- 1 A BLOCK FOR AN UDERWATER MATTRESS Technical Field 5 This specification discloses a block for an underwater mattress and an underwater mattress composed of a plurality of such blocks. The underwater mattress is of a type used to support a submerged conduit lying on an underwater floor. Background Art 10 Submerged conduits are used for many and varied purposes including to: convey fluids such as but not limited to oil and gas; and, carry communication channels such as metallic cables and optical fibres. The conduits are laid on an underwater floor such as a seabed. Action of water and water currents flowing adjacent to the conduit can 15 cause scouring of the floor leading to the creation of hollows in the floor beneath the conduit. This creates free spans in the conduit. Gravity and water pressure may act to push or bend the conduit into the hollow and currents may cause vortex induced vibrations. 20 In order to minimise or prevent scouring a mattress made of concrete blocks may be laid on the underwater floor to provide a base or support for an overlying conduit. The mattress is often comprises a plurality of square prism concrete blocks that are joined together by embedded cables or rods. 25 Summary of the Disclosure In one aspect there is disclosed a block for an underwater concrete mattress comprising: an outer housing made of a plastics material; and 30 a body contained within the housing comprising a settable material. In a second aspect there is disclosed block for an underwater mattress comprising: an outer housing made of a plastics material; and a body contained within the housing comprising a settable material; 35 wherein one or more outer surface portions of the housing are formed with different friction coefficients. 4254118_2 (GHMatters) P93439.AU 19/04/13 -2 In a third aspect there is disclosed block for an underwater mattress comprising: an outer housing made of a plastics material; and a body contained within the housing comprising a settable material; wherein the block has a substantially rectangular footprint of dimension X by Y mm 5 wherein Y > X. In one embodiment of the first or second aspects the block has a substantially rectangular footprint of dimension X by Y mm wherein Y > X. 10 In one embodiment Y = L.X wherein I is an integer > 2. In one embodiment X = 500mm. In one embodiment of the first or third aspect the block one or more outer surface 15 portions of the housing are formed with different friction coefficients. In one embodiment of any aspect a first portion of the outer surface that in use contacts an underlying floor or surface is formed with a higher friction co-efficient than a diametrically opposed second portion of the outer surface. 20 In one embodiment of any aspect the outer housing forms a mould into which the settable material which comprises the body can be poured. In one embodiment the outer housing comprises a plurality of keys configured and 25 located such that the keys are embedded in the settable material to effect attachment of the outer housing to the body. In one embodiment at least one of the keys is arranged to space one or more cables from an internal surface of the outer housing wherein settable material poured into the 30 housing can surround one or more cables supported on the at least one key. In one embodiment the at least one key is integrally formed with the outer housing. In one embodiment of any aspect the at least one key is in the general configuration of 35 a cross. In one embodiment the housing comprises separate first and second shells which can 4254118_2 (GHMatters) P93439.AU 19/04/13 -3 be brought together to form the mould. In one embodiment first and second shells have a peripheral face arranged to contact each other when the mould is formed, and wherein the peripheral faces are relatively 5 configured to form a plurality of openings enabling cables to extend through the block and extend from the block. In a fourth aspect there is disclosed a concrete mattress comprising a plurality of blocks in accordance with any one of the first, second or third aspects, wherein the 10 blocks are arranged in a planar array and coupled together by a plurality of cables that extend through the plurality of mutually adjacent blocks and are embedded in the settable material. In one embodiment the plurality of blocks are arranged such that the second portions 15 of their outer surfaces cooperate to form a mattress support surface enabling sliding support of an overlying conduit. This enables the mattress to support a conduit such as a pipeline during sliding which may for example arose due to thermal expansion. To this end the second portions of the outer surfaces of the blocks can be formed with a low coefficient of friction. 20 In one embodiment of the fourth aspect blocks in one row of the planar array are offset relative to blocks in adjacent row of the array. In one embodiment the plurality of blocks comprises blocks of two different footprint 25 dimensions Detailed Description of the Drawings Notwithstanding any other forms which may fall within the scope of the blocks and 30 mattress as set forth in Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 a is a side view of a first embodiment of the disclosed block for an underwater mattress; 35 Figure 1b is a bottom elevation of the block shown in Figure 1 a; Figure 1 c is an end elevation of the block shown in Figure 1 a; Figure 1d is an isometric view of a shell incorporated in the block; 4254118_2 (GHMatters) P93439.AU 19/04/13 -4 Figure 1 e is a partial cutaway view of the block depicted in Figure 1 a; Figure 1f is a top elevation of the block shown in Figure 1a; Figure 2a is a side elevation of a second embodiment of the block; Figure 2b is an end elevation of the second embodiment of the block 5 Figure 3 is an end elevation of third embodiment of the block; Figure 4 is an end elevation of fourth embodiment of the block Figure 5 is an end elevation of a fifth embodiment of the block; Figure 6a is a perspective view of an embodiment of a concrete mattress composed of a plurality of the blocks and supporting a conduit; 10 Figure 6b is a top elevation of the mattress shown in Figure 6a; Figure 6c is a side elevation of the mattress shown in Figure 6a; Figure 6d is an end elevation of the mattress shown in Figure 6a; and, Figure 6e is a view of detail A from Figure 6a. 15 Detailed Description of Preferred Embodiments Figures 1a - 1f depict a block 10 for an underwater mattress M (shown in Figure 6a). The block 10 has an outer housing 12 made of a plastics material. Contained within the housing 12 is a body 14. The body 14 comprises a settable material such as 20 cementitious material including but not limited to concrete. Extending through the block 10 is a plurality of cables 16 that facilitate the flexible coupling of adjacent blocks to form the mattress M. For the sake of clarity, not all of the cables that may be used or incorporated in each block 10 and the mattress M are depicted. 25 The housing 12 forms a mould into which the settable material may be poured to form the body 14. Further the housing 12 in the present embodiment is composed of a plurality, and in this particular instance two, shells 18a and 18b (hereinafter referred to in general as "shells 18"). The shell 18a is a relatively shallow shell and constitutes a bottom of the block 10 when the block 10 is being constructed and in particular when 30 the settable material is being poured into the housing 12. (However as will be explained later, when the mattress M is in use, the portion of the housing 12 comprising the shell 18a may form an upper surface of the mattress M.) The shell 18b is deeper than the shell 18a and of a different shape and configuration. 35 The shell 18b has a generally planar rectangular end wall like structure 20 from which extend four side walls 22. Each side wall 22 extends from a corresponding edge of the structure 20 and diverges at the same angle away from the structure 20. The side 4254118_2 (GHMatters) P93439.AU 19/04/13 -5 walls 22 form a continuous peripheral wall that lead to and terminates in a laterally extending flange 24. The flange 24 is formed with a plurality of semi-cylindrical recesses 26. 5 As shown in Figure 1f, the wall like structure 20 is formed with a plurality of holes 28 through which settable material can be poured into the housing 12. The holes 28 are defined by a plurality of keys 30 in the form of annular and planar webs that create the structure 20. 10 The structure of the shell 18a is shown most clearly in Figure 1d. A shell 18a is in the general form of a shallow rectangular open top box and is formed with a peripheral laterally extending flange 32 formed with a plurality of semi-cylindrical recesses 34. Further keys 36 are formed on an inside surface of the shell 18a. The keys 36 are in the general configuration of a cross (but may be in other configurations such as but not 15 limited to a T shaped section or button) and can conveniently be formed integrally with the shell 18a. The keys 36 serve two purposes. One is to space the cables 16 from the inside surface of the shell 18a so that when settable material is poured into the housing 10 it can surround the cable 16. A further purpose of the keys 36 is to embed in the settable material so that when set the shell 18a is positively locked into and 20 engaged by the body 14. This may be further facilitated by the provision of through holes (not shown) in the keys 36. When the shells 18a and 18b are brought together so that their respective flanges 24 and 32 are in face to face relationship, their respective semi-circular recesses 36 and 25 34 coincide. These recesses together form cylindrical passage ways for the cables 16 to pass through and extend from the block 10. Optionally, the respective flanges 24 and 32 can be connected together by way of for example the use of mechanical fasteners passing through the flanges. This may assist 30 in holding the shells 18 together against pressure exerted by the settable material. However such coupling is not in itself to hold the shells together when in use because the keys 30 and 36 become embedded in the body 14 upon setting of the settable material. 35 When the block 10 is being constructed shell 18a rests on the ground and the openings 28 in the shell 18b are presented for the pouring of the settable material. However when a plurality of such blocks 10 are interconnected to form the mattress M, 4254118_2 (GHMatters) P93439.AU 19/04/13 -6 the mattress M may be laid on a underwater floor in a reverse configuration so that the shell 18b sits on the underwater floor. As the housing 12 is made of a plastics material, it is a relatively easy process to treat 5 or otherwise work or machine the outer surface of the housing 12 to provide prescribed or desired friction coefficients. The block 10 has a maximum foot print of X by Y mm. In particular in this embodiment the footprint is of a rectangular configuration and accordingly X does not equal Y and more particularly X > Y. In one specific embodiment Y = 500mm and X = L.Y where I is an integer > 2. Thus for the situation 10 where I = 2 then X = 1000mm. Other dimensions of the block 10 are as follows: Height H = 200mm Distance D1 = 733mm (being the length of a long side of wall like structure 20) 15 Distance D2 = 231mm (being the length of the short side of wall like structure 20) With such dimensions the volume of the block 10 is approximately 0.525m 3 . 20 It should be understood that a mattress M composed of blocks 10 with the above dimensions will have a greater weight than a mattress M of the same footprint area but with blocks having dimensions X = Y = 500mm. This is because a single block 10 with dimensions 500mm x 1000mm provides a greater volume than two adjacent 500 x 500mm blocks. This arises because there is a space between connected blocks 10 25 and there is a loss of volume due to the tapering of the sides of the blocks. As a result a single block of 500 x 1000mm of the configuration shown in Figures 1 a - 1f will provide greater mass for the same overall footprint area of two adjacent 500 x 500mm blocks. 30 Figures 2a and 2b depicts an embodiment of the block 1 Oa which differs from the block 10 by way of a greater height dimension Ha > H and maintaining the dimensions X=1000mm and Y=500mm. In this instance Ha = 300mm. All of the additional height in the embodiment 1 Oa is accommodated by way of the shell 18a' which is higher or deeper than the corresponding shell 18a of the block 10. 35 The height H of the block 10 can be varied from the above embodiments to suit a particular application. For example the height may be 400mm or 500mm. The 4254118_2 (GHMatters) P93439.AU 19/04/13 -7 variation in height can be accommodated by varying the height of either one or both shells 18a and 18b. The heights of the shells 18a and 18b may be the same or different to each other. For example Figure 3 depicts a block 1 Ob which has the same height Ha as the block 1 Oa but where the increase in height from H to Ha is 5 accommodated solely by the shell 18b', with the height of shell 18a being unchanged from block 10 to block 1Ob.. . Figure 4 depicts an embodiment of a block 1Oc having a height dimension Hc > Ha, for example Hc = 400mm. In this embodiment the shell 18a2 of the same height as shell 10 18a' but is of a shorter height than the shell 18b2. Figure 5 depicts a further embodiment of a block 1Od having a height dimension Hd > Hc, for example Hd= 500mm. Here each of the shells 18a3 and 18b3 is of the same height or depth. 15 In the blocks 1Ob, 1Oc and 1Od shown in Figures, 3, 4 and 5 respectively, the footprint of the shells 18a' ,18a1, 18a2 and 18a3 is the same as that for block 10 and block 1Oa, namely X = 1000mm Y = 500mm. 20 Figures 6a - 6e depict a mattress M composed of a plurality of rectangular blocks 10, 1Oa, 1Ob or 1Oc and corresponding half blocks 1Oh. The half blocks 1Oh have the same general configuration and height as a corresponding the block 10-1Oc but are formed with a square footprint where X = Y = 500mm. As can be seen, the blocks 10h are provided only at the ends of alternating rows R1 - R5 to accommodate for the 25 staggered or offset nature of the blocks 10-1 Oc in each of the adjacent rows R1 - R5. The use of the blocks 10-1 Oc not only increases the mass of the mattress M in comparison to using say only blocks 1Oh, but also provides increased stiffness due to a lower number of flexible joints provided by the cable 16 interconnecting adjacent blocks 10. Figures 6a - 6e also depict a conduit 40 extending over the mattress M. In 30 the configuration depicted in these figures, the mattress M is orientated so that the shell 18b lies on a surface such as an underwater floor, while the conduit 40 extends across the shells 18a. The surfaces of the shells 18a cooperate to form a support surface of the mattress on which the conduit 40 lies. Further the surfaces of the shells 18a can be formed with a relatively low coefficient friction to assist in accommodating 35 sliding of the conduit 40 which may for example be caused by thermal expansion of the conduit or by other physical changes or events affecting the conduit or indeed the mattress. Conversely the surface of shells 18b which engage the sea bed may be 4254118_2 (GHMatters) P93439.AU 19/04/13 -8 formed with a relatively high coefficient of friction to assist in resisting lateral movement relative to the seabed. Whilst a number of specific embodiments have been described, it should be 5 appreciated that the blocks 10 and mattress M may be embodied in many other forms. Specifically, the blocks 10 may be formed of dimensions and indeed in shapes or configurations other than those depicted. For example the blocks 10 may be formed in a general configuration of a triangular or hexagonal prism, or of other configurations which enable a tessellation. Further, the outer housing 12 may be made from a 10 different number of shells other than the two currently described and depicted. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word 'comprise" and variations such as "comprises" or "comprising" are used in an inclusive 15 sense, i.e. to specify the presence of the stated feature but not to preclude the presence of additional or further features in various embodiments of the block and mattress as disclosed herein. 4254118_2 (GHMatters) P93439.AU 19/04/13