METHOD AND APPARATUS FOR DEPLOYING A STRUCTURE BELOW THE SURFACE OF A BODY OF WATER

Abstract: Flotation means, for example, buoys, are attached to a offshore structure, such as a pipeline under construction. The flotation means move with the structure towards the sea bot­ tom easing the descent. As the structure 1, 30 reaches a predetermined depth, the floating action of the flotation means 2, 35 on the structure is removed causing the structure 1, 30 to be heavier. To remove the floating action of the flotation means 2, the flotation means 2 may be constructed of prebuckled pipe segments 7,8, which collapse at said predetermined wa­ ter depth. In another embodiment of the invention the flotation means 35 are secured to the structure 30 by fastening means 38 comprising a prebuckled pipe segment which collapse at a predetermined water depth thereby releasing the flotation means 35 from the structure 30.

Description: METHOD AND APPARATUS FOR DEPLOYING A STRUCTURE BELOW THE SURFACE OF A BODY OF WATER In subsea construction and repair activities it is often required to apply flotation means, such as buoys, for facilitating subsea emplacement of heavy structures or pieces of equipment. In such a way the effective underwater weight is reduced to a manageable value, minimizing the chances of damage during subsea installation and minimizing surface crane requirements after launching and during lowering and placement of the structures. Connections between the buoys and the structure are often made with bolts, eyes, hooks and other hardware. If it is necessary to release the buoys, this can be done with the direct help of divers, but it is considered safer to release the connections remotely, using explosives or acoustically triggered release mechanisms. Unfortunately, actively triggered mechanisms can be triggered accidentally and can fail to trigger when needed. The use of flotation means is particularly attractive for laying a pipeline in deep water from a laybarge. It has been the general practice in the art to provide stability and support to a pi string during an offshore pipe-laying operation by means of a stinger, which is a mechanical structure extending from a hinged joint at the rear of the barge to or towards the submarine floor. However, a stinger must be quite long for laying a pipeline in deep water, for example 60 to 300 metres or more. As a result such a stinger is expensive, difficult to control, and vulnerable to weather damage. Accordingly, in laying a deep water pipeline, buoys are used as an alternative to stingers for supporting the piR string during the pipelaying operation. U.S. Patent Specification 3,727,417 discloses a system for laying underwater pipelines wherein buoyant spheres are applied connected by cables which are attached to the pipeline at calculated intervals. As the pipeline is laid, it forms a sloping curve, and as the pipeline reaches bottom, the floating action of the spheres on the pipeline is removed by releasing the spheres, which subsequently float to the surface. U.S. Patent Specification No. 3,620,028 discloses a similar system for laying underwater pipelines wherein cable-connected floats are attached to the pipeline at selected intervals; as the pipeline reaches the bottom the floating action of the floats on the pipeline is removed by disengaging the floats, and the floats rise to the surface where they are collected. It is evident that the prior art does not provide a method for removing the floating action of the flotation means on the pipeline which is both inexpensive and fully reliable. However, such a method is provided by the present invention, as will be evident from the following disclosure. A method is provided by the present invention for deploying a structure below the surface of a body of water comprising the sequential steps of attaching flotation means by fastening means to the structure to be deployed; allowing the structure to descend to a critical depth below the surface of the body of water; and removing the floating action of the flotation means on the structure by applying means susceptible to buckling at a pressure occurring at said critical depth. The structure may be a submarine pipeline, or subsea structural elements for oil/gas production or other subsea activities. The present invention further pertains to apparatus for use in the above-mentioned method. In a first embodiment of the invention the apparatus comprises flotation means and fastening means for securing said flotation means to a structure to be deployed, the flotation means comprising a partially buckled hollow vessel susceptible to further buckling at a preselected water pressure. At the preselected water pressure the flotation means will flatten and thus lose most of its buoyancy. The flotation means may be relatively long lengths of thin walled pipe with thicker walled, partially buckled end closures. The flotation means, after collapsing remain on the structure, adding weight to the structure for stability, but minimizing drag forces due to cross currents because of the flattened profile of the flotation means. Such flotation means can be used in offshore oil and gas exploration/ production activities including construction and repair, to trigger any type of process such as detonating an explosive at a predetermined water depth, or in laying a long string of buoyed pipeline while the string is tensioned by a tug or similar tow vessel. As a given segment of the pipeline reaches the predetermined depth, the flotation means attached thereto collapses, causing that segment of pipeline to become heavier. This, in turn, causes the adjacent upper segment of the pipeline to move lower, causing the next flotation means to collapse as the process is repeated continuously while the pipeline is being laid. Alternatively, the flotation means can be contin#us or in long strings. In a second embodiment of the present invention the apparatus for deploying a structure below the surface of a body of water comprises flotation means, fastening means for releasably securing said flotation means to the structure to be deployed, and partially buckled actuating means susceptible to further buckling at a preselected water pressure to selectively release said fastening means. In this embodiment of the invention use is made of a mechanism of external pressure buckling of a pipe or cylindrical container or pressure vessel as a trigger to release the flotation means. One use of the above-mentioned apparatus is to release buoys from a pipeline being deployed in relatively deep water. The buoys can be attached at intervals along the pipeline, and as water is allowed to enter the pipeline, the attached buoys move towards-the--bottom with the pipeline. As a preselected external pressure or water depth is sensed by pressure buckling of the pipe, etc., the trigger is activated and the mechanism releases, separating the buoys from the pipeline as each one in turn reaches the selected (same or different) pressure or water depth. The present invention also pertains to a method for preparing means susceptible to buckling at a preselected pressure. This method comprises denting the mid-section of a cylindrical container, propagating the dent with fluid pressure outside the cylindrical container to reduce the cross-sectional area of the mid-section of at least part of the container without substantially collapsing the container, cutting the container into two parts at the reduced mid-section area, and closing the cut ends of the two parts of the container. The container, such as a capped pipe may be mechanically dented near the centre, placed in a pressure vessel and fluid admitted thereinto to propagate the dent somewhat further by pumping hydraulic fluid or water into the vessel. The pressure required to propagate the dent or buckle is noted since this will be the pressure causing further buckling of the container. The pipe is then cut in half and the ends may be capped or otherwise welded shut. The present invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Figure 1 discloses a pipelay application of the invention; Figure 2 depicts a first alternative of the pipelay application disclosed in Figure 1; Figure 3 depicts a second alternative of the pipelay application disclosed in Figure 1; Figure 4 shows the release mechanism shown in Figure 3 on a larger scale; Figures 5A and 5B show a collar release mechanism; and Figures 6A, 6B and 6C show the steps involved in making means according to the invention, susceptible to buckling at a preselected pressure. In Figure 1 is shown a means for rapidly laying a submarine pipeline with collapsible pipe buoys. It will be understood that other structures may be substituted for the pipeline in practice of the invention. As pipeline 1 passes over the stern of a vessel (not shown), buoys 2 are attached, and the pipeline and buoys descend beneath the water surface 3 to a critical water depth h (designated by a dotted line) where collapse of the buoys occurs. Then the pipeline 1 settles down to the sea floor 5 with attached collapsed buoys 6, without the stresses which might otherwise occur with the use of a stinger for pipe laying. Each pipe buoy 2 has partially collapsed or buckled end closures 7 which are integrally welded or otherwise attached to an open-ended vessel 8 of cylindrical or near cylindrical shape. The open-ended vessel 8 is of relatively long length, e.g., from 1 pipeline diameter to continuous, cf thin-walled pipe, e.g., with a diameter to thickness ratio greater than 30 (for steel, lower for lighter metals). The end closures 7 have thicknesses the same as or thicker than the vessel 8. The partially collapsed or prepropagated end closures 7 facilitate collapse of the entire buoy 2 by propagating buckle mechanism at a selected water pressure. In Figure 2 there is shown a string tie-in pipe laying arrangement. A previously laid pipe string 10 is lifted up for tie-in to an additional pipe string 11 by a tie-in vessel 12. A hinge-type pipe connector 13, which seals as it is lowered, is attached to and connects lines 10 and 11. Partially lowered joint 14 (shown in Figure 2 in phantom only) is the sealed joint corresponding to connector 13 and held by a line 15 to vessel 12. The previously laid pipe string 10 then assumes the position of line 16 (shown in phantom) and the new pipe string 11 drops to the level of line 17 (shown in phantom). Buoys 18 are added to line 17 which is held by a tensioning tug 19 or similar means. Once the buoys 18 reach the critical depth 20 below the water surface 21, the buoys 18 substantially completely flatten and are lowered along with the pipeline to the sea floor 22. As a pipe buoy collapses from one end to the other, thez pneumatic pressure rises from ambient at the initiation of collapse to a value at which the pressure difference (external minus the internal pressure) is insufficient to "drive the buckle further. Then the progressive buckling will stop, leaving some air inside. To make the pipeline heavier on bottom, an opening in the pipe buoy can be placed near the position where buckling would stop. This opening could be covered by a cover, pressed against the opening by a band around the pipe buoy to effect a seal. When the buckle passes through the band, the opening moves away from the cover, and the air exhausts from the buoy by relative buoyancy forces. The buoy thus achieves its maximum weight in this manner. Other standard types of air removal means, such as spring-loaded valves, could be used as well. Reference is now made to Figure 3 depicting a second alternative of the pipelay method shown in Figure 1. As disclosed in Figure 3 a submarine pipeline 30 is laid from the stern 31 of a barge 32. Water is admitted to the pipeline 30 at a waterfill location 33 to maintain a water level 34 which varies somewhat during the pipelay operation. As the pipeline 30 passes over the stern 31 of the barge 32, buoys 35 are attached, and pipeline 30 and buoys 35 descend beneath the water surface 36 to a critical water depth 37 (designated by a dotted line) where release of the buoys 35 from the pipeline 30 occurs. When release mechanism 38 is activated, a buoy 39 is released and starts to rise to the surface of the water, leaving the release mechanism 38 still attached to the pipeline 30 where it will remain indefinitely. Then the pipeline gradually settles down to the sea floor 40 without the stresses which might otherwise occur with the use of a stinger only. If Figure 4 the release mechanism 38 described above is shown in more detail. A cable 50 is attached to a buoy (not shown in this figure) and to a cable end 51 which in turn is attached to a connecting ring 52 or similar means which is attached to trigger 53. The preflattened trigger 53, prior to being activated, is shown with sides 54 in solid outline. The final flattened shape of trigger 53, after being activated, is shown with sides 55 in phantom. Rigid arms 56 show the position thereof in solid out line before the release mechanism 38 is activated. Loose arms 57 shown in phantom configuration, show the position of arms 56 in dotted outline after the release mechanism is activated and the trigger 53 is fLattened. Thus, the water pressure causes: the trigger 53 to buckle, arms 56 pivot about hinges and arm tips 58 move apart, releasing cable end 59 which is in turn attached to the structure being deployed beneath the surface-of water, e.g., a pipeline or other oil production equipment -(not shown in this figure). A variation of the invention employing a collar release mechanism is shown in Figure 5. Flotation collar 60¯encircles a pipeline (or other structure) 61. Trigger 62 has a rigid arm 63 preloaded against the trigger and connected to the flotation 60. As the trigger 62 releases by flattening (see Figure 5B) the arm 63 opens the flotation collar 60 about hinge 6)4,'and' drops the pipeline 61 (or other structure, e.g., oil-production equipment). In Figures, 6A, 6B and 6C the steps for making a prebuckled pipe susceptible to further buckling at a selected pressure are shown. Firstly, a container such as a capped pipe 70- is -mechanically dented in the centre (see Figure 6A). Any cylindrical pressure vessel of arbitrary cross-section can be used and other mechanical techniques for denting the-eontainer may be utilized. Subsequently, the capped and dented pipe 70 is placed in a pressure vessel 71 (see Figure 6B), and a hydraulic fluid 72 under pressure is passed into the pressure vessel 71. The buckle initiated in the first step -is propagated by the hydraulic fluid 72. The pressure required to propagate the buckle is noted sinc-e this will be the release pressure causing further buckling. Finally, the pipe 70 is cut in half, and welded or otherwise closed at mid-section 73, see Figure 6C.

Claims: C L A I M S 1. Method for deploying a structure, for example a pipeline, below the surface of a body of water comprising the sequential steps of attaching flotation means by fastening means to the structure to be deployed; allowing the structure to descend to a critical depth below the surface of the body of water, and removing the floating action of the flotation means on the structure by applying means susceptible to buckling at a pressure occurring at said critical depth. 2. Apparatus for use in the method according to claim 1, comprising flotation means and fastening means for securing said flotation means to a structure to be deployed, the flotation means comprising a partially buckled hollow vessel susceptible to further buckling at a preselected water pressure. 3. Apparatus as claimed in claim 2, wherein the vessel is a cylinder with at least one partially buckled end closure. 4. Apparatus as claimed in claim 3, wherein the extent of partial buckling of the cylinder corresponds to a preselected water pressure which will propagate the partial buckling to substantially complete buckling. 5. Apparatus as claimed in any one of the claims 2-4, wherein means are provided for exhausing trapped gas from the buckled vessel. 6. Apparatus as claimed in claim 5, wherein the means for exhausing trapped gas is a spring-loaded valve. 7. Apparatus as claimed in any one of the claims 2-6, wherein the vessel ranges in length from about one pipeline diameter to the same length as the pipeline forming he structure to be deployed. 8. Apparatus as claimed in any one of the claims 1-7, wherein the vessel is made of steel and has a ratio of diameter-tothickness of greater than 30. 9. Apparatus as claimed in any one of the claims 2-8, wherein the diameter of the vessel ranges from about 1/10 to 10 times the diameter of the pipeline forming the structure to be deployed. 10. Apparatus as claimed in claim 3, wherein the end closures have a wall thickness of from equal to or greater than the side walls of the vessel. 11. Apparatus for use in the method according to claim 1, comprising flotation means; fastening means for releasably securing said flotation means to a structure to be deployed; and partially buckled actuating means susceptible to further buckling at a preselected water pressure to selectively release said fastening means. 12. Apparatus as claimed in claim 11, wherein said actuating means is a partially flattened container. 13. Apparatus as claimed in claim 12, wherein the container is a cylinder with capped ends. 14. Apparatus as claimed in any one of the claims 11-13, wherein the fastening means comprises a lever attached to the actuating means and narijpulative by further buckling of the actuating means. 15. A method for preparing means susceptible to buckling at a preselected pressure comprising denting the mid-section of a cylindrical container; propagating the dent with fluid pressure outside the cylindrical container to reduce the cross-sectional area of at least part of the mid-section of the container without substantially collapsing the container; cutting the container into two parts at the reduced mid-section area; and closing the cut ends of the two parts of the container. 16. Method as claimed in claim 15, including observing the pressure necessary to propagate the dent.

Inventors: AYERS, RAY ROLLAND
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1981
Main classification: F16L1
IPC codes: F16L1