OA11137A - Spar with features against vortex induced vibrations - Google Patents

Abstract

A spar platform comprising a deck (12), a buoyant tank assembly (14) including a first buoyant section (14a) connected to the deck, a second buoyant section (14b) arranged below the first buoyant section and a buoyant spacing structure interconnecting the first and second buoyant sections in a manner providing a horizontally extending gap (30) between the first and second buoyant sections, a counterweight arranged below the buoyant tank assembly and connected to the buoyant tank assembly by a counterweight (18) spacing structure.

Description

011137

The présent invention relates to a heave résistant,deepwater platform supporting structure known as a"spar." More particularly, the présent inventionrelates to reducing the susceptibility of spars to dragand vortex induced vibrations ("VIV").

Efforts to economically develop offshore oil andgas fields in ever deeper water create many uniqueengineering challenges. One of these challenges isproviding a suitable surface accessible structure.

Spars provide a promising answer for meeting thesechallenges. Spar designs provide a heave résistant,floating structure characterized by an elongated,vertically disposed hull. Most often this hull iscylindrical, buoyant at the top and with ballast at thebase. The hull is anchored to the océan floor throughrisers, tethers, and/or mooring Unes.

Though résistant to heave, spars are not immunefrom the rigors of the offshore environment. Thetypical single column profile of the hull isparticularly susceptible to VIV problems in thepresence of a passing current. These currents causevortexes to shed from the sides of the hull, inducingvibrations that can hinder normal drilling and/orproduction operations and lead to the failure of theanchoring members or other critical structuraléléments.

Helical strakes and shrouds hâve been used orproposed for such applications to reduce vortex inducedvibrations. Strakes and shrouds can be made to be 011137 effective regardless of the orientation of the current to the marine element, but us general shrouds and strakes materially increase the drag on such large marine éléments.

Thus, there is a clear need for a low drag, VIVreducing System suitable for deployment in protectingthe hull of a spar type offshore structure.

In accordance with the invention there is provideda spar platform comprising a deck, a buoyant tankassembly including a first buoyant section connected tothe deck, a second buoyant section arranged below thefirst buoyant section and a buoyant spacing structureinterconnecting- the first and second buoyant sectionsin a manner providing a horizontally extending gapbetween the first and second buoyant sections, acounterweight arranged below the buoyant tank assemblyand connected to the buoyant tank assembly by acounterweight spacing structure.

By the provision of the gap between the first andsecond buoyant sections the corrélation of vortexinduced vibrations between the individual sections issubstantially disrupted so that any remaining vortexinduced vibration is substantially reduced compared tothe situation in which no gap is présent between thebuoyant sections. A further réduction of vortex induced vibration isachieved if said first and second buoyant tank sectionshâve different outer diameters. The different buoyantsections then hâve different aspect ratios whichfeature further contributes to disturbing any cor-relation of vortex induced vibration between theindividual buoyant sections. 011137

The description above, as well as furtheradvantages of the présent invention will be more fullyappreciated by reference to the following detaileddescription of the illustrated embodiments which shouldbe read in conjunction with the accompanying drawingsin which: FIG. 1 is a side elevational view of an embodimentof a spar platform with spaced buoyancy in accordancewith the présent invention; FIG. 2 is a cross sectional view of the sparplatform of FIG. 1 taken at line 2-2 in FIG. 1; FIG. 3 is a side elevational view of an alternateembodiment of a spar platform with spaced buoyancy inaccordance with the présent invention; FIG. 4 is a cross sectional view of the spar platform of FIG. 3 taken at line 4-4 in FIG. 3; FIG. 5 is a cross sectional view of the spar platform of FIG. 3 taken at line 5-5 in FIG. 3; FIG. 6 is a cross sectional view of the spar platform of FIG. 3 taken at line 6-6 in FIG. 4; FIG. 7 is a schematically rendered cross sectionalview of a riser System useful with embodiments of theprésent invention; FIG. 8 is a side elevational view of a riser Systemdeployed in an embodiment of the présent invention; FIG. 9 is a side elevational view of anotherembodiment of the présent invention; and FIG. 10 is a side elevational view of asubstantially open truss in an embodiment of theprésent invention. FIG. 1 illustrâtes a spar 10 in accordance with theprésent invention. Spars are a broad class of floating,moored offshore structure characterized in that they 011137 are résistant to heave motions and présent an elongated, vertically oriented hull 14 which isbuoyant at the top, here buoyant tank assembly 15, andis ballasted at its base, here counterweight 18, whichis separated from the top through a middle orcounterweight spacing structure 20 .

Such spars may be deployed in a variety of sizesand configuration suited to their intended purposeranging from drilling alone, drilling and production,or production alone. FIGS. 1 and 2 illustrate adrilling spar, but those skilled in the art may readilyadapt appropriate spar configurations in accordancewith the présent invention for production operationsalone or for combined drilling and productionoperations as well in the development of offshorehydrocarbon reserves.

In the illustrative example of FIGS. 1 and 2,spar 10 supports a deck 12 with a hull 14 having aplurality of spaced buoyancy sections, here first orupper buoyancy section 14A and second or lower buoyancysection 14B. These buoyancy sections are separated bybuoyant section spacing structure 28 to provide asubstantially open, horizontally extending vertical gap30 between adjacent buoyancy sections. Here thebuoyancy sections hâve equal diameters and divide thebuoyant tank assembly 15 into sections of substantiallyequal length below the water line 16. Further, theheight of gap 30 is substantially equal to 10% of thediameter of buoyant sections 14A and 14B. A counterweight 18 is provided at the base of thespar and the counterweight is spaced from the buoyancysections by a counterweight spacing structure 20.Counterweight 18 may be in any number of con- 011137 figurations, e.g., cylindrical, hexagonal, square,etc., so long as the geometry lends itself to con-nection to counterweight spacing structure 20. In thisembodiment, the counterweight is rectangular andcounterweight spacing structure is provided by asubstantially open truss framework 20A.

Mooring Unes 26 secure the spar platform over thewell site at océan floor 22. In this embodiment themooring Unes are clustered (see FIG. 2) and providecharacteristics of both taut and catenary mooring Uneswith buoys 24 included in the mooring system (seeFIG. 1). The mooring Unes terminate at their lowerends at anchor system 32, here piles 32A. The upper endof the mooring lines may extend upward through shoes,pulleys, etc. to winching facilities on deck 12 or themooring lines may be more permanently attached at theirdeparture from hull 14 at the base of buoyant tankassembly 15.

In FIG. 1, a drilling riser 34 is deployed beneathderrick 36 on deck 12 of spar platform 10. Thedrilling riser connects drilling equipment at thesurface with well 36 at océan floor 22 through acentral moon pool 38, see FIG. 2. A basic characteristic of the spar type structureis its heave résistance. However, the typicalelongated, cylindrical hull éléments, whether thesingle caisson of the "classic" spar or the buoyanttank assembly 15 of a truss-style spar, are verysusceptible to vortex induced vibration ("VIV") in thepresence of a passing current. These currents causevortexes to shed from the sides of the hull 14,inducing vibrations that can hinder normal drillingand/or production operations and lead to the failure of 6 C11137 the risers, mooring line connections or other criticalstructural éléments. Prématuré fatigue failure is aparticular concern.

Prior efforts at suppressing VIV in spar hulls hâvecentered on strakes and shrouds. However both of theseefforts hâve tended to produce structures with havinghigh drag coefficients, rendering the hull more sus-ceptible to drift. This commits substantial increasesin the robustness required in the anchoring system.Further, this is a substantial expense for structuresthat may hâve multiple éléments extending from near thesurface to the océan floor and which are typicallyconsidered for .water depths in excess of half a mile or so.

The présent invention reduces VIV from currents,regardless of their angle of attack, by dividing theaspect ratio of the cylindrical éléments in the sparwith substantially open, horizontally extending,vertical gaps 30 at select intervals along the lengthof the cylindrical hull. A gap having a height of 10%or so diameter of the cylindrical element is sufficientto substantially disrupt the corrélation of flow aboutthe combined cylindrical éléments and this benefit maybe maximized with the fewest such gaps by dividing thecombined cylindrical éléments into sections of roughlyéquivalent aspect ratios. For typically sized truss-type spars, one such gap through the buoyant tankassembly may be sufficient relief as truss framework20A forming the counterweight spacing structure 20contributes little to the VIV response of the spar.

Providing one or more gaps 30 also help reduce thedrag effects of current on spar hull 14. FIGS. 3-5 illustrate a spar 10 in accordance with 7 011137 another embodiment of the présent invention. In thisillustration, spar 10 is a production spar with aderrick 36 for workover operations. Buoyant tankassembly 15 supports a deck 12 with a hull 14 havingtwo spaced buoyancy sections 14A and 14B, of unequaldiameter. A counterweight 18 is provided at the base ofthe spar and the counterweight is spaced from thebuoyancy sections by a substantially open trussframework 20A. Mooring lines 19 secure the sparplatform over the well site.

Production risers 34A connect wells or manifolds atthe seafloor (not shown) to surface complétions atdeck 12 to provide a flowline for producinghydrocarbons from subsea réservoirs. Here risers 34Aextend through an interior or central moonpool 38illustrated in the cross sectional views of FIGS. 4 and5.

Spar platforms characteristically resist, but donot eliminate heave and pitch motions. Further, otherdynamic response to environmental forces also contri-bute to relative motion between risers 34A and sparplatform 10. Effective support for the risers which canaccommodate this relative motion is critical because anet compressive load can buckle the riser and collapsethe pathway within the riser necessary to conduct wellfluids to the surface. Similarly, excess tension fromuncompensated direct support can seriously damage theriser. FIGS. 7 and 8 illustrate a deepwater riserSystem 40 which can support the risers without the needfor active, motion compensating riser tensioningSystems. FIG. 7 is a cross sectional schematic of adeepwater riser System 40 constructed in accordance 011157 with the présent invention. Within the spar structure,production risers 34A run concentrically withinbuoyancy can tubes 42. One or more centralizers 44secure this positioning. Here centralizer 44 is securedat the lower edge of the buoyancy can tube and isprovided with a load transfer connection 46 in the formof an elastomeric flexjoint which takes axial load, butpasses some flexure deformation and thereby serves toprotect riser 34A from extreme bending moments thatwould resuit from a fixed riser to spar connection atthe base of spar 10. In this embodiment, the bottom ofthe buoyancy can tube is otherwise open to the sea.

The top of the buoyancy tube can, however, isprovided with an upper seal 48 and a load transferconnection 50. In this embodiment, the seal and loadtransfer function are separated, provided by inflatablepacker 48A and spider 50A, respectively. However, thesefunctions could be combined in a hanger/gasket assemblyor otherwise provided. Riser 34A extends through seal48 and connection 50 to présent a Christmastree 52adjacent production facilities, not shown. These areconnected with a flexible conduit, also not shown. Inthis embodiment, the upper load transfer connectionassumes a less axial load than lower load transferconnection 46 which takes the load of the productionriser therebeneath. By contrast, the upper loadconnection only takes the riser load through the lengthof the spar, and this is only necessary to augment theriser latéral support provided the production riser bythe concentric buoyancy can tube surrounding the riser.

External buoyancy tanks, here provided by hardtanks 54, are provided about the periphery of therelatively large diameter buoyancy can tube 42 and 011137 provide sufficient buoyancy to at least float an unloaded buoyancy can tube. In some applications it raay be désirable for the hard tanks or other form of external buoyancy tanks 54 to provide some redundancy in overall riser support.

Additional, load bearing buoyancy is provided tobuoyancy can assembly 41 by presence of a gas 56, e.g.,air or nitrogen, in the annulus 58 between buoyancy cantube 42 and riser 34A beneath seal 48. A pressurecharging System 60 provides this gas and drives waterout the bottom of buoyancy can tube 42 to establish theload bearing buoyant force in the riser system.

Load transfer connections 46 and 50 provide arelatively fixed support from buoyancy can assembly 41to riser 34A. Relative motion between spar 10 and theconnected riser/buoyancy assembly is accommodated atriser guide structures 62 which include wear résistantbushings within riser guides tubes 64. The wearinterface is between the guide tubes and the largediameter buoyancy can tubes and risers 34A areprotected. FIG. 8 is a side elevational view of a deepwaterriser system 40 in a partially cross-sectioned spar 10having two buoyancy sections 14A and 14B, of unequaldiameter, separated by a gap 30. A counterweight 18 isprovided at the base of the spar, spaced from thebuoyancy sections by a substantially open trussframework 20A.

The relatively small diameter production riser 34Aruns through the relatively large diameter buoyancycan tube 42. Hard tanks 54 are attached about buoyancycan tube 42 and a gas injected into annulus 58 drivesthe water/gas interface 66 within buoyancy can tube 42 10 011137 far down buoyancy can assembly 41.

Buoyancy can assembly 41 is slidingly receivedthrough a plurality of riser guides 62. The riser guidestructure provides a guide tube 64 for each deepwaterriser System 40, ail interconnected in a structuralframework connected to hull 14 of the spar. Further, inthis embodiment, a significant density of structuralconductor framework is provided at such levels to tieconductor guide structures 62 for the entire riserarray to the spar hull. Further, this can include aplate 68 across moonpool 38.

The density of conductor framing and/or horizontalplates 68 serve to dampen heave of the spar. Further,the entrapped mass of water impinged by this horizontalstructure is useful in otherwise tuning the dynamics ofthe spar, both in defining harmonies and inertiaresponse. Yet this Virtual mass is provided withminimal Steel and without significantly increasing thebuoyancy requirements of the spar.

Horizontal obstructions across the moonpool of aspar with spaced buoyancy section may also improvedynamic response by impeding the passage of dynamicwave pressures through gap 30, up moonpool 38. Otherplacement levels of the conductor guide framework,horizontal plates, or other horizontal impingingstructure 11 may be useful, whether across the moon-pool, across substantially open truss 20A, as outwardprojections from the spar, or even as a component ofthe relative sizes of the upper and lower buoyancysections, 14A and 14B, respectively. See FIG. 7.

Further, vertical impinging surfaces such as theadditional of vertical plates 69 at various limitedlevels in open truss framework 20A may similarly 11 011137 enhance pitch dynamics for the spar with effective entrapped mass. Such vertical plates may, on a limited basis, close in the periphery of truss 20A, may criss- cross within the truss, or be configured in another multidirectional configuration.

Returning to FIG, 6, another optional feature ofthis embodiment is the absence of hard tanks 54adjacent gap 30,

Gap 30 in this spar design Controls vortex inducedvibration ("VIV") on the cylindrical buoyancysections 14 by dividing the aspect ratio (diameter toheight below the water line) with two, spaced buoyancysections 14A and 14B having similar volumes and, e.g.,a séparation of about 10% of the diameter of the upperbuoyancy section. Further, the gap reduces drag on thespar, regardless of the direction of current. Boththese benefits requires the ability of current to passthrough the spar at the gap. Therefore, reducing theouter diameter of a plurality of deepwater riserSystems at this gap may facilitate these benefits.

Another benefit of gap 30 is that it allows passageof import and export Steel catenary risers 70 mountedexteriorly of lower buoyancy section 14B to themoonpool 38. See FIG.6 and also FIGS. 3-5. Thisprovides the benefits and convenience of hanging these.risers exterior to the hull of the spar, but providethe protection of having these inside the moonpool nearthe water line 16 where collision damage présents thegreatest risk and provides a concentration of Unesthat facilitâtes efficient processing facilities.

Import and export risers 70 are secured by standoffsand clamps above their major load connection to thespar. Below this connection, they drop in a catenary - 12 - 0111 57 lie to the seafloor in a manner that accepts vertical motion at the surface more readily than the vertical access production risers 34A.

Supported by hard tanks 54 alone (without apressure charged source of annular buoyancy), unsealedand open top buoyancy can tubes 42 can serve much likewell conductors on traditional fixed platforms. Thus,the large diameter of the buoyancy can tube allowspassage of equipment such as a guide funnel and compactmud mat in préparation for drilling, a drilling riserwith an integrated tieback connector for drilling,surface casing with a connection pod, a compact subseatree or other valve assemblies, a compact wirelinelubricator for workover operations, etc. as well as theproduction riser and its tieback connector. Such othertools may be conventionally supported from a derrick,gantry crâne, or the like throughout operations, as isthe production riser itself during installationoperations.

After production riser 34A is run (withcentralizer 44 attached) and makes up with the well,seal 48 is established, the annulus is charged with gasand seawater is evacuated, and the load of the pro-duction riser is transferred to the buoyancy canassembly 41 as the deballasted assembly rises and loadtransfer connections at the top and bottom of assembly41 engage to support riser 34A.

It should be understood that although most of theillustrative embodiments presented here deploy theprésent invention in spars with interior moon pools 38and a substantially open truss 20A separating thebuoyant sections from the counterweight 18; it is clearthat the VIV suppression and drag réduction of présent 13 011137 invention is not limited to this sort of spar embodiment. Such measures may be deployed for sparshaving no moonpool and exteriorly run vertical accessproduction risers 34A or may be deployed in "classicspars" 10 in which the buoyant tank assembly 15,counterweight spacing structure 20, and counterweight18 are ail provided in the profile of a singleelongated cylindrical hull disrupted only by the gapsof the présent invention. See, for example, FIG. 9illustrating both these configuration aspects.

It should also be appreciated that dividing thebuoyant tank assembly into multiple buoyant sectionsfacilitâtes a modular approach to building spars inwhich facility requirements and attendant deck loadscan be accommodated by adding or changing one or moreof the buoyant sections rather than redesigning theentire spar as an intégral cylindrical unit as. e.g.,a "classic" spar.

Further, other modifications, changes and sub-stitutions are intended in the foregoing disclosure andin some instances some features of the invention willbe employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that theappended daims be construed broadly and in the mannerconsistent with the spirit and scope of the inventionherein.

Claims (10)

1. ΤΗ 0789 PCT 011137 - 14 - C L A I Μ S

   1. A spar platform comprising:a deck; a buoyant tank assembly including a first buoyantsection connected to the deck, a second buoyant sectionarranged below the first buoyant section and a buoyantspacing structure interconnecting the first and secondbuoyant sections in a manner providing a horizontallyextending gap between the first and second buoyantsections ; a counterweight arranged below the buoyant tankassembly and connected to the buoyant tank assembly bya counterweight spacing structure.
2. 2. The spar platform of claim 1, wherein the height ofsaid gap is about 10% of the width of the first buoyantsection.
3. 3. The spar platform of claim 1 or 2, wherein saidfirst and second buoyant tank sections, hâve differentouter diameters.
4. 4. The spar platform of claim 3, wherein the différence between the outer diameters of the first andsecond buoyant sections is between about 40 - 80% ofthe largest of said diameters.
5. 5. The spar platform of any one of daims 1-4, whereinan open moon-pool extends substantially verticallythrough the first buoyancy section, and wherein atleast one riser extends through said moon-pool to thedeck of the spar platform. 15 011157
6. 6. The spar platform of claim 5, wherein said openmoon-pool further extends substantially verticallythrough the second buoyant section. Ί. The spar platform of claim 5 or 6, wherein eachriser is connected to the spar platform by a buoyancycan assembly comprising an open ended buoyancy can tubethrough which the upper end part of the riser extends,a seal arranged in an upper part of the buoyancy cantube and closing the annulus between the riser an thebuoyancy can tube, a load transfer connection betweenthe riser and the buoyancy can tube, and a pressurecharging System in fluid communication with saidannulus below said seal.
7. 8. The spar platform of any one of daims 1-7, whereinthe counterweight spacing structure is a substantiallyopen truss framework.
8. 9. The spar platform of claim 8, further comprising aplurality of horizontal impinging structures across thetruss framework.
9. 10. The spar platform of claim 10, wherein the horizontal impinging structures are formed by riserguide structures.
10. 11. The spar platform substantially as describedhereinbefore with reference to the drawings.