WO2012076703A2

WO2012076703A2 - Riser coupling

Info

Publication number: WO2012076703A2
Application number: PCT/EP2011/072360
Authority: WO
Inventors: Steinar Wasa Tverlid
Original assignee: Statoil Petroleum As
Priority date: 2010-12-10
Filing date: 2011-12-09
Publication date: 2012-06-14
Also published as: WO2012076703A3; NO20130953A1; GB2500540A; GB2500540B; NO345157B1; GB201312060D0

Abstract

An apparatus for flexibly coupling two sections of a subsea riser comprising: a sleeve having first and second ends for slidably receiving ends of respective first and second riser sections, such that each section operates as a piston within the sleeve.

Description

RISER COUPLING

Technical field

This invention relates to riser coupling, for example as used in deep water floating production systems.

Background

A riser is a pipe extending from a drilling platform on the sea surface to the seabed and serves to effectively extend a subsea well to a surface drilling facility. The riser may be connected at the seabed to a blowout preventer and a wellhead. At the sea surface, the riser may be connected to a drilling ship or platform. The riser may be used for transporting drilling mud from the borehole to the drill ship and also provides a channel for a drill pipe and other tools extending from the ship to the well head. Another type of riser is a production riser which provides a channel for the transport of hydrocarbons to the platform or ship. As the ship is located on the sea surface, the ship will move up and down as well as sideways with the swell on the sea surface. The well head maybe exposed to high dynamic loads due to the riser tension and angle. One of the components of the load is momentum, which eventually may cause fatigue or even failure in the well head. The industry is moving into deeper sea, which increases these problems.

Figure 1 shows a riser 1 , which may move with respect the sea bed and the well head 2, and which may therefore move out of axial alignment with respect to the well. A blowout preventer 3 is provided between the well head 2 and the riser 1. A flex joint 4 is provided between the riser 1 and the blowout preventer 3.

These and related problems are considered in the following prior art patent publications;

WO20051 13929, WO2008051092, WO2009102220, US3643751 , US5615977, GB2273119, GB1260157, US4039177, US2008/0251257. Summary

According to an aspect of the invention there is provided an apparatus for flexibly coupling two sections of a subsea riser comprising: a sleeve having first and second ends for slidably receiving ends of respective first and second riser sections, such that each section operates as a piston within the sleeve.

The first and second ends of the sleeve may each comprise a first collar projecting radially inwards wherein the ends of the respective first and second riser sections may each comprise a second collar projecting radially outwards, and wherein the inner diameter of the first collar may be smaller than the outer diameter of the second collar.

Spaces may be provided between each of the riser sections and the sleeve and the spaces may be filled with an hydraulic fluid. These spaces may be in fluid communication with one another and the resistance in the fluid communication path may be varied for controlling the level of damping.

An inner sleeve may be provided within the first mentioned sleeve and surrounding the ends of the first and second riser sections such that the inner sleeve defines annular spaces between the inner sleeve and the ends of the first and second riser sections, and between the inner sleeve and the first mentioned sleeve.

At least one of the annular spaces may be filled with an hydraulic fluid and the annular space between the inner sleeve and the first mentioned sleeve may be filled with rubber, or an hydraulic fluid, or a mixture of rubber and an hydraulic fluid for damping .

An hydraulic control system may be provided for actively controlling fluid pressure in said annular spaces.

The inner sleeve may be moveable within the first mentioned sleeve in at least the transverse plane.

Extensions may be provided at respective ends of the first mentioned sleeve and each extension may be provided around the outer circumference of the corresponding end of the first mentioned sleeve. The axially outermost portion of each extension may be tapered or rounded.

Brief description of the drawings

Figure 1 shows a vertical cross section of a well head, blowout preventer and riser. Figure 2a shows a vertical cross section of a riser connected to a blowout preventer with a single flex joint.

Figure 2b shows a vertical cross section of a riser connected to a blowout preventer with distributed strain.

Figure 2c is a graph illustrating a reduction of the tension in the blowout preventer due to distributed strain.

Figure 3a shows a vertical cross section of an active damper connecting two riser sections.

Figure 3b shows a vertical cross section of an active damper connecting two riser sections wherein the riser sections are separated due to a tensile force.

Figure 4 shows a vertical cross section of an active damper connecting two riser sections with a metal-to-metal seal between the riser sections and the damper.

Figure 5a shows a vertical cross section of an active damper connecting two riser sections wherein the riser sections are at an axial angle with respect to each other.

Figure 5b shows a vertical cross section of an active damper connecting two riser sections wherein the damper has rounded or tapered outermost portions.

Figure 6a shows a vertical cross section of an active damper wherein the damper has an inner sleeve and an outer sleeve.

Figure 6b shows a vertical cross section of an active damper wherein the damper has an inner sleeve and an outer sleeve and wherein the upper and lower riser portions are at a relative angle.

Description

Wellheads may experience fatigue and failure due to the dynamic loads from the riser on top of the blowout preventer (BOP). An embodiment of the present invention is a tool to attach to the riser close to the BOP. The tool may replace a riser joint. A plurality of tools may be attached to the riser along the length of the riser. The tool has two objectives: first, to actively dampen the axial stress fluctuations in the riser, and second, to actively or passively allow for smaller riser deviations from axial alignment without causing the transfer of large unwanted momentums (like a flex joint).

Figure 2 a) shows the situation of a large riser deviation, whereby the riser is attached to the BOP with a flex joint. Figure 2 b) shows a situation whereby the two objectives set out above are met and the tension is distributed along the length of the riser. Figure 2 c) is an illustration showing that the stress caused by riser flexing at the BOP is much smaller than the stress caused by riser flexing at the BOP when using the riser coupling system according to an embodiment of the present invention.

Figure 3 shows an upper riser section 5, a lower riser section 6 and collars 7. A sleeve 8 is mounted outside the riser sections 5, 6 to slide on these collars 7. On each end, the sleeve 8 has equivalent collars 9 on the inside, making the annuli 10 between the sleeve 8 and the upper and lower riser sections 5 , 6 restricted volumes. In these restricted volumes, an hydraulic oil is allowed to flow in and out by a control system 11. All sliding surfaces are equipped with standard hydraulic piston sealing. The pressure in the hydraulic oil is measured and the control system 11 tries to hold the pressure constant. For a fluctuating axial force, the control system will compensate the fluctuating forces with oil flow to absorb these fluctuations, such that dynamic effects are filtered out.

The riser sections 5, 6 together with sleeve 8 form a channel through which hydrocarbons or other fluids may be transported.

A metal to metal sealing between two opposing collars 7 and 9 is formed if the opposing collars 7 and 9 are pressed together. This sealing will be activated if the damper is extended to the maximal length if exposed to extreme tensile forces (for example in a kick situation). The sealing will then seal to an ever greater extent, the more the riser is exposed to tensile forces. Figure 3a shows the upper and lower riser sections (almost) in contact with each other within the sleeve 8. Figure 3b shows the upper and lower riser sections separated further than in Fig. 3a due to a tensile force, but not extended to maximal length.

The collars 7 of the upper and lower riser sections may be shaped such that the metal to metal seal between these collars 7 and the collars 9 of the sleeve increases if the tensile strain on the upper and lower riser sections increases. This seal is illustrated in the inset of Fig. 4. The seal is a metal to metal seal in addition to other seals provided between the different sections of the joint. The surface of the collar 7 facing the surface of the collar 9 has a shape such that, without stress, a wedge shaped cavity between the surfaces is provided, with the narrow part of the wedge shaped cavity further away from the axis of the riser than the wider part of the wedge shaped cavity. When a large stress is applied, the metal will deform such that the cavity 'closes' and little or no cavity remains between the two surfaces.

In a situation where the upper and lower riser sections are maximally extended within the sleeve, the collars of the upper and lower riser sections 5, 6 are in direct contact with the collars of the sleeve as shown in Fig. 4. If the upper riser section 5 is at an angle with respect to the lower riser section 6 in this maximally extended situation, a gap between the collars may open up as shown in region 12 of Fig. 5a. A further problem in this situation may be the concentration of stress or metal fatigue at the region opposite to region 12 where collars 7 and 9 make contact.

Figure 5b shows an extension 13 to the sleeve to improve the seal and to distribute the stress. The end of the extension 13 may be rounded off, as shown in extension 13 of Fig. 6b, or may be tapered off as shown in the inset 14. The extension will maintain the seal when the upper and lower sections are at an angle and will distribute the stress.

The embodiment above can be modified by adding an inner sleeve 15 in the annulus between first mentioned sleeve 8 and riser sections 5, 6 in the embodiment disclosed above. Figure 6a illustrates the inner sleeve 15 placed within the first mentioned sleeve 8. The collars 7 of the upper and lower riser sections 5, 6 extend to the inner diameter of the inner sleeve 15, while the collars 9 of the first mentioned sleeve 8 remain as before. The clearances are such that the inner sleeve 15 is allowed to move into eccentric positions relative to the first mentioned sleeve 8. The riser sections 5, 6, now located in the first mentioned sleeve 8 and allowed small movement with respect to the inner sleeve 15, will then be allowed to take a small angle with respect to each other, as shown in Fig. 6b. The inner sleeve 15 is displaced in the plane perpendicular to the axis of the first mentioned sleeve 8. Tension and momentum on the well head is reduced by allowing the riser sections 5, 6 to adopt an angle relative to one another.

The annulus 16 between the inner sleeve 15 and the first mentioned sleeve 8 can be filled with a substance that allows some flexibility and even some damping. This could be pure rubber (similar to the rubber lining in joints in a car's suspension system), oil or even a mix between oil and rubber to improve damping.

The movement of oil in the annuli 10 between the riser sections 5, 6 and inner sleeve 15 as a consequence of the riser flexing, can be controlled as in the first embodiment. In this way the angle of the riser can be controlled in every joint where this tool is installed. A new level of riser control can thus be achieved and optimized for minimum well head fatigue due to dynamic external forces.

Figures 3 to 6 show a single riser coupling, but a plurality of riser couplings may be used along the length of the riser. The stress in the riser may be absorbed by the plurality of riser couplings and distributed over the plurality of riser couplings.

The system disclosed herein can be controlled actively and, in combination with the active damping of the axial loads, the total system has the potential to significantly reduce the dynamic stresses on the riser close to the BOP and therefore also the negative impacts of dynamic stresses.

Fatigue is a result of stress variation, and is a function of the accumulated number of cycles and the amplitude of the cycles. Damping will reduce the amplitude of the stresses and possibly also the number. This will reduce the occurrence of fatigue or failure of the structure.

An advantage of at least some embodiments of the present invention is that it can achieve better control of the stresses the riser would otherwise transfer to the well head via the BOP, which have a negative effect on the well head and the potential to lead to failure of the well.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.

Claims

1. An apparatus for flexibly coupling two sections of a subsea riser comprising: a sleeve having first and second ends for slidably receiving ends of respective first and second riser sections, such that each section operates as a piston within the sleeve.

2. An apparatus according to claim 1 , wherein the first and second ends of the sleeve each comprise a first collar projecting radially inwards wherein the ends of the respective first and second riser sections each comprise a second collar projecting radially outwards, and wherein the inner diameter of the first collar is smaller than the outer diameter of the second collar.

3. An apparatus according to any preceding claim, wherein spaces are provided between each of the riser sections and the sleeve and wherein the spaces are filled with an hydraulic fluid.

4. An apparatus according to claim 3, wherei n the spaces are in fluid communication with one another.

5. An apparatus according to claim 4, further comprising a control unit arranged to vary the resistance in the fluid communication path for controlling the level of damping.

6. An apparatus according to any one of the preceding claims, comprising an inner sleeve within the first mentioned sleeve and surrounding the ends of the first and second riser sections such that the inner sleeve defines annular spaces between the inner sleeve and the ends of the first and second riser sections, and between the inner sleeve and the first mentioned sleeve.

7. An apparatus according to claim 6, wherein at least one of the annular spaces is filled with an hydraulic fluid.

8. An apparatus according to claim 6 or 7, wherein the annular space between the inner sleeve and the first mentioned sleeve is filled with any of rubber, an hydraulic fluid, and a mixture of rubber and an hydraulic fluid for damping.

9. An apparatus according to claim 7 or 8 further comprising an hydraulic control system for actively controlling the pressure of the hydraulic fluid in said annular spaces.

10. An apparatus according to any one of claims 6 to 9, wherein the inner sleeve is moveable within the first mentioned sleeve in at least the transverse plane.

1 1. An apparatus accordi ng to any one of the precedi ng clai ms , fu rther comprisingextensions at respective ends of the first mentioned sleeve.

12. An apparatus according to claim 11 , wherein each extension is provided around the outer circumference of the corresponding end of the first mentioned sleeve.

13. An apparatus according to claim 1 1 or 12, wherein the axially outermost portion of each extension is tapered or rounded.