Launching elongate subsea structures

Abstract

A method of launching an elongate subsea structure such as a pipeline bundle unit into water provides buoyant support to the structure by displacing water with a hollow buoyancy unit that applies buoyant upthrust to the structure. By lowering the buoyancy unit in the water to bring a permanently open aperture of the buoyancy unit beneath a surface level of the water, the buoyancy unit is flooded with water through the or each permanently open aperture to reduce the buoyant upthrust applied to the structure. A corresponding elongate subsea structure is also claimed.

Claims

1. A method of launching an elongate subsea structure into water, the method comprising: providing buoyant support to the structure by displacing water with a hollow buoyancy unit that applies buoyant upthrust to the structure; and by lowering the buoyancy unit in the water to bring at least one permanently open aperture of the buoyancy unit beneath a surface level of the water, flooding the buoyancy unit with water through the or each permanently open aperture to reduce the buoyant upthrust applied to the structure; wherein the method further comprises dragging at least part of the structure along a seabed while launching the structure.

2. The method of claim 1, comprising lowering the buoyancy unit to flood the buoyancy unit by lowering at least part of the structure in the water.

3. The method of claim 1, wherein launching the structure comprises launching a leading end of the structure into the water, flooding the buoyancy unit and subsequently launching a trailing end of the structure into the water.

4. The method of claim 1, comprising bending the structure elastically along a length of the structure during launching.

5. The method of claim 1, comprising pulling the structure from land into sea during launching.

6. The method of claim 5, comprising sinking at least part of the structure onto a seabed after reducing the buoyant upthrust applied by the buoyancy unit to the structure.

7. The method of claim 1, followed by lifting the structure from a position of the structure on a seabed, towing the structure to an installation site and sinking the structure at the installation site.

8. The method of claim 1, wherein flooding the buoyancy unit with water through the or each permanently open aperture comprises flooding the buoyancy unit through at least one permanently open aperture in a side wall of the buoyancy unit.

9. The method of claim 1, wherein flooding the buoyancy unit with water through the or each permanently open aperture comprises flooding the buoyancy unit through at least one permanently open aperture in a top of the buoyancy unit.

10. The method of claim 1, wherein flooding the buoyancy unit with water through the or each permanently open aperture comprises flooding the buoyancy unit through at least one permanently open aperture at an upper end of a duct that terminates within the buoyancy unit.

11. The method of claim 1, comprising submerging at least a majority of the buoyancy unit before lowering the or each open aperture beneath the surface level of the water.

12. The method of claim 1, comprising lowering the buoyancy unit in the water by increasing a total weight of the structure launched into the water.

13. The method of claim 1, comprising lowering the buoyancy unit in the water by increasing a length of the structure launched into the water.

14. The method of claim 1, comprising lowering the buoyancy unit in the water by advancing the structure into deepening water.

15. The method of claim 1, wherein flooding the buoyancy unit with water through the or each permanently open aperture comprises flooding the buoyancy unit through a second permanently open aperture after flooding the buoyancy unit through a first permanently open aperture.

16. The method of claim 15, comprising lowering the buoyancy unit after flooding through the first open aperture and before flooding through the second open aperture.

17. The method of claim 1, comprising applying buoyant upthrust to a towhead of the structure.

18. The method of claim 1, comprising also supporting the structure by applying tension to a line attached to the structure.

19. The method of claim 1, wherein the structure is negatively buoyant without the buoyant upthrust applied by the buoyancy unit.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) To illustrate the context of the invention, reference has already been made to FIGS. 1 and 2 of the accompanying drawings in which:

(2) FIG. 1 is a schematic side view of a bundle unit comprising a pipeline bundle and two towheads, shown here being towed to a subsea installation site using the controlled depth towing method known in the prior art; and

(3) FIG. 2 is a schematic side view of the pipeline bundle of FIG. 1 now laid on the seabed.

(4) In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

(5) FIGS. 3a, 3b and 3c are a sequence of schematic side views of a pipeline bundle unit in accordance with the invention, being launched into the sea from a coastal spoolbase;

(6) FIGS. 4a, 4b and 4c are a sequence of schematic part-sectioned side views of a leading towhead of the bundle unit shown in FIGS. 3a to 3c, as buoyancy applied to the towhead by a buoyancy device of the invention is automatically decreased; and

(7) FIGS. 5a, 5b and 5c are a corresponding sequence of schematic part-sectioned side views of a leading towhead of the bundle unit shown in FIGS. 3a to 3c equipped with a variant of the buoyancy unit shown in FIGS. 4a to 4c.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring firstly to FIGS. 3a to 3c, like numerals are used for like features. Thus, a bundle unit 10 comprises a pipeline bundle 12, shown here shortened and interrupted, extending between a leading towhead 14 and a trailing towhead 16.

(9) In FIG. 3a, the bundle unit 10 is shown ready for launch at a coastal spoolbase 30 where the bundle unit 10 was previously fabricated, tested and commissioned. The spoolbase 30 extends inland from a beach 32. In the offshore direction, the beach 32 shelves into the sea beneath the surface 18 to form a shallowly-inclined seabed 20. Consequently, the depth of the sea increases with increasing distance from the beach 32.

(10) A line 22 connects a tug 24 to the leading towhead 14 to pull the bundle unit 10 from the spoolbase 30 into the sea. The pull of the tug 24 overcomes frictional forces that resist movement of the bundle unit 10. For additional control of the launch operation, a small degree of back-tension may be applied to the bundle unit 10. For example, a line 34 is shown attached to the trailing towhead 16 for this purpose.

(11) The bundle unit 10 is equipped with one or more floodable buoyancy devices 36 of the invention. In this example, a buoyancy device 36 is mounted on top of each of the towheads 14, 16.

(12) In other examples, the buoyancy devices 36 could be positioned differently with respect to the towheads 14, 16. For example, the buoyancy devices 36 could be beneath or beside the towheads 14,16. Such a lower, or lateral, position of the buoyancy devices 36 with respect to the towheads 14,16 would maintain the towheads 14,16 of other parts of the bundle unit 10 closer to the surface 18 or at the surface 18.

(13) In further examples, only one of the towheads 14, 16 could be equipped with a buoyancy device 36 or there could be more than one buoyancy device 36 on one or both of the towheads 14, 16. Similarly, the pipeline bundle 12 could be equipped with one or more buoyancy devices 36.

(14) The buoyancy devices 36 exemplified here are open-topped hollow caisson-like structures that confer buoyancy on the towheads 14, 16 by displacement upon immersion. The open top is an aperture through which each buoyancy device 36 may be flooded with seawater.

(15) Each buoyancy device 36 has continuous side walls that are contiguous with a base. The side walls and the base may, for example, be of circular or rectangular shape in plan view.

(16) The buoyant upthrust imparted by each buoyancy device 36 increases with increasing depth of immersion in the sea and hence with increasing displacement. Upthrust increases with increasing depth until the surface 18 overtops the side walls of the buoyancy device 36 and seawater begins to flood the hollow interior through the open top.

(17) Flooding of the buoyancy device 36 reduces buoyancy and initiates sinking, which increases flooding and hence accelerates sinking. The substantial buoyant upthrust imparted by the buoyancy device 36 at the outset is thereby reduced to a negligible level.

(18) In the early stages of the launch operation shown in FIG. 3b, the leading towhead 14 has entered the sea, followed by a portion of the pipeline bundle 12 which extends across the beach 32. The pipeline bundle 12 has bent within elastic limits, under the weight of the leading towhead 14 that is negatively buoyant. The leading towhead 14 is fully submerged, along with a leading portion of the pipeline bundle 12 that is also negatively buoyant. In this respect, the pipeline bundle 12 is typically launched with ballast such as the chains 28 shown in FIG. 1 already attached at intervals along its length.

(19) The buoyancy device 36 attached to the leading towhead 14 is partially submerged, thus imparting buoyant upthrust that offsets the negative buoyancy of the leading towhead 14 and of the pipeline bundle 12. The buoyant upthrust may be sufficient to confer positive buoyancy, as in this example, or at least to reduce the effect of negative buoyancy.

(20) The buoyant upthrust of the buoyancy device 36 supports most or all of the apparent weight of the submerged leading towhead 14 and of the submerged leading portion of the pipeline bundle 12, which both remain clear of the seabed 20 in this example. A minor part of the apparent weight may be supported by a vertical component of tension in the line 22 that connects the tug 24 to the leading towhead 14.

(21) As more of the bundle unit 10 is launched into the sea, an increasing portion of the negatively-buoyant pipeline bundle 12 becomes submerged. This exerts increasing apparent weight forces on the leading towhead 14 to which the buoyancy unit 36 is attached. Some of that increased apparent weight could be borne by tension in the line 22 that connects the tug 24 to the leading towhead 14. However, most or all of the weight is borne by increased displacement as more of the buoyancy unit 36 is pulled under the surface 18. The support of the buoyancy unit 36 controls the curvature of the pipeline bundle 12 and prevents over-stressing.

(22) Eventually, with submergence of an increasing length of the pipeline bundle 12 as shown in FIG. 3c, the buoyancy unit 36 is pulled down into the water to such an extent that the top of the buoyancy unit 36 drops under the surface 18. The buoyancy unit 36 then floods quickly though its open top and so ceases to contribute significant buoyancy, thus allowing the leading towhead 14 to settle onto the seabed 20. As the leading towhead 14 sinks, it pulls the leading portion of the pipeline bundle 12 away from the surface 18 and hence away from wave action that could increase fatigue.

(23) The launch operation can then continue as the tug 24 pulls on the leading towhead 14 via the line 22. Eventually the trailing towhead 16 will enter the sea, where temporary buoyant support may be provided by a second buoyancy unit 36 attached to the trailing towhead 16. That buoyant support may be supplemented by hold-back tension in the line 34. Reducing the hold-back tension in the line 34 allows the trailing towhead 16 to sink away from the surface 18 as the attached buoyancy unit 36 also floods. A second tug 24 can then be connected to the trailing towhead 16 to allow the bundle unit 10 to be lifted from the seabed 20 and towed in mid-water to an installation site as shown in FIG. 2.

(24) In another approach, it would be possible for both of the towheads 14, 16 to float simultaneously before the buoyancy unit 10 sinks as a whole onto the seabed 20.

(25) In the variants shown in enlarged detail in FIGS. 4a to 4c and FIGS. 5a to 5c, like numerals are again used for like features.

(26) FIGS. 4a to 4c show a variant of the buoyancy unit 36 in which apertures in the form of openings, slots or ports 38 penetrate the side wall 40. The top of the buoyancy unit 36 is open in this example but could be closed like that of the variant shown in FIGS. 5a to 5c. In this respect, the ports 38 are less susceptible than an open top to inadvertent or premature flooding due to overtopping of the buoyancy unit 36 by waves.

(27) In this example, the ports 38 are elongated horizontally, being rectangles that are wider than they are high. In plan view, the ports 38 are distributed around the side wall 40 and are all at substantially the same height above the base 42 of the buoyancy unit 36. Other examples could have differently-shaped ports, for example being elongated vertically, or more or fewer ports, or ports that are positioned or distributed differently along or around the side wall 40.

(28) FIG. 4a shows the buoyancy unit 36 empty of water and partially submerged to provide substantial buoyant upthrust to the attached leading towhead 14. This buoyant upthrust offsets the apparent weight of the leading towhead 14 and of the leading portion of the pipeline bundle 12, holding them clear of the seabed 20.

(29) The ports 38 promote gradual or progressive flooding of the buoyancy unit 36 when a sufficient length of the pipeline bundle 12 has been launched into the sea and so exerts a weight force on the leading towhead 14 that exceeds a predetermined threshold.

(30) Flooding begins when the buoyancy unit 36 sinks to an extent that brings the ports 38 beneath the surface 18, as shown in FIG. 4b. The inrushing water reduces buoyancy and causes the open top of the buoyancy unit 36 to sink beneath the surface 18, at which point flooding accelerates rapidly to completion. The leading towhead 14 and the leading portion of the pipeline bundle 12 then settle onto the seabed 20 by virtue of their negative buoyancy. If necessary, the descent of the leading towhead 14 and the leading portion of the pipeline bundle 12 onto the seabed 20 may be controlled by tension in the line 22 that connects the leading towhead 14 to the tug 24 (not shown).

(31) FIGS. 5a to 5c show a further variant of the buoyancy unit 36 in which openings or ports 38 do not penetrate the side wall 40. Instead, an aperture in the form of a port 38 is defined by the top of a stand pipe or tube 44 that extends upwardly from an inlet 46 at a lower end to the port 38 at the upper end. In this example, the tube 44 extends upwardly from the base 42 and the inlet 46 penetrates the base 42.

(32) The top of the buoyancy unit 36 is closed by a top panel 48 in the example shown in FIGS. 5a to 5c but could instead be open like that of the example shown in FIGS. 4a to 4c. Whilst contiguous with and attached to the side wall 40 around its periphery, the top panel 48 is suitably provided with one or more vents 50 to allow air to escape as the buoyancy unit 36 floods with water, as shown in FIG. 5c.

(33) FIGS. 5a to 5c also differ from the preceding drawings in that the leading towhead 14 remains in contact with the seabed 20 throughout. Thus, the seabed 20 supports some of the apparent weight of the leading towhead 14 and of the submerged leading portion of the pipeline bundle 12. It follows that the depth of the water between the surface 18 and the seabed 20 determines whether the buoyancy unit 36 is submerged sufficiently as to flood.

(34) Whilst the buoyant upthrust provided by the buoyancy unit 36 is not sufficient to confer positive buoyancy on the negatively-buoyant assembly comprising the leading towhead 14 and the leading portion of the pipeline bundle 22, that upthrust does reduce the apparent weight of that assembly. This reduces friction with the seabed 20 and also reduces tension in the line 22 that connects the leading towhead 14 to the tug 24 (not shown).

(35) FIG. 5a shows the buoyancy unit 36 empty of water and partially submerged to provide substantial buoyant upthrust to the attached leading towhead 14. This buoyant upthrust reduces the apparent weight of the leading towhead 14 and of the leading portion of the pipeline bundle 12. The tube 44 is partially filled with water admitted through the inlet 46 in the base 42 but the level of that water has not yet risen to spill out of the tube 44 and into the buoyancy unit 36 through the port 38.

(36) As a greater length of the pipeline bundle 12 is launched into the sea, the leading towhead 14 reaches sufficiently deep water that the port 38 at the top of the tube 44 sinks below the surface 18 as shown in FIG. 5b. Water rising from the inlet 46 up the tube 44 now spills through the port 38 into the buoyancy unit 36 as the water level rises above the top of the tube 44. This sharply reduces buoyant upthrust exerted by the buoyancy unit 36. Buoyant upthrust then continues to reduce progressively as the leading towhead 14 is pulled into deeper water and more of the buoyancy unit 36 is submerged.

(37) Flooding continues until the buoyancy unit 36 sinks fully beneath the surface 18, as shown in FIG. 5c, whereupon flooding is complete and the buoyancy unit 36 ceases to contribute significant buoyant upthrust.

(38) Many other variations are possible within the inventive concept. For example, two or more ports could be at a different heights above the base of the buoyancy unit, either penetrating the side wall like the ports shown in FIGS. 4a to 4c and/or defined by respective tubes like the port shown in FIGS. 5a to 5c. This would promote gradual or progressive flooding after the buoyancy unit sinks to an extent that brings the lower or lowest port beneath the surface, before one or more ports above that level also sink beneath the surface and hence accelerate flooding.