OA10366A - Tensioned riser compliant tower - Google Patents

Abstract

A tensioned riser deepwater platform suitable for offshore oil and gas applications is disclosed having a foundation (16) secured to the ocean floor, a topside facility above the ocean surface, and a vertically extending tower jacket (12) secured to the foundation and supporting the topside facility. A plurality of substantially vertically extending risers provide fluid communication between the wells and the topside facility. These risers are connected to riser supports near their upper ends and the riser supports provide the pricipal load transfer between the risers and the tower jacket. Thus, the conductor guides and attendant horizontal bracing of conventional deepwater design can be substantially eliminated.

Description

1 010366

TENSIONED RISER COMPLIANT TOWER

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

Traditional bottom-founded platforms having fixed or rigidtower structures are effective to support topside faciiities inrelatively shallow to mid-depth waters, but their underlying designpremises become economically unattractive in developments muchdeeper than 1000 feet 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 faciiities.

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

Thus, there remains substantial benefit to be gained fromimprovements that would safely further reduce the requirement forthe amount of steel or beneficially alter the performancecharacteristics demanded of the steel supplied for deepwateroffshore platforms, whether fixed or compilant.

In accordance with the invention there is provided a tensionedriser deepwater platform for support of hydrocarbon wells of anoffshore prospect, comprismg : 2 010366 a foundation secured to an océan floor;a topside facility above an océan surface; a vertically extending tower jacket secured to the foundation,supportmg the topside facility, and defining a riser suspensioncorridor therebetween; at least one substantially vertically extending productionriser suspended m the riser suspension corridor and providing fluidcommunication between the wells and the topside facility; and a riser support assembly supporting the risers near their upperends to provide the principal load transfer between the riser andthe tower jacket and thereby supporting the risers in tension.

The riser support provides the principal load transfer betweenthe riser and the tower jacket, and the conventional conductorguides and attendant horizontal bracing can thus be substantiallyeliminated from the design. This invention is particularlyapplicable to compilant tower designs.

The brief description above, as weil as further objects andadvantages of the présent invention will be more fully appreciatedby reference to the following detailed description of the preferrecembodiments which should be read in conjunction with the accompanying drawings in which:

Fig. 1 is an isométrie view of a tensioned riser deepwatertower constructed in accordance with the présent invention.

Fig. IA is a side élévation view of the upper end of thetensioned riser deepwater tower of Fig. 1.

Fig. IB is a close-up of a riser support in an embodiment otthe présent invention in accordance with Fig. IA.

Fig. IC is a cross section of the tensioned riser deepwatertower of Fig. 1 taken along line 1C-1C in Fig. 1.

Fig. 1D is a cross section of the tensioned riser deepwatertower of Fig. 1 taken along line 1D-1D in Fig. IA.

Fig. 1E is a partially cross sectioned view of a dualconcentric string high pressure drilling riser which facilitâtes thepractice of the présent invention. 3 010366

Fig. 1F is an end pian view of the embodiment of Fig. IG intransport.

Fig. IG is a horizontal cross section of the compliantframework of an alternate embodiment of the présent invention.

Fig. 2 is a perspective view of a compilant tower design notbenefiting from the présent invention.

Fig. 2A is a cross section of the compilant tower of Fig. 2taken at line 2A-2A in that figure.

Fig. 3A is a schematic illustration of the sway mode responsefor a compliant tower.

Fig. 3B is a schematic illustration of the whipping moderesponse for a compliant tower.

Fig. 3C is a schematic illustration of the sway mode responsefor a compliant tower having multiple top-tensioned, rigidly secured risers.

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 fatiguecharactenstics for two different compliant towers.

Fig. 5 is a side élévation view of an alternate embodiment ofthe présent invention is which a semisubmersible vessel conductsdrilling operations adjacent the compliant tower;

Fig. 5A is a side elevational view of the compliant tower ofFig. 5 after drilling operations are completed.

Fig. 1 illustrâtes one embodiment of a tensioned riserdeepwater tower 10 constructed in accordance with the présentinvention. The risers and topside facilities hâve been omitted fromthis figure for the sake of simplicity. This illustration is basedon a preliminary design for thirty wells in 3000 feet of water, witha topside payload of 22,605 tons which includes 6000 tons of risertension. This example deploys a lightweight, wide body stancecompliant framework for the illustrated embodiment of tensioned 4 010366 riser deepwater tower 10. Further, particular benefits of thisembodiment wi.il. also be discussed in further detail beiow.

In this embodiment, a compilant framework 12 of tower 10 isprovided m the forro of a compilant piled tower in which piles orpilings 14 not only provide foundation 16 secured to océan floor 22,but also extend a substantial distance above the mudline 24, along asubstantial length of the compilant framework and thereby contnbutesignificantly to both the righting moment and dynamic response ofthe overall compilant framework. Pilings 14 are slidingly receivedwithin sleeves 18 along legs 20 at the corners of compilantframework 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, compréhensive drillincfacilities 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 require the full wide body stance which isotherwise useful in contributing to the dynamic responsecharacteristics of compilant framework 12. A platform base 34 joinsthe topside facilities to the 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 its corners andmcludes a pyramid truss arrangement 40 through which the loads ofthe risers (not shown) are supported in tension from riser grid 42 5 0,0366 or frorn the deck and directed to legs 20.

Fig. IB i5 a close-up of an embodiment deploying a way ofsupporting a user 44 through an intermediate tension reliefconnection 106 at nser grid 42. In this embodiment, the supportSystem establishes a tension relieved backspan 108 in riser 44 whichincreases the flexibility of the riser as taught in U.S. patentapplication Serial Number 057,076 filed by Peter W. Marshall on May3, 1993 for a 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 tensionrelief connection 106. The riser grid comprises a grid of beams 120and spanning plates 122 which is supportea at the top of framework12 by pyramid tru3s arrangement 40. Plate inserts 124 support theintermediate tension relief connection, here comprising asemispherical elastomeric bearing 126, joining the riser and theinsert plates. The intermediate tension relief connection séparâtesthe 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,highly pressurized riser 44 between well guide or subsea wellhead116 and surface wellhead 112 and isolâtes the required flexure fromthe restrained termination adjacent the surface wellhead therebyfacilitating use of a fixed wellhead within a compilant tower.

Movement of the risers is suggested by the schematicreprésentation of compilant tower 12 in Fig. 3C, discussed furtherbelow.

This riser support System carries 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 inthe form of production casing or production tubing inside arelatively larger tube called a conductor or drivepipe, which isdriven into the seabed and thus acts as an independent pile which is 6 010366 supported withm the framework of the cowei by conductor guides which are spaced ar frequent intervals along the height of the tower. These conductor guides are necessary in the traditional support of user ioads to provide latéral support for conductors m order to prevent buckling and collapse.

The drivepipes/conductors of the conventional practice hâve amuch larger diameter than necessary for the suspended productionrisers in ordinary applications of the présent invention., e.g.traditionaliy these diameters hâve been on the order of 18-48 inchesas opposed to 9 5/8 inches or smaller for the later productionrisers. In part this diameter is needed in the conductors becausethe conductors of traditional design are set in place and used forboth dnlling and production operations.

In comparison, the présent invention éliminâtes the need forthe drivepipes or conductors and their conductor guides. This alsoéliminâtes the need for a great deal of the horizonal bracing whichwould conventionally be provided primarily to support thoseconductor guides, as well as vertical bracing to support thecathodic protection necessary for these éléments.

Fig. IC is a cross section of the compilant framework of thetower of Fig. 1, but includes risers 44 passing through a risersuspension corridor 56 of compilant framework 12. In the preferredembodiment, the riser suspension corridor is provided by a large,open interior of the compilant framework without the conventionalsupport at regular intervals. This allows a possibility for greaterrelative motion between the risers and riser interférence must be "A considered. However, the absence of conductor guides and thereduced need for horizontal bracing facilitâtes the économiedeployment of a wide body compilant framework. In the preferredembodiment, this wide body stance accommodâtes a clearance betweenrisers 44 that avoids interférence without having to provide theconventional supports at regular intervals. A "wide-bodied stance" is a relative relation between theheight of the tower and the spacing of the legs. The area of thetower cross section is a function of this spacing and, for 7 'i ii «.r.fiji.iij 010366 conventional geometnes, a preferred range of "wide-bodiness"provides that the ratio of the total height ("L") of the compilantframework to the square root of the overall plan area of a crosssection ("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/<A < 12 over at least 70% of the length of the compilant framework.

It is also desired to minimize the horizontal bracing whilemaximizing the relative size of the substantially open risersuspension corridor. This ''openness" can be expressed as a functionof the area of the substantially open riser suspension corridor inrelation to the total area of the cross section of the compilantframework 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.

The illustrated embodiment also provides a method for reducingthe 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 openinterior. The small diameter production risers are freely suspendedin a top tensioned relation through the substantially open interiorof 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.

Eliminating conventional conductors and conductor guides alsomeans that this infrastructure is not available to provide latéralsupport for conventional high pressure drilling risers that arevertically self-supporting but raust be restrained from latéralbuckling. This latéral support for such heavy drilling risers hasbeen required in the past to allow well access for drillingoperations through a surface blowout preventer ("BOP"). However, 8 ..«ί 010366

Fig. 1E illustrâtes a dual string concentnc high pressure user 140that facilitâtes drilling operations through a suspended drillingriser system in the practice of an embodiment of the présentinvention. A iightweight outer riser 142A extends frora above océansurface 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 tocommumcate with the well, preferably through a sealing engagementat 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 the top of dual string high pressure riser 140.

This system permits use of Iightweight outer riser 142A alonefor drilling initial intervals where it is necessary to run largediameter drilling assemblies and casing and any pressure kick thatcould be encountered would be, at worst, moderate. Then, forsubséquent intervals at which greater subterranean pressures mightbe encountered, high pressure inner riser 142B is installed anddrilling continues therethrough. The inner riser has reduceddiameter requirements since these subséquent intervals areconstrained to proceed through the innermost of one or morepreviously set casings 146 of ever sequentially diminishingdiameter. Further, outer riser 142A remains in place and isavailable 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 diametertubular 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 includingthe weight of Iightweight outer riser 142A. Further, the easyreplacability of the inner riser permits reduced wear allowances andfacilitâtes additional benefits by using tubular goods designed for 9

010366 casmg to forrc high pressure inner riser 142B.

Fig. 1E also illustrâtes an alternative for the stress relievedbackspan of Eig. IB with tensionmg systen 150 supporting productionriser 44 from a tree deck 365. However, this tensioning Systemresults in a raoving surface wellhead 152 connected to facilitiesthrough flexible hoses and is not conducive to hard-piped connections that are suitable for a fixed surface wellhead.

The dual concentric string high pressure riser System of Fig. 1E is described in greater detail in U.S. patent application SerialNumber 167,100 filed by Romulo Gonzalez on December 20, 1993, for a

Dual Concentric String High Pressure Riser, the disclosure of whichis hereby incorporated herein by reference and made a part hereof.

Figs. 2 and 2A illustrate another design for a compilant tower10A, 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 conductorguides and horizontal framing at frequent intervals. This designwas examined for a water depth on the order of 3000 feet and a setof conductor guides were provided at intervals of about every 60 to80 feet along this length. Fig. 2A is a cross sectional view takenat one of these conductor guide levels, showing the need foradditional horizontal bracing 58 in support of conductor guides 60within which conductors or drivepipes 44A are laterally constrained.Although these are not otherwise identical, a direct comparisor. ofFigs. IC and 2A does provide a rough indication of the materialsavings in steel afforded, directly and indirectly, by the présentinvention, e.g., preliminary estimâtes of 66,000 tons as cpposed toclose to 100,000 tons of steel, respectively, in these preiiminarytower designs for similar water depths. Each of these estimâtesexcluded the steel in the foundations.

Returning to Fig. IC, another steel saving design technique isillustrated in the preferred embodiment. Here temporary requirementfor loads to be encountered during installation operations such asoff-loading tower sections 13 from a barge are acconmodated by a"floating" launch truss 62. The launch truss includes bracing 53A 10 .-, :·.,ύ~ > ί*.* :**. -·-.>-*.<..#, a,.. 010366 and rails 64 and provides select reinforcement as an alternative testrengthemng the overall structure to accommodate these temporaryloads when the compilant fraraework is supported horizontally. Thissupport function is somewhat complicated in that rails 64 nay be setmboard, rather than vertically aligned with the corner legs durmgtransport. This narrowed rail spacing supports horizontal transportof a wide body stance platform having sides exceeding the beam ofavailable class transport barges. Further, this structuralreinforcement offers continued benefit by installing the tower intoan orientation such that launch truss 62 will reinforce thecompilant tower in the direction of the critical environmental loadshistoncally prévalent at the site of the prospect.

Figs. 1F and IG illustrate another alternate embodiment of theprésent invention. Fig. IG is a cross section of a compilant tower10 in which legs 20 are arranged for a trapézoïdal tower crosssection having minimal horizontal bracing 58 and defining asubstantially open triangular riser suspension corridor 56 throughwhich nsers 44 can run. This establishes an alternate intégrallaunch truss arrangement 62 with launch skids 64 which is alsodirectional in its structural reinforcement and can be oriented oninstallation such that it reinforces the compilant tower in thedirection of the prévalent critical environmental loads, referencedhere as Emax.

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 existing fleet of relatively narrow barges154. Preliminary analysis for this type of embodiment suggestssuitabie stability for the loaded and ballasted barge based on thealignment of the centers of buoyancy 160, gravity 158 and metacenter156 with the centre of gravity 156 sufficiently below the metacenter156.

As noted above, compilant towers are designed to "give” in acontrolled manner in response to dynamic environmental loads andthis requises that the structure hâve harmonie frequencies that 11 010366 avoid those produced in nature. Figs. 3A and 3B illustrateschematically the principle harmonie modes for a compilant framework12 that are of critical design interest, higher order modes beingfar removed front 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 fornaturel 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 characteristic naturel frequency for such motions. Further,a structure with non symmetrical response may hâve more than onesway mode harmonie frequency. The embodiment of Fig. 1, as analyzedin the preliminary design for a spécifie offshore prospect has areprésentative sway mode period of 41 seconds. This is considerabiylonger than the driving forces to be encountered in nature as isconventional in compilant tower design.

Fig. 3C illustrâtes schematically the effect of motion in thecompilant framework 12 of a compilant tower upon a plurality ofrisers 44. Thus, motion of the compilant tower will tend to slackensome risers 44A while simultaneously increasing the tension in 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 variationsin the riser tension will alter the dynamic response of respectiverisers, substantially complicating this analysis. Another aspectobservable in this exaggerated drawing is angular deflection in theriser terminations.

Fig. 3B illustrâtes the first flexural mode motion, also calledthe second, bow-shaped or whipping mode response for a compilanttower 10. Again, non-symmetry may resuit in a plurality of harmonie - i2 - 0 1 0366 frequencies 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. 43 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 afixed tower having a 140-foot wide stance at the waterline, by curve66 for a compilant tower with a sinxilar waterline geometry and bycurve 66 for a 245-foot wide tensioned riser compilant tower inaccordance with Fig, 1. Upward trends from low energy "valleys" inthese transfer functions are indicated at points 64A, 66A 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 thisembodiment of the présent invention is characterized by anadditional "valley” of reduced relative applied force with respectto a narrower stance compilant tower.

Tightly compacted "slim towers" with conventional conductorguides and having a narrow body stance hâve been explored foropportunities to lower Steel requirements. However, designing suchstructures has continued to require great amounts of structuralSteel, and often attempts to optimize these designs hâve resorted tohigher, more expen3ive grades of Steel. Even so, the dynamicresponse of these designs hâve been analyzed to be marginal due tohigh wave forces in résonance with their whipping mode response, Arecent preliminary design effort for a slim tower having a body only140 feet wide, for about 3000-foot water depth was analyzed to hâvea whipping mode natural penod of about 10 seconds. It should alsobe noted that, despite its slim stance, this tower design (excludingpiles) was estimated to require 125,000 tons of steel, in contrastto 66,000 tons in a preliminary design in accordance with theprésent invention in a similar application. A wide body stance has been pursued as one approach to keepingthe whipping mode natural penod from getting so long that dynamic 13 010366 amplification and fatigue become problème. However, such anapproach of widening the stance, i.e. the width of the body, of thetower in accordance with the conventional drivepipe oc conductorguide practice adversely affects the project économies due tosubstantial increases in the Steel requireraents. Even acceptingthis drawback, the dynamic response of such a compilant tower couldstill prove unacceptable in application to an otherwise suitableprospect if conventional conductors, topside arrangements, andwaterline dimensions are used. Such a case is illustrated with thedynamic response characteristics of curve 66 in Fig. 4B which wascalculated for the preliminary design of the compilant tower ofFig. 2. That design was for forty Wells in almost 3000 feet ofwater. This design attempt concluded with a whipping mode naturalperiod estimated at 10.6 seconds and required the conclusion thatthis could prove subject to dynamic amplification. See point 66B inrelation to the rising energy levels on curve 66 in Fig. 4B.

By contrast, the présent invention improves the dynamicresponse characteristics. Referring again to Fig. 3C, the motions oftop-tensioned risers 44 are shown to raove independently of compilantframework 12 in dynamic response. Thus, the présent invention notonly removes the unnecessary internai bracing from the mass of thecompilant framework along its length, it also effectively removesthe mass of the risers. This may prove significant as demonstratedby the illustrated example in which 40 conventional 30-inch drivepipes would hâve a combined effective mass of about 70,000tons which is comparable to the weight of the Steel in the towerjacket itself. The whipping mode response of compilant towers isrelatively insensitive to variations in the load at the topsidefacilities and allowing the risers to extend substantially freelythrough the compilant framework 12 effectively découplés the mass ofrisers 44 from that which defines the whipping mode response ofcompilant tower 10.

Further, eliminating the conductor guides and attendanthorizontal bracing facilitâtes the use of the substantially openinterior, wide-bodied compilant tower embodiment. These openings, 14 .bit «>*». 1,1 ί Λ. , 010366 in combination with a wide stance at the waterline, penuts 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 acompilant 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 fatiguecharacteristics of a compilant platform. Fig. 4C illustrâtes a hotspot 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 whippingmode period is used as an indication of relative fatigue life for anoffshore platform. Here curve 102 représenta a platform design thatwas preliminari1 y 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 curveand the whipping mode period, i.e., at point 102A. Compare thesignificantly 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 risersfrom the dynamic response of the tower and the benefits of enhancedwave cancellation can produce a significantly improved dynamicresponse for a compilant tower. Compare the response curves 68 and66 in Fig. 4B for otherwise substantially similar compilant towers,particularly noting rising wave force response curves at points 68Aand 66A, respectively. Towers with shorter whipping periods areresonantly excited by a reduced wave force.

Another aspect of the presently preferred embodiment issuggested by a comparÎ3on of tensioned riser deepwater tower 10 ofFigs. 1 and conventional wide-bodied compilant tower 10A of Figs. 2and 2A. The compilant tower design of Fig. 2 was calculated to hâve 15 010366 a whipping mode harmonie frequency at 10.1 to 10.6 seconds,depending upon the axis of the structure. This period was judgedunacceptable in that natural 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 therisers from the compilant framework provides significant impact inthe overall dynamic response of the compared designs.

The advantages of the tensioned riser deepwater tower of theprésent invention hâve been primarily illustrated with a compilantpiled tower design. However, a full range of compilant towers,including but not limited to, flextowers, flextowers with trappedmass (water), and buoyant towers, could benefit from the applicationof the présent invention. The présent invention is also shown tofacilitate other improvements of the preferred embodiment, includingthe eliminating the conductor or drivepipe guides, economicallyproviding a wide waterline geometry, and decoupling the conductormass from the distributed mass which participâtes in the whippingmode. Further, benefits may also be conferred, e.g., reducing Steelrequirements, to more conventional fixed platforms deployed in waterseveral hundred feet deep and deeper when deployed in the upperdepth limits to such designs.

Further, the benefits of top tensioned risers to deepwatertower platforms are not ail limited to wide-bodied embodiments. Forexample, a more slender deepwater tower could benefit by havingsuspended risers extending externally along the tower framework.

Figs. 5 and 5B illustrate one embodiment with a tensioned risercompilant tower 10B. In the illustration of such an embodiment, thecompilant tower is adapted to receive support for drilling operations from an auxiliary drilling vessel such as semisubmersiblevessel 160 which is temporarily restrained or docked to compilanttower 10B for drilling operations. Drilling then proceeds through adrilling riser 162 supported by the auxiliary vessel. The well iscompleted and the drilling riser is replaced by a production riser fa. i ' K. X.-- .fc.ifa. Îi..'. 16 10 15 010366 44A which may be transferred to the compilant platform and securedthereto in top tensioned relationship at riser support 164, see Fig.SA. No conductors or conductor guides are used and, in thisembodiment, no riaer suspension corridor is used. In the illustrated embodiment, riser support 164 is a rocker arm risertensioner which facilitâtes riser transfer operations and spaces theproduction rieers 44A from the side of the compilant tower.

Although illustrated with semisubmersible vessel 160 deflectingcompilant tower 10B with an exaggerated offset, it will beappreciated that it may be desired not to stress compilant framework12A by such an offset practice and an alternative auxiliary vesselof a class providing a cantilevered drilling deck may allow betteralignment while avoiding the need for such offset displacement.Further, in some instances it may be desired to allow the productionrisers to hâve somewhat of a catenary bow in their rise to toptensioned support in riser support 164.

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 spiritand scope of the invention herein. 20

Claims (18)

17

010366

1. A tensioned riser deepwater platform for support of hydrocarbonwells of an offshore prospect, comprising: a foundation secured to an océan floor;a topside facility above an océan surface; a vertically extending tower jacket secured to the foundation,supporting the topside facility, and defining a riser suspensioncorridor therebetween; at least one substantially vertically extending productionriser susper.ded in the riser suspension corridor and providing fluidcommunication between the wells and the topside facility; and a riser support assembly supporting the risers near their upperend3 to provide the principal load transfer between the riser andthe tower jacket and thereby supporting the risers in tension.

2. The tensioned riser deepwater platform of claim 1, comprising aplurality of said substantially vertically extending productionrisers.

3. The tensioned riser deepwater platform of claim 2 wherein thetower jacket provides a substantially open centre extendingvertically therethrough in which the risers are arranged withsufficient mutual clearance to avoid interférence in normaloperations.

4. The tensioned riser deepwater platform of claim 3 wherein theriser support assembly comprises a riser grid.

5. The tensioned riser deepwater platform of claim 4 wherein theriser grid is supported by a pyramid truss at the top of the towerjacket.

6. The tensioned riser deepwater platform of claim 1 wherein thedeepwater platform is a fixed platform.

7. The tensioned riser deepwater platform of claim 1 wherein thedeepwater platform is a compilant tower and the tower jacket is acompilant framework. 18 ___ 010366

8. The tensioned risex deepwater platform of claim T wherein: the foundation is provided by a plurality of piles having theirlower ends secured to the océan floor; and the compilant framework further comprises;vertically extending legs; minimal horizontal bracing interconnecting the legs; andthe upper ends of the piles extending a considérable lengthabove the océan floor and being interconnected to the legs m amanner contributing to the elastic response of the compilantframework.

9. The tensioned nser deepwater platform of any of daims 1-8wherein each riser has an upper end connected to the nser supportassembly, a lower end in direct communication with a hydrocarbonréservoir, and a free hanging running span extending through theriser suspension corridor from the riser support to the lower end ofthe riser.

10. The tensioned riser deepwater platform of any of daims 1-9,wherein the nser suspension corridor through which the risers passextends substantially from the foundation to the topside facility.

11. The tensioned riser deepwater platform of daim 9, wherein theriser suspension corridor through the tower jacket is asubstantially open vertically extending interior in which the risersare arranged with sufficient clearance to avoid interférence undernormal operating conditions.

12. The tensioned riser deepwater platform of daim 1, wherein thetower jacket is a compilant framework and wherein the risers arearranged externally about the periphery of the compilant frameworkwith sufficient clearance to avoid interférence in normaloperations.

13. The tensioned riser deepwater platform of daim 12, wherein thewells are slightly spaced from the foundation and the risers hâve acatenary spread in their substantially vertical suspension.

14. A method for reducing the natural period of the whipping modeharmonie response in a compilant tower having a vertically extendingcompilant framework secured to a foundation at an océan floor and 19 ' ............... i A -. 53 . *a A S .Sk'stK A -f ... .. . .A— A I 010366 supporting a topside facility above an océan surface and having aplurality of nsers coraunicating between the topside facility and aplurality of wells at the océan floor through a running span, themethod comprising: decoupling the mass of the risers from the vertically extendingcompilant framework by securing the risers in top tensioned relationin a plurality of riser supports which provide the principle loadtransfer between the risers and the compilant framework; whereby the running spans of risers are free to respond toenvironmental forces along their length independent from thecompilant framework.

15. The method of claim 14, wherein securing the risers in toptensioned relation in a plurality of supports comprises running therisers to the exterior of the compilant framework.

16. The method of claim 14, further comprising:separating the running spans of the risers within a riser suspension corridor with adéquate horizontal clearance in a substantially open interior of the compilant framework to preventinterférence between the risers during normal operations and fiexureof the compilant tower.

17. The method of claim 16, further comprising:establishing the compilant framework with a minimum of horizontal bracing and without conductor guides.

18. The method of claim 14 wherein securing the risers in a toptensioned relation comprises: relieving the axial load in the riser at an intermediate risersupport at a riser grid; passing angular rotation of the riser through the intermediateriser support to a backspan of the riser having a reduced axialload; and terminating the riser in a restraining fixture at the distalend of the backspan, spaced apart thereby from the intermediateriser support; - 20 - 010366 whereby che flexibilité/ of the riser is increased at the restraimng fixture.