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Subsea structure and methods of construction and installation thereofRelated Patent Categories: Hydraulic And Earth Engineering, Marine Structure Or Fabrication Thereof, With Anchoring Of Structure To Marine FloorSubsea structure and methods of construction and installation thereof description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070003374, Subsea structure and methods of construction and installation thereof. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to method and apparatus for buoyancy tensioning of offshore deepwater structures. It finds particular application in tensioning a slender, vertical or near-vertical, bottom-anchored, submarine structure, such as a riser or a bundle of risers (which may, or may not, include a structural member) or an umbilical. [0002] Tensioning is the act of ensuring that a marine structure doesn't experience excursions from its nominal upright position that would fall outside the acceptable limits, even in extreme weather conditions, the said limits being possibly defined with reference to the occurring seastate. There should always be sufficient tension to ensure stability, no matter the weight of the structure and the weight of the pipelines/risers hanging off the structure. [0003] 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, 18 Oct. 2001. Updated versions of such risers have been proposed in WO 02/053869 A1. [0004] GB-A-2346188 (2H-Offshore) presents an alternative to the hybrid riser tower bundle, in particular a "concentric offset riser" (COR). The riser in this case includes a single production flowline located within an outer pipe. Other lines such as gas lift, chemical injection, test, and hydraulic control lines are located in the annulus between the core and outer pipe. The main flow path of the system is provided by the central pipe and the annular space may be filled with water or thermal insulation material. Water injection lines, which are generally equal in diameter to the flowline, are not accommodated and presumably require their own riser structure. A simpler single line offset riser (SLOR) is also marketed by 2H Offshore Engineering Limited. [0005] Prior solutions are based on the use of a top buoy that gives part (or all) of the required tension to the slender structure, which, at its other extremity, is anchored to seabed. This top buoy can be provided with a restoring moment in order to thwart any kind of moments induced by other attached hardware, such as connecting jumpers, mooring cables, etc. . . . Furthermore, this top buoy can be tied to the other parts of the structure through a rigid connection (transmitting moments) or through a hinge (3D or at least 2D). The slender structure can also be endowed with distributed buoyancy, to provide support when horizontal (so that it can be towed, for example). [0006] Provision of buoyancy tanks and floating operations to connect to the main support structure are expensive and a major constraint on project planning. With the current designs, connection of the top buoy to the other parts of the structure, be it conducted onshore or offshore, is a difficult operation, and time and resource-consuming. [0007] Furthermore, if the buoy is rigidly linked to the other parts of the slender structure, the connecting part is likely to be subject to high stresses and/or fatigue, depending on the way the complete structure is installed, the environmental conditions on site, etc. . . . [0008] As a consequence, it is an object of the invention to provide a method of providing buoyancy to a floating structure that addresses one or more of the aforementioned disadvantages. [0009] In a first aspect of the invention there is provided a structure for subsea installation having buoyancy distributed substantially along its length in order to float said structure in horizontal orientation to a point of installation, whereby when said structure is upended, said distributed buoyancy will maintain said structure in a substantially vertical orientation and operational state, without the need for further, non-distributed buoyancy (for example, without substantial tension being provided by a top buoy or surface vessel). [0010] In a second aspect of the invention there is provided a method of installing an elongate subsea structure having buoyancy distributed substantially along its length in order to float said subsea structure to the point of installation, wherein when said structure is upended and anchored to the seabed, either directly or indirectly, said distributed buoyancy is sufficient to maintain said structure in a substantially vertical orientation without a substantial further separate buoyant structure. [0011] The invention yet further provides such a structure in its installed state. This technique removes the need for a top buoy, replacing it with distributed buoyancy on the risers and structure. Distributed buoyancy is of course known for the shaping of portions of risers and like structures, for example to control touchdown behaviour in so-called "wave" configurations. This is generally with the context of a catenary structure suspended from a substantial vessel. [0012] Said distributed buoyancy may be provided by buoyancy modules provided at regular intervals along said rigid or flexible risers. This may involve fixing the modules with spaces between them, for example to achieve a structure similar to a SLOR, or COR, but without a top buoy. [0013] The buoyancy modules may alternatively be free to slide longitudinally along the structure, so as to impart their buoyancy force through one another to the top of the structure, rather than being transmitted to the supported structure at points along the length thereof. This alternative is preferred in accordance with the invention of a further patent application having the same priority date as the present application. The content of that other application is incorporated herein by reference (GB 0227851.3, agent's ref 64312GB, published as WO ______). [0014] Said subsea structure may comprise at least one rigid riser conduit. Said structure in its operational state may be connected to a flexible jumper or similar flexible portion at its top end for connection to a source or destination of fluid. Only a lesser proportion of said flexible jumper may have distributed buoyancy. Said buoyancy may be distributed in such a way to give the flexible portion a "steep wave" shape, in order to apply tension co-linear with the structure, thus avoiding large bending moments in the subsea structure. [0015] Said subsea structure will usually be a riser, or a bundle of risers. Said bundle of risers may comprise a plurality of spaced risers surrounding a central riser or support. [0016] Said buoyancy modules may be spaced substantially regularly along said subsea structure to enable said structure to be floated to the deployment site in a substantially horizontal orientation prior to deployment. [0017] The distribution of buoyancy about the length of the riser may be selected to provide substantially normal orientation of said flexible portions as they attach to said structure, such that any bending moment induced by tension in said flexible risers is minimised. [0018] The distributed buoyancy referred to herein will generally be of a permanent type, such as syntactic foam. Adjustments to the buoyancy may be made temporarily by the addition of modules, or in particular by air-filling then flooding parts of the structure at different stages in the installation process. [0019] Dense material may be used to fill the structure during deployment in order to compensate for the extra buoyancy required by the invention. Said dense material may comprise seawater, or may be a material denser than the ambient seawater. In the case of an onshore-fabricated, towed and upended structure, the downward tension that must be applied during upending at the bottom of the structure must overcome the net buoyancy of the structure. In such a case, once in operation, the tension of the structure (as measured for example, at anchor) is given by this net buoyancy plus the difference in content weight between the upending and operating phases. For example, if a gas riser is installed full of water, when in operation, it will be further tensioned by the difference in density between gas and water. The use of even heavier fluids, such as drilling mud, may be envisaged during installation, in order to boost the net buoyancy provided once in operational conditions by adding a bigger quantity of buoyancy material than can be properly compensated during installation. [0020] The finished installation may further comprise flexible jumpers with distributed buoyancy, whose buoyancy contributes to the tension in the structure in its operational condition. By deferring the connection of these jumpers until after upending and anchoring the main part of the structure, the net buoyancy during upending can be further reduced. BRIEF DESCRIPTION OF THE DRAWINGS [0021] Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: [0022] FIG. 1 shows a known type of riser structure in an offshore oil production system; [0023] FIG. 2 shows two riser bundles each having a novel structure without top buoy, in accordance with an embodiment of the present invention; and Continue reading about Subsea structure and methods of construction and installation thereof... 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