OA11869A - Satellite separator platform (SSP). - Google Patents

Abstract

A floating platform with motion characteristics for offshore deepwater developments with vertical axial symmetry and decoupling of hydrodynamic design features. A motion-damping skirt (120) is provided around the base of the hull (1), which is configured to provide ease of installation for various umbilicals and risers. A retractable center assembly (300) is used in a lowered position to adjust the center of gravity and metacentric height, reducing wind loads and moments on the structure, providing lateral areas for damping and volume for added mass for roll resistance. The center assembly (300) is used to tune system response in conjunction with the hull damping skirt (120) and fins (121). The center assembly (300) also includes separators (350) below the floating platform deck which serve to add stability to the floating structure by shifting the center of gravity downward, the separators (350) capable of being raised and lowered vertical separators alone or as a unit.

Description

118 6 9 TITLE: SATELLITE SEPARATOR PLATFORM (SSP)

SPECIFICATION

FIELD OF THE INVENTION

The invention relates to a moored offshore or self-propelled floating platform with improved 5 motion characteristics for économie offshore deepwater developments. The floating platform ofthe présent invention is capable of being self propelled in mild environments or moored for usein 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 subseawells and a host platform. The extended lengths pose energy loss, pressure drop andproduction difficulties.

Costs of structures for deepwater applications are high and costs are frequently increased dueto the foreign locations at which they are fabricated. Other difficulties, associated with 15 deepwater offshore operations, resuit from floating vessel motions which affect personnel andefficiencies especially when related to liquid dynamics in tanks. The primary motion relatedproblem, associated with offshore petrochemical operations, occurs with large horizontalvessels in which the liquid level oscillâtes and provides erroneous signais to the liquid levelinstruments causing shutdown of Processing and overall inefficiency for the operation. 1 118 6 9

Prior art for deepwater has generally resulted in facilities that yield expensive solutions for anoffshore oil field development. Such prior art includes tension leg platforms (TLP's), which mayincorporate well drilling capabilities to several mini-TLP designs that perform simplifiedfunctions; SPARS which are configured much the same as a spar buoy and hâve dry wellheads, 5 as opposed to subsea trees; deepwater floating production storage facilities (FPSO’s) and theirseveral variations.

Présent 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 thesubsea wells. Systems that are moored by catenary lines can be placed within the deepwater 10 fields within fairly close proximity to the subsea wells. 2 118 6 9

References Cited:

US PATENT DOCUMENTS

Patent Number Issue Date Inventor US Class RE32119 April 22, 1986 Abbott, Philip A. 405/227 5722797 February 21,1996 Horton, Edward E. III 405/224 5704731 May 10 1996 Huang, Yen T. 405/223.1 5588387 March 14, 1995 Tellington, Wentworth J. 114/261 5536117 July 16,1996 Frame, Malcolm B. 405/202 5525011 June 11,1996 Huang, Yen T. 405/223.1 5375550 December27, 1994 Innis, Donald A. 114/267 5363788 November 15,1994 Delrieu, Jean-Luc 114/125 5215028 June 1, 1993 Hayakawa, Yasuhiro 114/267 5213447 May 25, 1993 Srock, Brian J. 405/219 5129347 July 14, 1992 Hill, Anthony E. 114/266 4987846 January 29, 1991 Yamashita, Seiya 114/265 4987846 January 29,1991 Yamashita, Seiya 114/265 4913591 April 3,1990 Steele, James E. 405/196 4906139 March 6,1990 Chiu, Hin 405/224 4886398 December 12,1989 Sparks, Charles 405/224 4834014 May 30,1989 Olsen, Fred 114/265 4746245 May 24,1989 Mork, Harald J. 405/224 4716972 January 5,1988 Makinen, Eero 175/008 4692065 September 8,1987 Suzuki, Yoshio 405/211 4685833 August11, 1987 Iwamoto, William T. 405/195 4674919 June 23,1987 Olsen, Olav 405/226 4626137 December 2,1986 Willemsz, John R. 405/224 4606673 August 19, 1986 Daniell, Alan F. 405/210 4604962 August 12, 1986 Guibault, Denis 114/266 4596291 June 24, 1986 Makinen, Eero 175/005 4576520 March 18,1986 Suh, Sung L. 405/224 4576520 March 18,1986 Suh, Sung L. 405/224 4576517 March 18,1986 McCann, James 405/195 3 118 6 9

Patent Number Issue Date Inventor US Class 4565150 January 21, 1986 Liden, Hadar 114/265 4554883 November 26,1985 Lane, Wallace W. 114/266 4519728 May 28, 1985 Oshima, Masanao 405/224 4498412 February 12, 1985 Liden, Hadar 114/264 4482274 November 13,1984 Brandi, Roberto 405/224 4481899 November 13, 1984 Einstabland, Tomas B. 114/265 4457250 July 3, 1984 Oshima, Masanao 114/265 4452165 June 5,1984 Bergman, Gunnar B. 114/125 4433941 February 28,1984 Gerwick, Jr., Ben C. 405/211 4409921 October 18,1983 Carroll, James P. 114/264 4406243 September 27,1983 Kim, Chung U. 114/264 4378178 March 29,1983 Roach, Richard T. 405/224 4310052 January 12, 1982 Rivertz, Johan A. 166/362 4286538 September 1,1981 Matsui, Atsushi 114/266 4281613 August4, 1981 Ray, Donald R. 14/230 4239417 December 16,1980 Slatten, Arvid 405/195 4217848 August 19, 1980 Meyer-Haake, Gerhard D. 114/264 4170186 October 9,1979 Shaw, Clarence W. 114/264 4168673 September 25,1979 Poeppel, Fritz O. 114/265 4168556 September 25,1979 Fink, Charles R. 009/008.R 4167147 September 11,1979 Bergman, Gunnar B. 114/122 4155674 May 22,1979 Martin, Yves 405/224 4155673 May 22, 1979 Yashima, Nobuyoshi 405/224 4155323 May 22,1979 Fïnsterwalder, Klemens 114/264 4117691 October 3,1978 Spray, Claude 405/205 4108102 August 22,1978 Linstrom, Karl Olof 114/264 4087984 May 9,1978 Mo, Olàv 061/094 4067285 January 10,1978 Jones, Robert M, 114/266 3982492 September 28,1976 Steddum, Riddle E. 114/000.5D 3978805 September 7,1976 Thomas, David G. 114/000.5F 3975784 August 24, 1976 Whitaker, Charles T. 009/008.R 3951086 April 20, 1976 Lown, Eldon C. 114/000.5D

Patent Number Issue Date Inventor US Class 4 113 6 9 3951085 April 20, 1976 Johnson, Don E. 114/000.5F 3949693 April 13, 1976 Bauer, Peter 114/000.5D 3885511 May 27, 1975 Wipkink, Johannes 114/000.5D 3870010 March 11, 1975 Wright, Everald V. 114/230 3788254 January 29,1975 Sheil, John E. 114/000.5F 3768463 October 16, 1973 Gassett, Paul L. 141/388 3739737 June 19, 1973 Baier, Robert J. 114/000.5D 3673975 July 4, 1972 Strauss, Erwin S. 114/043.5 3635182 January 18, 1972 Paffett, James Arthur Haines 114/000.5

The principal éléments which can be modified for improving the motion characteristics of amoored floating vessel are the draft, the water plane area and its draft rate of change, locationof the center of gravity (CG), the metacentric height about which smalt amplitude roi) andpitching motions occur, the frontal area and shape on which winds, current and waves act, theSystem response of pipe and cables contacting the seabed acting as mooring éléments, and thehydrodynamic parameters of added mass and damping. The latter values are determined bycomplex solutions of the potential flow équations integrated over the floating vessel’s detailedfeatures and appendages and then simultaneously solved for the potential source strengths. Itis only significant to note herein that the addition of features which allow the added mass and/ordamping to be ‘tuned’ for a certain condition requires that several features can be modified incombination, or more preferably independently, to provide the desired properties. Theoptimization is greatly simplified if the vessel possesses vertical axial symmetry as in theprésent invention which reduces the 6 degrees of motion freedom to 4, (i.e., roll=pitch=pendularmotion, sway=surge=lateral motion, yaw=rotational motion, and heave=vertiçal motion), lt isfurther simplified if hydrodynamic design features may be de-coupled to linearize the processand ease the idéal solution search.

An object of the présent invention is a floating platform which contains features which allow theplatform motions to be optimized for size and weight to specified hydrodynamic environmentsand to include features which reduce offshore oil and gas Processing operations and field 5 1186 9 development costs. A further object of the présent 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 présent invention is a more efficient self propelled or severe weathermoored deepwater floating platform called an SSP with focus upon providing improved vesselmotions in wind and wave conditions while exhibiting features which reduce offshore gas/oil fielddevelopment and operation costs.

SUMMARY OF THE INVENTION

The présent invention provides for an offshore floating facility with improved hydrodynamiccharacteristics and the ability to moor in extended depths thereby providing a satellite platformin deep water resulting in shorter flowlines, cables and umbilicals from the subsea trees to theplatform facilities. The design incorporâtes a rétractable center assembly which containsfeatures to enhance the hydrodynamics and allows for the intégral use of vertical separators in aquantity and size providing opportunity for individual full time well flow monitoring and extendedrétention fîmes.

The floating platform of the présent invention is capable of being self-propelled in mildenvironments or moored for use in extreme depths and sever wind and wave conditions. Thefloating platform may be configured to perform the functions of a well-gathering platform, anoffshore utility work platform; a remote power or communication transmission hub or relayplatform or a satellite separator platform (SSP). The floating platform is hereafter referred to asan SSP despite its adapted use. A principal feature of the SSP is a rétractable center assembly within the hull, which may beraised or lowered in the field to allow transit in shallow areas. The rétractable center assemblyprovides a means of pitch motion damping, a large volumétrie space for the incorporation ofoptional ballast, storage, vertical pressure or storage vessels, or a centrally located moon poolfor deploying diving or remote operated vehicle (ROV) video operations without the need for 6 added support vessels. 1186 9

Hydrodynamic motion improvements are provided by: the basic hull configuration; extendedskirt and radial fins at the hull base; a (lowered at site) center assembly extending therétractable center section with base and mid-mounted hydrodynamic skirts and fins; the mass ofthe separators below the hull deck of the SSP favorably lowering the center of gravity; andattachaient of the steel catenary risers, cables, umbilicals and mooring lines near the center ofgravity at the hull base. The noted features improve vessel stability and provide increasedadded mass and damping which improves the overall response of the System underenvironmental ioading.

Key field production items that are satisfied by the invention are; housing large and efficientvertical high-pressure separators with extended rétention times which can minimize multiphaseflow with upstream primary séparation doser to subsea wells which also improves réservoirrecovery ratios; providing vertical separators of such dimension that multiple sensors can beused to optimize the liquid gas interface level; providing more economical full pigging ability withindividual control of well flowlines; providing individual well flowline Chemical injection withoutadded subsea manifolding; simplification of operations and maintenance requirements; andproviding for reduced inspection costs below water by the incorporation of a moon pool on the SSP centerline.

The principal cost réductions evolve from: the ability to perform fabrication in relatively shailowwater sites; élimination of the necessity of costly offshore deck installation which is typical ofcertain deepwater alternatives; allowing for short transportation routes to the offshore field byminimizing draft and allowing use of domestic coastal fabrication facilities; providing duplicatefunctions for structural appendages to minimize fabricated weight and maximize the availableflotation per ton of fabrication; and providing pressure control as close to the field as practical forimproved économies of pipeline and flowline steel tonnage and installation cost réduction. 7 118 6 9

The prior art does not disclose methods for: the use of rétractable center sections which by theirposition, structure, appendages, and contents improve the in-place hydrodynamiccharacteristics while allowing shallow water access when fully raised. The prior art also doesnot disclose methods of providing increased extended-duration vertical vessels or separatorsbelow the floating platform deck that serve to add stabiiity of the floating structure by favorablyshifting the center of gravity downward while also increasing the roll and heave-added mass anddamping of the floating structure for reduced platform motions in wind and waves. The prior artalso does not disclose methods of raising and lowering vertical separators alone or as a unitwithin a center assembly to allow passage to or from shallow waters. The prior art does notdisclose methods of providing a one-atmosphere access zone around the operationalcomponents of separators suspended from a floating platform. The prior art does not disclosemethods of optimizing a vertical separator's performance by a nearly continuous array ofsensors that allow a variable liquid level. The prior art does not disclose extending a skirt at thebase of a hull with a diameter and configuration to ease the offshore attachaient of steelcatenary risers, umbilicals, and cables.

Because of the features which may be provided for, when utilized in the capacity of a floatingdeepwater oil and/or gas primary séparation platform, the SSP perforais bulk séparation andyields full time test capabilities of each attached well via the flowlines and well contraiumbilicals. The hull features of the SSP include objects of the invention such as the verticalcolumns which provide hull-stiffening while serving as mooring line conduits for above watermooring line tensioning, and tension monitoring; facilities for the installation / addition /maintenance of the long vertical separators offshore; benefits afforded by the hydrodynamic hulldamping skirt that doubles as a submerged towing rim and a load ring that distributes thetransverse mooring loads and provides a foundation for steel catenary risers; umbilicals andcables; the separator supported damping skirt which doubles as separator spacing restraints;and the separator raising and lowering frame Systems allowing shallow water fabrication yard 8 118 6 9 access.

Due to the size of the individual first stage vertical separators and slug catchers afforded by thecenter assembly space and the available extended résidence times which can beaccommodated, efficient high-pressure séparation can be accomplished for the purposes ofminimizing transportation of produced réservoir water over long distances. Vertical, as opposedto horizontal, separators minimize motion effects due to the reduced overall élévationsignificance of internai separator fluid waves without the need for baffles and utilize the nearlyunlimited available space within the water column without wasting deck space.

Other fonctions provided by the SSP, while in the rôle of an oil/gas platform, are pigging of theflowlines and outgoing gas/oil lines; metering of multiple wells; Chemical injection of theincoming and outgoing lines; manifolding to perform the required fonctions; quarters facilities forthe limited crew required for operations; instrument gas génération and Controls; satellite andother information transmissions to the host platform(s); electric power transmission; andumbilical control of the satellite wells.

While Individual prior art includes several of the above features, none hâve addressed theaspect of extended oil / gas séparation résidence times in vertical separators and variableélévations control of the liquid gas interface by numerous vertically-spaced sensors, which are asolution to the issue of improved high-pressure séparation without excessive treatments. Priorart does not address the issue of minimizing the floating structure draft beyond normal ballastingfor fabrication yard and tow conditions to site provided by the SSP. The SSP may therefore befabricated and/or outfitted at common shipyards. Once lowered, the separators and associatedequipment increase the draft beyond normal port access and while doing so, improves thefloating vessel stability by lowering the center of gravity and increasing roll and heave-addedmass and damping.

The manner of achieving improved hydrodynamic motion improvements within the présent 9 118 6 9 invention involves the incorporation of the following features: (1) Use of the vertical reactions of the mooring System and the steel catenary and otherrisers to achieve an operating draft for the facility which is sufficient for reducedmotions with minimal conventional ballast; 5 (2) providing a huit water plane area vs. draft ratio which, when tuned to the heave- added mass and damping, provides sufficient free board in design storms andprovides adéquate vertical heave damping to resist high-frequency response tonormal wind and waves; (3) making use of the différence of the vertical wave particle velocities at the surface and ÎO at the base of the center assembly to damp out the storm-induced heave motions; (4) use of the rétractable center section weight and its content in the full lowered positionto decrease the overall System center of gravity; (5) use of the center section in its lowered position to improve the latéral damping inorder to offset latéral motion of the primary hull section and minimize rail and sway ^5 motions; (6) use of the trapped hydrodynamic added mass due to the center assembly and itscontents as well as enhanced damping and added mass features of spacedsegments and the lower center section ‘skirts’; (7) use of the skirt extending around the lower circumference of the main hull to 20 decrease the floating roll response by added mass and damping; (8) use of vertical gussets supporting the hull ‘skirt’ as fins to provide added mass anddamping and to reduce yaw rotational motions. 10 118 6 9 (9) providing the opportunity to separately tune roll and heave response frequencies bydesigning the hull skirt to control roll damping and the hull plus center section hullskirts to control heave added mass and damping, thereby allowing the System to befinely tuned for a drier deck in a design limit seastate.

BRIEF DESCRIPTION OF THE DRAWINGS

For further understanding of the nature and objects of the présent invention, référencé shouldbe had to the following drawings in which like parts are given like référencé numerals and wherein:

Figure 1 présents the plan and élévation view of the preferred embodiment of the présentinvention;

Figure 2 présents a more detailed plan view of the deck 120;

Figure 3a présents the SSP with the rétractable section item 300 extended for shallow waterbeing towed by a boat 500 in an un-powered SSP configuration for transferred to/from a site;Figure 3b présents the SSP in a partially ballasted position as the rétractable center assembly300 is partially lowered as may be the case of a tow during heavy seas or unexpected storm;Figure 3c présents the SSP in an installed position with center section 300 fully deployed belowthe waterline 400 and mooring lines 200 and risers 160 extending to the seabed (not shown);Figure 4 is a plan view of the lowest portion separators 350 contained within the centerassembly 300;

Figure 5.1 A is a side view, partly in phantom line, of a separator of the preferred embodiment ofthe présent invention;

Figure 5.2 is an enlarged side view of the bottom of the separator of Figure 5.1 A;

Figure 5.3 is an englarged view of the center of the separator of Figure 5.1 A;

Figure 5.4 is a side view of the top of the separator of Figure 5.1 A; andFigure 5.5 is a plan view of the separator of Figure 5.1 A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the présent invention provide for an offshore floating structure (as shown in 11 118 6 9

Fig. 1) with augmented added mass damping and mooring tensioning features which provides amoored platform for the functions of primary process oil / gas / water séparation, pigging of wellflowlines and downstream pipelines, Chemical injection and control of satellite well functions,pressure control for outgoing gas and oil lines to the host(s), and communication links to other 5 locations. Other features associated with the object of the invention includes a floating hull 1with low draft for convenience of domestic construction, suitably broad beam for a stable tow,deep set ballast tanks to augment the tow to site, élimination for the need of deck installationoffshore, and the capability of lowering the center assembly with features for improved motion. A portion of hull 1 is the hull segment 100 that is above the water line 400. Hull segment 100 10 includes a top deck 110, having a helideck 120 to allow helicopter traffic. Top deck 110 furtherincludes a crâne 112 for allowing materials to be transported to and from the deck 110 andcargo boats (not shown). A plurality of piping and pigging éléments 113, 114 which connect theprocess éléments in the central section 300 to the attached wells on the seabed are alsolocated on top deck 110. A control umbilical and/or power cable 115 is also routed to the 15 attached wells on the seabed from deck 110. Piping and pigging éléments 116, 117 are alsoprovided on deck 110 to transfer liquids and gases away from the SSP to distant pipeline orplatform connections. A second portion of hull 1 below water line 400 includes a hull skirt 120. Eléments 121, 122,123 act as a stiffener 130 which also provides rotational damping. 20 Hull 1 has a central section 140 which is hollow and contains a rétractable section 300. The hull1 further includes radial mooring lines 200 depending from the hull 1. Catenary risers 160 areattached to the hull skirt 120. A riser guard 170 is provided for the risers 160 as they are routedfrom the hull skirt 120 to the top deck 110. Hull 1 also has a boat bumper 180 for allowing smallvessels (not shown) to berth during loading and unloading of equipment or personnel. 25 The naval architectural and marine engineering aspects of the SSP floating platform differ fromprior art in that reliance of the upper hull 100 of the floating structure 1 is augmented by the 12 1186 9 center assembly 300, which provides a dual service by providing a large space for variouspayloads and/or equipment and providing a lower center of gravity when in a lowered positionto add to the stability as discussed above. Mooring attachment port 210 pénétrâtes the upperhull 100 and acts as a restraint, thus allowing tensioning of the mooring system lines 200. Themooring System follows prior art in the use of a compound or alternate mooring system tominimize vertical reactions while providing latéral restoring force for station keeping.

Figure 4 shows a preferred embodiment of the separators 350 included within the centerassembly 300. Separators 350 may be much larger in volume and rétention time, due to theirlength, compared to prior art. Further, separators 350 can make up a more significant portion ofthe platform mass than the prior art, and provide extended rétention duration for improvedséparation at higher pressures for séparation of gas and liquids. The extended vertical length ofseparators extends below the water line 400, and one such separator may be provided for eachwell. The separators may be provided with a multitude of sensors 360 (as shown in Figure 5),which detect the State of the fluids at each élévation and that each separator may be equippedwith several outlets for gas 395, oil / condensate and water 380. Each separator 350 may beequipped with several inlet valves 390 for optimization of the incoming well stream, and theoverall control system (now shown) can détermine the required opening and closing of therespective valving with the object of optimizing the séparation process with the availableconstraints of inflow and rétention volume of the separators.

The economy of the design is enhanced by the fact that, if desired, limited equipment isnecessary on the top deck of the facility 110, as shown in Fig. 2, with the exception of the wellcontrol panels, emergency quarters 111, and helideck 190. The well flowline and outgoing gas,oil lines, pig launcher 116 (as shown in Figure 2), associated manifolding, and Chemicalinjection system are on the hull segment 100. The hull segment 100 also contains the normaloffshore éléments of boat landings 180 and safety devices typical of floating structures. 13 1186 9

Preferred embodiments of the process function are: to allow primary well séparation by long efficient vertical separators within the center assembly that may comprise a far more significant portion of the weight System than prior art,assisting the marine vessel stability; 5 to utilize vertical separators to minimize the variation of liquid surface variations relative to level settings and minimize the overall dynamic motion problème due to floating marinevessel motions; to provide individual separators for continuai well monitoring of gas and liquid wellproduction and satisfy the test séparation requirements for any well; ^0 to afl°w signal transfer simultaneously of multiple individual wells with the liquid and gas rates plus température and pressure allowing remote control capability to a host location bysatellite communication; to provide improved quality gas and oil / condensate pipelines at reduced energyconsumption to the shallow water host platform(s) by improved moderate to high pressure 15 primary séparation reducing pumping and compression and minimizing multi-phase conditions; to provide smaller diameter pipeline from the SSP to the host platform(s) due to waterremoval nearer the subsea well field and improving flow characteristics by minimizingmultiphase flow and slugging and reducing the viscosity increase associâtes with oil and waterflow. 20 Preferred embodiments of the marine engineering aspects are:

As shown in Fig. 3d., an embodiment to enhance the hydrodynamic damping of the System is ahull, skirt 120, which extends below the boat level draft around the circumference of the hull andalso acts to provide reinforcement for the mooring attachaient points and ease of attachment ofSteel catenary and umbilical risers 160 while offshore. The skirt 120 is modified at the outer rim 25 123 to increase the added mass and damping in roll. The skirt is stiffened by large gussets 121, 14 1136 9 122, which provide yaw damping and “pockets” for added roll mass. Such duplicity of features tends to reduce the needs for excess weight and increased fabrication costs.

The lowest portion of the separator 350 shown on Fig. 4 is contained within the center assembly300. The center assembly 300 incorporâtes a damping skirt 301 (as shown in Figure 3c) which,being well below the wave action of the surface, provides heave-damping and added mass andtransmits the reaction to the hull via the center assembly. Other skirts 302 not previousiyidentified, serving as reinforcements and guides for the center assembly, add to the dampingand added mass in heave.

Thus, the SSP contains physical design features which facilitate: fabrication; transfer to the finalsite; field mounting of éléments which are to be suspended from the platform to the seabedduring offshore construction; mounting of specialized equipment in manners to minimizecontamination from marine environments; improvement of offshore gas/oil processing fielddevelopment costs; mooring in the field; and underwater video inspection of the suspendedcomponents and mooring attachments while in operation on site.

The floating facility allows the use of naval architectural features which improve thehydrodynamic motion characteristics while allowing the overall weight of the structure to bereduced for fabrication économies.

When used in the rôle of a moored oil/gas satellite separator platform the center assembly isused to contain hydrocarbon gas/oil separators, utility storage, and ballast. Due to the availablespace, separators can be measurably larger in their capacity with resulting higher rétentiontimes for improved séparation of gas and liquids than normally achievable in the présentoffshore practice which provides much value and operational flexibiiity for offshore operators.Due to the large allowable vertical dimension in the center assembly, vertical separators may beimplemented which are less affected by marine motions and, with the benefits of largerallowable size, offset any inefficiencies of the reduced liquid-gas contact area within theseparator. Due to the space within the center assembly, a large number of vertical separators 15 118 6 9 may be installée) permitting individual wells to be continually monitored for their independentproperties. Continuai well monitoring greatly benefits the évaluation of the field productionconditions by réservoir engineers. In other applications, vessels within the center assemblymay provide opportunity for temporary bulk or liquid storage. 5 The central assembly , when fully raised, allows for the fabrication and outfitting of the SSP atnumerous coastal shallow water fabrication facilities providing cost and transportation savingsopportunities to nearby offshore fields. When the central assembly is fully lowered to itsoperational position, the center assembly enhances the motion characteristics of the floatingfacility by several means. 1ü The main hull of the SSP includes hydrodynamic features such as a huit damping skirt forimproved roll damping and increased roll added mass; radial plate hull damping skirt stiffenersacting as fins which provide yaw damping and increased yaw added mass; and a variable waterplane area to provide nonlinear heave stiffness. These features provide benefit in stormconditions. Other features include options for a clear deck to reduce wind force and moments, 15 locating heavy mooring and other equipment well below the top deck, and a bilge/ballast area atthe hull base which ail add to improve shallow water towing stability when the center assemblywould be elevated for seabed clearance.

To provide benefit in storm conditions, the center assembly, in its fully lowered position, extendswell below the normal water level of the floating facility providing significant improvement in the 2ô center of gravity and metacentric height; and the réduction of tall vertical separators above thehull deck further reduces wind loads and moments on the structure. The center assembly maycontain vessels provided strictiy for ballast or by their operations, contain production liquids thatprovide natural ballast. The center assembly, when lowered, provides latéral area for dampingand volume for added mass for roll résistance. The damping and added mass of the center 25 assembly differs in yaw and heave motion. The center assembly contains skirts and finsoriented to provide the degree of hydrodynamic characteristics in either of the motions and be 16 118 6 9 used to tune the final System response following the hull design. The use of the center assembly to tune response in conjunction with the hull form and hull damping skirt and fins is central to this invention.

Thus, the floating platform termed an ‘SSP’ that may be moored or self-propelled which may beused as an offshore facility for communications and/or power génération or utility platform orwork platform or to gather seabed products from outlying sources and contains a means toindividually “tune” the heave added mass and damping motions separately from motions of rolland yaw.

Further, the floating platform utilizes a centrally located assembly that can be raised andlowered or installed offshore to the base of the hull, having sufficient size and volume tofavorably affect the sea-keeping motions in wind and wave conditions by shifting the center ofgravity and providing added mass and damping to reduce the floating structure motions in waves and wind.

Such a floating platform may utilize high pressure vertical separators providing individual wellmonitoring that may extend significantly below the normal water line and which may berétractable for shallow water access, or maintenance.

Such a floating platform may utilize a damping skirt at the base of the structure, which doublesas structural reinforcement for a rétractable center assembly. It may also utilize a damping skirtat the base of the structure, containing uptumed and/or downturned edge appurtenances,yielding pockets to improve added mass and damping characteristics.

Such a floating platform has provision for Steel catenary riser (SCR) réceptacles andconnections which attach to the damping skirt at the base of the structure.

Such a floating platform incorporâtes a skirt continuous or segmented, extending laterally fromthe hull base to improve riser, umbilical, and cable installation ease by providing sufficient hullclearance for installation vessels and clearance for the installation rigging.

Such a floating platform utilizes mooring lines which extend from the interior dry space abovethe water line through vertical or near vertical hull supporting columns to vertically/or near 17 118 6 9 vertically mounted fairleads.

Such a floating platform provides a central moon pool for ROV and diver operations whichdoubles as a structural stiffening of a centralized raising and lowering assembly.

Such a floating platform utilizes a sériés of sensors in submerged or in one atmospheric 5 environment impervious to submerged service, to identify the phase and product at variouslevels within separators providing information for the optimization of the System operations forvaiving gas, oil or condensate, or water while avoiding excess separator height.

Such a floating platform with a rétractable center assembly utilizes vertical rotational symmetryto avoid the need to weathervane and reducing its overall mass requirements for acceptable tÛ motions by implementing rotationally symmetric added mass and damping appartenances.

The best mode and preferred embodiments of the invention hâve been described. It is to beunderstood that the invention is not limited, thereto, but rather is to be measured by the scopeand spirit of appended daims. 18

Claims (12)

118 6 9 What is claimed is:

1. An offshore floating structure for use with water, comprising:a floating low draft hull, said hull having a broad beam;said hull including ballast tanks; said ballast tanks being deep set;said hull having a hollow center; a center section reciprocally mounted in said hollow center.

2. The structure of Claim 1, wherein said center section includes at least one verticalseparator extending below the water.

3. The structure of Claim 2, wherein each separator includes an inlet valve.

4. The structure of Claim 1, wherein there is further included a hull skirt which extends below the water around the circumference of said hull.

5. The structure of Claim 4, wherein said skirt has added mass at its outer rim.

6. The structure of Claim 5, wherein said skirt is strengthened by gussets.

7. The structure of Claim 1, wherein said center section includes a damping skirt below thewater.

8. The structure of Claim 7, wherein said damping skirt is located at the bottom of saidcenter section.

9. The structure of Claim 8, wherein said damping skirt may hâve upturned edgeappurtenances.

10. The structure of Claim 8, wherein said damping skirt may hâve downturned edgeappurtenances.

11. The structure of Claim 1 to accommodate catenary risers, having a damping skirt, thecatenary risers attached to said damping skirt.

12. The structure of Claim 1, wherein said hollow center includes structural stiffening. 19