GB2458725A

GB2458725A - A marine chaser adapted for improved sliding over chain links

Info

Publication number: GB2458725A
Application number: GB0708199A
Authority: GB
Inventors: Peter Bruce
Original assignee: Brupat Ltd
Current assignee: Brupat Ltd
Priority date: 2007-04-27
Filing date: 2007-04-27
Publication date: 2009-10-07
Also published as: GB0708199D0

Abstract

A chaser 1, adapted for moveable engagement with a mooring cable 8 with a clutchable element (11, Fig 6), comprises a body member 2 having a suspension point 26 at a top end for attachment of a suspending line (6, Fig 6). The chaser further comprises a mooring cable engagement means (10, Fig 6) at a bottom end adapted for passage of the mooring cable, and an aft side (12, Fig 1) for facing away from an anchor attached to the mooring cable, and one-way locking means 13 comprising a tooth (14, Fig 1) for locking onto the clutchable element to arrest movement of the chaser away from said anchor along said mooring cable. The tooth is located at the aft side of the chaser adjacent to the mooring cable engagement means. A contact surface (18, Fig 1) of the tooth faces away from the mooring cable engagement means, to be lockable onto the clutchable element on arrival of the clutchable element at the contact surface, whereby passage of the mooring cable over the mooring cable engagement means is arrested. A straight line (24, Fig 11), parallel to a central plane of symmetry and lying on an upper surface (19, Fig 1) of the tooth, forms an angle wit a straight line (22, Fig 11), joining the suspension line attachment point to an aftermost point 20 on the tooth, not exceeding 60{, to promote sliding of chain links making contact therewith.

Description

IMPROVE3Ts IN MARINE CRMERS A marine chaser, hereinafter referred to simply as a chaser, is a line-suspended device for engaging and S moving in contact with a mooring cable including at least one of a rope and a chain extending between a moored floating marine structure and a marine anchor for the purpose of at least one of laying out and recovering the anchor. Chasers are of open hook-like construction for hooking onto a mooring cable or of closed-loop construction for encircling the mooring cable. The open hook-like chaser is used only for recovering an anchor whereas the closed-loop chaser, often referred to as a permanent chaser, is used for both laying out and recovering the anchor, Chasers including means f or locking on a clutchable element such as a chain link on a mooring cable to stop movement of the chaser along the mooring cable in a direction away from an anchor are known from US Patent No. 4,724,789. Such chasers allow most of the heaving tension in the suspending line of the chaser to be applied to the portion of the mooring cable between the chaser and the anchor to facilitate breaking out of the anchor during recovery. The known locking chaser presents a problem wherein the locking action prevents return of the chaser along the mooring cable to a floating marine structure following laying-out of the anchor. The hook-construction locking chaser is not disadvantaged by this problem since it is used only for anchor recovery and is not required to move away from the anchor while engaged on the mooring cable in order to be returned to the floating marine structure. Ir. can be disengaged from the mooring cable by movement transverse to the axis of the mooring cable following decking on an anchor handling vessel or by falling off the mooring cable as the mooring cable becomes nearly vertical at the anchor stowage rack on the floating marine structure following winching--in of the mooring cable. The closed-loop locking chaser cannot be disengaged in this manner. A solution for this problem permitting return of a closed loop chaser to a floating structure is proposed in US Patent No. 4,724,789 wherein canting ann means are provided for disabling the locking mechanism in the closed--loop chaser before laying out an anchor and subsequently enabling the locking mechanism before recovering the anchor. This solution has the disadvantage of requiring the chaser to be decked on an anchor handling vessel to disable the locking mechanism before anchor deployment and again to enable the locking mechanism before anchor recovery.

It is an object of the present invention to provide a chaser having means for enabling remote locking and unlocking from moveable engagement with a mooring cable.

According to the present invention, a chaser, adapted for moveable engagement with a mooring cable comprising at least one of a rope and a chain attached to an anchor with said mooring cable including a clutchable element fixed thereon, comprises a body member having a suspension point at a top end of said body member for attachment of a suspending line thereto, having mooring cable engagement means at a bottom end of said body member adapted for passage thereover of said mooring cable, and having an aft side for facing away from said anchor, and comprises One-way locking means including a contact surface for locking onto said clutchable & element to arrest movement of said chaser away from said anchor along said mooring cable, characterized in that said one-way locking means is located at said aft side of said chaser adjacent to said mooring cable engagement means and in that said contact.

surface faces away from said mooring cable engagement means to be lockable onto said clutchable element on arrival of said clutchable element at said contact surface whereby tassage of said mooring cable over said mooring cable engagement means is arrested.

Preferably said locking means comprises tooth means for locking onto said clutchable element.

Preferably a straight line parallel to a central plane of symmetry and lying in an upper surface of said tooth means forms an angle with a straight line joining said suspension line attachment point to an aftermost point on said tooth means not exceeding 60 and further preferably not exceeding 50 to promote sliding of chain links making contact therewith.

Preferably said tooth means includes slot means whereby contact of said surface with said clutchable element occurs at opposed sides of said mooring cable.

Preferably said slot means includes a slot diverging from said body member to accommodate a range of sizes of said mooring cable.

Preferably said aft side of said body member includes fairing means for directing said mooring cable towards said locking means.

Preferably said mooring cable engagement means includes an arcuate surface for smooth passage of said mooring cable thereon.

Preferably said arcuate surface comprises the periphery of a wheel mounted in said body member for smooth passage of said mooring cable thereon.

Preferably said arcuate surface includes a groove for receiving at least one of a rope and an upstanding link of chain.

Preferably said contact surface is arranged to lie substantially orthogonal to a straight line tangential to said arcuate surface.

Preferably said locking means is located relative to said mooring cable engagement means such that locking of said clutchable element onto said contact surface occurs only when said mooring cable is bent in a two-legged bight about said mooring cable engagement means to bring said clutchable element into contact with said contact surface.

Preferably the legs of said two-legged bight form an acute angle.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 shows a side elevation of a chaser according to the present invention; Figure 2 shows a forward end elevation of the chaser of Figure 1; Figure 3 shows an aft end elevation of the chaser of Figure 1; Figure 4 shows a plan view of the chaser of Figure 1; Figure 5 shows an underside view of the chaser of Figure 1; Figure 6 shows the chaser of Figure 1 sliding on a stud-link chain cable in a direction towards an anchor; Figure 7 shows the chaser of Figure i sliding on a stud*--link chain cable in a direction away from an anchor; Figure 8 shows the chaser of Figure 1 engaging a bight of stud-link chain cable; Figure 9 shows the chaser of Figure 1 engaging a bight of wire rope cable.

Figure 10 shows a further embodiment of a chaser for promoting sliding of chinlinks in the manner shown in Figure 6; Figure 11 shows the chaser of Figure 10 in a similar arrangement to Figure 9.

Referring to Figures 1 to 9, a chaser I comprises an S annular body member 2 having a top end portion 3 bearing a lug 4 providing a suspension point comprising a shackle hole 5 for attaching a suspending line 6 by means of a shackle 7 (Figures 6 to 9). Annular body member 2 is dimensioned to permit passage along a mooring cable 8 (Figures 6 to 9) and any connecting elements fitted thereto, connecting a floating marine structure to an embedded marine anchor, and has a bottom end portion 9 formed to include a curved engagement surface 10 adapted for moving engagement with mooring cable 8 which includes a clutchable element 11 (Figures 6 to 9) being a chain link or a rope socket when mooring cable S comprises a chain cable or a rope cable respectively.

Annular body member 2 has an aft side 12 for facing away from an anchor (not shown) attached to mooring cable 8. A one-way locking claw 13 having two fingers 14 and a slot 15 therebetween is formed in bottom end portion 9 of annular body member 2 aft of curved engagement surface 10. Slot 15 has a width W sufficient to accommodate mooring cable 8 between fingers 14 and may be conscrucced with width W increasing aft to be divergent from body member 2 to accommodate a range of sizes of mooring cable 8. Aft side 12 of annular body member 2 is curved between top end portion 3 and an after-most toe 16 of claw 13 to define a fairing 17 at each side of claw 13 serving to direct mooring cable 8 riding thereon into slot 15. Claw 13 has a contact surface 18, on each of fingers 14, which faces away from curved engagement surface 10 and which is curved for bearing on clutchable element 11 substantially at right angles to the axis of mooring cable 8 when mooring cable $ is bent in a bight about curved engagement surface 10 to bring mooring cable 8 into slot 15 whereupon movement of mooring cable 8 relative to curved engagement surface 10 brings clutchable element 11 into locking contact with contact surfaces 18. Claw 13 also has an upper surface 19 arranged to cause clutchable element 11 incident thereon to slip off claw 13 when moving aft from curved engagement surface 10. Toe 16 of claw 13 is spaced from the aft end of curved engagement surface 10 sufficiently to enable a clutchable element 11 on mooring cable 8 to make contact with contact surfaces 18 of claw 13 when the angle between the legs of the bight formed in mooring cable 8 is reduced sufficiently by heaving up on suspending line 6. Relative movement of mooring cable 8 in the bight in a direction through chaser 1 towards aft side 12 is unobstructed since clutchable element 11 slips off upper surface 19 of claw 13 (Figures 6) when incident thereon. However, relative movement of mooring cable 8 in the bight through chaser I from aft side 12 is arrested when clutchable element 11 bears on contact surfaces 18 of claw 13 (Figures 8 & 9). Once movement of mooring cable 8 relative to chaser 1 is thus arrested, tension T in suspending line 6 is almost completely transferred to the portion of mooring cable 8 located between chaser 1 and the embedded anchor attached thereto. This greatly facilitates breaking the embedded anchor out of the seabed.

It will be readily appreciated that a pulley wheel may advantageously be located centrally in bottom end portion 9 of chaser I to reduce friction due to mooring cable 8 sliding under load on curved engagement surface 10. In this case, curved engagement surface 10 would comprise the periphery of the pulley wheel. Further, it is envisaged that a groove (not shown) may be formed in curved engagement surface 10, if desired, to receive both wire rope mooring cable and upstandina links of a chain mooring cable.

in use, for deployment of an anchor attached by mooring cable 8 to a floating marine structure, chaser 1 is fitted to the mooring cable 8 adjacent to the anchor which is stowed on an anchor stowage rack on the floating marine structure. The suspending line 6 attached to chaser I is passed to art anchor handling vessel which pulls chaser I into contact with the anchor and hauls the anchor away from the floating marine structure by steaming to a desired drop point. The anchor is lowered to the seabed at the drop point by paying out the suspending line 6 which is then left slack. A mooring winch on the floating marine structure hauls in mooring cable 8 to set the anchor into the seabed and holds mooring cable 8 taut. The anchor handling vessel then turns about and steams back to the floating marine structure while dragging chaser I in direction A (Figure 7) along taut mooring cable 8 away from the anchor. Curved engagement surface 10 makes smooth sliding contact with mooring cable 8 on the way back to the floating marine structure with claw 13 held well clear of any clutchable element 11 on taut mooring cable 8. The suspending line 6 is then passed back to the floating marine structure for stowage thereon prior to completion of deployment of the anchor by further hauling in of mooring cable 8 by the mooring winch to embed the anchor to a desired S operating penetration below the surface of the seabed. Thus, chaser 1 functions in the manner of a conventional non-locking chaser during both deployment of an anchor and subsequent return of chaser I to a floating marine structure by virtue of claw 13 being located at an a.ft position on chaser 1 well clear of the line of passage of taut mooring cable 8.

In use, for recovery of an anchor, suspending line 6 attached to chaser i, now stowed on the floating marine structure, is passed to an anchor handling vessel which pulls chaser 1 in direction B (Figure 6) along mooring cable S towards the embedded anchor attached thereto. Curved engagement surface 10 makes smooth sliding contact with mooring cable 8 while being dragged towards the embedded anchor. A clutchable element Il on mooring cable 8 making contact with upper surfaces 19 slips off claw 13 and so does not impede movement of chaser 1. towards the embedded anchor. The anchor handling vessel stops pulling when chaser i reaches the seabed surface.

Tens ion in mooring cable 8 is then reduced by the winch on the floating marine structure to allow the anchor handling vessel to heave up on suspending line 6 to form a bight in mooring cable 8 (Figures 8 and 9). Further heaving causes the angle between the legs of the bight to reduce so that mooring cable 8 enters slot 15 in claw 13 and runs back over curved engagement surface 10 to bring a clutchable element 11 into locking contact with contact surfaces 18 on claw 13 thus locking chaser 1 temporarily onto mooring cable 8. Continued heaving brings suspension line 6 substantially into alignment with the portion of mooring cable 8 located between chaser I and the embedded anchor attached thereto and so applies almost all of the tension T (Figures 8 and 9) in suspension line 6 to breaking out the embedded anchor from the seabed. This greatly facilitates the breaking out operation and so allows a lower cost anchor handling vessel with a less powerful winch arid lower bollard pull to be utilised. Once broken out of the seabed, the anchor is returned to the floating marine structure while hanging from chaser 1. by hauling in on mooring cable 8 to pull both the anchor and the anchor handling vessel back to the floating structure. As mooring cable 8 adjacent to chaser 1 becomes nearly vertical on approaching the anchor stowage rack on the floating marine structure, the anchor handling vessel pays out suspending line 6 to allow gravity to cause marine chaser I to disengage from clutchable element 10 and fall down into engagement with the anchor. Further heaving in of mooring cable 8 brings chaser 1 and the anchor together into a final stowage position on the anchor stowage rack. Suspending line 6 is then passed back and secured on the floating marine structure to complete the anchor recovery operation.

Remote unlocking of chaser I from locked engagement with mooring cable 8 is achieved by reversing the locking procedure. Suspending line 6 is paid out by the anchor handling vessel and mooring cable 8 is heaved in by the winch on the floating marine structure to straighten the bight in mooring cable 8 to withdraw clutchable element 11 from locking contact with contact surface 18 on claw 13. Chaser 1 can then be returned along the now taut mooring cable 8 to the floating marine structure following, for example, repositioning of an embedded anchor.

Reference is now made to Figures 10 and 11 which illustrate a preferred arrangement of the claw 13 with respect to the body 2. Like parts to those of the earlier Figures will be given the same reference numeral to assist in clarity. In this embodiment the claw 13 is arranged to promote sliding of chain links 11 when they make contact with the upper surface 19 of the claw 13. The protruding point 20 of the claw 13 is arranged at an intersection of two lines 22, 24, ensuring that the angle formed does not exceed 60' and preferably does not exceed 50'. The first line 24 is parallel to a central plane of symmetry and runs through the point 20. The second line joins the point 20 with the suspension line attachment point 26, this being at the centre of the shackle hole 5. This arrangement provides for the claw 13 to lie below the uppermost surface 28 of the surface 10 2$ when the chaser is arranged as shown in Figure 6.

Thus a tangent line taken at the uppermost surface 28 of the contact surface 10 will lie on or above the aftermost point 20 of the claw 13.

Whereas the embodiment of the present invention particularly described herein comprises a chaser providing a solution to the hitherto unsolved problem set out in U.S. Patent No. 4,724,789, it will be apparent that other embodiments of different shape will be within the scope of the invention.

S

Claims

CLAIMS1. A chaser, adapted for moveable engagement with a mooring cable comprising at least one of a rope and a chain attached to an anchor with said mooring cable including a clutchable element fixed thereon, comprises a body member having a suspension point at a top end of said body member for attachment of a suspending line thereto, having mooring cable engagement means at a bottom end of said body member adapted for passage thereover of said mooring cable, and having an aft side for facing away from said anchor, and comprises one-way locking means 0 including a contact surface for locking onto IC) 15 said clutchable element to arrest movement of 0 said chaser away from said anchor along said Cv') mooring cable, characterised in that said one-way locking means comprises tooth means for locking onto said clutchable element and is located at said aft side of said chaser adjacent to said mooring cable engagement means and in that said contact surface faces away from said mooring cable engagement means to be lockable onto said clutchable element on arrival of said clutchable element at said contact surface whereby passage of said mooring cable over said mooring cable engagement means is arrested, and wherein a straight line parallel to a central plane of symmetry and lying in an upper surface of said tooth means forms an angle with a straight line joining said suspension line attachment point to an aftermost point on said tooth means not exceeding 600 to promote sliding of chain links making contact therewith.
2. A chaser, as claimed in claim 1, characterised in that said angle does not exceed 50°.
3. A chaser, as claimed in claim 1 or 2, characterised in that said tooth means includes slot means whereby contact of said surface with said clutchable element occurs at opposed sides 0') of said mooring cable.

If) 15
4. A chaser, as claimed in claim 3, characterised O in that said slot means includes a slot CY) diverging from said body member to accommodate a range of sizes of said mooring cable.
5. A chaser, as claimed in claims 1 to 4, characterised in that said aft side of said body member includes fairing means for directing said mooring cable towards said locking means.
6. A chaser, as claimed in claims 1 to 4, characterised in that said mooring cable engagement means includes an arcuate surface for smooth passage of said mooring cable thereon.
7. A chaser, as claimed in claim 6, characterised in that said arcuate surface comprises the periphery of a pulley wheel mounted in said body member for smooth passage of said mooring cable thereon.
8. A chaser, as claimed in claims 6 and 7, characterised in that said arcuate surface includes a groove for receiving at least one of a rope and an upstanding link of chain.0')
9. A chaser, as claimed in claims 1 to 4, 0 characterised in that said contact surface is I.f) 15 arranged to lie substantially orthogonal to a 0 straight line tangential to said arcuate Cr') surface.
1O.A chaser, as claimed in claims 1 to 4, characterised in that said locking means is located relative to said mooring cable engagement means such that locking of said clutchable element onto said contact surface occurs only when said mooring cable is bent in a two-legged bight about said mooring cable engagement means to bring said clutchable element into contact with said contact surface.
11. A chaser, as claimed in claim 10, characterised in that the legs of said two-legged bight form an acute angle.