GB2287297A - Undersea pipelines - Google Patents

Abstract

An undersea pipeline that carries gas or oil at high temperature and pressure can be subject to compressive loads that can cause the pipeline to buckle and fail. This can be avoided by forming the pipeline with alternately oppositely directed bends along its length since this allows lateral movement of the pipeline and so relieves some of the longitudinal compressive force. A bent pipeline is formed by preparing a pipeline from straight pipe sections and then imparting alternately oppositely directed bends of the pipeline which is then laid undersea. The bends can be imparted by pairs of rams 15ab, 16ab, 17ab arranged along the length of the pipeline 13 and acting on diametrically opposite sides of the pipeline. <IMAGE>

Description

UNDERSEA PIPELINES The invention relates to undersea pipelines.

Undersea pipelines are used, for example, to convey oil and gas from undersea wells. Increasingly, the oil and gas transported through these pipelines is at high pressures and temperatures. These high pressures and temperatures combine to impart an extremely large compressive force along the length of the pipeline which therefore has a tendency to buckle in an attempt to relieve the force. This buckling can lead to failure of the pipeline and is to be avoided at any cost.

One solution to this problem is to cover the pipeline with a layer of rock fragments which has the effect of restraining the pipeline from buckling. The cost of the rock and the cost of its placement over the pipeline is, however, a significant portion of the cost of developing the well and so there is a considerable incentive to eliminate this step.

One approach which has been proposed is deliberately to deform the pipeline into a series of alternately oppositely directed bends. The effect of the surpentine shape thus formed is to allow the pipeline to move a small distance laterally during operation, thus relieving some of the longitudinal compressive force and reducing significantly the tendency of the pipeline to buckle.

An example of this is given in the paper by Lannan, G.A. and Barry, D.W. entitled 'Mobile Bay Fairway Field Flowline Project", the Proceedings of the Offshore Technology Conference at Houston in 1992. In this case, the pipeline was formed of pipe lengths bent on shore and subsequently welded together on a vessel prior to being laid undersea.

According to a first aspect of the invention, there is provided a method of laying an undersea pipeline comprising forming a pipeline of straight pipe sections and then bending the straight pipeline to form a series of alternately oppositely directed bends along the length of the pipeline prior to laying the pipeline undersea.

The bends preferably lie in a common plane.

The method may comprise passing a length of said straight pipeline between two oppositely arranged sets of rams, actuating the sets of rams to form in said length a required bend and then advancing said bent length for laying undersea.

The curvature of each bend may be the same.

According to a second aspect of the invention, there is provided a machine for imparting bends to pipelines for laying undersea comprising means for passing a straight pipeline to a bending station and bending means at said bending station controlled to impart alternately oppositely directed bends to successive lengths of said pipeline.

The bending means may comprise two sets of rams, the rams of one set being arranged to act on one side of the pipeline and the rams of the other set being arranged to act on a side of the pipeline diametrically opposite said one side.

Control means are preferably provided for operating said bending means to form said successive bends. The machine may further comprise a welding station for welding straight pipe sections into a pipeline prior to said bending station.

The machine may be mounted on a semi-submersible vessel, or any form of ship-like vessel or barge.

The following is a more detailed description of an embodiment of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a plan view from above of a machine for imparting bends to pipelines including a welding station, a guiding station and a bending station and showing a length of a pipeline entering the bending station prior to bending, Figure 2 is a plan view from above of the guiding station and the bending station of the machine of Figure 1 showing the length of pipeline in the bending section formed with a bend, and Figure 3 is a similar view to Figure 2 but showing the length of pipeline bent in Figure 2 advanced from the bending station and a subsequent length of the pipeline bent with an oppositely directed bend.

Referring first to Figure 1, the machine comprises a welding station 10, a guide station 11 and a bending station 12. These stations 10,11,12 are mounted on a semi-submersible vessel (not shown) such as a pipe-laying ship and are followed by means (not shown) for deploying the pipeline onto the seabed.

At the welding station 10, straight pipe sections are welded together end-to-end in known fashion to produce a continuous pipeline 13. This pipeline 13 is guided and advanced by guides 14 at the guide station 11 to the bending station 12.

As shown in Figure 1, the bending station 12 comprises three pairs of oppositely arranged rams, 15a,15b,16a,16b,17a,17b.

The pairs of rams 15a,15b,16a,16b,17a,17b are spaced longitudinally along the path of the pipeline 13 through the bending station 12. The rams of each pair 15a,15b,16a,16b, 17a,17b are arranged on diametrically opposite sides of the pipeline path and lie in a horizontal plane.

The rams 15a,15b,16a,16b,17a,17b are controlled by a control device (not shown).

Referring now to Figure 2, when a first length of the pipeline 13 is in the bending station 12, the rams 15a,15b,16a,16b,17a,17b are controlled by the control device to adopt the disposition shown in that Figure. In this disposition, the ram 15a is retracted and the ram 15b extended, the ram 16a extended and the ram 16b retracted and the ram 17a retracted and the ram 17b extended. The effect of this is to form a bend in the pipeline 13.

All the rams 15a,15b,16a,16b,17a,17b are then retracted and the pipeline 13 is advanced until the bent length of the pipeline 13 is clear of the bending station and the next successive straight length is within the bending station 13.

The control device then controls the rams 15a,15b,16a,16b,17a,17b to the disposition shown in Figure 3.

In this disposition, the ram 15a is extended, the ram 15b retracted, the ram 16a retracted, the ram 16b extended, the ram 17a extended and the ram 17b retracted. The effect of this is to form in the length of pipeline 13 a bend which is in an opposite direction to the bend in the preceding length of the pipeline 13.

The rams 15a,15b,16a,16b,17a,17b are then all retracted and the pipeline 13 advanced again until a succeeding length of straight pipeline 13 is within the bending station. The rams are then configured as shown in Figure 2 and by successive such operations, the pipeline is provided with alternately oppositely directed bends along the length of the pipeline.

After this bending, the pipeline 13 is laid undersea.

As shown, all the bends lie in the same plane although this is not essential. In addition, as shown, all the bends have the same curvature although this is not necessary. The formation of the seabed may require that successive lengths have different curvatures. There could, of course, be lengths with no curvature intermediate the bent lengths.

It is not essential that the pipeline 13 is bent using hydraulic rams 15a,15b,16a,16b,17a,17b. These could be substituted by hydraulically-operated guide rollers, electrically-operated rams or any other suitable bending devices. There might not be three pairs of rams; there would be more pairs of rams.

Claims (11)

1. A method of laying an undersea pipeline comprising forming a pipeline of straight pipe sections and then bending the straight pipeline to form a series of alternately oppositely directed bends along the length of the pipeline prior to laying the pipeline undersea.

2. A method according to claim 1 wherein said bends lie in a common plane.

3. A method according to claim 2 comprising passing a length of said straight pipeline between two oppositely arranged sets of rams, actuating the sets of rams to form in said length a required bend and then advancing said bent length for laying undersea.

4. A method according to any one of claims 1 to 4 wherein the curvature of each bend is the same.

5. A machine for imparting bends to pipelines for laying undersea comprising means for passing a straight pipeline to a bending station and bending means at said bending station controlled to impart alternately oppositely directed bends to successive lengths of said pipeline.

6. A machine according to claim 5 wherein the bending means comprise two sets of rams, the rams of one set being arranged to act on one side of the pipeline and the rams of the other set being arranged to act on a side of the pipeline diametrically opposite said one side.

7. A machine according to claim 5 or claim 6 wherein control means are provided for operating said bending means to form said successive bends.

8. A machine according to any one of claims 5 to 7 and further comprising a welding station for welding straight pipe sections into a pipeline prior to said bending station.

9. A machine according to any one of claims 5 to 8 and mounted on a semi-submersible vessel, or any form of ship-like vessel or barge.

10. A machine substantially as hereinbefore described with reference to, and as illustrated by Figures 1 to 3.

11. A method substantially as hereinbefore described with reference to, and as illustrated by Figures 1 to 3.