity, and can extend into and along the entire length of the guide unit 102, such that a wire end 434 terminates below an opening at the lower end of the guide unit 102. The wire end 434 is attachable to the end of the power cable 10. The hoisting device 400 may reel in the wire 432, such that an attached power cable 10 is pulled towards the frame 410. The hoisting device 400 may be coupled to a controller, for example the controller 120, such that the hoisting device 400 is remotely operable. The rotatable drum 420 may further comprise a motor configured to reel the wire end 434 towards, or away from the upper end of the guide unit 102. The hoisting device 400 may be operable to pull the power cable 10 into and through the opening in the guide unit 102, thereby making the power cable 10 at least partially protected by the guide unit 102. The hoisting device 400 is further operable to lower the power cable 10 back through the guide unit 102 until the wire 432 emerges at an opening at the lower end of the guide unit 102.

[0038] Turning to FIG. 4, a detailed view of an example of the guide unit’s 102 lower end is illustrated. The means to provide the relative movement may comprise an actuator 300, for example arranged on the offshore structure 100. A side of the guide unit 102 adjacent to the supporting post 101 may be provided with a bracket 310 having a rack profile 312 extending along the length of the guide unit 102. A motor- driven toothed wheel 320 may be rotatably mounted on the guide unit 102 and may form an interlocking mesh with the corresponding teeth of the rack profile 312. The bracket 310 may be further provided with a guide roller 330 extending from the bracket 310 and contacting the supporting post 101. The toothed wheel 320 may be designed to rotate, via power supplied by the motor, to move the guide unit 102 relative to the supporting post 101. Alternatively, the bracket 310 may be provided on the supporting post 101, with the wheel 320 provided on the guide unit 102.

[0039] Turning to a method of operating the system in the figures, the method may comprise the following steps in any order. A power cable 10 is provided to the system such that a first portion of length is vertically along the offshore structure 100, and a second portion of length deviates out of the offshore structure 100 into a body of water and eventually laying on a sea bed. The power cable 10 may be initially held in place by the locking means 110. The locking means may be deactivated and the means to provide the relative movement may be activated, such that the power cable 10 is moved relative to the guide unit 102. Due to the relative movement, a different section of the power cable 10 is within the main fatigue region 104 and subject to interaction from the part of the guide unit 102 engagable with the transition area of the power cable 10. The means to provide the relative movement may be deactivated and the locking means 110 may be activated to lock the relative movement between the power cable 10 and guide unit 102. As an alternative to the manual activation and deactivation of the locking and / or means to provide the relative movement, the controller 120 may be provided to automate the process by providing the relative movement upon the condition of the main fatigue region reaching a predetermined threshold. Optionally, the step of activating the means to provide the relative movement may further comprise pulling the power cable 10 from the first locking position to the second locking position.

[0040] The method may further comprise detecting, via the controller 120, the condition of the main fatigue region. For example, a sensor may detect the condition of the power cable 10 in the main fatigue region 104. The deactivation of the locking means 110 and providing the relative movement may be executed upon the condition exceeding a predetermined threshold. In addition, or as an alternative, the activation of the locking means 110 may be executed upon the main fatigue region 104 being displaced by a predetermined length, for example the power cable 10 may have moved a predetermined length relative to the main fatigue region 104 of the guide unit 102.

[0041] The aforementioned solves shortcomings in prior art. As seen in past disclosures, mechanical stresses due to wind and / or tidal forces cause frequent interactions between the transition area of the power cable 10 and the interacting part of a protective tube. Such interactions result in the main fatigue region being exposed to frequent cyclic stresses, and therefore being more prone to failure. However, selectively varying the location of the main fatigue region along the length of the power cable 10 results in an increase to the power cable's 10 service life.

[0042] The power cable 10 may be an electric power cable. However, this disclosure is not limited to only electric power cables. The power cable 10 may be any flexible connecting member used in subsea applications, for example a flexible pipeline, a tension cable, an attachment cable, a communication cable, and / or a chain.

[0043] It will be apparent to those skilled in the art that various modifications and variations can be made to the system and method for increasing a service life of a power cable 10 attached to an offshore structure 100. 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

Claims:

1. A system for increasing a service life of a power cable (10) attached to an offshore structure (100), the system comprising: a guide unit (102) for guiding a power cable (10) from a body of water to the offshore structure (100) such that the power cable (10) is in a position to be electrically connected to the offshore structure (100), the power cable having a first portion of length arranged mainly vertically along the offshore structure (100), and a second portion of length deviating out from the offshore structure (100) to extend through the body of water; wherein a transition area between the first and second portions of length defines a main fatigue region (104); wherein the guide unit (102) further comprises a part engageable with the transition area of the power cable (10); characterised in that: the system further comprises a means for providing a relative movement along the first portion of length between the guide unit (102) and the power cable (10), and a locking means (110) for locking the relative movement between the guide unit (102) and the power cable (10), such that the part of the guide unit (102) engageable with the transition area of the power cable (10) can have a variable position along a longitudinal length of the power cable (10).

2. The system according to claim 1 , wherein the system further comprises a plurality of locking positions distributed in a longitudinal direction of the guide unit (102), each locking position engageable with the locking means (110) to prevent the relative movement.

3. The system according to claim 2, wherein the means to provide the relative movement comprises an actuator (300) or a hoisting device (400), and is further configured to move the power cable (10) from a first locking position to a second locking position of the plurality of locking positions.

4. The system according to claim 3, wherein the hoisting device (400) is arranged on the offshore structure (100) and further comprises a pulling elementattachable to the power cable (10), and wherein the hoisting device (400) is operable to raise or lower the power cable (10).

5. The system according to any one of the preceding claims, further comprising: a controller (120) communicatively coupled to the locking means (110) and the means for providing the relative movement; wherein the controller (120) is configured to monitor a condition of the main fatigue region, and generate instructions to provide the relative movement, and / or lock the relative movement via the locking means based on the condition of the main fatigue region.

6. The system according to claims 3 and 5, wherein the controller (120) is configured to generate instructions to activate the locking means (110) upon the power cable (10) being moved from the first locking position to the second locking position.

7. The system according to any one of claims 5 to 6, wherein the controller (120) is further configured to generate instructions to deactivate the locking means (110) and provide the relative movement upon the condition exceeding a predetermined threshold; and optionally wherein the condition comprises a duration of time relative to when the relative movement was last provided.

8. The system according to any one of claims 5 to 7, wherein the controller (120) is further configured to generate instructions to activate the locking means (110) upon the main fatigue region being displaced by a predetermined length.

9. The system according to any one of the proceeding claims, wherein the guide unit (102) is selected from a group consisting of: an l-tube, a J-tube, a telescopic tube, a short tube element, a bell mouth, and a connector unit.

10. The system according to any one of the preceding claims, wherein a bend stiffener (20) is attachable to lower end of the guide unit (102).

11. The system according to any one of the preceding claims, wherein the offshore structure (100) is a floating offshore structure, and optionally wherein the floating offshore structure comprises a wind turbine (30).

12. A method for increasing a service life of a power cable (10) attached to an offshore structure (100), the method comprising: providing a system according to any one of the preceding claims; deactivating the locking means (110) and providing the relative movement to move the power cable (10) relative to the guide unit (102); and activating the locking means (110) to lock the relative movement between the power cable (10) and the guide unit (102).

13. The method according to claim 12, when providing a system according to claim 3, wherein the step of providing the relative movement further comprises: pulling the power cable (10) from the first locking position to the second locking position.

14. The method according to claim 12, when providing a system according to claim 7, wherein the method comprises: detecting, via the controller (120), the condition of the main fatigue region (104); and the step of deactivating the locking means (110) and providing the relative movement is executed upon the condition exceeding a predetermined threshold.

15. The method according to claim 12, when providing a system according to claim 8, wherein the method comprises: detecting, via the controller (120), the condition of the main fatigue region (104); and the step of activating the locking means (110) is executed upon the main fatigue region (104) being displaced by a predetermined length.

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