Patent Application
20060002767
The present invention relates to method and apparatus for buoyancy distribution of offshore deepwater structures, in particular, but not restricted to, buoyancy distribution along a substantially vertical submarine structure, such as a riser, a bundle of risers, or any other structural member.
The structure may form part of a so-called hybrid riser, having an upper and/or lower portions (“jumpers”) made of flexible conduit. U.S. Pat. No. 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Insulating material in the form of syntactic foam blocks surrounds the core and the pipes and separates the hot and cold fluid conduits. Further background has been published in paper “Hybrid Riser Tower: from Functional Specification to Cost per Unit Length” by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, Oct. 18, 2001. Updated versions of such risers have been proposed in WO 02/053869 A1. The contents of all these documents are incorporated herein by reference, as background to the present disclosure.
Buoyancy of offshore structures is achieved by using temporarily or permanently attached buoyancy modules providing an upward thrust when submerged in the sea. Conventional devices such as stop-collars and clamps are used to transmit the buoyancy thrust from the buoyancy modules to the supported structure. The buoyancy thrust acts upon the structure where it is generated. The buoyancy modules are clamped around stop collars using straps or bolts.
In particular cases, such as a hybrid riser tower (bundle of risers, fabricated onshore), buoyancy may be required for the supporting of a structure in two (or more) completely different orientations, such as a horizontal orientation (during installation) and a vertical orientation (in operation).
The buoyancy thrust has to be transmitted in both orientations along two perpendicular directions, depending upon the orientation of the structure at the time. Having the buoyancy acting onto the structure where it is generated may be advantageous in one direction (supporting of horizontal risers during fabrication and installation), but a hindrance in another direction. Where two or more risers are bundled together it can be difficult to clamp buoyancy modules along each riser (due to differential thermal expansion, for example), or along one riser only (due to effective compression). To overcome this difficulty the modules are clamped to just one of the risers. However, in operation if the risers are hanging freely from the top structure of the bundle, the forces associated with weight compensation may induce a large compressive load on the riser to which the buoyancy modules are attached.
It is therefore an object of the invention to provide method and apparatus to transfer the substantial compressive forces provided by the buoyancy modules directly to the subsea structure, once installed, rather than via one or more of the risers.
In a first aspect of the invention there is provided a method of installing an elongate subsea structure, wherein said subsea structure is provided with a plurality of buoyancy modules, said buoyancy modules being slidably mounted to said subsea structure, such that when said subsea structure is deployed at sea in a substantially vertical orientation said buoyancy modules are free to adjust their positions up or down said subsea structure by sliding, the buoyancy force of each buoyancy module acting upon the buoyancy module above it rather than locally along the structure, and the cumulative buoyancy force from said buoyancy modules acting substantially against the top of said subsea structure.
This method allows all of the suspended weight of the subsea structure to be taken from its top, where all of the buoyancy force is applied. As a result, no single part of the structure, such as a riser, has to support the majority of the structure's weight.
For the above, substantially vertically can be taken to be some degree off true vertical, but orientated such that said elongate object rises substantially from seabed to surface. Also substantially acting from the top can be taken to mean acting some metres below the top, but such that the buoyancy is acting near the top taking into consideration the full length of the elongate object.
Said subsea structure will usually be a riser, such as a steel catenary riser, or a bundle of risers. Said bundle of risers may be a bundle of seven risers arranged with one in the centre and the rest spaced apart and distributed evenly around this. Instead of a riser, the central one may be a supporting core. In such a case, the transfer of the buoyant forces to the top of the structure is performed by the buoyancy modules, rather than by one core or core riser conduit. The excessive compressive loads otherwise imposed on that core conduit are thus avoided.
Each of said buoyancy modules may come in a number of sections, such that it can be fitted around all of the risers in a bundle of risers, as illustrated in WO '869, mentioned above.
Said buoyancy force may act from the top as a result of each buoyancy module acting on the one above, either directly or via an intermediary, the uppermost buoyancy module acting against a top plate. Alternatively the uppermost module could be fixed to said subsea structure. The intermediary may be a pad of compressible material.
Said buoyancy modules may, in their original configuration, be spaced substantially evenly along said subsea structure to enable said subsea structure to be floated to the deployment site in a substantially horizontal orientation prior to deployment.
The structure may be provided with a substantial top buoyancy. Alternatively, and in accordance with the invention of a further patent application having the same priority date as the present application, the structure may be supported completely by distributed buoyancy. The content of that other application is incorporated herein by reference (GB 0227850.5 agent's ref 64314GB, published as WO ______).
In further aspects of the invention there are provided a structure prepared for installation by the method as described above, and a structure installed according to the method.
Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:
In the upended configuration, the up-thrust is transmitted to the top plate 440 forming part of the riser top structure 400 to compensate for the weight of the risers. The up-thrust is transmitted through surface contact between vertically adjacent buoyancy modules, as indicated by arrows, 450, with optional compliant devices between modules, not shown in
The cross-sectional area of the buoyancy modules is such that the resulting stress in the uppermost module can be sustained, whereas if the up-thrust was transmitted via one of the pipes/risers then it would lead to unsustainable compressive loads.
As mentioned in the introduction, the top structure 400 may comprise a substantial buoy, as in the prior examples. Export of hydrocarbons is via flexible jumper hoses (not shown), one for each riser conduit.
Alternatively, the structure may rely entirely on distributed buoyancy, as described in our co-pending patent application (Agent's reference 64314GB).
Buoyancy for the risers is provided by the buoyancy already distributed along them for installation purposes, evenly distributing the complement required in operational conditions. It may be desirable to compensate for any surplus of buoyancy, during installation, by filling the structure with fluids heavier than those that will fill the conduits in operation. This will assist the process of sinking the lower part of the riser to the anchor point.
A consequence of the absence of a top buoy is that the structure supported in such a manner cannot withstand a large bending moment at top, since the only counteracting stiffness is given by the steel and is therefore very low, due to the slenderness of the structure. Using flexible sections 520 to connect to the top of the structure overcomes this problem, and providing them with a steep-wave shape, in order to apply tension co-linear with the structure, avoids a large bending, or rotational, moment. Each flexible section 520, at least in the region 530 above the junction 540 with the rigid portion 500/510, is also provided with distributed buoyancy for this purpose. The tension given by the flexibles, during operation, is taken into account when determining the buoyancy required along the structure itself.
In either type of installation, the installation process is broadly the same as that illustrated in U.S. Pat. No. 6,082,391, mentioned above. The height of the installed structure may for example be 500 m, or over 1 km.
The skilled person will further appreciate that the exact form of components and methods used can vary from the ones described herein without departing from the spirit and scope of invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0227851.3 | Nov 2002 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP03/14833 | 11/25/2003 | WO | 5/23/2005 |
