GB2279098A - Underwater structures - Google Patents

Abstract

An anchoring arrangement for securing a structure such as a subsea manifold to the seabed includes a vertical guide 26 forming part of the structure, a pile 54 driven through the guide into the seabed and locking means for locking the pile 54 to the guide 26. The locking means preferably consists of a locking pin (70, not shown) inserted through intersecting slots 56, 58 formed in the guide 26 and pile 54 respectively. The slots 56, 58 may be horizontal and vertical as shown or may both be arranged at 45 DEG to the vertical and at 90 DEG to each other. <IMAGE>

Description

"A Subsea Manifold" This invention relates to a subsea manifold for use in connecting a number of wells to a number of pipelines.

In earlier exploitation of offshore oilfields, it was the practice to bring each well to the surface.

Typically, a number of wells would be terminated on a single production platform, where a manifold arrangement was provided to connect the wells with pipelines such as export pipelines and gas lift lines.

More recently there has been pressure to exploit smaller fields and fields in deeper waters where that approach would not be economic.

There have been previous designs of subsea manifold in which the wells and the pipelines are interconnected in a manifold secured to the seabed. These designs, however, have been expensive to manufacture and install, and a further reduction in cost is desirable.

Prior art subsea manifolds have been based on mounting a manifold, essentially of the same design as would be used ashore or on a platform, on a base or skid, positioning the skid on the seabed, securing it in position, and fixing over it a protective structure intended to prevent impact damage. This procedure involves a series of cranage and diver or ROV operations. Moreover, the basic manifold and skid assembly is heavy and requires the use of an expensive crane vessel.

In accordance with one aspect of the present invention, a subsea manifold comprises a support structure and an assembly of pipework mounted on the support structure, the pipework including an array of headers, each header being connected in use to a respective pipeline, and each header having means for controlled connection to a plurality of wells, and in which the headers are arranged such that the longitudinal axis of any header is above or below the longitudinal axis of at least one of the other headers.

Preferably, the headers are arranged such that the longitudinal axis of each header is either above or below the longitudinal axis of any other header and typically, the longitudinal axes may define a nonhorizontal plane which is preferably vertical.

Preferably each header is connected to each well via a respective pipe loop which includes a first vertical section to one side of the header array, a top section passing across the header array, and a second vertical section to the other side of the header array, the means for controlled connection comprising at least one valve included in one of the vertical sections.

The pipe loop may suitably include a manually operable valve and/or a remotely actuable valve. Typically, the manually operable valve and/or the remotely actuable valve may be located in one or other of the vertical sections.

Preferably, the top section of the pipe loop forms part of a removable spool.

Preferably, the manifold includes a pigging loop in circuit between the pipelines and the headers, the pigging loop being disposed in a non-horizontal plane which is typically vertical and most preferably, transverse to the headers.

In one example the subsea manifold could be coupled to a pipeline bundle for use as a towhead or pullhead.

Typically, in this example, the subsea manifold would also comprise controlled buoyancy means to alter the buoyancy of the subsea manifold. Preferably, the buoyancy may be provided by tubular members in the support structure.

The invention from another aspect provides a subsea manifold comprising an assembly of pipework which includes an array of headers each of which is connected, in use, to a respective pipeline, each header having means for controlled connection to a plurality of wells; the pipework assembly being mounted within and supported by an integral structure forming a base and a protective frame.

In a further aspect, the invention provides a subsea installation comprising a space frame of elongate members joined together to form a base for engagement with the seabed, a top spaced from the base, and inwardly sloping sides between the base and the top; and in which each side is provided with a protective grating which extends from the top to a location above the base.

Preferably, the elongate members are tubular members welded together.

Preferably, the slides slope at an angle of about 550 to the horizontal.

Preferably, the grating on each side extends between a top horizontal tubular and an intermediate horizontal tubular positioned at about half height.

From another aspect, the invention provides an anchoring arrangement for securing a structure to the seabed, the arrangement comprising a vertical guide forming part of the structure, a pile driven through the guide into the seabed, and locking means for locking the pile to the guide.

The locking means is preferably formed by slots formed at an angle to each other in the guide and the pile which cooperate, once the pile is driven, to form a through aperture in which a locking device is inserted.

Preferably, the pile and the guide are slotted at angles of 45" to the vertical so that the slots in the pile and the guide are at right angles to each other.

However, other orientations may be used.

In a particularly preferred form the structure is rectangular in plan and is anchored by four piles, one adjacent each corner; the piles are tubular and the guides are tubular sleeves.

Examples of a subsea manifold in accordance with the invention will now be described with reference to the drawings, in which: Fig. 1 is a perspective view, partially exploded, of a first example of a subsea manifold; Fig. 2 is a front elevation of the manifold; Fig. 3 is a plan view of the manifold with lids removed; Fig. 4 is a cross-sectional end view of part of a header assembly of the manifold; Fig. 5a is a plan view of a pigging loop of the manifold; Fig. 5b is a section on A-A of Fig. 5a.

Fig. 6a is a side view illustrating a first example of a pile and pile sleeve of the manifold; Fig. 6b is a detail view of item 1 of Fig. 6a; Fig. 6c is a cross-section on A-A of Fig. 6b with a connecting pin in place; Fig. 7a is a side view illustrating a second example of a pile and pile sleeve of the manifold; Fig. 7b is a detail view of item 1 of Fig. 7a; Fig. 7c is a cross-section on A-A of Fig. 7b with a connecting pin in place; Fig. 8 is a perspective view of a second example of a subsea manifold with structural framework shown schematically; and Figs. 9a to 9c are an end elevation, side elevation and a plan view respectively of the manifold of Fig. 8 with structural framework details shown.

Referring to Figs. 1 to 3, the manifold is a unitary assembly in which a cage-like enclosure of welded steel tubulars, generally designated at 10, acts as both a structural support for pipework and valves, and a protective structure. The enclosure 10 is generally rectangular in plan and has a base 12, a top 14 and sloping sides 16. The top 14 is closed in normal use by removable lids 18 formed by welded tubulars 20 closed by corner plates 22 and gratings 24. The assembly is pinned to the seabed by piles inserted through pile sleeves 26, as will be described in more detail below.

An important feature of the invention is that the sides 16 of the enclosure 10 slope inwardly at an angle of 55 to the horizontal, and are provided with protective gratings 28 which extend from the top 14 only half-way to the base 12. It has been found that this combination is effective in protecting the pipework from trawl gear, the angulation of the sides 16 causing trawl boards to bounce or flip up over the lower portions of the sides 16. The elimination of gratings or plating over approximately half the surface area of the sides 16 gives a useful weight saving. It is preferred to stiffen the corners with plating 30, either in the upper part only as in Fig. 1, or to the full depth as in Figs. 2 and 3.

The pipework within the enclosure 10 in this embodiment connects five pipelines 32 with eight production flowlines 34 and gas lift flowlines 36. The pipelines 32 comprise two production lines 32a and 32b, a production/test line 32c, a gas lift line 32d, and a water injection line 32e. The pipelines 32a-3 communicate with respective headers 38a-38e to which the flowlines 34, 36 are connected.

The headers 38 are arranged in a vertical plane. This makes more economical use of space than does the customary horizontal array (given that a certain height of protective structure would be required in any event), thus allowing the volume of the protective structure to be reduced and hence reducing the weight of the assembly. It may be possible to obtain similar benefits or a portion of the benefits without requiring the headers 38 to be arranged in a vertical plane.

However, the central longitudinal axis of one header is above or below the central longitudinal axis of any other header.

Fig. 4 shows a typical tie-in, in this case to the production/test header 38c, comprising a pipe loop 40 passing around and across the top of the array of headers 38. The loop 40 includes a manual flow control valve 42 and a remotely actuated flow control valve 44, the valves 42 and 44 being disposed vertically on either side of the headers 38. This arrangement again optimises use of volume within the enclosure 10, allowing overall weight to be minimised; it also gives good access to the valves for a diver. The section of the loop 40 between the valves 42 and 44 is constructed as a spool connected by bolted flanges, and can thus be removed if it is necessary to carry out major work on the headers or valves. The manifold also incorporates a pigging loop generally designated at 46 in Figs. 1-3 and shown in greater detail in Figs. 5a and 5b.The pigging loop 46 is arranged in a vertical plane and across the pipelines 32 which, once again, maximises use of space and reduces overall weight. The pigging loop 46 includes a Y-piece 48, a remotely actuated control valve 50, and a removable spool portion 52.

Figs. 6a-6c illustrate a first example of a connection of a pile 54 with one of the pile sleeves 26. In this embodiment, the pile 54 and sleeve 26 are cylindrical tubes. The top of the sleeve 26 may be enlarged to form a guide for inserting the pile 54. The sleeve 26 is formed with a pair of aligned horizontal slots 56 and the pile 54 with a pair of aligned vertical slots 58. The slots 56, 58 are of the same width and cooperate to form a through aperture for receipt of a connecting pin 70 secured in position by a flange 72 at one end and a locking pin 74 through the other end.

The connecting pin 70 is omitted in Fig. 6b for clarity. This arrangement gives a simple manner of attaching the manifold to the seabed, and the crossed slots allow a degree of tolerance in the axial and rotational positioning of the pile when driven.

Figs. 7a to 7c illustrate a second example of a connection of a pile 54 with one of the pile sleeves 26. As in the first example, the pile 54 and sleeve 26 are cylindrical tubes. However, in this second example, the sleeve 26 is formed with a pair of aligned slots 75 at 45O to the vertical and the pile with a pair of aligned slots 76 (shown in phantom) also at 45O to the vertical. However, as shown in Fig. 7b, the slots 76 are at right angles to the slots 75 so that the slot 75 extends from the lower left to the upper right and the slots 76 extend from the upper left to the lower right.

The slots 75, 76 are of the same width and cooperate to form a through aperture for receipt of a connecting pin 70 secured in position by a flange 72 at one end and a locking pin 74 through the other end. The connecting pin 70 is omitted in Fig. 7b for clarity. This arrangement gives a simple manner of attaching the manifold to the seabed, and the crossed slots allow a degree of tolerance in the axial and rotational positioning of the pile when driven. In addition, the slots 75, 76 have the advantage that their orientations allow the manifold to be rigidly secured to the piles when all the pins 70 are inserted.

Fig. 8 shows a second example of a subsea manifold 100.

In this example the subsea manifold 100 is in the form of a flow line bundle pullhead (or towhead). As in the first example, the subsea manifold 100 comprises a structural framework 101 which encloses and supports five headers 102, 103, 104, 105 and 106. The headers 102-106 are mounted vertically within the structural framework 101, as shown in Fig. 8 and Fig. 9b. The arrangement of the headers 102-106 and the associated pipework is very similar to the arrangement shown in Figs. 1 to 5 and includes wellhead flowline tie-in flanges 107 for coupling the headers to the subsea wellhead flowlines and control valving 108 and associated pipework 109 for controlling flow of fluid to or from the headers 102-106 from or to the wellheads to which the headers 102-106 are coupled via the tie-in flanges 107.The pipework associated with the control valves 108 and tie-in flanges 107 for conveying fluid flow to or from the headers 102-106 includes removable spools 110 to facilitate maintenance of the valves 108.

Each of the headers 102-106 has a flange 111 at one end to permit extension of the manifold if necessary. At the other end the headers 102-106 extend into a flowline bundle carrier pipe 112 and the structural framework 101 is coupled to the flowline bundle carrier pipe 112 via a collar 113.

The structural framework 101 is shown in more detail in Figs. 9a, 9b and 9c. Extending from the collar 113 are two lower structural members 114 and two upper structural members 115. The structural members 114 are each coupled to a horizontal structural member 116 which extends along the lower length of the headers 102-106. At the end of the structural members 116 remote from the collar 113, the structural members 116 are coupled together via structural members 117. The members 115 extending from the collar 113 are coupled to large diameter tubular members 118 which extend along the upper edges of the framework and along the length of the headers 102-106. Extending between the large diameter tubular members 118 are four large diameter cross members 119.At the ends of the tubular members 118 remote from the pipeline bundle 112 two structural members 120 couple tubular members 118 to the members 117 and to a coupling point 121.

Extending between the structural members 116 are four cross members 122 and four vertical members 123 on each side of the framework 101 couple the lower members 116 to the upper tubular members 118.

The advantage of the second example of the subsea manifold shown in Fig. 8 and Figs. 9a to 9c is that the manifold can be used as a pullhead (or towhead) for a pipeline bundle. The large diameter tubular members 118 provide buoyancy for the manifold and have control means coupled to the members 118 for controlling the buoyancy of the manifold. A vessel for pulling the pullhead may be coupled to the structural framework 101 using the coupling point 121 in order to permit movement of the subsea manifold 110 to the desired location.

Improvements and modifications may be incorporated without departing from the scope of the invention.

Attention is drawn to our copending application No.

9324251.9, which claims the subsea manifold described herein.

Claims (18)

1. An anchoring arrangement for securing a structure to the seabed, the arrangement comprising a vertical guide forming part of the structure, a pile driven through the guide into the seabed, and locking means for locking the pile to the guide.

2. An anchoring arrangement according to claim 1, wherein the locking means comprises slots formed at an angle to each other in the guide and the pile, and a locking device, the slots cooperating, once the pile is driven, to form a through aperture in which the locking device is inserted.

3. An anchoring device according to claim 2, wherein the locking device comprises a connecting pin and a locking pin, the connecting pin being inserted through the slots and secured in position by a flange at one end and the locking pin through the other end.

4. An anchoring arrangement according to claim 2 or claim 3, wherein, the slots in the pile and the guide are at angles of 45O to the vertical, and are at right angles to each other.

5. An anchoring arrangement according to any of the preceding claims, wherein the pile is tubular and the guide is a tubular sleeve.

6. An anchoring arrangement according to any of the preceding claims, wherein the tubular sleeve comprises part of a subsea manifold.

7. An anchoring arrangement for securing a structure to the seabed substantially as hereinbefore described, with reference to any of the accompanying drawings.

8. A subsea manifold comprising a support structure and an assembly of pipework mounted on the support structure, the pipework including an array of headers, each header being connected in use to a respective pipeline, and each header having means for controlled connection to a plurality of wells, and in which the headers are arranged such that the central longitudinal axis of any header is above or below the central longitudinal axis of at least one of the other headers.

9. A subsea manifold according to Claim 8, wherein the central longitudinal axis of each header is either above or below the central longitudinal axis of any other header.

10. A subsea manifold according to Claim 9, wherein the central longitudinal axes of the headers define a non-horizontal plane.

11. A subsea manifold according to Claim 10, wherein the non-horizontal plane is a vertical plane.

12. A subsea manifold according to any of Claims 8 to 11, wherein each header is connected to each well via a respective pipe loop which includes a first vertical section to one side of the header array, a top section passing across the header array, and a second vertical section to the other side of the header array, the means for controlled connection comprising at least one valve included in one of the vertical sections.

13. A subsea manifold according to Claim 12, wherein the top section of the pipe loop forms part of a removable spool.

14. A subsea manifold according to any of Claims 8 to 13, wherein the manifold includes a pigging loop in circuit between the pipelines and the headers, the pigging loop being disposed in a non-horizontal plane.

15. A subsea manifold according to any of Claims 8 to 14, wherein the manifold is coupled to a pipeline bundle to form a towhead or pullhead.

16. A subsea manifold according to Claim 15, wherein the manifold also includes adjustable buoyancy means to alter the buoyancy of the manifold.

17. A subsea manifold according to Claim 16, wherein the buoyancy may be, at least partially, provided by tubular members in the support structure.

18. A subsea manifold substantially as hereinbefore described, with reference to the accompanying drawings.