GB2114700A

GB2114700A - A method of positioning a flowline below the surface of a body of water

Info

Publication number: GB2114700A
Application number: GB08303470A
Authority: GB
Inventors: James Truett Coleman
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1982-02-12
Filing date: 1983-02-08
Publication date: 1983-08-24
Also published as: AU1078883A; FR2521684A1; GB8303470D0; GB2114700B; JPS58149490A; NO830472L

Abstract

In a method of positioning a flowline bundle 10 below the surface of a body of water spaced apart flowline base structures 28, 34 are secured to the water bottom and the flowline bundle is towed to a position above the flowline base structures 28, 34. One end 16 of the flowline bundle 10 is then connected to a first cable 24 and the other end 18 of the flowline bundle 10 is connected to a second cable 30. The first cable 24 is passed through one of said flowline base structures 28 and the second cable 30 is passed through the other flowline base structure 34. The ends of the flowline bundle 10 are then simultaneously drawn down and subsequently connected to the flowline base structures 28, 34. <IMAGE>

Description

SPECIFICATION A method of positioning a flowline below the surface of a body of water The present invention relates to a method of positioning a flowline, in particular an oil pipeline, below the surface of a body of water.

Subsea oil wells are becoming more common as land-based oil reserves are depleted. Once a subsea oil well is drilled, the problem is encountered of transporting the oil from the well to a location where it may be loaded on a tanker and taken to a refinery. Generally, the transportation system includes several conduits or flowlines, each of which connects to a well and which are bundled together and laid on the ocean floor. Fowlines are therefore an important, and indeed an expensive, component in any subsea completion concept. Historically the costs of flowlines have exceeded 50% of total budget estimates for subsea completions.

One known method for deploying a flowline bundle involves initial assembly on land and then towing the bundle to the location where it will be used. One end of the bundle is then drawn down to the point where it is to be connected, whereafter the second end is lowered to the ocean floor and dragged to the point where it is to be connected. During the entire procedure, the bundle is subjected to great forces which can cause bending and collapse of the conduit bundle.

Neither of the two existing methods of laying pipeline, the conventional Lay barge and the more sophisticated Reel barge, are capable of laying a flowline in deep water whose length is less than water depth.

The problems encountered in laying oil pipelines are becoming increasingly pronounced as offshore exploration drilling moves into deeper water. Thus, using conventional pipe laying techniques, it is understood that pipe laying tension forces to control bending and collapse stresses are water depth related and therefore pose some finite practical water depth limitations.

In addition, diverless second end connections are extremely complicated in deep water.

The present invention provides a method of installing a flowline or a bundle of flowlines up to 12,000 feet in length for water depths in excess of 1,000 feet. A flowline bundle is defined as two or more flowlines in a common carrier with single piece multi-bore end terminations.

The inventibn resides in a method for positioning a flowline below the surface of a body of water comprising the steps of: (a) fixing first and second flowline base structures at spaced locations beneath said surface; (b) towing said flowline to said base structures; (c) connecting a first end and a second end of said flowline to said first and said second flowline base structures, each through a cable; (d) pulling said cables through said first and said second flowline base strucures simultaneously to draw said first and second ends to said first and second flowline base structures respectively; and (e) connecting said first end and said second end to said first flowline base structure and said second flowline base structure, respectively.

In the accompanying drawings, which illustrate one example of the invention, Figure 1 is a schematic illustration of one stage during deployment of a flowlie bundle, and Figures 2A, 28 and 2C are vector diagrams of forces encountered in the method shown in Figure 1.

Referring now to Figure 1, a flexible flowline bundle 10 is illustrated as having buoys 12 with associated depth indicators 14. Ends 16 and 18 of flowline bundle 10 are terminated in conical structures 20 and 22, respectively. Conical structure 20 is permanently affixed to tension cable 24 which traverses from bow to stern of vessel 26 and extends as draw down cable section 27 to flowline base structure 28 and to conical structure 20 as draw down section 29.

Similarly, conical structure 22 is permanently affixed to tension cable 30 which traverses from bow to stern of vessel 32 and extends as draw down cable section 33 to flowline base structure 34 and to conical structure 27 as draw down cable section 35. Flowline base structure 28 and 34 are permanently mounted on sea bed 36, although as an alternative a portable arrangement for the flowline base structures 28 and 34 may be used provided they are stable and capable of withstanding the stress exerted by the buoyancy of flowline bundle 10. Flowline base structures 28 and 34 have conical fittings 38 and 40, respectively, which are adapted to receive conical structures 20 and 22, respectively.

Flowline base structure 28 and 34 are passive subsea structural bases piled to the sea floor.

Flowline base structures 28 and 34 are designed to withstand pull in and alignment forces during draw down and connection of the flowline bundle 10. Conically shaped receiving structures 38 and 40 form the entry cone for conical structures 20 and 22 connected at two ends 16 and 18 of flowline bundle 10. Conical receiving structures 38 and 40 may include spring loaded dogs (not shown) to engage slots (not shown) in conical structures 20 and 22 to provide a positive mechanical connection between flowline base structures 28 and 34 and flowline bundle 10. A flowline connector (not shown) having two short vertical guide posts, one on each side, may be used to position the connection assembly comprising conical structures 20 and 22 and conical receivers 38 and 40, respectively.

Referring now to Figures 2A, 28 and 2C, the vector diagrams illustrate the real and imaginary forces exerted on flowline bundle 10 prior to the draw down procedure. TT is the tension force and TD is the draw down force. TR is illustrated as the resultant catenary tension force at each end 16 and 18 of flowline bundle 10. FB is the buoyancy force of both the flowline bundle 10 and buoys 12. For simplicity, the buoyancy force of flowline bundle 10 is illustrated at several locations although in practice it is evenly distributed throughout the length of flowline bundle 10.

By controlling the tension on each of the bundles and monitoring the bundle configuration, the pipe stresses are known and under control at all times. TT and TD must be precisely controlled to provide a resultant TR necessary to control bending stresses in the carrier pipe within reasonable limits. Gradual changes in magnitude and direction of the resultant catenary tension force (TR) transforms flowline bundle 10 from its straight line configuration at the surface as illustrated in Figure 2A to a full inverted catenary shape as illustrated in Figure 2C.

In Figure 2A, TH equal TT and the angle 6 between the vectors TR and TT is equal to zero since in its initial position, flowline bundle 10 is buoyant and no force is being exerted on draw down cable sections 29 and 35. The result is that TD is equal to zero. As force is increased on draw down cable sections 29 and 35, TD increase and angle a, the angle between TT and TR increases while angle 6, the angle between TD and TR decreases. The change continues until angle 4 equals zero and TR becomes equal to TD at the maximum for angle 0 where flowline bundle 10 assumes an inverted catenary shape illustrated in Figure 2C.

The flowline bundle tension and draw down forces necessary to achieve this transformation are a function of flowline bundle buoyancy and stiffness only and therefore are not related to water depth. In operation, the connections transmit the flowline traversing forces (see Figures 2A, 28 and 2C) into flowline base structures 28 and 34 during final connection of flowline bundle ends 16 and 18. Ends 16 and 18 of flowline bundle 10 are drawn to flowline structures 28 and 34 by draw down cables 29 and 35 respectively. Floats 12 maintain a buoyancy of the entire flowline bundle 10 and add the buoyancy necessary to produce a cantilever configuration of the flowline bundle 10 as it is being drawn to the ocean floor.During the entire draw down operation, depth detectors 14, which may be of any type known in the art, although acoustic depth detectors are preferred, constantly monitor the depth of flowline bundle 10 at each float 12 position. The depth positions monitored by depth gauges 14 may be fed into a computer having a cathode ray tube display to illustrate the configuration of flowline bundle 10 as it is being drawn toward the ocean floor 36.

In practice, the bundle 10 is fabricated onshore, fitted with the conical structures 20, 22 and pressure tested prior to deployment. The bundle is then launched and towed near the surface, but preferably below the zone of wave activity, to the installation site. The draw down cables pivoted on the ocean floor are then attached to each bundle end and to surface winches, whereafter the bundle ends are simultaneously drawn down and mechanically latched to the base structures 28, 34. The buoys 12 are then selectively released to allow the bundle to lay on the ocean floor and the bundle carrier may be flooded with sea water.

It will be appreciated that the method of invention can be used to lay a flowline or flowine bundle in water whose depth is greater than the length of the flowline or bundle. Equally, of course, the method of the invention is applicable to the laying of flowlines and flowline bundles of length exceeding water depth.

Claims

1. A method of positioning a flowling below the surface of a body of water comprising the steps of: (a) fixing first and second flowline base structures at spaced locations beneath said surface; (b) towing said flowline to said base structures; (c) connecting a first end and a second end of said flowline to said first and said second flowline base structures, each through a cable; (d) pulling said cables through said first and said second flowline base structures simultaneously to draw said first and second ends to said first and second flowline base structures respectively; and (e) connecting said first end and said second end to said first flowline base structure and said second flowline base structure, respectively.

2. The method according to claim 1 wherein said pulling step includes: forming an inverted catenary shape with said flowline bundle.

3. A method for positioning a flow line bundle below the surface of a body of water comprising the steps of: (a) securing spaced apart flowline base structures to the water bottom; (b) towing the flowline bundle to said flowline base structures; (c) connecting one end of said flowline bundle to a first cable and another end of said flowline bundle to a second cable; (d) passing said first cable through one of said flowline base structures an said second cable through anotherflowline base structure; (e) drawing down said one and said other end of said flowline bundle to said one and said other flowline base structures, simultaneously; and (f) connecting said one end to said one flowline base structure and said other end to said other flowline base structure.

4. The method according to claim 3 wherein said drawing down step (c) includes: forming an inverted catenary shape with said flowline bundle.

5. The method according to any preceding claim and including the step of removably fixing buoys at spaced intervals along said flowline or flowline bundle to provide buoyancy.

6. The method according to any preceding claim and also including the steps of: providing depth sensors at spaced intervals along said flowline or flowing bundle; and receiving and monitoring depth indications from each of said depth sensors.