OA10302A - Improved compliant tower - Google Patents

Abstract

A compliant tower is disclosed having a foundation (16) connected to a wide-bodied compliant framework (12) with a plurality of vertically extending legs (20) and a minimum of horizontal bracing. The compliant framework is configured to maintain a substantially wide, open riser suspension corridor. A topside facility is supported by the compliant framework and a plurality of freely suspended production risers extend through the riser suspension corridor from the vicinity of the topside facility to communicate with the reservoir. These production risers are spaced to provide clearance to prevent riser interference in response to normal flexure of the compliant tower and normal environmental loads on the risers. A riser support assembly is configured to accommodate relative motion between the risers and the topside facility, supporting the production risers in tension near their upper ends to provide the principal load transfer between the riser and the compliant framework.

Description

IMPROVED COMPLIANT TOWER

The présent invention relates to an improved design fordeepwater offshore platforms. More particularly, the présentinvention relates to an improved compilant tower design.

Traditional bottom-founded platforms having fixed or rigidtower structures are effective to support topside facilities inrelatively shallow to mid-depth waters, but their underlying designpremises become economically unattractive in developments muchdeeper than 300 m or so.

Compilant towers were developed as one alternative to providebottom-founded structures in deeper water which are designed to"give" in a controlled manner in response to dynamic environmentalloads rather than rigidly resist those forces. A basic requirementin controlling this response is to produce a structure havingharmonie frequencies or natural periods that avoid those encounteredin nature. This has produced designs which, when compared withtraditional rigid platforms, substantially reduce the total amountof Steel required to support topside facilities.

Various approaches to altering the frequency responsecharacteristics of compilant designs hâve been proposed which hâvesought to further reduce loads and Steel requirements with tightlycor.structed "slim" towers. Nevertheless, these applications requireçreat amounts of Steel, and often a high percentage of this steelmusc be selected from premium grades and alloys.

Thus, there remains substantial benefit to be gained fromrmprovements that would safely further reduce the requirement fortre amount of steel or beneficially aller the performancecharacteristics demanded of the Steel supplied for deepwateroffshore compilant towers.

In accordance with the invention there is provided a compilanttower for conducting hydrocarbon recovery operations from adeepwater offshore réservoir, comprising a foundation, a compilant -“'Λ. ' I· irameworx, coraprisinç a piuraiity o: vestjcuily extendmg lej:,secured to the foundation, and a minimum ot horizontal bracinginterconnecting the legs in a manner definmg a substantially openriser suspension corridor extending substantially the entire lengthof the compilant framework, and a topside facility supported by thecompilant framework.

Preferably at least one freely suspended production riserextends through the riser suspension corridor from the-vicinity ofthe topside facility to communicate with the réservoir. Theproduction risers are spaced to provide clearance to preventinterférence between the production risers and between theproduction risers and the horizontal bracing in response to normalflexure of the compilant tower and normal environmental loads on theproduction risers. A riser support assembly is configured toaccomodate relative motion between the risers and the topsidefacility, supporting the production risers in tension near theirupper ends to provide the principal load transfer between the riserand the compilant framework.

The brief description above, as well as further objects andadvantages of the présent invention will be more fully appreciatedby reference to the following detailed description of the preferredembodiments which should be read in conjunction with the accompanying drawings in which: FIG. 1 is an isométrie view of an improved compilant tower employing the method of the présent invention. FIG. IA is a side élévation view of the upper end of the improved compilant tower of FIG. 1. FIG. IB is a close-up of a riser support in an embodiment of the présent invention in accordance with FIG. IA. FIG. IC is a cross section of the improved compilant tower of FIG. 1 taken along line 1C-1C in FIG. 1. FIG. 1D is a cross section of the improved compilant tower of FIG. 1 taken along line 1D-1D in FIG. IA. FIG. 1E is a partially cross sectioned view of a dual concentric string high pressure drilling riser in accordance with a v vf υ ά practice of trie présent invention.

FIG. 1F is an end plan view of the compilant tower of FIG. IG in transport. FIG. IG is a horizontal cross section of the compilantframework of an alternate embodiment of the présent invention. FIG. 2 is a perspective view of a compilant tower design notbenefitting from the présent invention. FIG. 2A is a cross section of the compilant tower of FIG. 2taken at line 2A-2A in that figure.

IG FIG. 3A is a schematic illustration of the sway mode response for a compilant tower. FIG. 3B is a schematic illustration of the whipping j mode response for a compilant tower. FIG. 3C is a schematic illustration of the sway mode response for a compilant tower having multiple top-tensioned, rigidly securedrisers. FIG. 4A is a graphical représentation of wave frequencydistribution in storm and non-storm situations. FIG. 4B is a graphical représentation of the dynamic responsecharacteristic of preliminary designs for three different deepwaterstructures. FIG. 4C is a graphical représentation of the fatiguecharacteristics for two different compilant towers. FIG. 1 illustrâtes an improved compilant tower 10 constructedin accordance with the practice of one embodiment of the présentinvention. The risers and topside facilities hâve been omitted fromthis figure for the sake of simplicity m introducing the basictower structure. This illustration is based on a preliminary designfor tnirty wells in 1000 m of water, with a topside payload of22,605 tons which includes 6000 tons of nser tension.

This iilustrated embodiment combines stveral interrelatedaspects which contribute to the improved compilant tower of theprésent invention having a iightweight, wide body stance compilantframework, a substantially open interior without conductor guidesand with a minimum of horizontal bracing, and tensioned production 1 - ' · iiicst· éléments c»ru 11 but nuj to the benefits of the présent invention are djscussed in furtherdetail below.

In the illustrated embodiment, a compilant framework 12 oftower 10 is provided in the form of a compilant piled tower in whichpiles or pilings 14 not only provide foundation 16 secured to océanfloor 22, but also extend a substantial distance above the mudline24, along a substantial length of the compilant framework andthereby contribute significantly to both the righting moment anddynamic response of the overall compilant framework. Pilings 14 areslidingly received within sleeves 18 along legs 20 at the corners ofcompilant framework 12.

The tops of the pilings may be fixedly secured to the legs atpile receiving seats 27 by grouting or a hydraulically actuatedinterférence fit. Minimal relative motions from non-stormconditions may be accommodated with an elastomeric grommet orbearing at the intersection of the pilings and sleeves. Largermotions are accommodated by the sliding connection.

The upper end of this embodiment of tensioned riser deepwatertower 10 is illustrated in greater detail in FIG. IA, here includingtopside facilities 30 which are supported above océan surface 26.Topside facilities, as used broadly herein, may be as minimal as,e.g., a riser grid supporting Christmastrees or may include additional facilities, up to and including, comprehensive drillingfacilities and processing facilities to separate and préparéproduced fluids for transport. Legs 20 converge in a taperedsection 32 which is provided in this embodiment because the topsidefacilities do not recuire the full wide body stance which otherwisecontributes to the dynamic response characteristies of compilantframework 12. A platform base 34 joins the topside facilities tothe top of the tapered section.

In this embodiment, platform base 34 not only supports adrilling deck 36 and other operations decks in the topsidefacilities, but it also retains boat decks 38 at îts corners andincludes a pyramid truss arrangement 40 through which the loads of

i rom r λsc i ; r ! G or from the deck and directed to legs 2U. FIG. IB is a close-up of an embodiment deploying a way olsupporting a riser 44 through an intermediate tension relief 5 connection 106 at riser grid 42 through a plurality of riser supports. In this embodiment, the support system establishes atension relieved backspan 108 in riser 44 which increases theflexibility of the riser as taught in U.S, patent application SerialNumber 057,076 filed by Peter W. Marshall on May 3, 1993 for a 10 Backspan Stress Joint, the disclosure of which is hereby incorporated herein by reference and made a part hereof.

Riser 44 extends from a subsea wellhead 116 at sea floor 24 toriser grid 42 through a running span 118. The riser load issubstantially transferred to riser grid 42 at intermediate tension 15 relief connection 106. The riser grid comprises a grid of beams 120 and spanning plates 122 which is supported at the top of framework12 by pyramid truss arrangement 40. Plate inserts 124 support theintermediate tension relief connection, here comprising a semispherical elastomeric bearing 126, joining the riser and the 20 insert plates. The intermediate tension relief connection séparâtes the full tension running span 118 of riser 44 from tension relievedbackspan 108. The distal end of the backspan of the riser issubstantially fixed at a restrained termination 110 adjacent surfacewellhead 112. This arrangement allows flexure of highly tensioned, 25 highly pressurized riser 44 between well guide or subsea wellhead 116 and surface wellhead 112 and isolâtes the required flexure fromthe restrained termination adjacent the surface wellhead therebyracilitating use of a fixed wellhead within a compilant tower.

Movement of the risers is suggested by the schematic 30 représentation of compilant tower 12 in FIG. 3C, discussed further below.

This riser support system cames the load of risers 44 intension at or near the top of the risers. By contrast, well riserloads in offshore towers are traditionally carried in compression in 35 the form of production casing or production tubing inside a relatively larger tube called a conductor 01 dtiv.'prpe, which i sdriven into the seabed and thus acts as an iiidcpendent pile which issupported within the frarnework oi the tower by conductor guideswhich are spaced at frequent intervals along the height of the 5 tower. These conductor guides are necessary in the traditional support of riser loads to provide latéral support for conductor:; inorder to prevent buckling and collapse.

The drivepipes/conductors of the conventionaJ practice hav<· amuch larger diameter than necessary for the suspended production Ιΰ risers in ordinary applications of the présent invention, e.g. traditionally these diameters hâve been on the order of 0.45-1.? mas opposed to 0.25 m or smaller for the later production risers. Inpart this diameter is needed in the conductors because theconductors of traditional design are set in place and used for both 15 drilling and production operations.

In comparison, the practice of the présent invention éliminâtes the need for the drivepipes or conductors and theii. conductorguides. This also éliminâtes the need for a great deal of thehorizontal bracing which would conventionally be provided primaiily 20 to support those conductor guides, as well as vertical bracing to support the cathodic protection necessary for these éléments. FIG. IC is a cross section of the compilant frarnework of thetower of FIG. 1, but includes risers 44 passing thtough a risersuspension corridor 56 of compilant frarnework 12. A riser 2? suspension corridor is provided by a large, open interior of the compilant frarnework withouc the conventionaJ suppôtt at regularintervals. This allows a possibility for greuter lelative motionbetween the risers and riser interférence must be considered.However, the absence of conductor guides and the reduced need for 3? horizontal bracing facilitâtes the économie deployinent of a wide body compilant frarnework and this wide body stance accommodâtes ,>clearance between risers 44 that avoids interférence without havingto provide the conventional supports at regular intervals. A "wide-bodied stance" is a relative relation between the 35 height of the tower and the spacing oi the legs. The area of t.h»· .....-..- ,11-, ci il ü«, loi conventionai geometries, a preterred range of "wide-bodin^s:; ”provides that the ratio of the total height CL") of the compilantframework to the square root of the overall plan area of a cross 5 section ("A") of the compilant framework be less than 12:1.

However, this embodiment need not maintain this relation over theentire length of the compilant tower to achieve these benefits and apreferred range may be defined as meeting the relation of L/Va < 12 10 over at least 70% of the length of the compilant framework.

It is also desired to minimize the horizontal bracing while maximizing the relative size of the substantially open risersuspension corridor. This "openness" can be expressed as a lunctionof the area of the substantially open riser suspension corridor in 15 relation to the total area of the cross section of the compilant framework at that same horizontal level. A preferred degree ofopenness is achieved with the riser suspension corridor having across sectional area at least 22% that of the compilant frameworkalong the entire length of the tower. 2C The présent invention also provides a method for reducing the environmental loading for the compilant tower. The compilantframework is installed having a plurality of legs, a minimum ofhorizontal bracing between the legs and a substantially openmterior. The small diameter production risers are freely suspended 25 ir. a top tensioned relation through the substantially open intenor of the compilant framework. This construction enhances thetransparency of the compilant tower to wave action and attendantenvironmental loading. This benefits foundation design by reducingthe shear and moment requirements for the design sea States.

Eliminatmg conventional conductors and conductor guides alsor,eans that this infrastructure is not available to provide latéralsupport for conventional high pressure drilling risers that arevertically self-supportmg but must be restrained from latéraltuckling. This latéral support for such heavy drilling risers has 35 been required in the past to allow well access for drilling tnrougn a surxsce bi.iwoui ptevem ' " BOP " I . How> vt-1 , FIG. 1E illustrâtes a dual string concentnc hiqh pressure nset 140that facilitâtes drilling operations through a suspended drillingriser System in the practice of an embodiment of the présent 5 invention. A lightweight outer riser 142A extends from above océan surface 26 where it is supported by deck 36a of a deepwater platformto the vicinity of océan floor 22 where it sealingly engages asubsea wellhead or well guide 116A. A high pressure inner riser142B extends downwardly, concentrically through the outer riser to 10 communicate with the well, preferably through a sealing engagement at subsurface wellhead 116A. Installation of the outer riser can befacilitated with a guide System 148. A surface blow out preventer("BOP") 144 at the drilling facilities provides well control at thetop of dual string high pressure riser 140. 15 This system permits use of lightweight outer riser 142A alone for drilling initial intervals where it is necessary to run largediameter drilling assemblies and casing and any pressure kick thatcculd be encountered would be, at worst, moderate. Then, forsubséquent intervals at which greater subterranean pressures might 20 be encountered, high pressure inner riser 142B is installed and drilling continues therethrough. The inner riser has reduceddiameter requirements since these subséquent intervals areccnstrained to proceed through the innermost of one or morepreviously set casings 146 of ever sequentially diminishing 25 diameter. Further, outer riser 142A remains in place and is available to provide positive well control for retrieval andreplacement of inner riser 142B should excessive wear occur in theinner riser.

Providing the high pressure requirements with smaller diameter 30 tubular goods for inner riser 142B provides surface accessible, redundant well control while greatly diminishing the weight of theriser in comparison to conventional, large diameter, single stringhigh pressure risers. This net savings remains even after includmgthe weight of lightweight outer riser 142A. Further, the easy 35 replacability of the inner riser permits reduced wear allowances and facilitâtes additional benefits !>y usimj lubiil.ir ,|oods désignée) forcasing to form high pressure inner riser J 42B. FIG. 1E also illustrâtes an alternative for the riser supportof the stress relieved backspan of FIG, IB with tensioning System 5 150 supporting production riser 44 from a tree deck 36B. However, this tensioning system results in a moving surface wellhead 152connected to facilities through flexible hoses and is not conduciveto nard-piped connections that are suitable for a fixed surfacewellhead. 10 The dual concentric string high pressure riser system of FIG. ΙΞ is described in greater detail in U.S. patent application Serial l.'urtber 167,100 filed by Romulo Gonzalez on December 20, 1993, for aTuai Concentric String High Pressure Riser, the disclosure of whichis hereby incorporated herein by reference and inade a part hereof. 15 FIGS. 2 and 2A illustrate another design for a compilant tower ISA, also in the form of a wide body stance compilant piled tower.However, compilant tower 10A does not employ the.· présent inventionand is constrained to provide risers passing through conductortildes and horizontal framing at frequent intervals, thereby linking 20 the mass of the risers with that of the compilant framework in defining the dynamic response of the tower. This design wasexartined for a water depth on the order of 1000 m and a set ofcmcuctor guides were provided at intervals of about every 60 toSI feet along this length. FIG. 2A is a cross sectional view taken 25 at me of these conductor guide levels, showing the need for sndimenai horizontal bracing 58 in support oi conductor guides 60withm which conductors or drivepipes 44A are laterally constrained.Alt.tough these are not otherwise identical, a direct comparison ofFI32. IC and 2A does provide a rough indication of the material 1. savirgs in Steel afforaed by the présent invention, e.g., preliminary estimâtes of 66,000 tons as opposed to close to 100,000tms of Steel, respect!vely, in these preliminary tower designs forririlar water depths. Each of these estimâtes excluded the Steel mme foundations. 15 Returning to FIG. IC, another Steel saving design technique is · t» *' -t T ’ ' 10 15 20 24 30 35 * —«scratea wnicr, ma\ ce combined with inv pi e:.<-nt. invention. tie-r<·temporary requirement for loads to be encountexed dunnginstallation operations such as off-loading tower sections 13 from abarge are accommodateb by a "floating" launch truss 62. The launchtruss includes bracinç 58A and rails 64 and provides selectreinforcement as an alternative to strengthening the overallstructure to accommodate these temporary loads when the compilantframework is supportée horizontally. This support function issomewhat complicated in that rails 64 may be set inboard, ratherthan vertically aligned with the corner legs during transport. Thisnarrowed rail spacing . apports horizontal transport of a wide bodystance platform having sides exceeding the beam of available classtransport barges. Fur' ner, this structural reinforcement offerscontinued benefit by installing the tower into an orientation suchthat launch truss 62 wiil reinforce the compilant tower in thedirection of the critical environmental loads historically prévalentat the site of the prospect. FIGS. 1F and IG il.. ustrate alternate compilant frameworkconfiguration. FIG. IG .s a cross section of a compilant tower 10in which legs 20 are an '.nged for a trapézoïdal tower cross sectionhaving minimal horizonta.. bracing 58 and defining a substantiallyopen triangular riser su,pension corridor 56 through which risers 44can run. This establishus an alternate intégral launch trussarrangement 62 with launch skids 64 which is also directional in itsstructural reinforcement and can be oriented on installation suchthat it reinforces the compilant tower in the direction of theprévalent critical environmental loads, referenced here as EJI)ax. FIG. IG illustrâtes the compilant tower of FIG. 1F in bargetransport for installation. The trapézoïdal cross section providesan inclined launch truss which facilitâtes the deployment of widerbodied towers with an exis'.ing fleet of relatively narrow barges154. Preliminary analysis for this type of embodiment suggestssuitable stability for the loaded and ballasted barge based on theaiignment of the centers o buoyancy 160, gravity 158 and metacenter156 with the center of gra-ity 156 sufficiently below the metacenter

As noted above, compilant towers aie designed to "give" in a 10 15 20 controlled manner in response to dynamic enviroiunental loads andthis requires that the structure hâve harmonie frequencies thatavoid those produced in nature. FIGS. 3A and 3b illustrateschematically the principle harmonie modes for a compilant framework.12 that are of critical design interest, higher order modes beingfar removed from driving frequencies that might be produced by wind,wave and current. Such forces are typically encountered at periodsof 7 to 16 seconds in the Gulf of Mexico and designs strive fornatural periods less than about 6 seconds or greater than about 22seconds. A wave period distribution typical for portions of theGulf of Mexico is graphically illustrated in FIG. 4A. Région 70 isthat normally occurring and région 72 illustrâtes the shift indistribution for extreme storm events.

Returning to FIGS. 3A and 3B, FIG. 3A schematically illustrâtesthe first mode, also called the fundamental, rigid body, or swaymode motion for a compilant tower 10. A given compilant tower willhâve a charactenstic natural frequency for such motions. Further,a structure with non symmetrical response may hâve more than «nesway mode harmonie frequency. The embodiment of FIG. 1, as analyzedir. the preliminary design for a spécifie offshore prospect has areprésentative sway mode period of 41 seconds. This is considerablylonger than the driving forces to be encountered m nature as isccrventional in compilant tower design. FIG. 3C illustrâtes schematically the effect of motion m thecompilant framework 12 of a compilant tower upon a plurality olrisers 44. Thus, motion of the compilant tower will tend to slackensorte risers 44A while simultaneously increasing the tension m otherrisers 44C and leaving other risers 44B without a substantialchange. The clearance provided the risers must accommodate thismotion and accommodate dynamic response. Note also that variationsm the riser tension will alter the dynamic response of respectiverisers, substantially complicating this analysis. Another aspectobservable in this exaggerated drawing is angular deflection m the 'y·'-'»·v :;ser terminations. FIG. 3B illustrâtes the first flexural mode motion, also rul.ledthe second, bow-shaped or whipping mode responr.e tor a compilanttower 10. Again, non-symmetry may resuit in a plurality of harmoniefrequencies for this whipping mode response. Avoiding the naturalharmonie frequency of this response is often more of an engineeringchallenge than achieving a désirable sway mode. FIG. 4B is a generalized graph illustrating the applied waveforce characteristics of certain tower designs as a plot of anapplied wave force transfer function against frequency. Thisrelation is qualitatively represented in FIG. 4B by curve 64 for afized tower having a 50 m wide stance at the waterline, by curve 66for a compilant tower with a similar waterline geometry and by curve68 for a 80 m wide tensioned riser compilant tower in accordancewith FIG. 1. Upward trends from low energy "valleys" in thesetransfer functions are indicated at points 64A, 6i>A and 68A,respectively, on these response curves. The fatigue requirementsfor each of these platforms increases rapidly for tower naturalperiods longer than these points. However, the response of thisemhodiment of the présent invention is characterized by anadditional "valley" of reauced relative applied force with respect,to a narrower stance compilant tower.

Tightly compacted "slim towers" with conventional conductoiguides and having a narrow body stance hâve been explored foroppertunities to lower Steel requirements. However, designing r.uchstructures tas continuée: to require great. amonnts of structuralSteel, and often attempts to optimise these design.·; hâve resorted tohigter, more expensive grades of si.eei. Even so, the dynamicresponse of these designs hâve been analyzed to be marginal due tohigh wave forces in résonance with tbeir whipping mode response. Arecent preliminary design effort for a slim tower having a*body onlySC m. wide, for about 1CC0 m water depth was analyzed to hâve awhipping mode natural period of about 10 seconds. It should aise· bencted that, despite its slim stance, this tower design (excludinqpiles; was estimated to require 125,000 tons of stvel, in contrant ce 66,000 tons m a preliminary tie-sign in accordance with thvprésent invention in a similar application. A wide body stance has been pursued as one approach to keepingthe whipping mode naturel period from getting su long that dynamic 5 amplification and fatigue become problems. However, such an approach of widening the stance, i.e. the width of the body, of thetower in accordance with the conventional drivepipe or conductorguide practice adversely affects the project économies due tosubstantiel increases in the steel requirements. Even accepting C this drawback, the dynamic response of such a compilant tower could still prove unacceptable in application to an otherwise suitableprospect if conventional conductors, topside arrangements, andwaeerline dimensions are used. Such a case is illustrated with thedynamic response characteristics of curve 66 in FIG. 4B which was 5 calculated for the preliminary design of the compilant tower of FIG. 2. That design was for forty wells in almost 1000 m of water. Thisdesign attempt concluded with a whipping mode naturel periodescimated at 10.6 seconds and requrrc,'d the conclusion that thiscould prove subject to dynamic amplification. See point 66B in 0 relation to the rising energy levels on curve 66 in FIG. 4B.

By contrast, the présent invention improves thv dynamic response characteristics. Referring again to FIG. 3C, the motionscf cop-tensioned risers 44 are shown to move independently ofcompilant framework 12 in dynamic response. Thus, the présent. 5 invention effectively removes the mass of the risers from the mass zi che compilant framework. It aise facilitâtes further réductionsin che mass of che compilant framework by eliminatmg the need forconcuccor guides and associated internai bracing. This ma y provesiçnificant as demor.scrated by the illustrated ezample in which 40 e conventional 0.75 m drivepipes wouid hâve a combined effective mass ci about 70,000 tons which is comparable to the weight of the Steelin che tower jacket itself. The whipping mode response of compilanttowers is relatively insensitive to variations in the load at thetopside faciiities and allowing the risers to entend substantially ·> creely chrough the compilant framework 12 effectively _____uic ι«β»4 ci lüeis -;-i trou; tii.it whicl· defines the whipping mode response of compilant, tower 10.

Further, eliminating the conductor guides and attendanthorizontal bracing facilitâtes the use of the substantially open 5 interior, wide-bodied compilant tower embodiment. These openings, in combination with a wide stance at the waterline, permits waves topass through, impacting on the far side substantially out of phasewith the force of wave impact applied on the leading side. Thus,"wave cancellation” is another benefit to the dynamic response of a 10 compilant tower which is facilitated by the présent invention.

Strategie placement of wave impacting structure, such as by placingboat docks 38 in FIG. IA on the periphery, may further enhance thiseffect.

This enhanced wave cancellation can greatly improve the fatigue 15 characteristics of a compilant platform. FIG. 4C illustrâtes a hot spot stress analysis of two compilant platforms having similarnaturel whipping mode periods at approximated 8.50 to 8.75 seconds.Calculations in accordance with API methodology for "Allowable HotSpot Stress” as a function of base shear and at the naturel whipping 20 mode period is used as an indication of relative fatigue life for an offshore platform. Here curve 102 represents a platform design thatwas preliminarily analyzed which did not enhance wave cancellationthrough the practice of the présent invention. The allowable hotspot stress for shear is indicated at the intersection of this curve 25 and the whipping mode period, i.e., at point 102A. Compare the significantly higher allowable hot spot stress indicated by curve104 intersecting the natural period for whipping mode response atpoint 104A. The higher allowable stress permits a lighter design.

Combining the benefits of decoupling the mass of the nsers .*0 frem the dynamic response of the tower and the benefits of enhanced

wave cancellation can produce a significantly improved dynamicresponse for a compilant tower. Compare the response curves 68 and6c in FIG. 4B for otherwise substantially similar compilant towers,particularly noting rising wave force response curves a: points 68A 55 and 66A, respect!vely. Towers with snorter whipping periods are

Another aspect of the presently prefivred vmbodiment issuggested by a companson of tensioned riser compilant tower 10 ofFIGS. 1 and conventional wide-bodied compilant tower 10A of FIGS. 2ar.d 2A. The compilant tower design of FIG. 2 was calculated to hâvea whipping mode harmonie frequency at 10.1 to 10.6 seconds,depending upon the axis of the structure. This period was judgedunacceptable in that naturel environmental forces could becomeamplified in harmonie response. By contrast, the lightweight, wide-bodied compilant tower of FIG. 1 is calculated in an application tohâve a substantially improved 8.5 second whipping mode period.Although these cases are not otherwise identical, decoupling thensers from the compilant framework provides significant impact inthe overall dynamic response of the compared designs.

The advantages of a compilant tower of benefiting from themethod of the présent invention hâve been primarily illustrated witha compilant piled tower design. However, a full range of compilanttowers, including but not limited to, flextowers, flextowers withtrapped mass (water), and buoyant towers, could benefit from theapplication of the présent invention.

Other modifications, changes and substitutions are intended inthe forgoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that the appended daimsbe construed broadly and in the manner consistent with the spiritar.c scope of the invention herein. C’.TT'T’·'

Claims (12)

1. TH POU 1_!>CT 10 15 10

   1. A compilant tower for conducting hydrocarbon recovery operations from a deepwater offshore réservoir, comprising afoundation (16), a compilant framework (12), comprising a pluralityof vertically extending legs (20) secured to t.he foundation (16), ahorizontal bracing interconnecting the legs (20), and a topsidefacility (30) supported by the compilant framework (12), characterized in that the horizontal bracing interconnects the legsin a manner defining a substantially open riser suspension corridorextending substantially the entire length of the compilant framework.
2. 2. The compilant tower of claim 1, further comprising at least onefreely suspended riser (44) extending through the riser suspensioncorridor from the vicinity of the topside facility (30) tocommunicate with the réservoir.
3. 3. The compilant tower of claim 2, comprising a plurality oi therisers (44) .
4. 4. The compilant tower of claim 3, further comprising a risergrid (42) positioned between the compilant framework (12) and thetopsides (30) and vertically aligned with the riser suspensioncorridor, and a pyramid truss (40) connecting the riser grid (42) tothe legs (20) of the compilant framework (12). c. The compilant tower of claim 3, further comprising a pluralityof the riser suspension corridors. Z. The compilant tower of claim 2, wherein the compilant framework (12) further comprises a longitudinally extending launchtruss ( 62) . The compilant tower of claim 1, wherein the foundationcomprises the lower ends of a plurality of piles (14) anchored inthe sea floor and wherein the compilant framework (12) furthermcicdes the upper ends of the piles (14) extending adjacent and 10 15 engagea to the legs (20) along a subsldiiti.ü pi’ttion of the heightof the compilant framework (12).
5. 8. The compilant tower of claim 2, wherein a plurality of saidfreely suspended risers (44) extend through the riser suspensioncorridor from the topside facility (30) to cummunicate with theréservoir, said risers (44) being spaced apart to provide clearanceto prevent interférence between the risers (44) in response tonormal flexure of the compilant tower. S. The compilant tower of claim 1, wherein the riser suspensioncorridor vertically extends substantially the entire length of thecompilant framework (12) without interruption by the horizontalbracing and having a horizontal cross sectional area of at least 22of that of the compilant framework (12), and a plurality of freelysuspended risers (44) extends through the riser suspension corridorfrom the topside facility (30) to communicate with the réservoir,said risers (44) being spaced to provide clearance to preventinterférence between the risers and between the risers and thehorizontal bracing in response to normal flexure of the compilanttower or normal environmental loads on the risers.
6. 11. The compilant tower of any of daims 1-9, wherein the ratio ofthe height of the compilant framework (12) to the square root of tharea between the legs (20) of the compilant tower adjacent thefcundation is less than 12:1. II. The compilant tower of any of daims 1-10, wherein thecompilant framework (12) has a wide body configuration such that thfcllowing relation is met over at least 20- of the length of th,·compilant framework: L/\‘A < 12 L is the total length of the compilant framework, andA is the area of a cross section of the compilant framework i selected élévation, and the substantially open vertically extending riser suspensioncorridor is braced cn.ly at its periphery. at
7. 12. The compilant tower oi any cl claimn li, wh.-rein theriser (44) is suspended in a top-tensioned relation from a riseïsupport at the top end of the compilant tramework (12), and therunning spans (118) of the risers extend through the riser h suspension corridor without intermédiare supports ,it regular intervals along the compilant framework (12).
8. 13. The compilant tower of any of daims 1-12, wherein thecompilant framework (12) comprises a launch truss (62), orienter! soas to directionally strengthen the compilant framework (12) in the 12 direction of the prevailing environmental loads upon installation of the compilant tower.
9. 14. The compilant tower of any of daims 2-13, wherein thesubstantially open riser suspension corridor extends through thecompilant framework (12) at least the full depth of the wave zone 15 and each riser (44) is freely suspended through the riser suspension corridor, the tower being arranged so that waves impact a first aideof the compilant tower, pass through the risers and the interioi ofthe compilant framework (12) substantially unitnpeded, and impact asecond side of the compilant tower with a force which is significant 22 in relation to the force of the impact on the first side.
10. 15. The compilant tower of any of daims 2-14, further comprising ariser support assembly supporting each production riser (44) neat.its upper end to provide the principal load transfer between theriser (44) and the compilant framework (12) and thereby supporting 25 the production riser in tension, said riser support assembly being cor.figured to accomodate relative motion between the productionriser t44) and the topside facility (30).
11. 16. The compilant tower of daim 15 wherein each productionriser (44) comprise a running section (118) and a backspan 31 section (108) and the riser support assembly comprises an intermédiare tension relief connection (126) operably connecting theproduction riser (44) to the riser grid (42) to support a significant portion of the tension carried by the running span (118)cf the production riser m a manner that passes angular rotation to 35 the backspan section (108) of the production riser; and a restra'.ned connection (110) at the topside iuciltty ( iU) that sccuies tlu· <-nd of the backspan section (108) of the production user.
12. 17. The compilant tower of any oi claim 1-16, wherein the well.··. areprovided with subsea wellheads (116), further comprising a drilling 5 facility presented by the topside facility (30), a high pressure drilling riser connecting the drilling facility to one of the Wellsfor conducting drilling operations, the high pressure drilling risercomprising a lightweight outer riser (142A) suspended from thedrilling facility and extending to the subsea wellhead (116), a high 10 pressure inner riser (142B) extending inside the lightweight outer riser (142A) from the drilling facility downwardly to communicat.ewith the well, and a surface BOP providing well control at the topof the inner riser (142B). MCS11/TH0001PC ? ÆI·· vwï-''"·* ..........— · -