OA11139A

OA11139A - Spar platform with vertical slots

Info

Publication number: OA11139A
Application number: OA9900149A
Authority: OA
Inventors: Donald Wayne Allen
Original assignee: Shell Int Research
Priority date: 1996-12-31
Filing date: 1999-06-30
Publication date: 2003-04-16
Also published as: GB9912465D0; NO992536D0; NO992536L; WO1998029298A1; GB2334005B; GB2334005A; BR9714113A; ID21673A

Abstract

The present invention is a spar platform (10) having a deck (12) supported by a buoyant tank assembly (22) having a buoyant chamber outside wall (18) and a buoyant chamber inside wall (20) which defines a vertically extending open moonpool (26). A plurality of buoyant chamber side walls connect the buoyant chamber inside and outside walls and defining a plurality of vertical slots (28). A counterweight and a counterweight spacing structure depend from the buoyant tank assembly.

Description

011139

SPAR PLATFORM WITH VERTICAL SLOTS

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

Efforts to economically develop offshore oil and gasfields in ever deeper water create many uniqueengineering challenges. One of these challenges isproviding a suitable surface accessible structure. Sparsprovide a promising answer for meeting these challenges.Spar designs provide a heave résistant, floatingstructure characterized by an elo.ngated, verticallydisposed hull. Most' often this hull is cylindrical,buoyant at the top and with ballast at the base. Thehull is anchored to the océan floor through risers,tethers, and/or mooring Unes.

Though résistant to heave, spars are not immune fromthe rigors of the offshore environment. The typicalsingle column profile of the hull is particularlysusceptible to VIV problems in the presence of a passingcurrent. These currents cause vortexes to shed from thesides of the hull, inducing vibrations that can hindernormal drilling and/or production operations and lead tothe failure of the anchoring members or other criticalstructural éléments.

Helical strakes and shrouds hâve been used orproposed for such applications to reduce vortex inducedvibrations. Strakes and shrouds can be made to beeffective regardless of the orientation of the current to 011139 - 2 - the marine element. But shrouds and strakes materiallyincrease the drag on such large marine éléments.

Thus, there is a clear need for a low drag, VIVreducing System suitable for deployment in protecting the 5 hull of a spar type offshore structure.

In accordance with the invention there is provided a spar platform comprising:a deck; a buoyant tank assembly, comprising: 10 a buoyant chamber outside wall; a buoyant chamber inside wall defining a verticallyextending open moonpool; a plurality of buoyant chamber side walls, connecting thebuoyant chamber inside and outside walls and defining a 15 plurality of vertical slots in the buoyant tank assembly; a counterweight; anda counterweight spacing structure.

The invention will be described further in moredetail by way of example and with reference to the 20 accompanying drawings in which: FIG. 1 is a side elevational view of a spar platform in accordance with the présent invention; FIG. 2 is a cross sectional view of the spar platform of FIG. 1, taken at line 2-2 in FIG. 1; 25 FIG. 3 is a side elevational view of another alternate embodiment of a spar platform in accordancewith 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; 30 011139 - 3 - FIG. 6 is a side elevational view of yet another alternate embodiment of a spar platform in accordance with the présent invention; and FIG. 7 is a cross sectional view of the spar platform of FIG. 6, taken at line 6-6 in FIG. 6.

In the figures like reference numerals relate to likecomponents. FIGS. 1 and 2 illustrate one embodiment of theprésent invention. Here spar 10 présents a deck 12 aboveocéan surface 14. Deck 12 is supported at the top of spar hull 22. The hull is elongated and verticallyoriented with a buoyant top section and a ballasted lowersection. The hull has an outside wall 18 and an insidewall 20. The inside wall defines a moonpool 26. A plurality of mooring Unes 24 are connected to aspread of anchors (not shown) set in the océan floor tohelp hold spar 10 in place over wells or subsea manifolds(not shown). In other embodiments, a plurality of risers16 may act alone as tethers to form the anchoring System securing hull 22 in place while providing conduits forconducting produced oil and gas.

Risers 16 extend from the océan floor to the deck forconducting well fluids from wells or subsea manifold.The upper end of risers 16 are connected to productionfacilities supported by deck 12 and, after initialtreatment, the hydrocarbons are directed through anexport riser to a subsea pipeline, not shown. In thisembodiment, risers 16 are arranged within moonpool 26along the interior periphery of hull 22. See alsoFIG. 2.

Spar 10 is résistant to heave motions and has anelongated, vertically oriented hull 22 which is buoyantat the top, here buoyant tank assembly 21, and is 011139 ballasted at its base, here' counterweight 23, which isseparated from the top through a middle or counterweightspacing structure 25 . Such spars may be deployed in avariety of sizes and configuration suited to theirintended purpose ranging from drilling alone, drillingand production, or production alone. A basic characteristic of the spar type structure isits heave résistance. However, the typical elongated,usually cylindrical hull or caisson 22 is verysusceptible to vortex induced vibration ("VIV") in thepresence of a passing current. These currents causevortexes to shed from the sides of the hull 22, inducingvibrations that can hinder normal drilling and/orproduction operations and lead to the failure of therisers, 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 moresusceptible 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 orso.

The présent invention employs a plurality of verticalslots 28 through the buoyant tank assembly 22 which aredefined by side walls 29 connecting inner wall 20 andouter wall 18. Slots 28 are aligned in pairs on opposingsides of hull 22. These paired slots allow current to 011139 pass through the spar hull to the moonpool and out again,thereby reducing drag and preventing the highlycorrelated flow around the hull which leads to VIVproblème. The size of the slot, its orientation andconfiguration may be determined by the spécifieapplication. For instance, effectiveness may beincreased across a greater angle of attack by the currentby beveling, or double beveling the relative orientationof side walls 29 which conduct current through spar 10.

In the embodiment of FIGS. 1 and 2, the verticalslots are aligned to bisect the hull with a flow pathdirectly through the centre of the moonpool and extendsubstantiâlly the entire length of hull 22. Slots ofthis length provide an opportunity to pass risers 16 froman auxiliary drilling and completion vessel (not shown)to the moonpool within the spar. This facilitâtes use ofa smaller, production only spar platform. However,slots of this length may raise particular needs forstructural reinforcement. Here a plurality of beams orstruts 30 are deployed to join the respective halves ofthe bisected spar hull 22. Such beams may be used withminimal interférence to direct drilling and workoversupport through the use of auxiliary vessels.Alternatively, removable struts 32 may join across slots28 intermittently along the slots. Struts 32 may beremoved for riser passing operations. Further, beams 30and/or removable struts 32 may be formed fromsubstantially open space frames or presented in astreamlined configuration to minimize obstruction.

This reinforcement may be particularly importantwhere more than one pair of slots 28 are deployed. Thismay be desired to accommodate misalignment of the currentto the nominal design orientation. Further, some 011132 locations may hâve secondary as well as primary design nominal current orientations, e.g., prévalent seasonal shifts.

However, vertical slots 28 need not extend the entire 5 length of hull 22 in order to provide significant drag réduction and VIV suppression. See, e.g., the alternateembodiments of FIGS. 3 and 6. FIGS. 3-5 illustrate anembodiment having two pairs of vertical slots 28A and28B, here arranged with different azimuth orientations 10 represented by arrows 34 in FIG. 4. Each pair is horizontally aligned, but the pairs need not be on thesame level, nor of the same orientation. The two levelshere can provide for greater déviation from the nominaldesign current orientation. Alternatively, a given 15 location may be routinely be subjected to different prevailing currents as a function of depth in the watercolumn. In the later circumstance, different prevailingcurrents could be optimally addressed with pairedvertical slots 28 deployed at various levels which are 20 designed for the orientation, magnitude, and projected variance expected along the spar hull. Note also theriser placement in FIG. 4. Risers too, may be subjectedto VIV and the need for VIV suppression within the hullmay be controlled by positioning the risers out of 25 alignment with the current path through the moonpool.

Also asymmetrically connected mooring Unes may be used to reorient the spar in response to déviations inthe current. FIG. 6 illustrâtes a configuration of the présent 30 invention in which the buoyant tank assembly 21 alone is the hull 22 which defines a buoyant chamber with a pair of slots 28 extending therethrough. A truss or open space frame provides a low drag counterweight spacing 011139 7 structure 25 connecting counterweight 25 to buoyant tankassembly 21. Further, it should be noted that themoonpool need not be circular, e.g., it may be square orrectangular and the current path may laterally or 5 diagonally bisect the moonpool.

Other modifications, changes, and substitutions are intended in the foregoing disclosure, and in someinstances some features of the invention will be employedwithout a corresponding use of other features. 10 Accordingly, it is appropriate that the appended daims be construed broadly and in a manner consistent with thespirit and scope of the invention herein.

Claims

011139 AMENDED CLAIMS

1. A spar platform comprising:a deck ( 12); a buoyant tank assembly (22), comprising:a buoyant chamber outside wall (18);a buoyant chamber inside wall (20) defining a vertically extending open moonpool (26); a plurality of buoyant chamber side walls (29), connecting the buoyant chamber inside and outside walls(18, 20) and defining a plurality of vertical slots (28)in the buoyant tank assembly; a counterweight (23); and a counterweight spacing structure (25), characterizedin that the slots (28) extend substantially the entirelength of the buoyant tank assembly (22).
2. The spar platform in accordance with claim 1, whereinthe vertical slots (28) are arranged on opposite sides ofthe buoyant tank assembly (22) and are aligned throughthe centre of the moonpool (26).
3. The spar platform in accordance with claim 2, whereintwo said vertical slots (28) are provided.
4. The spar platform in accordance with any one ofclaims 1-3, further comprising a plurality of removablestruts (32) connected across the vertical slots (28) toallow passage of substantially unimpeded current flowwhen in place and to allow riser passage into themoonpool (26) when temporarily removed.
5. A spar platform in accordance with any one ofclaims 1-4, further comprising a plurality of struts (30)interconnecting the buoyant tank assembly (22) across thesubstantially open moonpool -(26). - 9 - 011139
6. The spar platform in accordance with claim 2 or 3,wherein the vertical slots (28) are arranged in aplurality of horizontally aligned pairs, the verticalslots (28) of each pair being arranged on opposite sides 5 of the buoyant tank assembly (22) and aligned through the centre of the moonpool (26).
7. The spar platform in accordance with claim 6, whereinthe horizontally aligned pairs of vertical slots (28) arearranged across a plurality of azimuthal orientations(34) . 10