Method and system for mitigating cable wear in a hoisting system

Abstract

A method for mitigating the effects of cable wear in an active heave compensated hoisting system of an offshore vessel in a locked to bottom mode of operation is disclosed. The method comprises supporting an upper end of a string which is connected to a subsea well from a travelling block of the hoisting system, wherein the travelling block is suspended from a crown block via a cable. The method further comprises operating an active heave compensation system to control a drawworks of the hoisting system to pay in and out the cable to compensate for motion of the offshore vessel and maintain a target overpull in the string. The method further comprises adjusting a ballast system of the offshore vessel to vary the draft of the vessel, and controlling the drawworks in accordance with the variation in the draft of the vessel to cause a length of cable to slip through the hoisting system and maintain the target overpull in the string.

Claims

1. A method for mitigating the effects of cable wear in an active heave compensated hoisting system of an offshore vessel in a locked to bottom mode of operation, the method comprising: connecting an upper end of a string which is connected to a subsea well to a travelling block of the hoisting system, wherein the travelling block is suspended from a crown block via a cable; applying a target overpull in the string; operating an active heave compensation system to control a drawworks of the hoisting system to pay in and out the cable to compensate for motion of the offshore vessel and maintain the target overpull in the string; adjusting a ballast system of the offshore vessel to vary the draft of the vessel while operating the heave compensation system to compensate for motion of the offshore vessel; and controlling the drawworks in accordance with the variation in the draft of the vessel to cause a length of cable to slip through the hoisting system and maintain the target overpull in the string.

2. The method according to claim 1, comprising adjusting the ballast to cause a length of cable to move through the hoisting system without disconnecting the string from the travelling block.

3. The method according to claim 1, comprising determining a requirement to slip the cable through the heave compensated hoisting system prior to adjusting the ballast system.

4. The method according to claim 3, comprising determining a condition of the cable and adjusting the ballast system of the vessel in accordance with the determined cable condition.

5. The method according to claim 4, comprising determining the condition of the cable based on measured parameters including at least one of applied load, elongation, strain and physical inspection.

6. The method according to claim 4, comprising determining the condition of the cable analytically including at least one of software tools, numerical analysis, modelling and computational simulation; wherein the analytical determination includes using physical parameters including at least one of historic use, loading, cable travel, relative movement between the travelling block and the crown block, historic use of the heave compensation system, ton-miles utilization of the cable, and geometry of the hoisting system.

7. The method according to claim 6, comprising determining a required adjustment of the ballast system in accordance with the predetermined length of cable to be slipped through the hoisting system.

8. The method according to claim 1, comprising performing offshore operations associated with the subsea well prior to the vessel entering the locked to bottom mode.

9. The method according to claim 1, comprising slipping the cable through the hoisting system prior to the vessel entering the locked to bottom mode to replenish the cable within the hoisting system.

10. The method according to claim 1, comprising performing a slip-and-cut operation prior to the vessel entering the locked to bottom mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the present disclosure will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 is a diagrammatic illustration of a hoisting system provided on an offshore vessel; and

(3) FIGS. 2 and 3 illustrate a sequence of replenishing a length of cable in an offshore hoisting system.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) FIG. 1 diagrammatically illustrates a portion of an offshore vessel 10 which includes a derrick 12 and a hoisting system, generally identified by reference numeral 14, which might typically be used to support operations associated with the offshore oil and gas industry, for example to provide support to a payload 16. For exemplary purposes the vessel 10 may be a drilling vessel (e.g., a drill ship). The hoisting system 14 includes a drawworks 18 (or a winch) for controlling a hoisting cable (or drill-line) 20, a crown block 22 and a travelling block 24 suspended from the crown block 22 via the cable 20, wherein the travelling block 24 can be connected to the payload 16. A portion of the hoisting cable 20 extends between the drawworks 18 and the crown block 22 and is typically referred to as the fast line 20a. A portion of the hoisting cable 20 extends from the crown block 22 to a fixed anchor 26 and is typically referred to as the deadline 20b. The cable 20 may terminate at the anchor 26, or as in the illustrated example may optionally also extend to a deadline cable storage drum 28.

(5) The drawworks 18 is controlled to pay in/out the cable to adjust the height of the travelling block 24. Further, the cable 20 may pass a number of times between sheaves (not individually shown) of the crown and travelling blocks 22, 24 to provide a desired lifting mechanical advantage.

(6) The hoisting system 14 in the present example is an active heave compensated hoisting system, specifically an active heave compensated drawworks hoisting system, in which the drawworks 18 is controlled to pay in and out the cable 20 in accordance with vessel motions, such as caused by wave motion, tidal variation and the like.

(7) During operation of the hoisting system 14 it may be necessary to mitigate the effects of wear on the cable 20, for example due to wear and/or cable fatigue such as caused by the cable running over the sheaves of the crown and travelling blocks 22, 24. An example method for mitigating the effects of wear on the cable will now be described with reference to FIGS. 2 and 3, which illustrate a specific example of the vessel 10 in a locked to bottom mode of operation. However, prior to the vessel 10 entering a locked to bottom mode of operation a slip-and-cut operation may be performed to replenish the cable 20 within the hoisting system 14, such that a fresh portion of cable 20, at least relative to the stress points in the hoisting system 14, is provided prior to initiating the locked to bottom mode of operation.

(8) Referring initially to FIG. 2, the hoisting system 14 of FIG. 1 is again illustrated, omitting the top end of the derrick 12 and thus the crown block 22 (FIG. 1). Furthermore, in this case the cable storage drum 28 (FIG. 1) has been omitted as being optional. The hoisting system 12 in the present example is illustrated in use supporting a tubular string 16 (e.g., a riser) which is secured at its lower end to a subsea wellhead 32 at the seabed 34. The upper end of the tubular string 16 is supported by the travelling block 24 via a topdrive 36 and a tension frame 38. As such, the tubular string 16 is connected between the wellhead 32 and the travelling block 24, thus establishing the locked to bottom mode. The hoisting system 14 is controlled to apply a desired or target substantially constant overpull in the tubular string 16.

(9) A well control tree 40 is mounted within the tension frame 38 and is secured to the top end of the tubular string 16. In the present example the hoisting system 14 supports the various components and equipment during a well testing operation (or any other well operation), which might require a number of days to complete. The present disclosure provides methods and systems which can mitigate or account for cable wear within the hoisting system 14 without interrupting such operations.

(10) In the present example an active heave compensation system 42 is provided which controls the drawworks 18 in accordance with known principles to pay in and out the cable 20, illustrated by double-headed arrow 44, to compensate for motion of the offshore vessel 10, illustrated by double-headed arrow 46. Such compensation may be performed to maintain the target overpull condition within the tubular string 16. The active heave compensation system 42 may utilise a number of input parameters for suitable operation, such as the “hook load” at the travelling block 24, illustrated by broken line 47. In some examples maintaining the target overpull condition within the tubular string 16 may be such that the travelling block 24 and tension frame 38 are maintained at a substantially fixed position, for example at a fixed height 48 relative to the seabed 34. Such adjustment of the drawworks 18 may result in the position of the tension frame 38 varying relative to a deck surface 50 of the vessel 10, as illustrated by double-headed arrow 52. The cyclical and reciprocating movement of the cable 20 through the hoisting system 14 may accelerate wear and/or fatigue within the cable 20.

(11) During use an operator may undertake suitable monitoring and analysis to determine a requirement to address cable wear. This determination may be based on operator experience, and/or on data associated with the hoisting system 14, such as historic use of the hoisting system 14, operation time, ton-miles utilisation of the cable 20, cable movement, usage of the heave compensation system 42, loading applied, and the like. The condition of the cable 20 may be monitored and/or determined, for example based on measured or sensed parameters, such as applied load, elongation, strain, physical inspection and the like. Alternatively, or additionally, the condition of the cable 20 may be determined analytically, for example via software tools, numerical analysis, modelling, computational simulation and the like. Such analytical determinations may utilise physical parameters, such as is historic use, loading, cable travel, relative movement between the travelling block 24 and the crown block 22 (FIG. 1), historic use of the heave compensation system 42, ton-miles utilisation of the cable 20, geometry of the hoisting system 14 and the like.

(12) With reference now to FIG. 3, when it is determined that cable wear needs to be addressed a ballast system 54 of the vessel 10 is deliberately adjusted to vary the draft of the vessel 10. In the specific example of FIG. 3 the ballast system 54 is adjusted to lower the vessel 10 relative to the waterline 56, illustrated by arrow 58. The lowering of the vessel 10 will thus seek to also lower the height of the travelling block 24 and tension frame 38 relative to the seabed 34, and thus cause a variation in the applied overpull in the tubular string 16. However, the active heave compensation system 42 reacts to the lowering of the vessel to control the drawworks 18 (for example in response to one or more input/sensed parameters, such as hook load 47) and pay in a necessary length of cable 20, illustrated by arrow 60, to adjust the relative position of the travelling block 24 (e.g., relative to the seabed 34, deck 50, crown block 22 (FIG. 1) etc.) to maintain the target overpull condition in the tubular string 16. The adjustment of the hoisting system 14 may thus increase the distance 52 between the tension frame 38 and deck surface 50.

(13) Such control of the drawworks 18 in response to adjusting the ballast system 54 thus causes a length of cable 20 to slip through the hoisting system 14 to reposition any worn, stressed or fatigued cable portions to a more favourable position, such as away from the sheaves of the travelling block 24 and crown block 22 (FIG. 1). The length of cable 20 slipped through the hoisting system 14 in response to adjusting the ballast system 54 may be determined in advance to be sufficient to reposition those cable sections which might have been subject to the greatest wear and/or fatigue. This advance determination may be based upon operator experience, sensed parameters, analytical techniques, and the like. This predetermined cable adjustment may then inform the required adjustment of the ballast system 54.

(14) In the present example the travelling block 24 and crown block 22 (FIG. 1) each includes multiple sheaves to provide a number of cable passes and thus a desired lifting mechanical advantage. However, this also requires that the cable 20 must move at a far greater rate than the travelling block 24. This factor may provide advantages in the present disclosure in that a larger length of cable 20 will be required to slip through the hoisting system 14 than the variation in the draft of the vessel 10 in order to maintain the target overpull in the tubular string 16. This may allow a smaller adjustment of the ballast system 54 and variation in vessel draft to provide the required length of cable slippage through the hoisting system 14 to accommodate for cable wear/fatigue. In some examples the draft of the vessel 10 may only need to be varied by 1 m to require a 16 m slippage of the cable 20 through the hoisting system 14 to maintain the target overpull in the tubular string 16.

(15) In the example presented above the ballast system 54 is adjusted to lower the vessel 10 within the water. However, a similar cable slippage effect may be achieved by raising the vessel 10 in the water, causing the drawworks 18 to pay out cable 20. Furthermore, while the present example utilises the heave compensation system 42 to react to the adjustment of the ballast system 54, a separate dedicated system may also be provided.

(16) The example presented above illustrates a single adjustment of the ballast system 54. However, the process may be repeated as required, and as permitted by the draft of the vessel 10. Further, the disclosed method may adjust the ballast system 54 in a series of discrete stages, such that a number of cable replenishment operations are provided. Further still, the ballast system 54 may be adjusted continuously, to continuously and gradually slip the cable 20 within the hoisting system 14.

(17) In the example presented an active heave compensation system 42 is provided. However, in other examples a passive heave compensation system may also be provided, for example interposed between the topdrive 36 and tension frame 38, incorporated within the tension frame 38, and/or the like.

(18) The example described above exemplifies a benefit of the methods and systems in mitigating the effects of cable wear without interrupting operations. However, once the operations (e.g., flow testing) are completed a more conventional cable replenishment operation may take place, such as a cut-and-slip operation.