Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
2001-10-22
2004-07-13
Will, Thomas B. (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S205000, C114S264000
Reexamination Certificate
active
06761508
ABSTRACT:
SPECIFICATION FIELD OF THE INVENTION
The invention relates to a moored offshore or self-propelled floating platform with improved motion characteristics for economic offshore deepwater developments. The floating platform of the present invention is capable of being self propelled in mild environments or moored for use in extreme depths and severe wind and wave conditions.
BACKGROUND
In the development of offshore energy systems such as deepwater oil and/or gas production, long flowlines, power cables and control umbilicals are frequently required between subsea wells and a host platform. The extended lengths pose energy loss, pressure drop and production difficulties.
Costs of structures for deepwater applications are high and costs are frequently increased due to the foreign locations at which they are fabricated. Other difficulties, associated with deepwater offshore operations, result from floating vessel motions which affect personnel and efficiencies especially when related to liquid dynamics in tanks. The primary motion related problem, associated with offshore petrochemical operations, occurs with large horizontal vessels in which the liquid level oscillates and provides erroneous signals to the liquid level instruments causing shutdown of processing and overall inefficiency for the operation.
Prior art for deepwater has generally resulted in facilities that yield expensive solutions for an offshore oil field development. Such prior art includes tension leg platforms (TLP's), which may incorporate well drilling capabilities to several mini-TLP designs that perform simplified functions; SPARS which are configured much the same as a spar buoy and have dry wellheads, as opposed to subsea trees; deepwater floating production storage facilities (FPSO's) and their several variations.
Present tendon technology limits the common TLPs to approximately 5,000-ft water depths, which causes them to be stationed long distances from planned deepwater fields and the subsea wells. Systems that are moored by catenary lines can be placed within the deepwater fields within fairly close proximity to the subsea wells.
The principal elements which can be modified for improving the motion characteristics of a moored floating vessel are the draft, the water plane area and its draft rate of change, location of the center of gravity (CG), the metacentric height about which small amplitude roll and pitching motions occur, the frontal area and shape on which winds, current and waves act, the system response of pipe and cables contacting the seabed acting as mooring elements, and the hydrodynamic parameters of added mass and damping. The latter values are determined by complex solutions of the potential flow equations integrated over the floating vessel's detailed features and appendages and then simultaneously solved for the potential source strengths. It is only significant to note herein that the addition of features which allow the added mass and/or damping to be ‘tuned’ for a certain condition requires that several features can be modified in combination, or more preferably independently, to provide the desired properties. The optimization is greatly simplified if the vessel possesses vertical axial symmetry as in the present invention which reduces the 6 degrees of motion freedom to 4, (i.e., roll=pitch=pendular motion, sway=surge=lateral motion, yaw=rotational motion, and heave=vertical motion). It is further simplified if hydrodynamic design features may be de-coupled to linearize the process and ease the ideal solution search.
An object of the present invention is a floating platform which contains features which allow the platform motions to be optimized for size and weight to specified hydrodynamic environments and to include features which reduce offshore oil and gas processing operations and field development costs.
A further object of the present invention is to provide a platform which allows the roll hydrodynamics to be determined and optimized and by other features allows tuning of the frequency response for the vertical heave.
An additional object of the present invention is a more efficient self propelled or severe weather moored deepwater floating platform called an SSP with focus upon providing improved vessel motions in wind and wave conditions while exhibiting features which reduce offshore gas/oil field development and operation costs.
SUMMARY OF THE INVENTION
The present invention provides for an offshore floating facility with improved hydrodynamic characteristics and the ability to moor in extended depths thereby providing a satellite platform in deep water resulting in shorter flowlines, cables and umbilicals from the subsea trees to the platform facilities. The design incorporates a retractable center assembly which contains features to enhance the hydrodynamics and allows for the integral use of vertical separators in a quantity and size providing opportunity for individual full time well flow monitoring and extended retention times.
The floating platform of the present invention is capable of being self-propelled in mild environments or moored for use in extreme depths and sever wind and wave conditions. The floating platform may be configured to perform the functions of a well-gathering platform, an offshore utility work platform; a remote power or communication transmission hub or relay platform or a satellite separator platform (SSP). The floating platform is hereafter referred to as an SSP despite its adapted use.
A principal feature of the SSP is a retractable center assembly within the hull, which may be raised or lowered in the field to allow transit in shallow areas. The retractable center assembly provides a means of pitch motion damping, a large volumetric space for the incorporation of optional ballast, storage, vertical pressure or storage vessels, or a centrally located moon pool for deploying diving or remote operated vehicle (ROV) video operations without the need for added support vessels.
Hydrodynamic motion improvements are provided by: the basic hull configuration; extended skirt and radial fins at the hull base; a (lowered at site) center assembly extending the retractable center section with base and mid-mounted hydrodynamic skirts and fins, the mass of the separators below the hull deck of the SSP favorably lowering the center of gravity; and attachment of the steel catenary risers, cables, umbilicals and mooring lines near the center of gravity at the hull base. The noted features improve vessel stability and provide increased added mass and damping which improves the overall response of the system under environmental loading.
Key field production items that are satisfied by the invention are: housing large and efficient vertical high-pressure separators with extended retention times which can minimize multiphase flow with upstream primary separation closer to subsea wells which also improves reservoir recovery ratios, providing vertical separators of such dimension that multiple sensors can be used to optimize the liquid gas interface level; providing more economical full pigging ability with individual control of well flowlines; providing individual well flowline chemical injection without added subsea manifolding; simplification of operations and maintenance requirements; and providing for reduced inspection costs below water by the incorporation of a moon pool on the SSP centerline.
The principal cost reductions evolve from: the ability to perform fabrication in relatively shallow water sites; elimination of the necessity of costly offshore deck installation which is typical of certain deepwater alternatives; allowing for short transportation routes to the offshore field by minimizing draft and allowing use of domestic coastal fabrication facilities; providing duplicate functions for structural appendages to minimize fabricated weight and maximize the available flotation per ton of fabrication; and providing pressure control as close to the field as practical for improved economics of
Madan Mossman & Sriram P.C.
Mayo Tara L.
Ope, Inc.
Will Thomas B.
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