GB2452961A

GB2452961A - Riser pipe

Info

Publication number: GB2452961A
Application number: GB0718388A
Authority: GB
Inventors: Stephen Anthony Hatton; Timothy Eyles; Glen Jewell
Original assignee: 2H Offshore Engineering Ltd
Current assignee: 2H Offshore Engineering Ltd
Priority date: 2007-09-21
Filing date: 2007-09-21
Publication date: 2009-03-25
Also published as: GB0718388D0

Abstract

A riser has first and second sections 20, 22 joined by a swivel joint 24 through which flow call pass. One section 20 is suspended from a surface vessel 12 and tile other section 22 is attached to the bottom of the first section through a swivel joint 24, so that the opposite end of the second section can vary its position on the seabed through the presence of the swivel which allows the free end of the second section (usually attached to a piece of subsea equipment 14,) to move around without having to move the surface vessel.

Description

RISER PIPE

BACKGROUND

a. Field of the invention

The present invention relates to sub-sea pipelines and risers for use in the elevation of materials from the seabed to the surface and in particular to use in sub-sea mining applications.

b. Related Art Risers are dynamic tubular submerged structures used to elevate fluids and materials from the seabed to the surface, and are particularly used for the

extraction of oil and gas from offshore fields.

For some seabed applications, such as but not limited to a seabed mining application, the seabed equipment (e.g. a mining tool) will not stay in a single fixed position on the seabed but will be required to move around. This will encompass both horizontal and vertical motions, due to changes in seabed elevation. This requirement provides a limitation on the vessel which will be :. unable to follow the movement of the mining tool directly, due to the watch circle associated with the Dynamic Positioning (DP) or mooring line capabilities of the ***i vessel. It will also be important to isolate the mining tool from any quasi-static or :: 25 dynamic loads imparted by environmental loading on the riser or the surface vessel. * S. * S *

S.. * . . . A dynamically positioned surface vessel may move by small amounts (typically e tens of metres) from its nominal position, and a moored vessel may move by typically �5% of the total water depth. The riser may also be displaced laterally due to the drag loading imparted on the riser by current. For applications such as seabed mining, it is also anticipated that the seabed equipment will be required to move around on the seabed, and it will be desirable to do this without the need to accurately relocate the surface vessel. This is particularly true for a moored vessel, where the relocation of the vessel is a complex task requiring the assistance of additional support vessels.

A design solution is required that allows the transport of fluids and materials from the seabed equipment (mining tool) to the surface vessel. The design solution should have a high degree of compliance to accommodate the relative displacements at the top, bottom and along the length of the pipe. Flexible pipe technology does exist which allows a pipe to exhibit a low bending stiffness.

This flexible pipe may be used for all or just a section of the riser length.

However due to its construction method it is costly particularly when a deepwater rating (up to 2500m) is required. Flexible riser technology may also be unable to meet the high internal wear requirements of a subsea mining application in which the slurry may contain abrasive particles travelling at high speeds.

SUMMARY OF THE INVENTION

:. According to the invention, there is provided a riser pipe having separate first and second sections, each section being in the form of a rigid metal pipe, and * *S.

the sections being joined end to end by a swivel joint which allows flow between 25 the pipe sections and allows the sections to take up positions where one section is substantially at right angles to the other section. * **

:.:.* A riser pipe of this type can be used with a first section being supported at its upper end on a surface vessel, with that first pipe section extending vertically towards, but not reaching, the sea bed and the other, second section being connected, through the swivel joint, at the bottom of the first section and extending generally parallel to the sea bed, le at right angles to the first section.

However as the sea bed will not be flat, the swivel will allow the second section to alter its vertical angle relative to the first section. The swivel joint will also allow the second section to move in a circle around the axis of the first section.

The riser thus provides a means of allowing fluid transfer from equipment landed on the seabed, along the second pipe section to the base of the free-hanging first pipe section, whilst accommodating relative motions of the seabed equipment and the free-hanging pipe section.

Preferably both pipe sections will have substantially the same flow cross-section.

The riser system will usually be used with the base of the free-hanging riser at a distance above the seabed, such that the second pipe section is orientated at a shallow angle to the horizontal plane. Any change in relative position of the seabed equipment, or the base of the riser, will be accommodated by a small change in the angle of the second pipe section. It is noted that the significant length to diameter ratio of both the pipe sections will allow large deflections of the riser to be accommodated, as although the pipe sections are described as :. "rigid" in long lengths they will be able to bend to some degree. For example, the first pipe section may have a length of 2500 meters, and current and other *S..

loadings can cause this section to significantly move out of a straight line configuration.

S. Si** * S * *, The second pipe section may be fitted with external buoyancy modules to make it neutrally buoyant in its installed position. This reduces the in-place stress *** * loading on the second pipe section. As the second pipe section will be self-supporting, the stress on the free-hanging first pipe section will also be decreased.

The swivel joint allows articulation and rotation between the two pipe sections.

A further swivel joint may be fitted between the end of the second pipe section and the seabed equipment, to allow for movement of the seabed equipment (for example this equipment may be an excavator which travels on the sea bed).

The swivel joint is preferably symmetrical about a plane containing the axes of both the first and second pipe sections. To allow this symmetry, the flow passage through the swivel joint may be divided into two passages, one either side of the plane.

Thus rotation can be allowed in both the vertical and horizontal planes at the interfaces with the seabed equipment and with the free-hanging first pipe section. A short stiffened or tapered joint section may be included at one or both ends of the second pipe section in order to reduce the stress concentration at the interfaces with the seabed equipment and the free-hanging first pipe section.

:. Subsea connections at both ends of the second pipe section will allow for easy installation and retrieval of that section. The use of standard commercially **a available pipe allows prompt and cost-effective replacement of sections of pipe with spare joints if necessary. * *

* ** BRIEF DESCRIPTION OF THE DRAWINGS * * * * **.

The invention will now be described, by way of example only, with reference to the following Figures in which: Figure 1 shows a schematic depiction of a riser pipe according to the present invention; and Figure 2 shows a schematic depiction of a swivel which could be used at one or both ends of the second pipe section.

DETAILED DESCRIPTION

Figure 1 shows a schematic depiction of a riser 10 connected between a surface vessel 12 and seabed equipment 14. The sea surface is at 16 and the seabed at 18.

The riser 10 has a first pipe section 20 and a second pipe section 22 connected to one another through a swivel joint 24. The first section 20 is suspended vertically from the surface vessel 12, and the second section extends generally horizontally from the bottom of the section 20 to the seabed equipment which is offset from the first section. There is no fixed connection between the bottom of the first pipe section and the seabed, and the bottom of the vertical pipe section can typically be positioned about 50m above the seabed.

:. The seabed equipment 14 (for example, a mining tool) is located on the seabed :.::: at a position offset from the base of the free-hanging section. *S..

25 The pipe sections can be made up from sub-sections joined by standard rigid pipe joints. The pipe sections will normally be metal tubes. * I.

:.:.* The use of a lengthy rigid transfer pipe, with a lateral offset between the riser ** * and the seabed equipment, allows the system to accommodate a significant amount of relative motion between the seabed equipment and the riser base.

This will allow the seabed equipment to change location as required, as well as isolating the seabed equipment from any movement of the free-hanging riser.

The base assembly of the free-hanging riser may move laterally, due to current loading on the riser and offset of the surface vessel, and vertically due to vessel heave motions. The subsea equipment may move laterally across the seabed, and there may be some local changes in elevation also due to the nature of the seabed. The anticipated vertical motions are small in comparison with the expected horizontal motions.

Rotational devices 24 can be used at one or both ends to allow rotation of the second pipe section relative to the seabed equipment or relative to the bottom end of the free-hanging pipe. This could be one or more simple articulation elements or swivel joints allowing a limited amount of rotation in a single plane.

The joint should allow the bores of the two pipe sections to communicate with one another, so that whatever is to be collected from or beneath the sea bed can flow from the seabed to the sea surface.

Figure 2 shows a schematic representation of a suitable swivel joint 24. This joint allows 360 degrees of rotation in both the horizontal and vertical planes, while maintaining fluid transfer through a near symmetrical dual-pipe system through the joint.

The swivel joint 24 connects the first pipe section 20 with the second pipe section 22. Each pipe section is connected into a forging 26, 28. In the forging : 25 28, the flow passage of the pipe section 22 is split into two symmetrical flow channels 30, 32, each of which has a flow cross section equal to at least half * that of the pipe sections 20, 22. The channels 30, 32 then feed into two vertical swivels 34, 36 at the ends of the T-shaped forging 26, and the flows are **.

* recombined into one and then pass through a single horizontal swivel 38 and into the pipe section 20.

It will be understood that the preceding references to vertical risers are not intended to act as a geometrical limitation but as defining a functional difference over a catenary riser. In use, a vertical riser will define a vertical or substantially vertical path. S. * * * *** *S*. * . *5*.

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Claims

1. A riser pipe having separate first and second sections, each section being in the form of a rigid metal pipe, and the sections being joined end to end by a swivel joint which allows flow between the pipe sections and allows the sections to take up positions where one section is substantially at right angles to the other section.

2. A riser pipe as claimed in Claim 1, wherein the swivel joint allows the second section to alter its vertical angle relative to the first section.

3. A riser pipe as claimed in Claim I or Claim 2, wherein the swivel joint allows the second section to move in a circle around the axis of the first section.

4. A riser pipe as claimed in any preceding claim, wherein a first of the sections is adapted, in use, to be suspended vertically from the sea surface and the second of the sections is adapted to extend generally horizontally along the seabed.

5. A riser pipe as claimed in any preceding claim, wherein the second :. section has a swivel at both ends. * *s* * S*. * . * *..

6. A riser pipe as claimed in any preceding claim, wherein both pipe 25 sections have substantially the same flow cross-section.

**.*** * * * .*

7. A riser pipe as claimed in any preceding claim, wherein the second pipe * * * * * section is fitted with buoyancy modules to make it neutrally buoyant in its installed position.

8. A riser pipe as claimed in any preceding claim, wherein the swivel joint allows articulation and rotation between the two pipe sections.

9. A riser pipe as claimed in any preceding claim, wherein the swive' joint is symmetrical about a plane containing the axes of both the first and second pipe sections.

10. A riser pipe as claimed in Claim 9, wherein the flow passage through the swivel joint is divided into two passages, one either side of the plane.

11. A riser pipe as claimed in any preceding claim, wherein a short stiffened or tapered joint section may be included at one or both ends of the second pipe section.

12. A riser pipe substantially as herein described with reference to the accompanying drawings. * * * *.* ***. * S S... *

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