GB2182695A - Multi tube riser flexjoint - Google Patents

Abstract

A flexjoint arrangement for a multi tube riser for offshore oil or gas production is described. In its centre, there is a riser pipe containing multiple production tubes (2). The inner tubes are bent to a minimum radius as the outer pipe (1) deflects at the flexjoint. The flexjoint pivot point is offset from the centre line of the riser to compensate for the length differences between the bent inner pipes (2) and the kinked outer pipe when the riser is deflected at an angle. The pivot point, when the riser is deflected, may be between one of a ring of bearing pads (7) and pads (8) on the underside of a guidance assembly (5, 6) including a guide funnel (10) which acts to stop the tubes (2) being bent below a certain radius of curvature. <IMAGE>

Description

SPECIFICATION Multi tube riser flexjoint The present invention relates to the use of risers for the offshore oil and gas industry. A riser is the structural pipe that connects equipment on the seafloor to a floating vessel at the surface. Because the surface vessel can move in its mooring due to environmental forces the riser also moves, resulting in an angular deflection. A flexible joint is therefore usually used at the base of the riser to allow for this angular movement. Several types of flexible joint have been used, such as ball joints, universal joints, and layered elastomer/ steel joints (a Murdock joint). These provide satisfactory solutions when a single pipe is used. A problem is created when several pipes are involved. The usual application is when several tubes are housed inside a larger pipe such as when a riser is used for a subsea well workover or for production.Well access is through the internal tubes, and structural loads are taken by the outside pipe. The present invention specifically relates to the flexing of a riser with internal tubes.

One solution is to use a version of the Murdock joint, but with multi bores. However, this adds complications in that extra moving parts are required to keep the elastomer in compression. There is also a kink produced which restricts the movement of tools through the tubes. The device is restricted to small angles since the tubes are displaced relative to one another which increasingly restricts through access as the angle increases.

Another solution is to avoid using a flexing joint and allow the pipe and tubes to deflect.

In this case the outside pipe is gradually strengthened towards its base to enable it to take high bending movements. These are referred to as stress joints. Their significant disadvantage is the large bending moment that they place into the structure below the stress joint.

The theoretical considerations are easier appreciated diagrammatically than in words, and for clarity of explanation reference should be had to the accompanying drawings. In these drawings:, Figure 1 shows the large-bending moment referred to above which is created by a rising deflecting at its base Figure 2 diagrammatically shows an arrangement including an outer case Figure 3 shows the relative length change between inner tubes and outer case when a riser is deflected Figure 4 shows how the relative length change shown in Fig. 3 may be compensated and, Figure 5 is a schematic perspective view of a riser flexjoint in accordance with the present invention connected to a riser base.

As just noted, Fig. 1 shows the disadvantage of the large bending moment at the bottom of the riser. This high bending moment can be reduced by putting flexunits in the outside pipe or outer case. The radius of curvature of bend can therefore be made smaller to suit that allowed by the smaller diameter internal tubes. This can be accomplished in two ways. Firstly, multiple flexjoints can be used so that the outside pipe approximates with several straight sections, the curve of the inside tubes. This may be accomplished as described and illustrated in British Patent Specification 2135748A. A second alternative is to have a single flexjoint at the base of the riser and allow the internal tubes to flex inside a large diameter riser above the flexjoint as shown in Fig. 2. Provided the flexjoint is kept at the lowest position there is no offset and thus no induced bending moment.A problem arises, however, with a relative length change between the internal tubes and the external pipe. This is illustrated in Fig. 3. When the riser is in a vertical position the length of the tubes and pipe are the same. When the riser is deflected, it is important that the inner tubes be kept within an acceptable stress level. Thus there will be a minimum radius of curvature to which the tubes can be bent. One geometry is shown in Fig. 3 where the tubes bend away from the vertical towards the centre line of the angled outer pipe and then go through a reverse bend, of the same minimum radius, to become concentric and parallel to the centre line of outer pipe. Because the centre line of tubing travels a longer path than the centre line of the outer pipe, the top of the tubing will be in a lower position relative to the pipe.The only way to overcome this directly is to put a flexjoint in the tubes which defeats the objective; or a sliding joint arrangement for the tubes can be used - but these usually give rise to problems in practice.

Alternatively, the relative length change can be accommodated by shortening the outer case. This has to be done while still being able to transmit the riser structural loads. Fig.

4 illustrates the geometry of how this is accomplished. Instead of a central pivot, a circular disk is attached to the bottom of the riser outer pipe. When the riser moves at an angle the outer edge of the disk becomes the pivot point. As the riser deflects, point 'E' on the outer pipe moves below point 'A' on the inner tubing, effectively shortening the length of the pipe relative to inner tubes. The radius of the disk determines the amount of relative length change. It is therefore possible to compensate exactly for the difference in path lengths between the curved tubes and the straight riser pipe. The exact compensation is true for the shapes shown in Fig. 4, however, for only one selected angular position.

Having picked a particular radius for the pivot disk at the bottom of the riser to suit a particular angle, any other angle will not exactly compensate for relative length changes.

However, the main objective is to keep the pipes above a minimum bend radius. This is achieved by setting the geometry described above to the maximum expected riser angle.

This is the case where the inner tubes will be deflected the most and will be at the predetermined stress level. For smaller angles the reverse phenomenon happens and the inner tubes have a longer length. The geometry of the inner tubes will change by having an additional reverse curve at either end of the tubes.

Although various shapes of curves for the inner tubes have been suggested, these will not necessarily happen of their own accord. Bend restrictors and additional stiffening can be added to ensure that the minimum bend radius is not exceeded and the flexure is not concetrated in local lengths.

According to a first feature of the present invention there is provided a method of providing a flexjoint in a riser pipe containing multiple production tubes whereby the inner tubes are bent to a minimum radius as the outer pipe deflects at a flexjoint, which comprises providing a pivot point for the flexjoint offset from the centre line of the riser such that when the riser is deflected from the vertical, the outer pipe pivots about the flexjoint pivot point whereby to compensate for length differences between the inner pipes (which then assume a bent configuration) and the outer pipe.

The present invention also provides an improved flexjoint assembly for a multi tube riser having a riser base, a set of multiple production tubes extending. vertically from the base, and an outer pipe surrounding the inner tubes, the outer pipe being arranged to pivot, when the riser moves from a vertical position, about a point laterally offset from the centre of the inner tubes.

A preferred embodiment is shown, by way of illustration of how the invention can be put into effect, in Fig. 5. A riser outer pipe 1 houses internal tubing 2. The tubing terminates at the base in a connector assembly 3.

The arrangement is shown attached by the connector 3 to a riser base 4. A guidance assembly consisting of a guide cone and temporary locking assembly 5 and a sliding guide 6 is provided. The riser outer pipe 1 is attached to the connector assembly 3 via bearing pads 7 and 8. Pad 7 is attached to the outer pipe 1 and pad 8 is attached to the connector assembly 3. Bearing pads are used to avoid point contact which would create high local stresses. The pads therefore distribute the load over a large area reducing the contact stress. They are made out of a suitable bearing material.

With the riser at an angle, only one bearing pad takes the riser tension load. If the riser deflects in one plane then it pivots about the center of radius of that one bearing pad. If the plane of deflection changes then contact changes to the next bearing pad. To provide this capability the bearing pads have a spherical contact surface. Thus the riser can deflect in any direction and the load will automatically be taken by the appropriate pad in the ring of bearing pads. The bearing pads, being offset from the centre line of the riser, will provide the outer pipe length shortening described previously to compensate for the difference in lengths between the outer pipe and the inner tubes.

In order to avoid differential length problems between individual inner tubes they can all be rotated as an assembly through at least 180 degrees of twist. To avoid torque problems the pipes can be divided into groups, with one set twisting one way and the other twisting the other way. Also the pipes can all be joined together by plates such as 9 to ensure they all act together. A guide funnel 10 is provided to prevent the tubes being bent tighter than the minimum bend radius as described earlier.

The guide assembly consisting of 5 and 6 illustrates how the flexjoint assembly can be used in conjunction with a riser guidance and locating device described in Specification 2135748A. In Fig. 5, a riser base 11 illustrates a typical riser base for a floating production system where flowlines from satellite wells are gathered from individual wells to flow up the riser.

Although Fig. 4 shows a floating production system riser application for the flexjoint where angles over 30 degrees are required, it can also be used for workover risers and extended well testing risers. In this case smaller angles are required, which will make the joint much smaller in diameter, but more importantly, will allow through bore access which is essential.

The deflected inner tubes of the present invention are ideal for this use.

Claims (10)

1. A method of providing a flexjoint in a riser pipe containing multiple production tubes, whereby the inner tubes are bent to a minimum radius as the outer pipe deflects at a flexjoint, which comprises providing a pivot point for the flexjoint offset from the centre line of the riser such that when the riser is deflected from the vertical, the outer pipe pivots about the flexjoint pivot point whereby to compensate for length differences between the inner pipes (which then assume a bent configuration) and the outer pipe.

2. A method according to claim 1 which comprises providing, around the inner tubes, a ring of bearing pads and providing on the outer pipe a ring of bearing pads co-acting therewith.

3. A method of providing a flexjoint in a riser pipe substantially as hereinbefore de scribed with reference to Fig. 5 of the accompanying drawings.

4. A flexjoint arrangement for a multi tube riser having a riser base, a set of multiple production tubes extending vertically from the base, and an outer pipe surrounding the inner tubes, the outer pipe being arranged to pivot, when the riser moves from a vertical position, about a point laterally offset from the centre of the inner tubes.

5. A flexjoint arrangement according to claim 4 wherein the inner tubes are surrounded by a bearer ring and the outer pipe is adapted to bear against a position on the bearer ring when the outer pipe is deflected from the vertical.

6. A flexjoint arrangement according to claim 5 wherein the outer pipe comprises a ring of individual bearer pads each adapted to contact the bearer ring surrounding the inner tubes.

7. A flexjoint arrangement according to claim 6 wherein the bearer pads each have a convex or part spherical bearing surface.

8. A flexjoint arrangement according to any one of claims 5 to 7 wherein the bearer ring includes a guide funnel located about the lower end of the inner tubes to prevent the inner tubes being bent tighter that a given minimum bend radius.

9. A flexjoint arrangement according to any one of claims 4 to 8 and including one or more plates joining the inner tubes together to maintain them in a close spaced arrangement.

10. A riser flexjoint arrangement substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawings.