GB2337542A

GB2337542A - Riser pipes

Info

Publication number: GB2337542A
Application number: GB9811118A
Authority: GB
Inventors: Kevin Burton
Original assignee: UWG Ltd
Current assignee: UWG Ltd
Priority date: 1998-05-23
Filing date: 1998-05-23
Publication date: 1999-11-24
Also published as: GB9811118D0

Abstract

A well riser is made up of a number of sections connected end to end. Each section 30 has a helical tube or tubes 40 wound around a central cylindrical member 32, to suppress vortex-induced vibration. The helical tube or tubes can carry fluids both fom the wellhead and to the wellhead.

Description

Riser Pipes 2337542 This invention relates to riser pipes for subsea wells

and for floating production facilities (FPF). A riser pipe is used between a drilling rig or FPF above the water surface and a wellhead or wellhead manifold on the seabed. Drilling of the well is carried out through a riser and, after the drilling phase, production from the well passes up a riser to the rig platform.

In the drilling and use of oil and gas wells, a wellhead is set in the seabed, and the rig is positioned directly above the wellhead. If the rig is a floating rig, the position of the rig over the wellhead can be accurately set by paying out or taking in on the rig mooring lines or by use of dynamic positioning techniques.

The riser then extends vertically down through the water to the wellhead, and tension is put in this riser to pull it straight between the wellhead and the rig. In some cases, sections of the riser are fitted with buoyant jackets or the like below the water surface to assist in tensioning the riser between the well and the rig.

Where there is a current running in the water where the rig is moored, the current will create stresses in the riser as it flows past the riser. The water current may be the result of a tidal flow, in which case the direction 30 of flow will reverse at regular intervals, or may be a permanent current. It is known that the f low of water past a cylindrical object which is positioned at right angles to the flow results in vortices building up alternately on opposite sides of the cylindrical body. 35 Each vortex builds up to a size where it detaches from the body and a new vortex then builds up on the other -side of SP214 6. A2 22 May, 1998 the body. This process is known as vortex- shedding, and it leads to the cylindrical body (in this case the riser) vibrating from side to side as vortices are shed first from one side, then from the other.

It is an object of this invention to reduce the damaging effects of vortex shedding on well risers.

According to the invention, there is provided a well riser comprising a central elongate member with at least one auxiliary tube following a helical path surrounding the main elongate member.

The central elongate member is preferably cylindrical, and 15 the riser is preferably made up of a number of elongate riser sections joined end to end.

The invention also provides a well riser section comprising a central elongate member with at least one auxiliary tube following a helical path surrounding the main elongate member, and means for joining the section to adjacent sections with the or each tube communicating with a corresponding tube in an adjacent section.

The main elongate member may be a pipe forming the main path along the riser, when the riser is a drilling riser. Alternatively the member may be an elongate strength member to take the tensile forces introduced when the riser is tensioned.

Particularly in the case of a drilling riser, the applicants have found that the use of one or more helical tubes surrounding the main pipe produces a reduction in vortex-shedding and thus a reduction in vibration, apparently because the current incident on the riser encounters an asymmetric profile, from whichever direction SP2146.A2 22 May, 1998 the current flows. This avoids the situation where a vortex builds up first on one side, then detaches and then another vortex builds up on the other side. detaches and so forth. Because this situation is avoided, the riser is no longer subject to rapid fluctuations in hydrostatic pressure and it is therefore not vibrated from side to side.

The riser is preferably made up from a number of elongate 10 sections which can be joined end to end with their tubular bores communicating with one another.

The auxiliary tubes surrounding the main tube may carry fluid between the rig and the wellhead and can function as choke and kill lines, as production flow lines or as injection flow lines.

There may be one or more auxiliary tubes. The all follow substantially the same helical different helical paths between the ends of the riser section. The tubes may be secured to cylindrical member at points long their length. may all follow a helical path of one hand, or twist in one direction and others in the direction.

tubes may path, or riser or the main The tubes some may opposite The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a drilling rig in position above a wellhead; Figure 2 is a perspective view of a riser section in accordance with the invention; SP214 6. A2 22 may, 1998 1 i g 1 1 4 Figure 3 is a cross-section through a riser section joint; and Figure 4 is a cross-section through an alternative riser section joint.

The drilling rig 10 of Figure 1 floats on the sea surface 12 and is moored by mooring lines 14. The rig 10 is supported by buoyancy chambers 16 in a conventional manner.

Figure 1 also shows the seabed at 18, with a wellhead schematically indicated at 20. The wellhead 20 is fitted with a blowout preventer 22 and a lower marine riser package 24. A riser pipe 26 then extends between the wellhead 20 and the rig 10.

The riser is shown in Figure 1 following a curved path. However drilling engineers endeavour to arrange the positions of the rig and the wellhead, and the tension in the riser so that the riser follows as nearly as possible a straight line between the rig and the wellhead.

The riser 26 is conventionally assembled from a number of separate riser sections. The water depth where this type of arrangement is used may be as much as 3000 metres, and each riser section will be about 15 - 18 metres long. Any particular riser pipe 26 may therefore be made up of 10 or more riser sections.

A drilling riser section 30 in accordance with the invention is shown in Figure 2. The riser section has a pipe 32 with a hollow bore which forms the main passage from the rig to the wellhead. At each end the pipe 32 has a flange 34,36 with bolt holes 38 by means of which it can S P214 6. A2 22 May, 1998 be connected to a corresponding flange of an adjacent pipe section.

Around the pipe 32, a smaller diameter tube 40 follows a helical path. This tube ends in sockets 42 in the flanges 34, 36, and when two tubes are connected together, the socket 42 will be in fluid communication with a corresponding socket on the adjacent flange, as will be described with reference to Figures 3 and 4.

In Figure 2, only one helical tube 40 is shown. In practice there are likely to be at least two and possibly as many as four such helical tubes extending between the flanges 34 and 36 and arranged so that they communicate with corresponding tubes in adjacent riser sections, when the sections are coupled together. The tubes may all lie directly adjacent to one another so that there is effectively only one helical body wound around the pipe 32, or they may be regularly spaced around the pipe 32.

The tubes may all follow the same helical path, ie., they can be parallel to one another or they can follow different helical paths and may even follow differently handed helical paths.

The tubes may be welded or otherwise fixed to the pipe 32 at intervals, to support the tubes.

Figures 3 and 4 show two alternative details of a joint between riser pipe sections. In these figures, components on one section are indicated by reference numerals with the suffix "'a"and corresponding components on the adjacent riser section are denoted by reference numerals with the suffix "b" SP2146.A2 22 May, 1998 - 6 It is important that the small diameter pipes 40 have some freedom of movement relative to the flanges 36. This allows a degree of relative movement on changes in pressures within the different pipes. The tubes 40 are therefore located in stepped bores 44 in the flanges 36 and are sealed in these bores by means of resilient O-ring seals 46. The flanges 36a, 36b are connected by a bolt 48 with a nut 50 and a sealing gasket 52 is fitted between the flanges.

Figure 4 shows an alternative arrangement where the lower riser section 132b has a male end which locates within a f emale socket at the lower end of the upper riser section 132a. A segmented dog 54 drops into recesses in both sections to secure them together. The small diameter tube 40b on the lower riser section is located in a flange 56, and the corresponding tube 40a from the upper riser section is housed within the female socket 58 of the upper riser section 132a. On assembly, the upper end of the lower tube 40b is received in the bore in the female socket 58, and the seals 46a, 46b ensure the necessary pressure tightness of the joint.

In use, a riser pipe 26 will be assembled from the necessary number of sections 30 to cover the distance from the rig to the wellhead. It is known that sea currents can be different at different water depths. If desired, different configurations of the small diameter tubes 40 may be used for parts of the riser pipe at different depths, and indeed some of the sections in a particular riser pipe may have small diameter tubes 40 which are parallel with the main pipe 32, rather than following a helical path.

It is conventional in riser pipes to have a main pipe 32, and also to have choke and kill lines running alongside SP2146.A2 22 May,1998 - 7 the main pipe. These choke and kill lines are used to either take out drilling mud from the well, to pump additional mud into the well or, in the form of mud booster lines, to pump additional mud down the well in an 5 emergency.

Conventionally, these choke and kill lines run in straight lines parallel to the pipe 32, but the auxiliary tubes 40 running around the main pipe 32 can carry the choke and kill flows.

The provision of an irregularity in the circular circumference of the tube 32 will be effective in preventing the establishment of regular sequential vortexes on opposite sides of the tube, and thus will prevent the conventional problems associated with vortex shedding. Because the irregularity follows a helical path, whatever direction the current comes from the current will see a non-symmetrical shape.

The riser described thus far is used for drilling. Once the wellhead is complete, the drilling riser can be replaced by a production riser which can have a single central strength member surrounded by a number of helical production pipes up which the production fluid flows. There may be as many as 6 or 8 or more of these helical production pipes.

The riser sections described allow a riser pipe to be put together with inherent resistance to vortex-inducedvibration, using the choke and kill lines which are conventionally present in such structures.

SP214 6. A2 22 May, 1998

Claims

8 Claims

1. A well riser comprising a central elongate member with at least one auxiliary tube following a helical path surrounding the main elongate member.

2. A well riser as claimed in Claim 1, wherein the central elongate member is cylindrical.

3. A well riser as claimed in Claim 1 or Claim 2, made up of a plurality of elongate riser sections joined end to end.

4. A well riser section comprising a central elongate member with at least one auxiliary tube following a helical path surrounding the main elongate member, and means for joining the section to adjacent sections with the or each tube communicating with a corresponding tube in an adjacent section.

5. A well riser section as claimed in Claim 4 for a drilling riser, wherein the main elongate member is a cylindrical pipe forming the main path along the riser.

6. A well riser section as claimed in Claim 4 for a production riser, wherein the main elongate member is an elongate strength member to take the tensile forces introduced when the riser is tensioned.

7. A well riser as claimed in any one of Claims 1 to 3, 1 or a well riser section as claimed in any one of Claims 4 to 6, wherein the auxiliary tubes surrounding the main tube are for carrying fluid between the rig and the wellhead and act as choke and kill lines SP214 6. A2 22 may, 1998 - 9 8. A well riser as claimed in any one of Claims 1 to 3, or a well riser section as claimed in any one of Claims 4 to 7, wherein the auxiliary tubes surrounding the main elongate member are for carrying fluid between the rig and the wellhead and act as production flow lines.

9. A well riser as claimed in any one of Claims 1 to 3, or a well riser section as claimed in any one of Claims 4 to 7, wherein the auxiliary tubes surrounding the main elongate member are for carrying fluid between the rig and the wellhead and act as injection flow lines.

10. A well riser as claimed in any one of Claims 1 to 3, or a well riser section as claimed in any one of Claims 4 to 9, wherein the auxiliary tubes all follow substantially the same helical path between the ends of the riser or riser section.

11. A well riser as claimed in any one of Claims 1 to 3, or a well riser section as claimed in any one of Claims 4 to 10, wherein the auxiliary tubes are secured to the main elongate member at points long their length.

10. A well riser section substantially as herein described with reference to the accompanying drawings.

SP214 6. A2 22 May, 1998