GB2243390A

GB2243390A - Protective blanket

Info

Publication number: GB2243390A
Application number: GB9108670A
Authority: GB
Inventors: Charles Michael Waters
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1990-04-26
Filing date: 1991-04-23
Publication date: 1991-10-30
Also published as: WO1991016503A1; GB2258261B; GB2258261A; GB9108670D0; GB9220548D0; AU7759691A

Abstract

In a flexible blanket comprising a two-dimensional array of shells (14) interconnected by ropes (13) to facilitate flexibility, and which shells may be filled with ballast material (19) to provide a weighted blanket for use underwater, each shell (14) is of a two-part construction comprising upper and lower parts which can be interlocked with one another, eg as a snap-fit, to locate the ropes therebetween. <IMAGE>

Description

FLEXIBLE BLANKET This invention relates to flexible blankets and in particular, though not exclusively, to flexible blankets for the protection or stabilisation of underwater structures and pipelines, sea and river beds, embankments and other situations subject to instability and erosion.

It is common practice to lay pipelines on the sea bed between, for example, oil production platforms and on-shore processing facilities. However, as a result of strong currents the sea bed is a very unstable environment. Scouring can remove the sea bed from beneath any object which interrupts the flow of the currents, and sand waves can erode the entire sea bed over a considerable area. Furthermore the temperature and viscosity of the product conveyed may induce regular movement and oscillation of the pipeline.

One effective method for the protection of a sea bed pipeline is described in the specification of Patent Publication GB 2156941B. This describes and claims an innovative form of flexible blanket especially adapted for use for example to protect a sea bed pipeline. The flexible blanket described therein comprises a plurality of segments arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least at a median zone of their height dimension, each segment tapering substantially regularly from a maximum cross-section at said median zone to a minimum cross-section at one or both ends thereof and each segment being bonded to each adjacent segment in the region of their respective median zones such that segments are held substantially contiguously but are free to pivot one relative to another about the bonded interconnection insofar as is permitted by said taper, the arrangement of the tapers being such that each segment is capable of pivoting with more than one degree of freedom.

In a preferred form of that invention the segments comprise a blow-moulded polyethylene plastics shell which is filled with a dense material such as concrete to give the required weight and density.

The plastics shells are each formed with apertures at their median zones for the passage of ropes therethrough in the case where adjacent segments are bonded to one another by rope means, and each shell is open at one, upper, end to facilitate filling with concrete. The use of plastics shells is particularly advantageous in contrast to the provision of simple concrete blocks in that the shells facilitate accurate location of ropes prior to embedding in concrete, serve as moulds which do not require removal from the concrete when set, and provide a relatively smooth surface for more sympathetic contact with adjoining pipelines, structures or equipment than raw concrete.

A further advantage of the shells is that they readily facilitate on-site fabrication of the flexible blanket, it being necessary simply to pour concrete into an array of shells which have previously been interconnected by ropes threaded through the median zone apertures.

Whilst this assembly technique works well, it does require considerable factory time and care to thread the ropes to form an array of interconnected segments, the pre-assembled array albeit of light weight is bulky to transport and care is needed to ensure that when filling the individual shells filling material does not spill over to spaces between neighbouring segments and thereby potentially inhibit subsequent flexibility of the blanket or cause damage to the segments or ropes.

The present invention seeks to provide a flexible blanket of a kind which is at least as effective in use as a flexible blanket of the kind the subject of GB 2156941B and which may be fabricated more easily.

The invention seeks also to provide an improved shell for use in forming a flexible blanket.

In accordance with one of its aspects the present invention provides a shell for use in forming a segment of a flexible blanket of a kind in which a plurality of segments are arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least at a median zone of their height dimension and each segment being connected to each adjacent segment in the region of their median zones such that the segments are held substantially contiguously but are free to pivot relative to one another about the interconnections, said shell comprising two shell portions for joining one to the other at a joint line which includes regions of the median zone through which extends rope means for forming an interconnection between neighbouring segments of a blanket and the two shell portions of a shell comprising confronting edge formations adapted for interlocking with one another to resist separation of the portions when interlocked.

The term "median zone" is used to mean a zone between ends of a shell and not necessarily a zone lying exactly mid-way.

At least one of the shell portions of a segment may be of a kind which can be nested within another shell portion, said another shell portion being for example either the other shell portion for forming a segment shell or a corresponding, e g upper or lower, shell portion of another segment shell.

Preferably the two shell portions are of substantially identical construction, one portion being formed with a closed end and the other with an open end to permit a pair of assembled shell portions to be filled with concrete.

It is further preferred that the two shell portions of a segment each be of a generally tapered form and more particularly but not exclusively that when one shell portion is inverted from the position which it adopts when assembled with a second shell portion it can nest within that second shell portion (and additional shell portions) so that two shell portions in a nested configuration occupy a height substantially less than the height of two shell portions when assembled together.

Where the segments comprise shell portions whcih can be stacked in a nested configuration, preferably the height of two nested shell portions is less than 150% and more preferably less than 130% of the height of a single shell portion.

When in a nested configuration preferably only rim regions of successive shell portions lie in contact with one another.

The shell portions may have radially outer rim regions which provide said confronting edge formations whereby said portions may be interlocked together.

The confronting edge formations may interlock in a snap-fit manner. Preferably the confronting edge formations are of identical form. The two shell portions for forming a shell preferably are provided with marks alignable to ensure correct orientation of the respective halves for snap-fitting or otherwise securing together where it is necessary that the two halves be arranged in a prescribed relative orientation.

A shell portion having an open end for enabling a pair of shell portions to be filled with concrete may have a radially inner rim region which comprises an upstanding collar portion. This strengthens the open end.

Median zone apertures for accommodating ropes that extend through the shell for interconnection with neighbouring shells may be formed by open-ended cutouts in one of the shell portions of a size sufficient to accommodate a rope. Where, however, the shell portions are substantially identical, each shell portion may have smaller open-ended cut-outs each sufficient to accommodate approximately half of the cross-sectional area of a rope.

Preferably confronting edge formations of two shell portions form a close fit assembly thereby to prevent undesirable leakage of filler material such as concrete from within the shells. Where a rope aperture is defined by open-ended cut-outs in each of the shell portions, preferably the end walls in the region of said cut-outs are each slightly inclined so that when the shell portions are assembled together, the wall regions of the respective portions lie at opposite small angles of inclination to a plane perpendicular to the direction of the length of a rope passing through the aperture defined by those cut-outs.

In consequence the edge formations of the assembled shell portion may be arranged to adopt an overlapped configuration in the region of each median zone aperture to resist undue deformation in the event of pulling of a rope through the shells and also to minimise seepage of unset concrete.

Preferably as considered in a cross-sectional plane perpendicular to the wall of a shell portion, the edge of the shell portion material defining a cutout for a rope is rounded thereby to minimise risk of abrasion of a rope.

The edge portion of each shell portion for confronting another shell portion preferably is shaped to provide an outwardly extending rib which, in the assembled condition of two shell portions, extends around the shell at the median zone thereof. That rib may define the maximum cross-section of the shell and each shell may taper substantially regularly from a maximum cross-section at said median zone to a minimum cross-section at one or both ends thereof.

The cut-outs for accommodating at least a portion of a rope preferably lie inwards of said rib portion.

In consequence when the rib portions of two segments lie in contact with one another a small length of the rope extending therebetween lies exposed and is able to deflect to facilitate relative movement of the shell portions without undue strain to itself or the segments.

The shell portions may be formed by blow-moulding of polyethylene or like material. Preferably, however, they are formed of plastics by an injection moulding or like technique which results in a greater uniformity of dimension than can be achieved with blow-moulding.

The accuracy attainable by injection moulding the shell portions on the one hand enables provision of shell portions which can be click-fitted together.

Secondly it results in the overall dimension of an array of segments forming a blanket being more accurately predictable than if the shells have been formed by e g blow-moulding.

The present invention further provides, in accordance with another of its aspects, a flexible blanket comprising a plurality of segments arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least in the median zone of their height dimension and each segment being flexibly interconnected to each adjacent segment in the region of their respective median zones such that the segments are held substantially contiguously but are free to pivot one relative to another about the interconnections, each segment comprising a shell defined by a pair of co-operating interengaged shell portions and each shell being filled with a dense material.

Each shell as considered in plan view may be a regular polygon, such as a square or regular hexagon, the latter having the advantage of facilitating triaxial flexibility. Alternatively the segments as considered in plan view may be of circular, elliptical or other curved shape, or may be of an irregular polygonal form.

The assembly of two shell portions may taper from the median zone to one end only, the other end being parallel-sided, or alternatively it may taper from the median zone toward each end. If the shells are tapered to only one end, the resulting blanket made of similar segments will only be able to flex well in one direction from the plane of the blanket.

If all the segments taper from the median zone to each end, the resulting blanket may flex in two directions from the plane of the blanket.

The rope means may be a textile type rope for example of plastics material such as nylon, propylene or polyester. Alternatively it may be a wire rope comprising a plurality of metal filaments twisted about one another either into a cord or cabled construction.

The shell portions preferably have wall portions which taper at an included angle in the range 25 to 500. An included angle of 400 is considered desirable for many applications.

Preferably the blanket comprises an array of ropes selected in size to closely fit the shell portion apertures through which they extend, when the shell portions are assembled to form complete shells. The blanket may comprise some ropes, in particular those which may be required for lifting of the blanket, of a greater size than other ropes, and in that case the rope apertures at the median zone of each shell may be correspondingly sized.

The invention further provides a method of forming a flexible blanket in which a plurality of first shell portions are laid in an array, each shell portion having uppermost an edge region formed with a plurality of open-ended cut-outs for accommodating rope means, arranging rope means to extend through aligned cutouts of the shell portions of the assembled array, fitting a second shell portion to each lower shell portion, said second shell portion having an open end opposite its end for interengaging with the lower shell portion, and filling each shell with settable material through the open end of the associated upper shell portion.

Preferably the method comprises use of shell portions of a kind which are accurately dimensioned, for example which have been formed by an injection moulding technique, and the array of interconnected upper and lower shell portions being filled with concrete by providing over the array a jig member formed with a -plurality of filling apertures arranged to lie in register with predetermined positions of the open ends of the shells whereby settable compound may be poured into each shell without substantial risk of compound spilling to spaces between neighbouring shells.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows in situ on a pipeline a multi layer assembly of blankets of the present invention; Figure 2 is a schematic plan view of a blanket layer of Figure 1; Figure 3 is an outline perspective view of one of the segments forming the blanket of Figure 2; Figure 4 is a vertical sectional view of the segment of Figure 3; Figure 5 is a photographic view of a side of an outer plastics shell of the segment of Figures 3 and 4; Figure 6 is a photographic view of an upper, outer end of the plastics shell of Figures 3 and 4; Figure 7 is a photographic side view in a first direction of a half portion of the shell of Figures 5 and 6; Figure 8 is a photographic side view corresponding to that of Figure 7 but of an adjacent side edge; ; Figure 9 is a photographic view of an inner end of the shell portion of Figures 7 and 8; Figure 10 is a side view of two of the shell portions of Figures 7 and 8 when in a nested configuration; Figure 11 is a sectional representation of a rope aperture in the plane of a median zone, and Figure 12 is a vertical sectional view of a pair of interconnected corner formations.

Figure 1 shows a sea bed pipeline 10 protected by a multi-layer assembly 11 of flexible blanket layers 12.

The lowermost layer 12a is the largest and is a primary layer by which the assembly of layers may be lifted. The other two layers 12b,12c provide weight at required regions, as described in GB 2156941B.

Each layer has an array of ropes 13 which flexibly interconnect the individual segments 14 of each layer.

The array of segments 14 in each layer 12 is shown in outline by Figure 2. In plan each segment is of a regular hexagonal shape and ropes are arranged to extend in each of three symmetrically orientated directions generally to hold neighbouring segments in an abutting relationship. Each rope extends through openings in an opposing pair of side faces of an aligned row of segments. Ropes 13 are arranged to extend directly between side edges 9 of the blanket in the direction 13a and are secured to rigid edge lifting bars 15 by means of which the blanket may be raised or lowered. Other, smaller section ropes extend in directions 13b,13c at + 1200 relative to the direction 13a; rope portions may extend in a zig-zag manner in each of the directions 13b,13c.

Each segment is tapered from a median zone 16 to each outer end, an upper outer end 17 and a lower outer end 18 and comprises an injection moulded polyethylene two-part shell filled with concrete 19 which directly embeds the ropes 13.

The construction of each shell will now be described in more detail.

Each shell comprises two substantially identical shell portions 20 which snap-fit together at the median zone 16 to form a substantially unitary structure.

The two shell portions differ only in that one of the portions, for comprising an upper portion has a circular section removed from its outer end whereby the upper end 17 of the assembled shell has an opening 8 which facilitates filling with a settable compound. This opening 8 is defined by an upstanding collar portion 17a the purpose of which is described below. The inner end 26 of each shell portion, being the end lying at the median zone 16, has an outwardly extending rib formation 21 (not shown in Figure 3) having alternately male and female formations 22,23 (described in more detail below) at successive corners; the formations are of complementary shape whereby two appropriately orientated shell portions may be snap-fit together, to be held together at each of the six corners 24.

At the middle of the length of each side 25 of the inner end of each shell portion an open "cut-out" shape 27 is provided to receive a rope 13. The cutout 27 is provided in a wall portion which lies inwards of the outer extremity of the rib formation 21 and in the assembled shell the confronting cut-outs of the two shell portions co-operate to define a rope opening which lies recessed in the rib formation.

The "cut-out" shape may be formed as a result of moulding in a suitably shaped mould or may be formed after moulding by drilling or otherwise removing shell wall material.

Each cut-out 27 lies in a local wall region 28 which is inclined at a small angle, typically 5 to 100 relative to the side of the shell portion (and thus at that angle relative to a plane perpendicular to the direction of the length of a rope extending through the rope opening when in an assembled condition). In each shell portion the local wall regions in successive edges 25 of the hexagonal rib formation lie at opposite angles of inclination, and these wall regions are arranged to overlap with corresponding wall regions of a second shell portion when in an assembled condition; by virtue of the opposite -angle of inclination of confronting cut-outs those local wall regions nest together in a cross-over configuration as depicted by Figures 5 and 11.

Each cut-out also has a smooth, rounded edge 30 as considered in cross-section of the wall of the shell portion to prevent abrasion of a rope by the shell wall material. In the embodiment shown in Figure 11 that edge 30 has a profile of semi-circular shape.

The male and female formations 22,23 provided alternately at the successive six corners of each shell portion are shown in more detail in Figure 12 which is a cross-sectional view through the shell wall.

The rib formation 21 of a lower shell portion is of an inwardly directed hook-shaped profile 31 having an inwardly and downwardly facing abutment 32. The rib formation of the other, upper shell portion is of an outwardly directed hook-shaped profile 33 of U-shape form having an abutment 34 which co-operates with the abutment 32 to hold the two shell portions locked relative to one another in an assembled condition. The U-shape form of profile 33 allows the outer limb 35 thereof to deflect inwards and thus, in combination with the flexibility of the limb portion 36 of the profile 31, the outer face 37 of limb 35 can slide over the end head portion 38 of limb 36 to snap-fit on assembly to lie in the configuration of Figure 12. Opening 39 is not relied upon for assembly; it arises merely from a need to facilitate mould removal during injection moulding.

The side walls of each shell portion are tapered, whereby shells may be stacked in a nested configuration for ease of transport. Figure 10 shows two shells in a nested configuration.

In use of the shell portions to construct a flexible blanket an array of shell portions of the type having closed ends is formed on a flat work surface, the hexagonal shell portions being arranged close packed with rib portions in contact. Ropes are then arranged to extend between opposite wall faces of each shell portion, to extend through the respective cut-outs of all shell portions lying in a row in each of the directions 13a,13b,13c. For the direction 13b,13c ropes are laid in a to and fro manner as shown in Figure 2 and with four rope ends being secured by end knots 40 at the corner segments 41. The ropes laid in the direction 13a are larger in diameter than the zig-zag ropes and are secured at each end to lifting bars 15. Upper shell portions are then aligned and fitted to each lower shell portion to snap-fit therewith. A layer of reinforced rubber sheeting is then tightly fitted around the aforedescribed end collars 17a of the open ends of the array of shells thereby to prevent concrete falling between the shells during a subsequent operation in which the shells are filled with concrete.

Subsequent to filling the concrete is allowed to set and the rubber sheet is removed.

Claims

CLAIMS:

1. A shell for use in forming a segment of a flexible blanket of a kind in which a plurality of segments are arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least at a median zone of their height dimension and each segment being connected to each adjacent segment in the region of their median zones such that the segments are held substantially contiguously but are free to pivot relative to one another about the interconnections, said shell comprising two shell portions for joining one to the other at a joint line which includes regions of the median zone through which extends rope means for forming an interconnection between neighbouring segments of a blanket and the two shell portions of a shell comprising confronting edge formations adapted for interlocking with one another to resist separation of the portions when interlocked.

2. A shell according to claim 1 wherein the confronting edge formations of the two portions of a shell may be interlocked with one another in a snapfit manner.

3. A shell according to claim 1 or claim 2 wherein said confronting edge formations are of identical form.

4. A shell according to any one of the preceding claims wherein at least one of the shell portions of a shell is of a generally tapered form whereby one portion may be nested within another shell portion.

5. A shell according to claim 4 wherein the shell comprises two shell portions each of a generally tapered form whereby one portion may be nested within the other portion.

6. A shell according to claim 4 wherein a shell portion has a rim region and is shaped such that when two of said shell portions are arranged in a nested configuration the rim regions of the two shell portions lie in contact with one another.

7. A shell according to claim 6 wherein said rim region is a radially outer rim region of a shell portion.

8. A shell according to claim 6 or claim 7 wherein only said rim region of a shell portion contacts another of said shell portions when said two shell portions lie in a nested configuration.

9. A -shell according to any one of the preceding claims wherein said two shell portions are of substantially identical construction.

10. A shell according to any one of the preceding claims wherein one shell portion is formed with a closed end and the other with an open end to permit a pair of assembled shell portions to be filled with concrete.

11. A shell according to any one of the preceding claims wherein regions of confronting edge formations of the shell portion defining a median zone aperture for rope means lie at small angles of inclination to a plane perpendicular to that of the normal direction of the length of rope means passing through the aperture.

12. A shell according to claim 11 wherein said regions adopt an overlapped configuration in the region of the median zone aperture.

13. A shell according to any one of the preceding claims wherein as considered in a cross-sectional plane perpendicular to the wall of a shell portion, the edge of shell portion material defining an aperture for rope means is rounded.

14. A shell according to any one of the preceding claims wherein an edge portion of a shell portion for confronting another shell portion is shaped to provide an outwardly extending rib which extends around the shell at the median zone thereof.

15. A shell according to claim 14 wherein cut-outs for accommodating at least a portion of rope means extending through the shell lie inwards of said rib portion.

16. A shell according to any one of the preceding claims wherein median zone apertures for accommodating ropes to extend through the shell are formed by openended cut-outs in at least one of the shell portions.

17. A shell according to any one of the preceding claims wherein the shell portions are provided with alignment marks for correct orientation prior to joining together.

18. A shell according to any one of the preceding claims and which is a plastics shell formed by injection moulding.

19. A shell constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

20. A flexible blanket comprising a plurality of segments arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least in the median zone of their height dimension and each segment being flexibly interconnected to each adjacent segment in the region of their respective median zones such that the segments are held substantially contiguously but are free to pivot one relative to another about the interconnections, each segment comprising a shell defined by a pair of co-operating interengaged shell portions and each shell being filled with a dense material.

21. A flexible blanket according to claim 20 wherein said pair of co-operating shell portions are portions of a shell according to any one of claims 1 to 19.

22. A flexible blanket constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

23. A method for forming a flexible blanket in which a plurality of first shell portions are laid in an array, each shell portion having uppermost an edge region formed with a plurality of open-ended cut-outs for accommodating rope means, arranging rope means to extend through aligned cut-outs of the shell portions of the assembled array, fitting a second shell portion to each lower shell portion, said second shell portion having an open end opposite its end for interengaging with the lower shell portion, and filling each shell with settable material through the open end of the associated upper shell portion.

24. Method according to claim 23 wherein use is made of injection moulded plastics shell portions.

25. Method of forming a flexible blanket substantially as hereinbefore described with reference to the accompanying drawings.

25. Method according to claim 23 or claim 24 wherein a multi-apertured jig member is provided over an array of interconnected upper and lower shell portions to assist filling of those portions with settable compound.

26. Method according to claim 25 wherein use is made of a jig member comprising a layer of reinforced rubber sheeting, each upper shell portion comprising an upwardly extending collar around which an aperture of the rubber sheeting is tightly fitted.

27. Method of forming a flexible blanket substantially as hereinbefore described with reference to the accompanying drawings.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS.

1. A shell for use in forming a segment of a flexible blanket of a kind in which a plurality of segments are arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least at a median zone of their height dimension and each segment being connected to each adjacent segment in the region of their median zones such that the segments are held substantially contiguously but are free to pivot relative to one another about the interconnections, said shell comprising two shell portions for joining one to the other at a joint line which includes regions of the median zone through which extends rope means for forming an interconnection between neighbouring segments of a blanket and the two shell portions of a shell comprising confronting edge formations adapted for interlocking with one another in a snap-fit manner to resist separation of the portions when interlocked.

2. A shell according to claim 1 wherein said confronting edge formations are of identical form.

3. A shell according to any one of the preceding claims wherein at least one of the shell portions of a shell is of a generally tapered form whereby one portion may be nested within another shell portion.

4. A shell according to claim 3 wherein the shell comprises two shell portions each of a generally tapered form whereby one portion may be nested within the other portion.

5. A shell according to claim 3 wherein a shell portion has a rim region and is shaped such that when two of said shell portions are arranged in a nested configuration the rim regions of the two shell portions lie in contact with one another.

6. A shell according to claim 5 wherein said rim region is a radially outer rim region of a shell portion.

7. A shell according to claim 5 or claim 6 wherein only said rim region of a shell portion contacts another of said shell portions when said two shell portions lie in a nested configuration.

8. A shell according to any one of the preceding claims wherein said two shell portions are of substantially identical construction.

9. A shell according to any one of the preceding claims wherein one shell portion is formed with a closed end and the other with an open end to permit a pair of assembled shell portions to be filled with concrete.

10. A shell according to any one of the preceding claims wherein regions of confronting edge formations of the shell portion defining a median zone aperture for rope means lie at small angles of inclination to a plane perpendicular to that of the normal direction of the length of rope means passing through the aperture.

11. A shell according to claim 10 wherein said regions adopt an overlapped configuration in the region of the median zone aperture.

12. A shell according to any one of the preceding claims wherein as considered in a cross-sectional plane perpendicular to the wall of a shell portion, the edge of shell portion material defining an aperture for rope means is rounded.

13. A shell according to any one of the preceding claims wherein an edge portion of a shell portion for confronting another shell portion is shaped to provide an outwardly extending rib which extends around the shell at the median zone thereof.

14. A shell according to claim 13 wherein cut-outs for accommodating at least a portion of rope means extending through the shell lie inwards of said rib portion.

15. A shell according to any one of the preceding claims wherein median zone apertures for accommodating ropes to extend through the shell are formed by openended cut-outs in at least one of the shell portions.

16. A shell according to any one of the preceding claims wherein the shell portions are provided with alignment marks for correct orientation prior to joining together.

17. A shell according to any one of the preceding claims and which is a plastics shell formed by injection moulding.

18. A shell constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

19. A flexible blanket comprising a plurality of segments arranged in one or more layers each one segment in height, the segments of the or each layer being disposed substantially contiguously at least in the median zone of their height dimension and each segment being flexibly interconnected to each adjacent segment in the region of their respective median zones such that the segments are held substantially contiguously but are free to pivot one relative to another about the interconnections, each segment comprising a shell according to any one of the preceding claims and each shell being filled with a dense material.

20. A flexible blanket constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

21. A method for forming a flexible blanket according to claim 19 or claim 20 in which a plurality of first shell portions are laid in an array, each shell portion having uppermost an edge region formed with a plurality of open-ended cut-outs for accommodating rope means, arranging rope means to extend through aligned cutouts of the shell portions of the assembled array, snap-fitting a second shell portion to each lower shell portion, said second shell portion having an open end opposite its end for interengaging with the lower shell portion, and filling each shell with settable material through the open end of the associated upper shell portion.

22. Method according to claim 21 wherein use is made of injection moulded plastics shell portions.

23. Method according to claim 21 or claim 22 wherein a multi-apertured jig member is provided over an array of interconnected upper and lower shell portions to assist filling of those portions with settable compound.

24. Method according to claim 23 wherein use is made of a jig member comprising a layer of reinforced rubber sheeting, each upper shell portion comprising an upwardly extending collar around which an aperture of the rubber sheeting is tightly fitted.