WO1981001402A1

WO1981001402A1 - Heave compensator

Info

Publication number: WO1981001402A1
Application number: PCT/GB1980/000196
Authority: WO
Inventors: R Mccallum; R Graham
Original assignee: Caley Hydraulics Ltd; R Mccallum; R Graham
Priority date: 1979-11-17
Filing date: 1980-11-12
Publication date: 1981-05-28
Also published as: IT1129864B; IT8068757A0; EP0040210A1; BE886205A

Abstract

A heave compensator is proposed for use in various offshore situations where it is desired to handle loads being moved between two locations which are movable relative to each other. A typical application of the invention is where the first location is a ship and the second location is the sea bed and the load is a diving chamber (28) suspended from the ship and which is desired to be maintained at a substantially constant height above the sea bed regardless of the heaving motion of the ship. An effective connection between the ship and the sea bed is made via a heave line (2) by lowering a relatively heavy anchor block (11) onto the sea bed and the heaving motion of the ship produces a paying-out and paying-in movement of the heave line (2) relative to the ship. An arrangement of pulleys is provided on the ship, about which arrangement is looped a load-handling line (22). Tension is maintained in the load-handling line (22) by pressurised hydraulic means. Paying-out and paying-in of the load-handling line (22) is effected by shortening and lengthening the looped formation therein and control means are provided responsive to the paying-out and paying-in of the heave line (2) to ensure a sympathetic paying-out and paying-in of the load handling line (22). Examples of the invention are disclosed suitable where the diving chamber is not suspended directly over the anchor block and taking into account the different amounts of heaving motion of the suspension points for the diving chamber and the anchor block.

Description

HEAVE COMPENSATOR

DESCRIPTION

This invention relates to a system for the handling of loads being moved between two locations which are movable relative to each other and for compensating for such relative motion. According to the invention there is provided such a system which comprises a heave line for establishing an effective connection between a first location and second location movable relative to the first location, means at the first location for paying-out and paying-in the heave line from and to the first location in response to relative motion between the locations, control means responsive to the paying-out and paying-in of the heave line, a winch fixedly mounted at the first location, a load-handling line for connecting a load to the winch, an arrangement of pulleys rotatable about two axes at the first location, the loadhand ling line being looped about the arrangement of pulleys and one axis being movable towards and away from the other axis under the control of the control means for paying-out and paying-in the load-handling line in sympathy with the paying-out and paying-in of the heave line, and pressurised hydraulic means for maintaining tension in the load-handling line by tending to lengthen the looped formation in the load-handling line.

In one application of the invention, the first location may be a ship and the second location may be the sea bed and the effective connection between the ship and the sea bed may be made by lowering from the ship onto the sea bed a relatively heavy anchor block by means of the heave line. The weight of the anchor block would be such that the heave line could be regarded as being connected to the sea bed itself. The load may be a diving chamber or a chamber carrying a self-propelled submersible craft. By means of the invention the chamber may be lowered to a desired height above the sea bed and remain substantially at that height even though the ship may be rising and falling due to sea swell. Other applications of the invention include the movement of loads between two ships and between a ship and a fixed structure such as an offshore platform. In the firs application the first location would be a first ship and the second location would be a second ship with the heave line being connected to the second ship. In the second case the first location would be a ship and the second location would be e.g. an offshore platform. Alternativel in the second case the first location would be e.g. an offshore platform and the second location would be a ship.

In any event a load being raised from or lowered onto the second location would be substantially unaffected by the relative movement between the two locations due to sea swell and therefore would be more easily controllable from the second winch at the first location.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 illustrates a first embodiment, Fig. 2 illustrates a second embodiment, and

Fig. 3 illustrates a modification of the embodiment of Fig. 2.

In Fig. 1 a first winch means is shown as an anchor winch 1. A heave line 2 extends from the anchor winch 1 to a pulley 3 journalled for rotation on a fixedly mounted block (not shown). From the first pulley 3 the heave line 2 extends to a pulley 4 journalled for rotation on another block 5 which is movable towards and away from the fixedly mounted block. From the pulley 4 the heave line 2 extends to a pulley 6 also journalled for rotation on the fixedly mounted block. The heave line 2 continues to a pulley 7 journalled for rotation on the block 5. The heave line 2 then continues to a pulley 8 journalled for rotation at a fixed location and from the pulley 8 around two pulleys 9 and 10 journalled for rotation on an anchor block 11 to a pulley 12 journalled for rotation at a fixed location coaxially with the pulley 8. From the pulley 12 the heave line 2 extends to a pulley 13.rotatably journalled on the block

5, a pulley 14 rotatably journalled for rotation on the fixedly mounted block and two further pulleys (not visible in Fig. 1) respectively rotatably journalled on the block 5 and the fixedly mounted block and finally to a fixed point 15. The pulley arrangement just described has a velocity ratio of 4:1.

The block 5 is mounted on a carriage (not shown) and is also attached to the piston rod 16 of a hydraulic ram the cylinder 17 of which is fixedly mounted and is connected into a hydraulic circuit comprising a pressure control 18 and a bag-type gas-loaded accumulator 19 to which may be connected one or more back-up gas bottles 20.

A second winch means is shown as a lift winch 21. A load-handling line 22 extends from the lift winch 21 to a pulley 23 rotatably journalled on the fixedly mounted block and from the pulley 23 to a pulley 24 rotatably journalled on the movable block 5. From the pulley 24 the load-handling line 22 extends to a pulley 25 rotatably journalled on the fixedly mounted block and from the pulley 25 to a pulley 26 journalled on the. block 5. The pulley arrangement just described also has a velocity ratio of 4:1. From the pulley

26 the load-handling line 22 extends over a pulley 27 disposed between the pulleys 8 and 12 to a load in the form of a diving chamber 28. From the opposite sides of the diving chamber 28 extend wings 29 having eyes 30 through which pass the lengths of the heave line 2 extending between the pulleys 8 and 9 and between the pulleys 10 and 12.

The anchor block 11 is lowered from the ship to the sea bed and raised from the sea bed to the ship with the movable block 5 and its carriage locked to the ship's deck. While the anchor block 11 is resting on the sea bed and the heave line 2 provides an effective connection between the ship and the sea bed the. movable block is unlocked and moves towards and away from the fixed block in sympathy with the movement of the ship due to sea swell, the hydraulics associated with the movable block 5 maintaining tension in the heave line 2. Because the load-handling line 22 is looped around a pulley arrangement having the same velocity ratio as the pulley arrangement around which is looped the heave line 2 and both pulley arrangements are journalled on the same fixed and movable blocks the movement of the ship due to sea swell is compensated relative to the suspended diving chamber 28 so that the diving chamber 28 moves relative to the ship but is substantially stationary relative to the anchor block 11 and the sea bed. Thus, when the diving chamber 28 is lowered to a convenient height above the sea bed it stays substantially at that height regardless of the ship's movement due to sea swell. Also because the diving chamber 28 is guided between the lengths of the heave line 2 extending between the pulleys 8 and 9 and between the pulleys 10 and 12 lateral movement of the diving chamber 28 is minimised.

It will be seen that by using a similar technique, e.g. when transferring a load of drill pipes from a ship to an offshore platform, the heave line may be connected from the ship to the deck of the offshore platform instead of to the sea bed via the anchor block so as to compensate movement of the ship due to sea swell relative to the platform so that transfer of the load of drill pipes is controlled from the ship in such a way that the load is not suddenly deposited on the deck of the platform or suddenly lifted from such deck by movement of the ship relative to the platform due to sea swell. In some applications it may be suitable to duplicate the heave winch, the heave line and the pulley arrangement around which the heave line is looped for a more satisfacto connection between the two locations. It may also be suitable to duplicate the load-handling winch, the loadhandling line and the pulley arrangement around which the load-handling line is looped. In these cases all four pulley arrangements may be accommodated on the fixed and movable blocks. Where the load is suspended by two load-handling lines the position of the fixed block may be adjustable by hydraulic means so as to level the load. Differences in the lengths of the load-handling lines paid-out or paid-in by the respective load-handling winches resulting in the load being suspended in an attitude other than level may be detected by footage counters and corrected either automatically or by reference to visual indication of the differences.

In some situations it is not convenient for the diving chamber or other load to be suspended directly over the anchor block in which case it is necessary to take into account the different amounts of heaving movement of the suspension points for the load and the anchor block in order that the suspended load remains substantially stationary in a vertical sense relative to the anchor block and the sea bed.

Fig. 2 illustrates schematically a system for solving the above problem. In Fig. 2 a lift winch 31 has a load- handling line 32 reeved about six pulleys, three of which 33 are journalled for rotation on a fixed block 34 and the other three of which 35 are journalled for rotation on a movable block 36. From the pulleys 33, 35 the line 32 extends over a pulley 37 journalled for rotation at a fixed location from which a load (not shown) such as a diving chamber is suspended. The block 36 is mounted on piston rod 38 of a hydraulic ram the cylinder 39 of which is fixedly mounted. The side of the cylinder 39 remote from the block 36 is connected to a hydraulic pressure source (not shown) via line 40, a variable pressure regulator 41, line 42, a shut-off valve 43, and lines 44 and 45. The hydraulic pressure is applied against a bag-type gas-loaded accumulator 46 having a pressure gauge 47 and connected via a shut-off valve 48 to a back-up gas bottle 49. The gas bottle 49 is connected via a variable pressure regulator 50 to a gas supply (not shown). The line 42 is connected to a hydraulic reservoir 51 via a variable pressure regulator 52 so that excess hydraulic fluid can be drained off. In addition, the line 45 is connected to the reservoir 51 via a shut-off valve 53. A small anchor winch 54 has a heave line 55 extending therefrom over a pulley 56, journalled for rotation at a fixed location, to an anchor block (not shown) resting on the sea bed. Tension is maintained in the line 55 by any suitable means for example an inertia reel (not shown).

The winch 54 can drive in both directions a two-direction variable displacement pump 57 connected in a closed circuit with a two-direction fixed displacement hydraulic motor 58. The closed circuit is supplied with hydraulic fluid from a source (not shown) via a variable pressure regulator 59 in parallel with a check valve 60. The motor 58 can drive a second small winch 61 of the same size as the winch 54. Extending from the winch 61 in an auxiliary line 62 reeved about six pulleys, three of which 63 are journall for rotation on a fixed block 64 and the other three of which 65 are journalled for rotation on a block 66 mounted on a trolley 67. The line 62 extends to a fixed point 68. Tension is maintained in the line 62 by means of an inertia reel 69 connected to the trolley 67 by a line 70. On the trolley 67 is pivotally mounted a two-arm lever 71. One end of the lever 71 is articulated to a sleeve (not shown) slidable with friction about an elongate rod 72. The rod

72 serves as a guide for translational movement of the trolley 67 and is freely slidable at one end through an opening in a fixed bracket 73. On the other end of the rod 72 is mounted the movable part of a three-position threeport throttled directional control valve 74 which is urged towards the centre position by a centring spring 75. In the centre position of the valve 74 a line 76 leading to the side of cylinder 39 nearer the block 36 is isolated from a line 77 from a source (not shown) of hydraulic fluid under pressure and from an exhaust line 78 including a check valve 79. In the left hand position in Fig. 2 of the valve 74 the line 76 is connected with the line 77 and in the right hand position the line 76 is connected with the line 78. The other end of the lever 71 has a follower 80 for alternative contact with two cams 81 and 82 slidably mounted on a sleeve 83 against respective compression springs 84. The sleeve 83 is freely slidable about an elongate rod 85. One end of the rod 85 is fixed to an arm 86 extending from the block 34 and the other end is also fixed at 87. The sleeve 83 is mounted on an arm 88 extending from the block 36. The items 71 to 88 constitute a hydraulic servo mechanism with mechanical feedback.

Before the system shown in Fig. 2 is put into operation the ratio of heaving movements at the pulleys 37 and 56 is ascertained and the displacement of the pump 57 is adjusted according to this ratio and the fixed displacement of the motor 58. Since the velocity ratios of the two sets of pulleys is the same the movement of the trolley 67 to and fro will provide a reference for the required movement of the block 35 to and fro for compensated movement of the diving chamber to maintain it at a fixed height above the sea bed, the ratio of heaving movements being apparent from the different rotational speeds of the winches 54 and 61.

On upward heaving movement at the pulley 56 line 55 is paid-out causing the pump 57 to rotate in one direction and drive the motor 61 in one direction so that trolley 67 is moved to the left in Fig. 2. This causes the lever 71 to pivot in a clockwise direction relative to the trolley 67 because of the frictional drag on the rod 72. The follower 80 then contacts the cam 81 and the resiliently yielding resistance of the cam 81 causes the rod 72 and thus the valve 74 to be moved to the left. Thus pressurised hydraulic fluid is fed through lines 77 and 76 into the cylinder 39 so that the block 36 is moved to the left. If the block 36 moves to the left faster than the trolley 67 the follower 80 is contacted by the cam 82 so as to pivot the lever 71 clockwise relative to the trolley 67. This, in turn, allows the valve 74 to regain its centre position thus interrupting delivery of pressurised hydraulic fluid through the line 76 and stopping the block 36 until the trolley 67 catches up and the follower 80 regains contact with the cam 81. Movement of the block 36 to the left paysout the line 32.

On downward heaving movement at the pulley 56 line 55 is paid-in causing the pump 57 to rotate in the opposite direction and drive the motor 61 in the opposite direction so that the trolley 67 is moved to the right in Fig. 2. This causes the lever 71 to pivot in an anticlockwise direction relative to the trolley 67 because of the frictional drag on the rod 72. The follower 80 then contacts the cam 82 and the resiliently yielding resistance of the cam 82 causes the rod 72 and thus the valve 74 to be moved to the right. Thus hydraulic fluid is exhausted from the cylinder 39 through the lines 76 and 78 so that the block 36 is moved to the right. If the block 36 moves to the right faster than the trolley 67 the follower 80 is contacted by the cam 81 so as to pivot the lever 71 anticlockwise relative to the trolley 67. This, in turn, allows the valve 74 to regain its centre position thus interrupting the exhausting of hydraulic fluid through the line 76 and stopping the block 36 until the trolley 67 catches up and the follower 80 regains contact with the cam 82. Movement of the block 36 to the right pays-in the line 32.

The winch 61 is driven at a rotational velocity relative to the rotational velocity of the winch 54 in either direction at the ratio previously adjusted between the displacement of the motor 58 and the pump 52. Since the movement of the trolley 67 is determined by the rotation of the winch 61 and since the movement of the block 36 is governed by the movement of the trolley 67 the paying-out and paying-in movement of the line 32 relative to the paying-out and paying-in movement of the line 55 is also at the pre-adjust ratio so that the diving chamber is maintained at the same height above the sea bed.

Fig. 3 illustrates a modification of the embodiment of Fig. 2, in which the hydraulic feedback control loop is replaced by an electrical one. Those elements of Fig. 3 which are the same as in Fig. 2 are denoted by like reference numerals and will not be further described.

In Fig. 3, the pulley 56 is coupled (if necessary via suitable gearing, not shown) to drive a position transducer 90 whose output is a voltage V proportional to paying-out and paying-in of the heave line 55. The voltage V is applied across a potentiometer 92 whose slider 93 may be set to obtain an output signal A which is a given fraction of the signal V. The setting of the slider 93 is chosen in a similar manner to the setting of the pump 57 in Fig. 2 to accord with the ratio of heaving movements at the pulleys 37 and 56.

A second position transducer 91 is driven by arm 88 on the block 36, and produces an output signal B. The signals A and B are fed to a difference amplifier 94 to produce a signal D = A-B. The signal D is then passed in parallel through a variable gain amplifier 95 and a variable gain differentiator 96 to a summation amplifier 97 which produces the signal E = G₁D + G₂. dD/dt, where G₁ and G₂ are selected gain factors. This last signal E is used to drive the coils 98, 99 of the solenoids of an electrical servo valve generally designated at 100 via a driving amplifier 101. The amplifier is so arranged that when E is positive coil 98 is driven and when E is negative coil 99 is driven, a common return being provided. The driving amplifier 101 may for example be a push-pull amplifier.

Thus, the servo valve 100 replaces the valve 74 of Fig. 2,. and the circuits 94-98 and 101 replace the mechanical feedback system formed by the items 71-88 of Fig. 2. Apart from being electrically operated, the valve 100 is identical with the valve 74, being likewise urged towards its centre position.

In the quiescent state, A - B = D is zero and the valve 100 is centred. On the occurrence of heave, the output of transducer 90 changes to drive D positive or negative according to the direction of heave. The signal E thus changes from zero to a positive or negative value which is proportional to the difference between the heave at pulley 56 and that at pulley 37, and to the rate of change of this difference. At the onset of heave, the two terms reinforce each other and result in a large signal to the appropriate coil of valve 100, and a maximum action to correct the heave. Any tendency to overcorrection is mitigated by the term G₂.dD/dt becoming of opposite sign to the term G₁D. By suitable setting of G₁ and G₂ the compensation can be tuned to suit the mechanical characteristics of the apparatus, and any tendency either to lag or to overshoot held to acceptable limits.

Claims

1. A system for the handling of loads being moved between two locations which are movable relative to each other and for compensating for such relative motion, the system comprising a heave line for establishing an effective connection between a first location and second location movable relative to the first location, means at the first location for paying-out and paying-in the heave line from and to the first location in response to relative motion between the locations, control means responsive to the paying-out and paying-in of the heave line, a winch fixedly mounted at the first location, a load-handling line for connecting a load to the winch, an arrangement of pulleys rotatable about two axes at the first location, the load- handling line being looped about the arrangement of pulleys and one axis being movable towards and away from the other axis under the control of the control means for paying-out and paying-in the load-handling line in sympathy with the paying-out and paying-in of the heave line, and pressurised hydraulic means for maintaining tension in the load-handling line by tending to lengthen the looped formation in the load-handling line.

2. A system according to claim 1, wherein the means for paying-out and paying-in the heave line comprises a second arrangement of pulleys rotatable about two axes at the first location, the heave line being looped about the second arrangement of pulleys and one axis being movable towards and away from the other axis, the control means being constituted by the velocity ratios of the two arrangements of pulleys being the same and both arrangements of pulleys being journalled for rotation on two common blocks, one block being movable towards and away from the other block, and the pressurised hydraulic means also maintains tension in the heave line by tending to lengthen the looped formation in the heave line.

3. A system according to claim 1, wherein the means for paying-out and paying-in the heave line comprises a second winch, and the control means comprises a third winch, an auxiliary line extending between the third winch and a fixed point on the first location, via a looped formation in the auxiliary line about a second arrangement of pulleys journalled for rotation on two blocks, both arrangements of pulleys having the same velocity ratio and one block being movable towards and away from the other block, the movable block journalling pulleys of the second arrangement being movable by rotation of the third winch in response to rotation of the second winch caused by paying-out and paying-in of the heave line in response to relative movemen between the first and second locations, and a hydraulic servo mechanism with mechanical feedback interposed between the movable block journalling the pulleys of the first arrangement to move with the movable block journalling the second arrangement.

4. A system according to claim 3, wherein the hydraul servo mechanism with mechanical feedback comprises a two- arm lever pivotally mounted on the movable block journallin pulleys of the second arrangement, one end of the lever being frictionally connected with a three-position directional control valve biassed towards a centre position which isolates hydraulic fluid so as to prevent movement of the movable block journalling pulleys of the first arrangement, and also having a first and position for applying pressurised hydraulic fluid to move the movable block journalling pulleys of the first arrangement in a direction to shorten the looped formation in the load-handling line, and a second end position for exhausting hydraulic fluid to allow the pressurised hydraulic control means to lengthen the looped formation in the load-handling line, and the other end of the lever being engageable with a lateral extension of the movable block journalling the pulleys of the. first arrangement whereby movement of the movable block journalling pulleys of the second arrangement in a direction towards the fixed block journalling pulleys of the second arrangement causes a mechanical reaction to be set up in one sense between the other end of the lever and the said lateral extension to move the directional control valve by frictional slippage from its centre position to its first end position and if the resultant movement of the movable block journalling pulleys of the first arrangement is faster than the movement of the movable block journalling pulleys of the second arrangement the said mechanical reaction is altered to the opposite sense so as to allow the directional control valve to regain its centre position and stop the movable block journalling pulleys of the first arrangement whereby movement of the movable block journalling pulleys of the second arrangement in a direction away from the fixed block journalling pulleys of the second arrangement causes a mechanical reaction to. be set up in the said opposite sense between the other end of the lever and the said lateral extension to move the directional control valve by frictional slippage from its centre position to its second end position and if the resultant movement of the movable block journalling pulleys of the first arrangement is faster than the movement of the movable block journalling pulleys of the second arrangement the said mechanical reaction is altered to the said one sense so as to allow the directional control valve to regain its centre position and stop the movable block journalling pulleys of the first arrangement.

5. A system according to claim 1, wherein the means for paying-out and paying-in the heave line comprises a second winch, and the control means comprises a first position transducer whose output is a voltage proportional to the paying- out and paying-in of the heave line, a potentiometer having the voltage applied thereacross and a slider which can be set to obtain an output signal which is a given fraction of the voltage, a movable block journalling pulleys of the said arrangement having a lateral extension for driving a second position transducer whose output is a voltage proportional to the movement of the said movable block, a difference amplifier for receiving both the given fraction of the voltage output from the first position transducer and the voltage output from the second position transducer and producing an output voltage which is proportional to the difference between the two voltages fed into the difference amplifier a variable gain amplifier and a variable gain differentiator mutually connected in parallel each for receiving the difference amplifier output voltage, and respectively for producing an output voltage proportional, by a selected gain, to the difference amplifier output voltage and an output voltage proportional, by a selected gain, to the rate of change of the difference amplifier output voltage, a summation amplifier for receiving both the variable gain amplifier and variable gain differentiator output voltages and producing an output voltage which is the sum of the variable gain amplifier and variable gain differentiator output voltages, a driving amplifier for receiving the summation amplifier output, voltage and having two output lines respectively energisable by the summation amplifier output voltage when positive and when negative and a common return line, a three-position directional control valve biassed towards a centre position which isolates hydraulic fluid so as to prevent movement of the movable block, and also having a first end position for applying pressurised hydraulic fluid to move the movable block in a direction to shorten the looped formation in the load- handling line, and a second end position for exhausting hydraulic fluid to allow the pressurised hydraulic means to lengthen the looped formation in the load-handling line, a first solenoid energisable to move the valve into its first end position, a second solenoid energisable to move the valve into its second end position, the coils of the solenoids both being connected to the common return line and respectively being connected to the two output lines.

6. A system according to claim 1, wherein the means for paying-out and paying-in the heave line and the control means comprise a second winch and a second arrangement of pulleys rotatable about two axes at the first location. movable one axis being/towards and away from the other axis and the velocity ratios of the two arrangements being the same.

7. A system according to claim 1, wherein the means for paying-out and paying-in the heave line comprises a second winch, and the control means comprises a three- position directional control valve biassed towards a centre position which isolates hydraulic fluid so as to prevent movement of a movable block journalling pulleys of the arrangement of pulleys, and also having a first end position for applying pressurised hydraulic fluid to move the movable block in a direction to shorten the looped formation in the load-handling line, and a second end position for exhausting hydraulic fluid to allow the pressurised hydraulic means to lengthen the looped formation in the load-handling line, the operation of the valve being responsive to the paying-out and paying-in of the heave line and also to the resultant movement of the movable block.