WO2001088277A1

WO2001088277A1 - An underwater covering

Info

Publication number: WO2001088277A1
Application number: PCT/GB2001/002171
Authority: WO
Inventors: Steven Howlett
Original assignee: PIPESHIELD SERVICES Ltd
Current assignee: PIPESHIELD SERVICES Ltd
Priority date: 2000-05-18
Filing date: 2001-05-18
Publication date: 2001-11-22
Anticipated expiration: 2002-11-18
Also published as: GB0229422D0; GB2379472A; GB2379472B; AU2001256523A1

Abstract

A mattress (10), for use in underwater environments to cover and protect structures such as pipelines, comprises a number of blocks, linked together flexibly. At least one edge block (70) has a leading surface which comprises a substantially wedge-shaped flow diverting portion (74) and a flow disrupting portion (73). The flow disrupting portion (73) links the upper surface (71) of the block (70) with the flow diverting portion (74). The blocks can be linked together to form a rectangular array. The flow diverting portion (74) can have a profiled apex (77), such a curve inducing turbulence in water flow across the upper surface of the block (70). The angled subtended between the flowing diverting portion (74) and the base surface (72) of the block (70) is from 20-45°.

Description

AN UNDERWATER COVERING

Field of the Invention

The present invention relates to a protecting covering such as a mattress for use in covering and protecting underwater structures, particularly pipeline section and couplings. The invention more particularly relates to a profiled edge block for use as part of such a mattress.

Background to the Invention

Protective coverings such as mattresses are widely used in sub sea environments to protect pipelines, cables, umbilicals and other structures from damage,, for example due to debris falling from above. The mattresses typically comprise individual substantially cuboidal or rectangular cuboidal concrete blocks held together by rope made of, for example, polypropylene. The mattresses are therefore sufficiently flexible to fold or drape over the structure which is to be protected.

Apart from the mass of the mattress, no additional means are normally employed to ensure that it remains in position. The strength of water currents on or in the region of the seabed acting on the mattress are, however, sufficient to produce lateral and lifting forces on the blanket causing the blanket to move from its desired position. Once a mattress has been lifted, the individual blocks can lock together to form a more rigid mattress configuration. In the more rigid configuration, the mattress presents a more rigid surface to the water current and is therefore more susceptible to being moved by the current. The problem of drag and lift of the mattress has been addressed in the prior art but with limited success. For example, tapered edge blocks which present a streamlined profile to a water current have been employed to reduce drag forces by reducing turbulent flow.

The inhibition of turbulent flow can however exacerbate the lifting problem, as^λ laminar flow across the upper surface is enhanced. The laminar flow can cause lifting forces - often referred to as Bernoulli forces - which act on the blanket causing it to lift and subsequently move. Furthermore, tapered edge blocks are sensitive to changes in water current direction which individually makes their resistance to lift variable. Moreover, when the leading edge of a block is lifted the rotational axis for that block is its trailing edge. Tapered blocks therefore also have the disadvantage in that their centre of gravity lies closer to the trailing edge than normal untapered blocks. Hence the moment of the centre of gravity about the trailing edge, in order to restore or counteract any lifting force, is reduced.

A known solution to lifting blocks is to use denser concrete to form the individual blocks and therefore increase the overall mass of the mattress. Whilst the increased mass stabilises the mattress, the higher density concrete required is more expensive than normal concrete.

It is an object of the present invention to provide a mattress for use in protecting underwater structures having increased resistance to lifting and lateral motion. It is a further object of the present invention to provide an edge block for use as part of such underwater mattresses.

Summary of the Invention

According to the invention there is provided a mattress for use in underwater environments, the mattress comprising a plurality of blocks linked together by linkage means, wherein one or more edge blocks of the mattress has a leading surface comprising a flow diverting portion and a flow disrupting portion, the flow diverting portion being substantially wedge-shaped portion and being joined along a first edge to the in use upper surface of the edge block and along a second edge to the long edge of the wedge of the flow diverting portion. The apex of the wedge-shaped flow disrupting portion is advantageously curved, the curved apex acting to reduce lift on the edge block caused by water flow.

The blocks are conveniently arrayed in rows forming a rectangular mattress, each row being linked to its neighbour.

The linkage means is preferably a rope or cable set into each block, to maintain the individual blocks in a flexible relationship within the mattress, but to prevent them from sliding along the linkage means.

The wedge-shape comprises a base surface, and an in-use surface, angled to divert water, and sides linking the base to the upper in-use surface. The upper in-use surface preferably subtends an angle of 20-45° with the base of the edge block, to decrease drag forces on the mattress.

The flow disrupting surface of the edge block is preferably planar. The flow disrupting surface is advantageously at right angles to the in use upper surface of the block. The flow disrupting surface causes turbulence to disrupt laminar flow across the block's upper surface.

The width of the flow disrupting surface is approximately 1/7 - 1/5 that of the total thickness of an edge block, the width being insufficient to give rise to drag forces which would affect the stability of the edge block.

The radius of curvature of the flow diverting portion is preferably from 25 - 100mm. It is particularly preferably from 40 - 60mm to minimise drag forces and introduce turbulence into the laminar flow of the current. The curvature can advantageously vary across the flow disrupting surface. The surface of the flow diverting portion connecting its apex with the flow disrupting surface is optionally flat. Alternatively it can have a positive outward curvature to further disrupt laminar flow.

The trailing surface of the edge block is preferably curved. It is optionally N- shaped, with the base of the N projecting away from the edge block. The curvature of the flow diverting portion can be continuous across its length. Alternatively, it may vary along the length of the surface.

The invention includes within its scope a profiled edge block substantially as herein described for use as part of an underwater mattress.

Brief Description of the Drawings

The present invention will now be described more particularly with reference to the accompanying drawings which show, by way of example only, five embodiments of underwater mattresses and six embodiments of edge blocks for use as a constituent thereof. In the drawings,

Figures la and lb to 4a and 4b are plan views and corresponding sectional elevations of four embodiments of underwater mattresses in accordance with the invention;

Figure 5 is an isometric view of a first embodiment of edge blocks of a mattress;

Figures 5a and 5b are, respectively, a plan view and corresponding sectional view of a fifth embodiment of an underwater mattress;

Figure 6 is a section through a second embodiment of an edge block;

Figures 7-9 are further sections through further embodiments edge blocks;

Figure 10 is a plan view of a sixth embodiment of an edge block;

Figure 11 is a section through a block according to the prior art; and

Figure 12 is a section through a tapered block according to the prior art.

Detailed Description of the Invention

Referring initially to Figures 1A and IB, a mattress 10 comprises a number of profiled concrete blocks 11 arranged in rows in the centre of the mattress 10. Smaller blocks 12 are incorporated into alternate rows to offset the position of blocks 11 in adjacent rows. Profiled concrete edge blocks 13, 14 are secured to the outer rows of blocks 11 with the tapered surface 15 of the block 11 facing away from the main body of the mattress 10.

The concrete blocks 11-14 are held together by a polypropylene rope 16, 17 which passes through and is set into the blocks 11-14. As the blocks 11 are not rigidly held together, the mattress 10 possesses a degree of flexibility which enables it to bend to follow the contours of the structure being protected.

Figures 2A, 2B and 4A and 4B illustrate rectangular mattresses 20 having edge blocks 21, 22 on two opposing sides only. The blocks are located along the long edges of the mattress 20. Such a mattress would be suitable for protecting an elongate structure such as a pipe carrying, for example, gas or oil.

Figures 3 A, 3B illustrate a rectangular mattress 30 having edge blocks 31, 32 along only the two shorter sides of the rectangular mattress 30.

Figure 5 illustrates the spatial relationship between the edge blocks 50, 51 and also a corner block 52. The corner block has two notches 53, 54 and two flow diverting surfaces 55, 56. The edge blocks 50 differ from the edge block 51 merely in that they are of greater width. A notch 57 of the edge block 50 is a flat planar surface having one edge 58 common with the upper surface 59 of the edge block 50, the planar surface of the notch 57 being at right angles to the upper surface 59. The flow diverting surface 60 connecting the leading edge 61 of the edge block 50 and the notch 57 is outwardly convex.

Figures 5 A and 5B illustrate a mattress 40 having corner blocks 41 having two notches, in accordance with the corner block 52 shown in Figure 5.

The side walls 62, 63 and rear walls 64 each comprise two planar, outwardly extending portions which meet along a common edge 65, the shape formed giving the edge block a more hydrodynamic profile. Figure 6 illustrates the profile of an edge block 70 in detail. The edge block 70 has an upper surface 71 and a lower surface 72. The edge block 70 further comprises a notch 73, substantially at right angles to the upper surface 71. A flow diverting surface 74, which connects the notch 73 with a leading edge 75 of the edge block 70, has a planar portion 76 and a curved nose section 77. A polypropylene rope 78 is located within the edge block 70 and secures the edge block 70 to its nearest neighbours. The rear surface 79 connecting the upper surface 71 with a trailing edge 80 is formed from two planar portions 81, 82 which unite to form an apex 83.

Figures 7-9 illustrate the shapes of edge blocks of different thicknesses. For example, the edge block 90 illustrated in Figure 7 is typical of an edge block having a thickness of approximately 450mm. In Figure 8, is shown an edge block 100 having a thickness of approximately 300mm, and in Figure 9 the edge block 110 is typical of an edge block of approximate thickness 150mm.

In Figure 10, the edge block 120 has sides 121, 122 and a rear surface 123 which slopes outwardly away from the upper surface 124 and from the flow diverting surface 125, to give the edge block a more hydrodynamic shape.

In use, the mattress is laid over the structure to be protected with an edge having a profiled edge block facing towards the flow of current (indicated by "E" in Figure 1), the edge block or blocks resting on the sea or river bed. The tapered profile of the block reduce the drag force on the block, and hence the mattress by diverting water flow upwards and over the top surface of the mattress. The water flow at this point is substantially laminar and would normally, due to Bernoulli forces, cause lifting forces to act on the upper surface of the block, should it be able to continue flowing over that surface. However the flow of the water over the upper surface is disrupted by the presence of the notch which introduces boundary layer separation and flow reversal thus causing turbulence in the water flow, and reducing the Bernoulli effect. Together therefore, the tapered surface and the notch first divert and then reduce two of the forces which would cause the mattress to move from its desired position.

Furthermore, where the flow diverting surface has a curved surface, this further disrupts the laminar flow of the water passing over the block, which again reduces the lift force on the block. Disruption of the laminar flow is enhanced by the edge blocks having a nose portion 77, as shown in Figure 6, of high curvature. The nose portion causes a reduction of both lift and drag forces on the block.

The provision of a rounded nose portion and a notch also causes the centre of gravity of the edge block to move away from the trailing edge 80 towards the leading edge 75 of the edge block. The turning moment of the centre of gravity about the trailing edge 80 is thus increased in comparison with, for example a tapered edge block and the greater turning moment acts against edge up lift of the edge block.

The notch, as illustrated, is substantially planar. The notch can, where appropriate however, have an outwardly convex or a concave shape to further disrupt flow. The width of the notch is typically 1/7 to 1/5 of the total width of the block.

The flow diverting surface can be substantially planar. Alternatively, the flow diverting surface can have an outwardly convex surface. The angle subtended by the flow diverting surface with the lower surface 72 can be from 20-45°, with an angle of 30° being preferred.

Where the apex of the wedge-shaped flow diverting portion is curved, the radius of curvature can be from 25- 100mm. A radius of curvature of 40-60mm typically causes a minimisation of drag forces whilst introducing turbulence into the laminar flow.

An assessment and comparison of the stability of edge blocks towards current flow can be made by calculating the stability factor, Sf. The stability factor, S_f, is defined as the ratio of submerged weight per meter width of the edge block to the lift force at unit current flow, and a higher number indicates that an edge block is more stable.

For an edge block, according to the prior art, having the shape shown in Figure 11, the submerged weight per meter width is 0.049, and the lift force 0.08 which gives an S_f of 0.61. For a straight taper edge block, according to the prior art, as shown in Figure 12, the submerged weight per meter width is 0.064, and the lift force 0.08 which gives an S_f of 0.8.

For the edge block 70 according to the invention, shown in Figure 6 the submerged weight per meter width is 0.09, and the lift force 0.06, giving an S_f of 1.5.

The stability of the edge block 70 is therefore greater than for standard or tapered edge blocks. The greater stability enables edge blocks to be formed from less dense concrete than usual. As denser concrete is more expensive than the more usual standard concrete, the edge blocks can be made more cheaply but with the same performance.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.

Claims

1. A mattress (10) for use in underwater environments, the mattress (10) comprising a plurality of blocks linked together by linkage means (16), wherein one or more edge blocks (70) of the mattress (10) has a leading surface comprising a flow diverting portion (74) and a flow disrupting portion (73), the flow diverting portion (74) being substantially wedge- shaped, the flow disrupting portion (73) being joined along a first edge to the in-use upper surface (71) of the edge block (70) and along a second edge to the long edge of the wedge of the flow diverting portion (74).

2. A mattress (10) according to Claim 1, wherein the blocks are arrayed in rows forming a rectangular mattress (10), each row being linked to its neighbour.

3. A mattress (10) according to Claim 1 or Claim 2, wherein the linkage means (16) is a rope or cable set into each block, to maintain the individual blocks in a flexible relationship within the mattress (10).

4. A mattress (10) according to any preceding claim, wherein the wedge- shaped flow diverting portion (74) comprises a base surface (72), an upper surface (55), angled to divert water, and sides linking the base surface (72) to the upper surface (55).

5. A mattress (10) according to any preceding claim, wherein the upper surface (55) of the flow diverting portion (74) subtends an angle of 20-45° with the base surface (72) of the edge block (70).

6. A mattress (10) according to Claim 5, wherein the upper surface (55) of the flow diverting portion (74) subtends an angle of 30° with the base surface (72) of the edge block (70).

7. A mattress (10) according to any preceding claim, wherein the flow disrupting portion (73) of the edge block (70) is planar.

8. A mattress (10) according to any preceding claim, wherein the flow disrupting portion (73) is at right angles to the upper surface (71) of the edge block (70).

9. A mattress (10) according to any preceding claim, wherein the width of the flow disrupting portion (73) is approximately 1/7 - 1/5 that of the total thickness of an edge block (70).

10. A mattress (10) according to any preceding claim, wherein the apex (77) of the wedge-shaped flow diverting portion (74) is curved, the curved apex (77) acting to reduce lift on the edge block caused by water flow.

11. A mattress (10) according to Claim 10, wherein the radius of curvature of the apex (77) is from 25 - 100mm.

12. A mattress (10) according to Claim 11, wherein the radius of curvature of the apex (77) is from 40 - 60mm.

13. A mattress (10) according to any preceding claim, wherein the surface (76) of the flow diverting portion (74) connecting its apex (77) with the flow disrupting portion (73) is flat.

14. A mattress (10) according to any one of Claims 1 to 12, wherein the surface (76) of the flow diverting portion (74) connecting its apex (77) with the flow disrupting portion (73) has a positive outward curvature to further disrupt laminar flow.

15. A mattress (10) according to any one of Claims 1 to 12, wherein the curvature of the surface (55) of the flow diverting portion (74) varies across the surface (55) of the flow diverting portion (74).

16. A mattress (10) according to any preceding claim, wherein the trailing surface (79) of the edge block (70) is curved.

17. A mattress (10) according to any one of Claims 1 to 15, wherein the trailing surface (79) of the edge block (70) is "V"-shaped, with the base of the "V" (83) projecting away from the edge block (70).

18. A profiled edge block (70), for use as part of an underwater mattress, the profiled block being substantially as herein described with reference to the accompanying drawings.