IE43180B1

IE43180B1 - Apparatus for protecting an elongate installation such as a pipeline on the floor of a body of water

Info

Publication number: IE43180B1
Application number: IE2073/76A
Authority: IE
Original assignee: Larsen Ole Fjord
Priority date: 1974-12-30
Filing date: 1976-09-17
Publication date: 1980-12-31
Also published as: IT1075998B; NO754395L; DK591775A; FR2296733A2; IN144156B; NZ179670A; SE7514652L; IE43180L; FR2296733B2; NL7515045A

Abstract

The system if for depositing and protecting sediment on the bed of a water-course etc, using an oblong plate of stiff or flexible material placed on the bed. The plate (2) has side sections running in the lengthwise direction and whose upper surfaces diverge from each other in the direction of the bed at a gradient between 1:2 and 1:5 to the horizontal, means being provided to hold them in position. The equipment has perforated portions. The plate can be of impermeable material, the perforations or openings being obtained by placing the side sections adjacent to each other with spaces in between.

Description

The invention relates to apparatus for protecting an elongate installation such as a pipeline, on the floor of a body of water.

According to this invention, I propose, apparatus for protecting an elongate installation such as pipeline, on the floor of a body of water, which comprises an elongate structure which is adapted to be fitted, over the installation, and has two longitudinally extending side portions the upper surfaces of which, in use, slope downwardly to a level below the central portion to form a ridgelike structure, said structure being so heavy that no anchoring of the structure is necessary to retain the apparatus in position. Other features of this invention are defined in the appendent claims.

The ridge-like structure traps the drifting sediment and protects it from scouring. Thereafter deposition of sediment on top and along both sides of the structure gradually builds up a shoal extending away from the device in all directions. This shoal refracts the waves and spreads their ene rgy. 2. 318 οTo protect lengthy hydraulic structures such as submarine pipelines, cables, or foundations fromundermining ιhe apparatus is placed on top of the structure to be secured· and thereby causes -n accumulation of sediment along hoth sides of the structure.

Embodiments of the invention are described below.with reference to the accompanying drawings of which: Fig. 1 is a cross-section of one embodiment according to the invention the structure; it is tiade of concrete and stiffened by ties 45; Fig. 2 is a plan view of a concrete structure primarily for protection of cables or small diameter pipelines; it is provided with holes 68 at the top; Fig. 3 is a large-scale cross-section taken along the line XII-XII of Fig. 2; Fig. 4 is a large-scale cross-sect ion taken along the line XIJI-XIII of Fig. 2; Fig. 5 is a large-scale cross-section taken- along the line XIV-XIV of Fig. 2; Fig. 6 is a plan view of a pipeline 15 protected by concrete elements 61 or 62 which contain channels 58 and are supported on the pipeline by means of clamps 59. The edges of the elements are formed with tongues and grooves 66, so that Lhe elements form a continuous, coherent sheet on boLh sides of Lhe pipeline; Fig. 7 and Fig. 8 are alternative cross-sections- taken along the line XV-XV of Fig. 6.

The ridge formed by the meeting of the two side portions should be rounded (Figs. 1, 3, 7, 8). The two side portions should not be too steep. 3.

To obtain the desired deposition and to prevent too over straining the structure, the surface of the side portions should not he steeper than between 1:2 and 1:5 preferably between 1:4—1:3. The angle of slope may decrease gradually from the middle toward the edges of structure to prevent local scour at the edges (Fig. 1).

The structure may be made of flexible or rigid material or may include both. Rigid materials such as concrete, steel aluminium, glassfibre-reinforced polyester or polyethylene are preferred where the vertical component of the orbital motion of the waves is significant. To increase the stiffness and strength, sheet material may he corrugated. The corrugation may be orientated parallel with or perpendicular to the lateral direction of the structure. To avoid creation of turbulence, the surface of the top of the sheet should he even, without corrugation.

Suitable flexible materials are for example polyethylene, polyester, nylon or natural fibres such as jute and sisal.

Depending on the purpose of the structure and the prevailing conditions which are expected to prevail, it may be impermeable, it may have apertures located at the top of the ridge (Fig. 1), or a part of or all of the surface of the structure may be perforated evenly or differentially.

The structure may consist of two or more separate layers of per orated sheet material. If the percentage area of the apertures is rai1 er small, the upward flow through the apertures at the leeside of the dev ce hereby can be increased. In such cases the upper layer(s) may be very flexible and a little wider or longer than the lower layer and be fastened to this in certain points only, so that it can rise a little distance above the lower layer for upward flow. If, on the otherhand, the sheet consists of open net material so that it cannot retain the particles of the sediment, the extra, larger layer(s) of net may be attached at certain points to the underside of the upper, stretched layer of net, and preferably be made of buoyant material. The upward flow will press the lower layer(s) against the upper layer, whereby the sediment will be prevented from passing through the nets. The same effect as obtained with such extra layers of net can be achieved by a sheet consisting of open net that is supplied on its upper- respectively underside with strips of buoyant, flexible material attached by one end to the surface.

To prevent local scour under the edges of the device, the percentage area of the perforation may increase gradually from the ridge toward the edges of the device. In practice such gradual differentiation may be replaced by joining several layers of perforated sheets together and stepping down the number of layers toward the edges.

An opening along the ridge may be the only aperture of the device. It may be obtained by spacing the two interconnected side portions apart.

To prevent the coarser part of the sediment from becoming conducted up through the apertures, these may be covered with strainers made, for instance, of nylon mesh.

Wiιh the aims oi increasing the rate of deposition of sediment and of preventing overloading, of the apparatus due to the impact of waves or due to deposition of sediment on top bf the apparatus, the Sheet may be formed in special ways. The apertures may be provided with sheltering pockets which prevent the current above the apparatus from passing dtSwn through the apertures, but allow for falling down of sediment deposited on the device. The pockets, made of flexible or stiff material, may be attached to the surface or be formed in the sheet itself by slitting and folding it up like the raised teeth of a shredder, if necessary stiffened by attached forms. . 313° The pockets may be placed on Lho top side and/or the underside of the sheet.

The build up underneath the device may be accelerated by a successive regulation of the effective flow area of the perforation. For example, it may he appropriate to start with a comparatively large flow area at the top of the device, allowing for a filling up of the hollow underneath the device within a minimum of time. When the hollow is almost filled, part of the apertures may be closed - e.g. by means of strips of flexible or stiff material attachable to the surface of the sheet - so that the deposition .thereafter can grow all the way up to the underside of the sheet.

The join of the widths of material making up a flexible sheet may be made in several ways. In cases of staying of the sheet by means of rigid longitudinal and/or cross-stays for prevention of flapping up and down of the sheet, the stays may be placed in sleeves in the sheet material, or they may be formed as assembling bars joining neighbouring widths together. For this purpose two opposite sides of each stay may contain a groove which embraces the bended edge or a bead moulded in the edge of the width of sheet material.

A flexible sheet may be supported at its middle by inflatable spheres or by a continuous inflatable hose, allowing for a convenient way of installing the system and maintain a perfect stretching of the sheet under all conditions.

Structure protecting submarine installations such as pipelines, cables, foundations, and the like, may be supported directly on the installation by spheres or by a continuous hose.

Tiie side portions may bo supported on lattice girders made of sich materials as steel, aluminium or glassfibre-reinforced polyester. The 6. three points of a skeleton consisting of three rigid members are connected by a rope on which the sheet is supported. By the use of rope instead of rigid members, bending forces are avoided.

In systems protecting submarine structures the girders may consist of two symmetrical halves supported at the middle of the structure or on each other a distance above the structure.

In cases where the bending moments are not too great, the lattice girders may be replaced by beams but where the vertical motions of the water are negligible, e.g. at great depths of water, the side portions may need no support.

Heavy structures, especially those made of concrete normally need no anchoring. Figs. 2 to 5 shows a structure suitable for the protection of cables and small diameter pipeline and which are easily handled by the divers. To enhance the stability of these structures the component elements may be interlocked by means of tilted end surfaces 19-22 and/or tongues 23 and grooves 24.

Larger devices of concrete, Figs. 6 to 7, require utilization of buoyancy to enable the divers to handle them under water. Appropriately, each element 61 or 62 contains a system of interconnected air-filled channels 58 making up such portion of the total volume that the element is weightless under water. If necessary, lightweight concrete may be used. During the installation, the channels 58 are closed by a valve or a cork closing the one or two holes connecting outwardly the system of channels. When the elements are in place, the valves are opened, respectively thel corks removed, so that the channels become filled with water.

Alternatively, water-absorbent lightweight concrete with or without channels may be used. For instance, a surface of the structure may be 7. 4319° of water-tight concrete, whereas the interior may be of lightweight concrete that is only connected outwardly through one or two openings that can be closed. Alternatively, the structure may consist exclusively of absorbent lightweight concrete, the surface of which during the installation is covered by a detachable sheet of impermeable plastic.

The structure (e.g. as shown in Fig. 1) may be assembled in complete sections on the beach and thereafter rolled, e.g. on inflatable plastic rollers, and floated on the rollers to the site where the system is to be installed. Here the rollers are detached, and the section sinks to the bottom.

Besides accumulation of sediment around a submarine installation, the object of a system often is to prevent ships' anchors, dragnets, etc., from catching hold of and damaging the structure. A structure such as shown in Figs. 6 to 8 is ideal in this respect. Even if an anchor should catch hold of the lower edge of an element and even if it should be able to remove the coherent assembly of elements, the upper edge of the element Would rise, thereby raising the anchor-chain and the anchor-shank so that the anchor in any case would slip over the pipeline.

To cause the anchor-blade or arms to he lifted as high above the pipeline as possible, projections 69 on the underside of the element may bi appropriate. When a removed element slides across the pipeline, and Li.e projection 69 reaches the pipeline, the edge of the element carrying the anchor will be lifted further upwards, so that the blade or arms of the anchor are prevented from damaging ihe pipeline. The projections 69 may be pipes embedded in the concrete or mounted through holes in the concrete after positioning of the element.

Alternatively, such lifting of the anchor-blade or arms may be brought about by corresponding projections 70 and 71. When a removed element 8. or 62 slides across the pipeline, the projections 70 will reach the pipeline and then cause the element to rotate around the centerline of the pipeline until the former upper edge of the element reaches the surface of the element on the other side of the pipeline. Located properly in relation to 70, the projections 71 will now catch hold of the former upper edge of the removed element and cause the element to turn about this edge, thereby lifting even higher the anchor carried by the former lower edge of the element.

A smaller device as shown in Figs. 2 to 5 may turn about the edge opposite die edge caught by the anchor or fishing gear and· thereby lift the anchor-shank. To call forth such turning, the element should he provided with downward projections to ensure sufficient resistance 'from the bed sediment. Appropriately, such projections may be obtained by corrugating the lower edge in both sides of the element, the wave of the corrugations being perpendicular to tha pipeline and the waveheight gradually decreasing from maximum at each edge to zero along the ridge of the element. Alternatively, the underside of the element may be provided with projections descending into the bed sediment, or the edges may end as vertical skirts, or the element may include inclined or vertical pins 69 embedded in the element or stuck into the seabed through holes in the element after positioning of this.

In many cases the elongate installation needs protection on one sile only, so that the exposed side of the installation is protected by the eti responding half of the symmetrical system concerned. For example, a pi 't'line crossing a river only needs protection on its upstream side, if the current is always uni-directional, and the wave action is inessential. Similarly, a steep seawall, mole or quay may be protected from the undermining effect of waves and currents by such sheet.

Claims

1. Apparatus for protecting an elongate installation such as a pipeline, on the floor of a body of water, comprising an elongate structure which is adapted to be fitted over the installation, and has two longitudinally extending side portions the upper surface of which, in use, slope downwardly to a level below the central portion to form a ridge-like structure said structure being so heavy that no anchoring of the structure is necessary to retain the apparatus in position.

2. Apparatus according to Claim 1, wherein the central portion is rounded.

3. Apparatus according to Claim 1 or 2, wherein the side portions slope downwardly at between 1:2 and 1:5 to the horizontal.

4. Apparatus according to any preceding claim, wherein the side portions slope downwardly at an angle which decreases towards the outer edge thereof.

5. Apparatus according to any preceding claim, wherein the undersides of the side portions are horizontal or nearly horizontal,

6. - Apparatus according to any preceding claim, wherein the structure is divided info symmetrical halves which are hinged together along the longitudinal centreline of the structure.

7. Apparatus according to any preceding claim, wherein in use, the structure is permeable to a vertical flow of water.

8. Apparatus according to Claim 7, wherein the centre portion of thi- structure contains a longitudinally extending slit providing a control opening for the vertical flow.

9. Apparatus according to Claim 7 or Claim 8, wherein at least part of the structure is perforated by apertures. 10.

10. /ipparatus according to any preceding claim, wherein the structure is corrugated.

11. Apparatus according to Claim 10, wherein the corrugations are perpendicular to the longitudinal direction and wherein the height of the corrugations decreases gradually from a maximum at the lateral edges to zero toward the centerline of the ridge-like structure.

12. Apparatus according to any preceding claim, wherein the structure comprises a composite of flexible and rigid members.

13. Apparatus according to Claim 12, wherein the flexible members consist of one or more, separate or joined layers of plastic mesh.

14. Apparatus according to Claim 13, wherein flexible strips of buoyant tnalerial are attached by one end (o the surface of lhe mesh.

15. Apparatus according to Claim 13, wherein the flexible members are supported on a framework of longitudinally and transversely extending beams spanning the structure, or divided into symmetrical halves hinged together along the centerline of the structure.

16. Apparatus according to any preceding claim, wherein the structure consists of a grid with vertical and/or slanting sidewalls.

17. Apparatus according to any preceding claim, wherein the undersides of the side portions of the structure have projections. ,

18. Apparatus according to any preceding claim, where the structure f, contains cavities for buoyancy purposes.

19. Apparatus according to Claim 18, wherein the content of water £’ or air of the cavities is regulated by corks or valves.

20. Apparatus according to any preceding claim, wherein at last J part of Lhe structure is formed of concrete. ’ , £ ,· i 4 ' >,.< -’ > 1 , « 11. -* · V* * «7 rf

21. Apparatus for protecting an elongate installation, such as a pipeline, on lhe floor of a body of water constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.