GB2483162A

GB2483162A - Ring for stabilizing a riser

Info

Publication number: GB2483162A
Application number: GB1114722.0A
Authority: GB
Inventors: Simon Luffrum; Stephen Hatton
Original assignee: Subsea Riser Products Ltd
Current assignee: Subsea Riser Products Ltd
Priority date: 2010-08-25
Filing date: 2011-08-25
Publication date: 2012-02-29
Also published as: GB201114722D0; GB201014182D0

Abstract

A stabilizing attachment for a subsea riser or conduit (3, figure 1) comprising an incomplete ring 8 which is arranged to engage an outer surface of a subsea riser. The ring is resiliently expandable to increase an inner diameter to enable the ring to move along the riser longitudinally. The ring may define a gap 7, there may be several rings attached to a riser, and the riser may have a flexible coating (10, 12, figure 3) which may also be resilient. There may be an additional coating (13, figure 3) which covers the rings and exposed outer surface of the riser. The ring or rings may be secured to the riser by an interference fit, exerting a compressive force on the riser.

Description

Method and Apparatus for Riser Performance Optimisation

FIELD OF THE INVENTION

The present invention relates to risers, pipelines and/or conduits used in the offshore oil and gas industry for conveying fluids between the seabed and a host vessel at the water surface. In particular, the present invention relates to an attachment for stabilising a section of a subsea conduit, an assembly comprising a subsea riser and plurality of attachments for stabilising a section of the subsea riser and a method of stabilising a subsea riser.

BACKGROUND TO THE INVENTION

Various types of risers exist in the offshore oil and gas industry; amongst them are Steel Catenary Risers, commonly abbreviated as SCR's. A SCR 3, depicted in Figure 1, is a continuation of the seabed pipeline that rises up from the seabed 5 and is hung from the host vessel 1 at or near the surface 2 forming a natural free hanging catenary shape. Typically SCR's are fabricated from steel pipe with an outer diameter of 6 -2Oinches and are typically used in water depths ranging from a few hundred metres up to two thousand metres or more.

These risers are typically designed to be permanently installed for 25 years or more and structural and pressure integrity of the riser is paramount. Steel Catenary Risers, whilst presenting a simple continuous pipe design, have a complex structural response due to the applied environmental loading and the interaction with the seabed.

The catenary shape between the surface vessel and the seabed gives a compliancy that can accommodate movements of the host vessel due to wind, wave and current action. But movements of the host vessel are transmitted down the riser and induce stresses in the riser. Two sections of a SCR's are more highly loaded and hence more highly stressed and fatigue sensitive than others, at the top of the riser at the connection to the host vessel and at the seabed, in the area close to where the riser touches down on the seabed, this area is commonly referred to as the Touch Down Point or TDP 6.

A method for optimising the structural response of an SCR is to add additional weight to the riser locally over particular regions of the riser. Typically a section of weighted riser 4 will be used above the riser TDP region to control the structural response and reduce stresses to improve riser fatigue life at the TDP region.

Optimisation is achieved through varying the position and length of the weighted section as well as the additional weight added per unit length of riser pipe.

However, it is noted that simply adding weight can cause other problems in that the selected weights can add to the local stiffness of the pipe and increase hydrodynamic loading on the riser which can be detrimental to performance.

Additionally, if the weights are attached in a manner that causes a high point load the result can be to increase stresses locally.

It is an aim of the present invention to overcome at least one problem associated with the prior art whether referred to herein or otherwise.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a stabilising attachment for a subsea conduit comprising an incomplete ring having an inner diameter and having a first circumferential end and a second circumferential end, wherein an inner surface of the incomplete ring is arranged to engage an outer surface of the subsea conduit and to secure the incomplete ring to the subsea conduit, the incomplete ring being resiliently expandable to increase the inner diameter of the incomplete ring and to enable the incomplete ring to be longitudinally movable along the subsea conduit.

Preferably the first circumferential end is spaced from the second circumferential end to define a gap therebetween.

Preferably the incomplete ring comprises an incomplete annular member.

The incomplete ring may comprise a split ring.

The incomplete ring may comprise a "C" shaped ring.

Preferably the incomplete ring comprises a planar member such that the first circumferential end is directly aligned with the second circumferential end.

The incomplete ring may comprise a metal incomplete ring. Preferably the incomplete ring comprises a steel incomplete ring.

Preferably the incomplete ring is arranged, in use, to be secured to the subsea conduit through an interference fit.

Preferably the incomplete ring is arranged, in use, to exert a compressive force on the outer surface of the subsea conduit.

Preferably the subsea conduit comprises a section of a subsea riser.

The attachment may comprise a plurality of incomplete rings wherein each of the incomplete rings is preferably secured, in use, to the outer surface of the subsea conduit.

The incomplete ring may have a longitudinal dimension extending in the longitudinal direction of the subsea conduit, in use.

The longitudinal dimension may be less than 200mm and preferably less than 100mm and more preferably less than 70mm. The longitudinal dimension may be greater than 10mm and preferably greater than 20mm and more preferably greater than 40mm. The longitudinal dimension may be substantially 50mm.

The incomplete ring may comprise a (radial) thickness and wherein the radial thickness may be substantially 20mm. The (radial) thickness may be less than 100mm and preferably less than 50mm and more preferably less than 30mm. The (radial) thickness may be greater than 5mm and preferably greater than 10mm and more preferably greater than 15mm.

The inner diameter of the incomplete ring may be less than the diameter of the outer surface of the subsea conduit.

The inner diameter may be less than 1000mm and preferably less than 750mm and more preferably less than 500mm. The inner diameter may be greater than 100mm and preferably greater than 300mm and more preferably greater than 400mm. The longitudinal dimension may be substantially in the region of 475mm to 480mm.

The necessary increase in riser weight is achieved through attachment of rings, typically made from metal, with the size, spacing and number of rings selected to give the required increase in riser weight to improve its structural performance.

The weight of the incomplete ring, in air, may be greater than 5kg and may be less than 20kg. The weight of the incomplete ring, in air, may be substantially in the region of 11kg to 12kg.

Preferably, the rings have a nominal interference fit with the coated riser pipe outer diameter and are attached by temporarily expanding the ring and slipping it over the pipe, such that thereafter the ring compresses the riser pipe to secure the ring onto the riser pipe.

Preferably the stabilising attachment comprises a coating over which the incomplete rings are secured. Preferably the coating comprises a flexible coating which may enable the incomplete rings to move relative to the subsea conduit.

Preferably the coating comprises a resilient coating.

Preferably, the rings are attached with a compliant coating beneath the rings and a spacing between adjacent rings and preferably this spacing means that the rings provide negligible additional stiffness to the riser and when the riser is bent the rings do not interfere with each other.

The spacing between the rings may be less than 50mm and preferably less than 20mm and more preferably less than 10mm. The spacing between the rings may be greater than 1mm and preferably greater than 3mm and more preferably greater than 4mm. The spacing between the rings may be substantially 5mm.

Preferably the stabilising attachment comprises a plurality of incomplete rings.

The stabilising attachment may comprise greater than 50 incomplete rings and preferably comprises greater than 100 incomplete rings and more preferably comprises greater than 150 incomplete rings.

The stabilising attachment may comprise less than 1000 incomplete rings and preferably comprises greater than 500 incomplete rings and more preferably comprises greater than 300 incomplete rings.

The stabilising attachment may comprise substantially 200 incomplete rings which may be located longitudinally adjacent to each other along a length of the subsea conduit.

The incomplete rings may be secured over a section of the subsea conduit only.

The incomplete rings may be secured over a section of the subsea conduit only wherein the section has a longitudinal length which may be less than l000m and preferably less than 500m and more preferably less than 400m. The incomplete rings may be secured over a section of the subsea conduit only wherein the section has a longitudinal length which may be greater than 50m and preferably greater than lOOm and more preferably greater than 200m.

Preferably the rings are manufactured from metal.

Preferably the rings are attached to the subsea conduit (riser) in order to improve the overall structural performance of the subsea conduit (riser).

Preferably the rings are temporarily expanded to slide over the riser pipe, thereafter the rings compress the riser pipe to secure the rings on the riser pipe.

Preferably the rings are temporarily expanded to slide over a coated riser pipe, thereafter the rings compress the coating to secure the rings on the riser pipe.

Preferably the rings are placed on a compliant coating that allows the ring to move independently of the riser pipe and do not significantly increase the overall bending stiffness of the riser.

Preferably the rings do not significantly increase the overall bending stiffness of the riser pipe.

Preferably the rings are spaced on a riser pipe sufficiently to prevent the rings from interfering with each other when the subsea conduit (riser pipe) is bent.

The stabilising attachment comprises a weighted sleeve or a weighted sheath.

According to a second aspect of the present invention there is provided an assembly comprising a subsea conduit an attachment, the attachment comprising an incomplete ring having an inner diameter and having a first circumferential end and a second circumferential end, wherein an inner surface of the incomplete ring is arranged to engage an outer surface of the subsea conduit and to secure the incomplete ring to the subsea conduit, the incomplete ring being resiliently expandable to increase the inner diameter of the incomplete ring and to enable the incomplete ring to be longitudinally movable along the subsea conduit.

The subsea conduit may comprise a section of a subsea riser.

The subsea conduit may have a coating on an outer surface thereof.

The coating may comprise a compliant coating.

Preferably the coating comprises a resilient coating. The coating may comprise a flexible coating.

The attachment may comprise a plurality of incomplete rings. The plurality of incomplete rings may be longitudinally spaced apart along a length of the subsea conduit.

The incomplete rings may be evenly spaced apart along the length of the subsea conduit.

The incomplete rings may be spaced apart by different distances along the length of the subsea conduit.

The subsea conduit may comprise a non linear section of a subsea riser.

The assembly may comprise an outer coating which covers both the incomplete rings and the exposed outer surface of the subsea conduit located between adjacent incomplete rings.

According to a third aspect of the present invention there is provided a method of increasing the weight of a section of a subsea conduit comprising securing an attachment to the section of the subsea conduit, the method comprising securing a plurality of rings around the outer surface of the subsea conduit wherein each ring is secured by temporarily expanding an inner diameter of the ring and securing the ring to the outer surface of the subsea conduit through an interference fit.

Preferably the method comprises longitudinally spacing the rings along a section of the subsea conduit.

The method may comprise providing a compliant coating on the subsea conduit and securing the rings over the compliant coating.

The method may comprise applying an outer coating over the rings and the surface of the subsea conduit exposed between adlacent rings.

Preferably the present invention provides a method for locally increasing riser weight by attachment of rings to the outside of the riser pipe.

Preferably the method comprises increasing the weight of the subsea conduit (riser pipe) by the attachment of rings to the outside of the subsea conduit (riser pipe).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows the general arrangement of a steel catenary riser; Figure 2 is a perspective view of a preferred embodiment of an incomplete ring; and Figure 3 is a perspective view of a plurality of preferred embodiments of incomplete rings installed on a riser joint.

DESCRIPTION OF THE PREFFERRED EMBODIMENTS

The present invention provides a method for locally increasing the weight of a subsea conduit (riser pipe) 9 used in the offshore oil and gas industry in order to improve the structural performance of the conduit (riser) 9. The term conduit is used in this specification but the preferred embodiment will now be described with specific reference to a subsea riser pipe 9.

One aim of the present invention is for the weight of the subsea riser to be locally increased over a selected length of the subsea riser pipe 9 with the use of a stabilising attachment comprising metallic rings 8. In particular, the rings 8 are expanded, slid over the pipe 9 and then released to provide a gripping force that secures the ring 8 to the riser pipe 9. Accordingly, the stabilising attachment effectively provides a weighted sheath or a weighted sleeve which is secured around a section of a subsea riser.

The rings 8 may be placed directly over the riser pipe 9 or over corrosion or thermal coatings 12 already placed on the pipe 9. The metallic rings 8 may then be overcoated with a plastic or rubber material 13 to provide corrosion protection or additional thermal insulation over the riser.

The preferred embodiment of the present invention will now be described. As shown in Figure 2, the present invention provides an attachment in the form of an incomplete ring 8. The incomplete ring 8 comprises a first end 20 and a second end 22. The incomplete ring 8 is planar such that the first end 20 is aligned directly opposite the second end 22 and defines a gap 7 therebetween.

The incomplete ring 8 has a longitudinal length 30 and a radial thickness 32. The incomplete ring 8 has an inner surface 24 and an outer surface 26. In use, the inner surface 24 of the incomplete ring is arranged to engage with the outer surface of a subsea riser pipe 9.

In addition, the incomplete ring 8 has a diameter 34 which substantially corresponds with the diameter of the outer surface of the subsea riser 9. In particular the diameter 34 of the incomplete ring 8 is slightly smaller than the diameter of the outer surface of the subsea riser 9. Accordingly, in use, the incomplete ring 8 exerts a compressive force on to the outer surface of the subsea riser 9 and this force secures the incomplete ring 8 to the subsea riser 9.

Accordingly, the present invention comprises a simple device which does not require any unnecessary securement devices or adhesives/cements which may fail. Such securement devices or adhesives/cements would be particularly problematic due to the harsh conditions where subsea risers are located.

In use, the diameter 34 of the incomplete ring 8 is temporarily increased in order for the diameter 34 to be slightly greater than the outer diameter of the subsea riser 9. The diameter 34 of the incomplete ring 8 may be increased using any suitable means. For example, the incomplete ring 8 may be expanded by thermal means or the diameter of the incomplete ring 8 may be temporarily increased by mechanical or even manual means.

The incomplete ring 8 comprises a resilient material such that on removal of the temporary expansion force, the incomplete ring naturally returns towards the natural rest position and initial diameter 34. This natural and inherent returning force is sufficient to retain the incomplete ring 8 to the outer surface of the subsea riser 9.

In the preferred embodiment the incomplete ring 8 comprises a metal.

As shown in Figure 2, a preferred embodiment of an incomplete ring 8 has a radial thickness 32 of 20mm and a longitudinal length of 50mm. The inner diameter 34 of the incomplete ring 8 is 476.3mm. The incomplete ring 8 is manufactured from steel. As shown in Figure 3, the stabilising attachment comprises a number of incomplete rings 8 secured around the subsea conduit and in one preferred example, the stabilising attachment comprises 200 incomplete rings or 194 incomplete rings. The incomplete rings 8 are spaced apart longitudinally over a length of a section of the subsea conduit. The longitudinal length of the section of the subsea riser may be in the region of 200m to 400m. In this longitudinal section, there may be approximately 200 incomplete rings or, in one example, 194 incomplete rings 8. The incomplete rings 8 may be spaced from each other by a spacing 11 wherein this spacing may be 5mm. The stabilising attachment may have a weigh, in air, of 2293kg or a weigh, in water, of 2000kg.

The incomplete rings 8 are used to locally increase the weight of a riser by attaching them to the outside of the riser pipe, 9. In particular, a number of incomplete rings 8 are secured along a predetermined section of the subsea riser 9. The incomplete rings 8 are secured spaced apart along the longitudinal length of the section of the subsea riser 9.

The necessary increase in riser weight per unit length is achieved by varying the material density, number and size of the rings and their spacing 11. Accordingly, a user can calculate the required or desired increase in the weight of the subsea riser 9 in a particular section. The user can then vary several parameters to achieve the required weight benefits. For example, in order to relatively increase the weight per unit length, the user could select incomplete rings 8 of greater radial thickness 32, greater longitudinal length 30 or select a material of a greater density from which to manufacture the incomplete rings 8.

The attachment of the rings 8 to the riser pipe 9 is typically performed onshore, with the pipe, typically in 4Oft lengths, prior to being welded together. At this time the pipe 9 is not subject to bending loads beyond those induced by its own self weight or pressure or tension loading. The welding of the pipe 9 may also be performed offshore or partially onshore and partially offshore. The rings 8 may also be attached offshore either before or after the pipe is welded together to form a continuous riser.

The rings 8 are typically manufactured from a stiff and high density material like metal to achieve both a preload on the outside diameter of the pipe and the required increase in weight with a small volume of material ensuring a minimum increase in overall riser drag dimensions.

The rings will typically have a cut out or gap 7, giving a "c" shape profile to the ring 8 that allows the ring 8 to be temporarily expanded over the riser pipe, moved along the pipe 9 and positioned before releasing. The rings 8 may be applied directly to the riser pipe 9, or alternatively over an existing riser pipe coating 10 that has been applied to provide thermal insulation or corrosion protection (or both) to the riser pipe 9. The thermal insulation coating 10 may comprise a foam insulation which may have a radial thickness of substantially 101.6mm.

The inner diameter 34 of the ring 8 is sized to give a nominal interference fit with the riser pipe 9 or its coating 12, such that after expansion and release on the pipe 9 the ring 8 provides a compressive force on the riser pipe 9 or coating 12, securing the ring 8 on the pipe 9. This compressive force may be varied by changing the dimensions of the ring 8 the size of the cut out or gap 7 and the nominal interference fit.

The temporary expansion of the incomplete rings 8 may be provided by a number of means including manual, mechanical and thermal.

A compliant coating 12, such as rubber or polypropylene may be first placed on the riser pipe 9 (or over the insulation coating 10) before application of the rings 8.

The compliant coating 12 allows the rings 8 to move independently of the riser pipe 9, thus preventing the rings 8 from wholly or partially contributing to an increase in structural stiffness of the riser and also prevents any rubbing or fretting between the pipe body and ring 8.

The rings 8 may also be placed at a spacing that prevents the rings 8 from touching or abutting each other when the riser pipe 9 is bent into a catenary after installation of the riser and further bent due to applied environmental loading and movement of the host vessel.

After application the rings 8 may be overcoated 13, to provide corrosion protection to the rings 8 or to provide additional corrosion or thermal insulation to the riser 9.

The overcoat covering comprises a protective coating which completely covers the outer surfaces of the incomplete rings 8 and also covers the spaces between adjacent incomplete rings 8 where the subsea riser would otherwise be exposed.

This thereby provides a continuous protective coating. -14-

Claims

CLAIMS1. A stabilising attachment for a subsea conduit comprising an incomplete ring having an inner diameter and having a first circumferential end and a second circumferential end, wherein an inner surface of the incomplete ring is arranged to engage an outer surface of the subsea conduit and to secure the incomplete ring to the subsea conduit, the incomplete ring being resiliently expandable to increase the inner diameter of the incomplete ring and to enable the incomplete ring to be longitudinally movable along the subsea conduit.
2. A stabilising attachment for a subsea conduit according to Claim 1 in which the first circumferential end is spaced from the second circumferential end to define a gap therebetween.
3. A stabilising attachment for a subsea conduit according to Claim 1 or Claim 2 in which the incomplete ring comprises a planar member such that the first circumferential end is directly aligned with the second circumferential end.
4. A stabilising attachment for a subsea conduit according to any preceding claim in which the incomplete ring may comprise a metal incomplete ring.
5. A stabilising attachment for a subsea conduit according to any preceding claim in which the incomplete ring is arranged, in use, to be secured to the subsea conduit through an interference fit.
6. A stabilising attachment for a subsea conduit according to any preceding claim in which the incomplete ring is arranged, in use, to exert a compressive force on the outer surface of the subsea conduit.
7. A stabilising attachment for a subsea conduit according to any preceding claim in which the attachment comprises a plurality of incomplete rings wherein each of the incomplete rings is secured, in use, to the outer surface of the subsea conduit.
8. A stabilising attachment for a subsea conduit according to any preceding claim in which the incomplete ring has a longitudinal dimension extending in the longitudinal direction of the subsea conduit, in use and wherein the longitudinal dimension is in the region of 10mm to 100mm.
9. A stabilising attachment for a subsea conduit according to any preceding claim in which the incomplete ring comprises a radial thickness and wherein the radial thickness I sin the region of 5mm to 50mm.
10. A stabilising attachment for a subsea conduit according to any preceding claim in which the inner diameter of the incomplete ring is less than the diameter of the outer surface of the subsea conduit.
II. A stabilising attachment for a subsea conduit according to Claim 10 in which the inner diameter is in the region of 100mm to 1000mm
12. A stabilising attachment for a subsea conduit according to any preceding claim in which the weight of the incomplete ring, in air, is in the region of 5kg and 20kg.
13. A stabilising attachment for a subsea conduit according to any preceding claim in which the stabilising attachment comprises a coating over which the incomplete rings are secured.
14. A stabilising attachment for a subsea conduit according to Claim 13 in which the coating comprises a flexible coating which enables the incomplete rings to move relative to the subsea conduit.
15. A stabilising attachment for a subsea conduit according to Claim 13 or Claim 14 in which the coating comprises a resilient coating.
16. A stabilising attachment for a subsea conduit according to any preceding claim in which the stabilising attachment comprise a plurality of incomplete rings which are attachable with a compliant coating beneath the incomplete rings and a spacing between adjacent incomplete rings and the spacing ensures that the incomplete rings provide negligible additional stiffness to the sunbeam conduit and when the subsea conduit is bent the incomplete rings do not interfere with each other.
17. A stabilising attachment for a subsea conduit according Claim 16 in which the spacing between the rings is in the region of 1mm and 50mm.
18. A stabilising attachment for a subsea conduit according to any preceding claim in which the stabilising attachment comprises a plurality of incomplete rings.
19. A stabilising attachment for a subsea conduit according to Claim 18 in which the stabilising attachment may comprise between 50 and 1000 incomplete rings.
20. A stabilising attachment for a subsea conduit according to Claim 18 or Claim 19 in which the incomplete rings are secured over a section of the subsea conduit only.
21. A stabilising attachment for a subsea conduit according to Claim 20 in which the incomplete rings are secured over a section of the subsea conduit only wherein the section has a longitudinal length in the region of 50m and l000m.
22. An assembly comprising a subsea conduit a stabilising attachment, the stabilising attachment comprising an incomplete ring having an inner diameter and having a first circumferential end and a second circumferential end, wherein an inner surface of the incomplete ring is arranged to engage an outer surface of the subsea conduit and to secure the incomplete ring to the subsea conduit, the incomplete ring being resiliently expandable to increase the inner diameter of the incomplete ring and to enable the incomplete ring to be longitudinally movable along the subsea conduit.
23. An assembly according to Claim 22 in which the subsea conduit comprises a section of a subsea riser.
24. An assembly according to Claim 22 or Claim 23 in which the subsea conduit has a coating on an outer surface thereof.
25. An assembly according to Claim 24 in which the coating comprises a compliant coating.
26. An assembly according to any one of Claim 22 to Claim 25 in which the stabilising attachment comprises a plurality of incomplete rings which are longitudinally spaced apart along a length of the subsea conduit.
27. An assembly according to any one of Claim 22 to Claim 26 in which the subsea conduit comprises a non linear section of a subsea riser.
28. An assembly according to any one of Claim 22 to Claim 27 in which the assembly comprises an outer coating which covers both the incomplete rings and the exposed outer surface of the subsea conduit located between adjacent incomplete rings.
29. A method of increasing the weight of a section of a subsea conduit comprising securing a stabilising attachment to the section of the subsea conduit, the method comprising securing a plurality of incomplete rings around the outer surface of the subsea conduit wherein each incomplete ring is secured by temporarily expanding an inner diameter of the incomplete ring and securing the incomplete ring to the outer surface of the subsea conduit through an interference fit.
30. A method of increasing the weight of a section of a subsea conduit according to Claim 29 in which the method comprises longitudinally spacing the rings along a section of the subsea conduit.
31. A method of increasing the weight of a section of a subsea conduit according to Claim 29 or Claim 30 in which the method comprises providing a compliant coating on the subsea conduit and securing the incomplete rings over the compliant coating.
32. A method of increasing the weight of a section of a subsea conduit according to any one of Claim 29 to Claim 31 in which the method comprises applying an outer coating over the rings and the surface of the subsea conduit exposed between adjacent rings.
33. A stabilising attachment for a subsea conduit substantially as herein described with reference to, and as shown in, any of the accompanying Figures.
34. An assembly comprising a subsea conduit a stabilising attachment substantially as herein described with reference to, and as shown in, any of the accompanying Figures.
35. A method of increasing the weight of a section of a subsea conduit substantially as herein described with reference to, and as shown in, any of the accompanying Figures.