WO1993022592A1

WO1993022592A1 - Method of laying a pipeline from a laybarge, on an uneven seabed

Info

Publication number: WO1993022592A1
Application number: PCT/NO1993/000066
Authority: WO
Inventors: Sverre Lund
Original assignee: Den Norske Stats Oljeselskap A.S
Priority date: 1992-04-27
Filing date: 1993-04-27
Publication date: 1993-11-11
Also published as: NO175223C; NO175223B; NO921628L; NO921628D0; AU4273293A

Abstract

Method of laying a pipeline on an uneven seabed. The invention is characterized therein that the pipeline (5) at the working deck (2) of a laybarge (1) is provided with one or more pre-curved pipe elements (6, 7), so that along a predetermined subsea trajectory the pipeline (5) will follow the irregularities of the seabed, whereby the pipe elements (6, 7) are welded into the pipe string together with other, straight pipe sections and are lowered to the seabed from the laybarge.

Description

METHOD OF LAYING A PIPELINE FROM A LAYBARGE, ON AN UNEVEN SEABED

The invention relates to a method of laying pipelines, for example gas and oil pipelines on the seabed, in parti- cular in areas having a very uneven bottom.

Subsea pipelines are used for the transport of oil and gas between offshore productions installations and onshore plants for production, storing and terminal functions, or between various types of production, transit and terminal installations offshore.

In one of the most frequently used methods of laying subsea pipelines the laying takes place from a laybarge which is progressively hawled forwards by means of anchors or by means of propellor propulsion if anchorless positioning is employed, whereby the pipeline is deployed while exerting tensional force in the pipe, so that the curvature is restricted to that portion of the pipe which is freely sus¬ pended down to the seabed. In the region between the working deck and the freely suspended portion of the pipe the pipe is bent in the vertical plane and is here supported by a laying ramp, also called stinger. Thus, during laying the pipeline assumes an S-shape, and the laying method is denoted S- laying. Any cross-section of the pipeline is subjected to being bent twice in the vertical plane, first to an upward convex curvature over the stinger, and subsequently to an upward concave curvature along the freely suspended portion.

During S-laying the pipeline is welded together from straight pipe sections into a rectilinear pipe string on the horizontal or a little aft-sloping working deck. Usually, each pipe section has a length of about 12 meters (single sections) or about 24 meters (double sections) . The bendings to which the pipeline is subjected during laying, take place in an elastic manner, so that when laid on a flat seabed the pipeline is rectilinear and will be laying with a smooth engagement along the seabed.

When common S-laying is used in regions where the seabed is broken or hilly, the support of the pipeline on the seabed can be uneven or partially be lacking where the pipeline has free spans over depressions in the seabed. In the case of large irregularities on the- seabed, involving to a large extent free spans of the pipeline, which accordingly will loose its support on the bottom, for example because of trench formations, plow markings from icebergs or from projecting rock portions, resulting static bending stresses or stream and wave induced dynamic stresses can have effects that are unacceptable from a mechanicl strength viewpoint. Under the influence of its own weight the pipeline will be deformed in the vertical plane in relation to a rectilinear trajectory over the peaks of the uneven seabed. The degree of deformation depends upon the tensional force being applied to the pipe during laying, in such a way that a lower tension gives larger deformation, and thereby a smaller length and height of the free span. When planning the installation of a subsea pipeline investigations of the structure and topography of the seabed along the trajectory desired, are made. In the case of high degrees of irregularity at the sea bottom, calculations of expected free spans are performed. If the irregularities of the sea bottom result in free spans involving unacceptable effects with respect to mechanical strength of the pipeline, and better alternative trajectories are not available, the project will include specific foundation arrangements for the pipeline. Such foundation arrangements can consist in level- ling and gravel filling along the trajectory before laying the pipeline, and trenching, gravel filling and providing supports along the pipeline after the laying thereof. The extent of such foundation operations can increase strongly with increasing length and height of free spans, and such operations -are very expensive.

When S-laying is employed on an irregular bottom it will be an advantage to exert an axial tension as low as possible, so that the length and height of free spans will be as small as possible and the foundation works correspondingly restricted. The lower limits of tension, however, are given from the pipe weight, the elastic curvatures to which the pipe is bent, and properties of the laybarge and stinger.

Norwegian patent 158,234 describes a method of laying and providing a foundation of a pipeline on the seabed, characterized thereby that the pipeline is provided with pre- curved, angular bends in the horizontal plane and/or vertical plane, so that the pipeline will have a trajectory which follows the natural irregularities of the bottom. The laying is based upon the assembly of the pipeline from one or more sections which are manufactured at another location than where the pipeline is laid, and these sections are towed, from the manufacturing site to the laying location while suspended by means of buoyancy means. The purpose of the present invention is to obtain a method of laying a pipeline with pre-curved angular bends in the vertical plane, from a vessel being preferably equipped for S-laying. The pre-curved bends are formed by bent pipe elements having a lenght corresponding to the length of the straight pipe sections which are used during the S-laying.

The pre-curved pipe elements are welded into the pipe string where it is intended to rest on the seabed in points having slope variations which would lead to a situation where a rectilinear pipeline after laying without pre-curved pipe elements, would have undesired free spans. Thus, the result is that the pipeline follows the slope variation of the seabed without leading to high bending moments, and with a substantial reduction of the length and height of the free span, compared to the laying of a straight pipe. In view of the fact that the pipeline is not installed as a long, straight beam, but as a long beam having permanent directional changes in the vertical plane adapted to the seabed topography, the need for foundation works along the trajectory before installing the pipeline, will be eliminated or reduced to a minimum. Upon installation of the pipeline the foundation conditions must be checked in the usual manner, and if necessary additional foundations must be provided for. Stabilization of the pipeline by means of trenches, concrete mats, asphalt mats, stone or gravel filling can also be effected if this is necessary in view of wave and stream forces or forces resulting from temperature or internal pressure in combination with the axial direction change of the pipeline. The extent of such supplementary works will be significantly reduced in relation to the case where the pipeline had been laid without the pre-curved pipe elements.

The invention is explained more closely in the following description: Fig. 1 shows five steps during the laying of a pipe having two pre-curved pipe elements, from a laybarge, adapted to an irregularity on a seabed being otherwise flat.

Fig. 2 in four steps shows the welding of a pre-curved pipe element on the working deck at the laybarge, and the passing of the pre-curved pipe element through the tension machinery.

In figs. 1 a)-e) there is shown a laybarge 1 having a working deck 2 comprising supporting rollers for the pipeline 5, machinery 3 for exerting a tensional force on the pipe and a laying ramp (stinger) 4. Moreover, the figures show a pipeline 5 during laying, where 6 and 7 are pre-curved pipe elements being adapted to irregularities of a seabed being otherwise flat. Through lines or cables 10 and 11 auxiliary vessels 8 and 9 are connected to the pre-curved pipe elements in order to be able to transfer a vertical force between the auxiliary vessel and the pipe. Further, reference numerals 12 and 13 denote a flat and even seabed, whereas 14 desig¬ nates a depression of the seabed. 15 and 16 denote points where the seabed changes from a flat to an uneven or irregu- lar region.

The pre-curved pipe element is installed in the pipeline by welding to the end of the pipe at the working deck of the laybarge, at a position which makes the distance from the pre-curved pipe element along the pipe string being under laying, down to a point of the seabed, equal to the distance rom the location at the sea bottom where the bend is intended to be placed, to the same point. Additional, straight pipe sections are then welded to the pipe at the same time as the laybarge is moved and the pre-curved pipe element passes over the stinger and down towards the seabed where the pipe is freely suspended.

Most suitably, the pre-curved pipe elements 6 and 7 will be of the same lenght as the remaining pipe sections being welded into the pipeline, usually about 12 meters or 24 meters. Fig. 2 shows how a.double pipe element is welded into the pipe string 5. Such a double member consists .of a pre-curved single pipe element and a straight single section having been welded together beforehand. The radius of the pre-curved pipe element can suitably be the radius to which a corresponding straight tube can be elastically bent, but the radius can also be both larger and smaller than this. For example the limit of elastic bending of a 20 inch pipe will be a radius of about 120 meters. A 20 inch pipe element of lenght 12 meters which is bent to a 120 meters permanent radius of curvature will have an axial direction change of 0,1 radian (about 6°). This angle is illustrative for the order of magnitude of the axial direction changes made possible by the invention. Before the^•pre-curved pipe element 6 is welded to the end of the pipeline on the working deck 2, it will be necessary to lift the end of the pipeline and the pre-curved pipe element with a temporary support thereof, so that the pipe axis will have the same direction as the axis of the pre-curved pipe element at the welding seam, see Fig. 2b.

Correspondingly, the pipe axes must be oriented at the same direction when the next straight pipe section shall be welded to the pre-curved pipe element. With respect to the proce¬ dure for welding straight pipe sections into the pipeline, which is based on simultaneous welding operations at several welding stations along the working deck 2, it may be required to fulfill the welding of the pre-curved pipe element at the first welding station, on account of extra bending stresses in the pipe. After the welding of the pre-curved pipe element into the pipeline, the temporary support can be removed. The portion of the pipe string which rests on the working deck has no longer a rectilinear shape, and a part of the pipe string at either side of the pre-curved pipe element 6 will be elevated in relation to the straight working deck 2. This part of the pipe which is elevated is loaded as a beam by the own weight of the pipe, and this load contributes to a partial straightening of the pipe string, see Fig. 2c. The length and the height of the elevated part will depend on the pipe rigidity and the axial direction change along the pre-curved pipe element. The axial direction change can be restricted in view of practical circumstances at the laybarge concerned.

The machinery 3 which exert a tensional force on the pipe, is usually in the form of endless belts arranged in pairs and are adapted to be pressed from below and from above against the pipeline 5, at the same time as the longitudinal belt force is transferred to the pipe as a tensional force. The compressive force from above will contribute to an additional straightening of the pipe string, see Fig. 2d, so that the pipe string with the pre-curved pipe element welded into it will have an approximate rectilinear shape during its passage through the tensioning machinery 3.

The stinger 4 has the same direction of curvature as the pre-curved pipe element 6, and therefore the pre-curved pipe element will be subject to a smaller bending moment at the passage over the stinger than the straight part of the pipe¬ line.

Along the freely suspended part of the pipe string from the end of the stinger towards the seabed, the bending moment and the curvature of the pipe will be reversed. At the lower part the direction of curvature is concave upwards. In order to reduce the tendency for the pre-curved pipe element when reaching the region of upward concave curvature, to have a torque instability by adapting its direction of curvature to the direction of curvature of the freely suspended part of the pipestring, a preferred embodiment provides for a suppor¬ ting wire 10, 11 connected between the pre-curved pipe element and an auxiliary vessel 8,9. The tension in the wire is provided for by a winch on the auxiliary vessel, and shall correspond to the vertical component of unbalanced tension in the pipe in consequence of the axial change of direction along the pre-curved pipe element. As an alternative buoy¬ ancy tanks can be attached to the pre-curved pipe element, with a buoyancy adjusted to the vertical component of the pipe tension resulting from the directional change, or the pipeline can be arranged with a positive buoyancy at a local region on either side of the pre-curved pipe element, for example by coating the pipe in this region with a concrete weight jacket. The vertial. tension is maintained until the pre-curved pipe element rests on the seabed, whereby the seabed exerts a vertical force on the pipe at the pre-curved pipe element corresponding to the vertical force exerted during the lowering.

Without this force the pre-curved pipe will have a tendency to rotate 180° for assuming the same direction .of curvature as the natural curvature during the transfer to the seabed. Moreover, this force will prevent the generation of large bending stresses resulting from an eccentric effect of the pipe tension in the region at the pre-curved pipe element. In the case of small directional changes over the pre-curved pipe element, it may be possible that application of an upwardly directed force is not required, since a suffi- cient degree of rotational stability is obtained by restis- tance to rotation in the tension machinery on the laybarge, and the bending moment will be small because of an eccentric effect of the pipe tension.

It is a necessary pre-requisit according to the inven- tion that a very accurate surveying of the bottom conditions in the region concerned is made before laying of the pipeline 5. During the laying of the pipeline it may be necessary to navigate with respect to navigational aids being positioned at the seabed. When the pipeline is installed on the seabed, the foun¬ dation conditions are checked and any required additional foundation is provided for, as well as any required stabi¬ lizing of the pipeline by means of trenching, rock filling, asphalt mats, concrete weights is performed. Although the method has been described by the employment of a vessel equipped for S-laying, the method in principle can also be implemented with a vessel equipped for J-laying. In the case of J-laying the straight working deck has such a strong slope that when the pipeline leaves the working deck it has the angle of inclination which is required for the lowering to the seabed, without passing through a region where the pipe is bent to an upwardly directed convex curvature. The method has been described above for the laying of a pipeline with upwardly directed convex bends. The laying of a pipeline with upwardly directed concave bends may also be able to contribute to reduce free spans and a smaller extent of foundation operations on an uneven seabed, and in prin¬ ciple can be effected in the same way as the laying of a pipeline having upwardly directed convex bends. The condi¬ tions of bending or flexure over the stinger can impose restrictions as to practical changes of direction during the laying of a pipeline having upwardly directed concave bends in the case of S-laying. For the laying of a pipeline having such pipe elements, the balancing of the resultant of the axial force at either side of the pipe element in the freely suspended part of the pipeline, will require a downwardly directed force.. If balancing of this force is necessary, this can be arranged for in practice by means of additional weights being integrated into or attached to the pipe element having the upwardly directed concave bend.

Claims

C .1 a i m s

1. Method of laying a pipeline on an uneven seabed, c h a r a c t e r i z e d i n that the pipeline (5) at the working deck (2) of a laybarge (1) is provided with one or more pre-curved pipe elements (6,7), so that along a predetermined subsea trajectory the pipeline will follow the irregularities of the seabed, whereby the pipe elements (6,7) are welded into the pipe string together with other, straight pipe sections and are lowered to the seabed from the lay¬ barge.

2. Method according to claim 1, c h a r a c t e r i z e d i n that the pre-curved pipe elements have a- length corresponding to the other pipe sections of which the pipeline is composed.

3. Method according to claim 1 and 2, c h a r a c t e r i z e d i n that the pre-curved pipe elements (6,7) during welding are oriented with the convex side facing vertically upwards.

4. Method according to claim 1 and 2, c h a r a c t e r i z e d i n that the pre-curved pipe elements (6,7) during welding are oriented with the concave side facing vertically upwards.

5. Method according to claims 1 to 3, c h a r a c t e r i z e d i n that the end of the pipe¬ line (5) and the end of the pre-curved pipe element (6) are lifted with a temporary support, so that the pipe axis has the same direction as the axis of the pre-curved pipe element (6) at the welding seam.

6. Method according to claims 1 to 3, c h a r a c t e r i z e d i n that the pre-curved pipe elements (6,7) are welded into the pipeline (5) at positions which after laying will be located where the seabed has changes in its slope, and where a straight pipe without pre- curved pipe elements would rest on the shoulders on free spans.

7. Method according to claim 6, c h a r a c t e r i z e d i n that the axial change of direction along a pre-curved pipe element is adjusted accor¬ ding to the change of slope of the seabed.

8. Method according to claims 1 to 3, c h a r a c t e r i z e d i n that the pre-curved pipe elements having an upwardly directed convex curvature during lowering to the seabed, are subjected to an upwardly directed vertical force corresponding to an unbalanced axial tension resulting from directional changes along the pre-curved pipe element.

9. Method according to claim 8, c h a r a c t e r i z e d i n that the upwardly directed vertical force is applied through a wireline (10,11) from an auxiliary vessel (8,9) which is positioned vertically above the pre-curved pipe element during the lowering, or as an alternative by designing the pipeline adjacent the pre-curved pipe element with a positive buoyancy, or by attaching buoy¬ ancy tanks to the pre-curved pipe element.