WO1988005842A1

WO1988005842A1 - Installing frond mats on the seabed

Info

Publication number: WO1988005842A1
Application number: PCT/GB1988/000040
Authority: WO
Inventors: Peter Alsop
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-01-30
Filing date: 1988-01-21
Publication date: 1988-08-11
Anticipated expiration: 1989-07-30
Also published as: CN88100543A; ZA88561B; DK542188D0; GB8702131D0; DK542188A; AU1151688A

Abstract

A pack of frond mats (10a - 10f) are deployed on the seabed by first laying the mats flat and securing them independently to a rectangular frame (12) via break ties (21) and linking chains (11). The frame (12) is then hoisted and lowered to the seabed. When the frame (12) is resting on the seabed, the lowermost mat (10f) of the pack is detached from the frame (12) and anchored. The frame (12) is then indexed to the next location, and the lowermost mat (10e) again detached and anchored. The process is repeated until all mats have been deployed.

Description

INSTALLING FROND MATS ON THE SEABED

This invention relates to a method and apparatus for installing a flexible scour control frond mat on the seabed.

A recently developed frond mat for controlling scour or erosion of the seabed, particularly around structures such as oil rigs and pipelines, is described in my UK Patent 2166478. The mat consists of an array of spaced generally parallel frond lines traversed by spaced generally parallel cross-straps providing an open-grid mat structure, each frond line comprising a substantially continuous curtain of elongate buoyant fronds. Additional anchoring straps depend from the grid, each anchoring strap being provided with a ground anchor for anchoring the mat to the seabed.

One method of deploying such mats on the seabed is to progressively unwind the mats from reels, as described for example in my published UK Patent application 2171130. However, due to the size of the mats, these reels are somewhat bulky and can be difficult to handle and transport.

According to the present invention a flexible, open grid, frond mat is deployed on the seabed by laying the mat flat, releasably securing the mat to a frame at least partially bounding the mat, lowering the frame to the seabed, anchoring the mat, and releasing the mat from the frame.

The mat is preferably released from the frame in response to forced relative movement between the mat and the frame. This relative movement is preferably downward and/or lateral movement of the mat relative to the frame during the anchoring of the mat. Alternatively, it may comprise upward and/or lateral movement of the frame after the mat has been anchored.

The relative movement may release the mat by severing a link in the connection between the mat and the frame.

In one particular preferred embodiment of the invention, a plurality of the mats are superposed to form a stack, and each mat is independently secured to the frame. Once the frame has been lowered to the seabed, the mats are then sequentially deployed by indexing the frame and successively anchoring and detaching the lowermost mat of the pack. The connection between each mat and the frame preferably includes a plurality of severable links, each link preferably being severed in response to the relative movement between the mat and the frame during or after the anchoring of the mat. Where each mat includes a plurality of anchors spaced around its perimeter, the severable links are progressively severed as each anchor is driven into the seabed.

A particular example of the invention will now be described with refere'nce to the accompanying drawings in which:

Fig. 1 is a general plan view of a frame assembly for installing a pack of frond mats on the seabed,

Fig. 2 is an end view of the assembly of Fig. 1,

Fig.3 is a section on the line B-B in Fig. 1, Fig. 4 is a view in the direction of arrow A in Fig. 1,

Fig. 5 is a section through one of the anchor boxes carried by the frame assembly and showing a sequence of anchors in the box,

Fig. 6 is a diagrammatic sectional view of the frame secured to a mat pack and resting on the ground.

Fig. 7 is a detail of the portion ringed at the left hand end of Fig 6,

Fig. 8 is a detail showing attachment of the break- ties to the frond mat,

Fig. 9 is a side elevational view of a typical pin joint between two frame members in the frame assembly of Fig. 1, and

Fig. 10 is a sectional end view of the pin joint shown in Fig. 9.

Referring to these figures, a pack 10 of six frond mats lOa-lOf each measuring 5 x 5 metres is assembled and presented to a rectangular, lightweight put-down frame 12. The mats are linked to one another by common chains 11 (Figs 7 and 8) disposed at intervals around the pack 10. Each mat is independently connected to each of the chains 11 using independent break ties 21. The top and bottom link of each chain

11 is then shackled to respective upper and lower D- shaped handles 22a, 22b welded at corresponding intervals around the frame 12 so that the chains 11 lie opposite the handles 22 and the frame bounds the stack of mats. A typical break tie 21 is shown in Fig 8 as piercing one edge of the mat 10a and then being wrapped around a link of the chain 11.

Each mat consists of rows of discrete frond lines disposed generally parallel to one another, the lines being traversed by spaced generally parallel cross- straps to form an open-grid mat structure. Each frond line provides a dense substantially continuous curtain of fronds for filtering particulate material from underwater currents. Alternate frond lines are provided at both ends with anchoring straps 14 having associated ground anchor plates 15. The cross-straps are also provided at both ends with similar anchoring straps and anchor plates.

The detailed structure of the mat does not form part of the present invention and is not therefore illustrated in the drawings. Such a mat is however more fully described and illustrated in my aforesaid UK Patent 2,166,478. Very briefly, once anchored to the seabed, the function of the frond lines is to permanently retain waterborne particles carried by underwater currents so that over a period of time a permanent sandbank is created. The fronds consist of discrete fibrils and while water is able to pass through the mass of fibrils, the viscous drag is so great that particles of sand carried by the water are trapped and retained. By laying an array of such mats alongside underwater pipelines, or around the feet of oil rigs, erosion of the seabed beneath the pipeline or feet is prevented.

Before being presented to the put-down frame 12, the mats 10 may be rolled into a compact roll for ease of transport.

When the vessel carrying the mats reaches the required location, the mats are laid flat and the generally rectangular frame 12 is dropped onto the mat pack. The top and bottom links of each chain 11 are then shackled to respective upper and lower D-handles 22. The frame 12 with its attached mat pack 10 is then lifted and suspended from a main lift wire of a hoist mechanism on the vessel (not shown). The liftwire is connected to a central lifting column 30, the lifting column 30 taking the whole load of the frame 12, and mat pack 10, and being shackled to the lift wire through an integral lifting eye 31.

The rectangular frame 12 consists of hollow tubular frame members and provides a rigid base from which to deploy the 5 metre square mats. The frame members are welded to respective corner plates 36 at each corner of the frame. The frame is hung from the central lifting column 30 through four tubular angled support struts 32. Parallel cheek plates 33 are welded to the upper end of each of these tubular struts 32, the cheek plates fitting over four lugs 34 welded at right angles at the bottom of the central lifting column 30. Fitted shoulder bolts 40 are used to retain these parts together. This construction prevents the central lifting column from twisting and/or falling over when tension on the lift wire is released.

The lower end of each strut 32 includes a single slotted end plate 35 which is shackled to a respective corner plate 36 of the main frame 12. When the frame 12, complete with the mat pack 10, is lifted off the deck of the support vessel, the mats are prevented from sagging under their own weight by a removable support in the form of a length of webbing 37 which is stretched from one side of the frame to the other and passes underneath the mat pack. This webbing is passed around the frame and returned over the top of the mat pack. It is attached to the underside of a side frame member by a carbine hook and "D" clamp (not shown) and is tensioned by attaching its other end to the upper side of the frame member using a hook 38 and catch 39 mechanism.

When the frame and mat pack are next swung out over the side of the support vessel and lowered onto the surface of the sea, the mats tend to billow whilst entrapped air is forced out. The natural buoyancy of the fronds subsequently creates the same condition underwater. To prevent unrestrained billowing which might otherwise break the mats away from the frame, four additional bands of webbing 41 are stretched across the frame in two directions. These webs 41 supplement the effect of the central support webbing 37, and additional restraint is provided by a frond retention net 42 retained within the frame and superposed on the uppermost mat 10a of the pack. The net 42 prevents the fronds of the uppermost mat assuring their natural buoyant upright position while the frame is secured to the mat pack.

The anchors 15 associated with each of the mats 10 are stowed in anchor boxes 44 disposed at intervals around the frame 12. Each anchor box holds up to six anchors 15 complete with anchor straps 14. A spring_v loaded pusher 45 in each box maintains a forward force so that the leading anchor is urged to a forward position where it can be engaged by a driving tool and driven into the seabed. The forward force is provided by two extension springs 46 (only one of which is shown in the drawing) acting through two levers 47,48 keyed to a common shaft 49. The levers pass through the top of the anchor box and are joined at the bottom by a spindle 50. A wheel is fitted at each end of the spindle. After a leading anchor has been driven into the seabed, the remaining anchors in the box move forward to present the next anchor in the insertion posi ion.

The anchors are of the type more fully described in my UK Patent 2,162,562. Briefly, each anchor consists of a flat plate 15 with transverse slots for receiving and retaining the anchoring strap 14. The tail end of the anchor plate includes a resiliently biased hinged flap 51 which is retained in a first cocked position offering minimum drag resistance when a driving spigot 53 of a hammer gun 54 is engaged in a socket 55 welded to the plate. When the driving spigot 53 is withdrawn from the socket after driving the anchor plate into the ground, the tail flap 41 is released for movement into the more angled position shown in the drawing, to engage the side of the channel formed by the downwardly moving plate. It then provides a fulcrum about which the plate turns into a skew position resisting extraction of the plate from the ground when the anchoring strap is subsequently tensioned.

As shown in Fig. 6, the anchors 15 are installed in the respective boxes 44 face forwards so that the socket 55 of the leading anchor projects through a front flap of the box. A half cone 56 is welded to the outside of the front flap so that the narrow end 57 finishes just above the socket 55. This assists the diver during poor visibility conditions since location of the spigot 53 in the half cone 56 directs the spigot into the anchor socket 55 ready for insertion .

The anchor box 44 is shaped to follow the anchor profile. The anchors 15 sit on rails welded on the inside of the box, and straddle a slot for receiving the anchor straps 14. This reduces the friction to be overcome as the anchors move forward.

The front flap of the box is hinged at the top so that it can be lifted to permit loading of the box with anchors. When the flap is down in its normal position it is retained by toggle latches mounted on either side of the box.

An angled face at the front of the box is cut away to allow the leading anchor to drop through the bottom of the box inside the front flap. The cut-away is covered by rubber pads attached to the underside of the box and provide support for the leading anchor whilst the frame is being moved. The pads bend away to allow the leading anchor to be driven out of the anchor box into the seabed.

The hydraulic hammer gun 54 is fitted with a driving spigot 53 which engages the anchor socket 55. The gun is hung from a pivot arm 60 by a chain 61 shackled over a bar 62, the shackle 63 being slidable along the bar. This allows the gun to be positioned vertically above the anchor boxes 44 while the central lifting column 31 acts as a spindle around which the pivot arm 60 can rotate. This arrangement eliminates side forces on the gun 54 and therefore maintains perpendicular insertion while the weight of the gun and spigot are offset by a buoy (not shown) attached to the upper side of the outboard end of the pivot arm 60.

Two gun cradles 70 are attached to the frame 12. One s used to hold the gun 54 and spigot 53 whilst the frame is lowered to the seabed. The second cradle is available to hold a spare gun, or as a spare position should circumstances require a speedy removal.

Two opposed members of the frame 12 each comprise five discrete tubular sections 12a-12e hinged to one another at pin joints 19. This feature permits a closer fit. to the pipeline and seabed topography when the frame 12 is resting on the seabed. In particular pin joints 19 allow further downward movement of the appropriate section or sections of the frame whenever the section or sections is/are overhanging a sloping bank on the seabed. When the frame is subsequently hoisted clear of the seabed, the pin joints 19 lock with the frame sections 12a-12e aligned horizontally with one another. One of the pin joints 19 is illustrated in Figs 9-10. Pairs of side plates 80,81 are welded to opposite sides of the respective tubular frame sections 12a, 12b, the plates overlapping and having aligned openings for receiving a bolt 82 secured by an end nut 83. A spacer tube 84 fits over the shank of bolt 82 between the pairs of plates 80,81.

The weight of the complete frame 12 when resting on the seabed is taken by skids 71 fitted to the hollow tubular frame members. The skids 71 are provided with small keels to maintain directional stability when the frame is moved across the seabed under power from a thruster unit. They are mounted on legs 72 through a pin joint 73 to permit angular movement necessitated by the seabed topography.

At least one hydraulic thruster unit 74 provides motive power. The thru≤ters are driven hydraulically from the same supply as the hammer gun, the drive being through a simple normally closed spring-return spool valve (not shown). The valve is mounted in a handle arrangement incorporating an operating lever. The thruster mounting 75. permits directional adjustment using a swivel coupling. The thruster is held rigidly in the desired direction using bolts fitted through slots in the coupling.

In operation, once the frame 12 with its attached mat pack 10 has been lowered to the seabed, the retaining straps 37 and 41 are removed and the lowermost mat lOf is anchored by driving the respective anchors 15 and anchor straps 14 into the seabed using the hammer gun 54 operated by a diver. The diver swings the gun 54 into position over each of the anchor boxes 44 in turn, and at each box engages the spigot 53 in the socket 55 of the leading anchor before energising the gun to force the anchor out of the box and into the ground.

The frame 12 and the mat pack 10 are supported by the skids 71 and legs 72 just above the seabed. As each anchor 15 is driven into the seabed, the corresponding section of the lowermost mat lOf is therefore pulled down on to the seabed and laterally away from the frame 12 by the respective anchoring strap 14 secured to that section. This downward and lateral movement of the mat relative to the frame 12 tensions the break tie 21 associated with that section of the mat, the increased tension being sufficient to sever the tie. Accordingly, as each anchor is driven into the seabed, the mat lOf is progressively detached from the frame 12.

The thrusters 74 are then energised to advance the frame over the seabed to the next location, and the lowermost mat lOe is then anchored and detached from the frame in the same manner. The sequence is repeated for each of the remaining mats lOd-lOa.

Claims

1. A method of installing a flexible, open-grid frond mat on the seabed, the method comprising laying the mat flat, releasably securing the mat to a frame at least partially bounding the mat, lowering the frame to the seabed, anchoring the mat, and releasing the mat from the frame.

2. A method according to claim 1 in which the mat is automatically released from the frame in response to forced relative movement between the mat and the frame.

3. A method according to claim 2 in which the forced relative movement is a downward and/or lateral movement of the mat imparted during the anchoring of the mat.

4. A method according to claim 3 in which a plurality of anchors are secured to the mat by anchoring straps at spaced intervals around the mat, and in which the mat is anchored by sequentially driving each of the anchors and the associated anchoring straps into the seabed, the mat being progressively detached from the frame as the anchors are inserted.

5. A method according to claim 4 in which the mat is detached by progressively severing a plurality of severable links between the mat and the frame.

6. A method of installing a sequence of flexible, open-grid frond mats on the seabed, the method comprising laying the mats flat and superposing the mats to form a stack, releasably securing each of the mats independently to a frame at least partially bounding the stack, lowering the frame to the seabed, anchoring the lowermost mat of the stack, detaching the anchored mat from the frame, indexing the frame to the next succeeding location and thereafter repeating the anchoring, detaching and indexing steps until the complete sequence of mats have been installed.

7. A method according to claim 6 in which each mat is automatically detached from the frame in response to forced relative movement between the mat and the frame .

8. A method according to claim 7 in which the anchoring of the lowermost mat imparts a downward and/or lateral movement to the mat relative the frame, the mat being detached from the frame in response to the said movement.

9. Apparatus for deploying a plurality of open-grid flexible frond mats on the seabed, each mat having a plurality of associated anchors at the respective free ends of anchoring straps secured at intervals around the perimeter of the mat, the apparatus comprising a plurality of rigid frame members providing a frame adapted to at least partially bound a stack of the said mats, means carried by the frame for holding the said anchors, means including a severable link for connecting each mat independently to the frame at spaced intervals around the stack, means for connecting the frame to a hoist mechanism for lowering the frame to the seabed, and means for advancing the frame over the seabed after anchoring the lowermost mat of the stack.

10. Apparatus according to claim 9 further comprising means for supporting a powered driving tool for driving the anchors into the seabed, the support means extending horizontally over the frame and being

5 pivotable about a central vertical axis of the frame.

11. Apparatus according to claim 10 in which the support means further comprises a flexible line hanging from a horizontal arm, the upper end of the

10 flexible line being slidable along the arm.

12. Apparatus according to claim 9 in which the anchor holding means comprises at least one holder for holding a sequence of anchors, and means for

15 - resiliently urging the sequence toward one end of the holder such that the leading anchor in the sequence is aligned with an exit opening.

13. Apparatus according to claim 9 in which the 20 frame is generally rectangular and in which at least one pair of opposed sides of the frame each comprise a plurality of corresponding frame members hinged to one another, the hinged members being free to pivot downwardly from the horizontal.

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