OA11118A

OA11118A - Spar structure

Info

Publication number: OA11118A
Application number: OA9900097A
Authority: OA
Inventors: Donald Wayne Allen; Dean Leroy Henning
Original assignee: Shell Int Research
Priority date: 1996-11-15
Filing date: 1999-05-12
Publication date: 2003-04-04
Also published as: GB9909038D0; NO992357D0; WO1998022336A1; GB2332396A; NO992357L; ID21930A; BR9713058A; GB2332396B

Abstract

A floating spar structure for offshore hydrocarbon recovery operations is disclosed having a vertically oriented elongated floating hull (22) with a buoyant upper section (21) and a ballasted lower section (23) and an anchoring system (24) connecting the hull to the ocean floor. The hull of the spar is provided with a vertically oriented, fairing shaped profile section (30) whereby low drag vortex induced vibration suppression protects the spar structure.

Description

011118 - î -

SPAR STRUCTURE

The présent invention relates to a spar structure for offshore hydrocarbon recovery operations.

Production of oil and gas from offshore fields hascreated many unique engineering challenges. One ofthese challenges is dealing with effects of currents oncylindrical marine éléments employed in a variety ofapplications, including, e.g., subsea pipelines;drilling, production, import and export risers, tendonsfor tension leg platforms, legs for traditional fixedand for compilant platforms, other mooring éléments fordeepwater platforms, and the hull structure of spartype structures. These currents cause vortexes to shedfrom the sides of the marine éléments, inducingvibrations that can lead to the failure of the marineéléments or their supports.

For short cylindrical éléments that are adjacentconvenient means for secure mounting, the marineéléments and their supports can be made strong enoughto resist significant movement by the forces created byvortex shedding. Alternatively, the marine elementcould be braced to change the frequency at which theelement would be excited by vortex shedding.

However, strengthening or bracing becomesimpractical when the application requires that theunsupported segments of marine element extend for longruns. Deepwater spar structures are typical of suchapplications.

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 011118 -Β-ίο the marine element. But shrouds and strakesmaterially increase the drag on such large marineéléments.

Thus, there is a clear need for a low drag, VIVreducing System for protecting the hull of a spar typeoffshore structure.

In accordance with the invention there is provideda spar structure for offshore hydrocarbon recoveryoperations having a vertically oriented elongatedfloating hull with a buoyant upper section and aballasted lower section and an anchoring systemconnecting the hull to the océan floor, the sparstructure being provided with a vertically orientedfairing shaped profile section connected to the hull.

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.

Fairings can provide low drag VIV suppression forcylindrical members. However, these hâve been bestsuited for relatively small diameter éléments such asoffshore risers. For a number of reasons, fairingshâve not been thought applicable to large marineéléments. One reason is the corrélation of the need foreffective VIV suppression to Reynolds number. TheReynolds number for a stationary cylinder within afluid moving perpendicular to the axis of the cylinderis approximated with the following expression:

Re = VD/v where:

Re is the Reynolds number; V is the current velocity; D is the outside diameter; andv is the kinematic viscosity 011118 - 3 -

Thus, in a given medium, here seawater, theReynolds number is proportional to the velocity timethe diameter and the hull of a spar is several ordersof magnitude greater in diameter than typical riserswhere fairings hâve been thought appropriate. Typicalof prior applications are offshore production risersdesigned on the basis of Reynolds numbers on the orderof 50,000 to 100,000 and drilling risers at one to twomillion. By contrast, spar structures would anticipateReynolds numbers on the order of five to fifty million,and perhaps more, depending upon the size and con-figuration. Further, it has been common wisdom that thewell correlated vortex shedding along a cylinderexhibited at high Reynolds numbers would require thateffective VIV suppression also addresses reducingspanwise corrélation. However, conventional fairingsare not the choice in applications defined with in thismanner when compared with helical strakes or shroudswhich disturb such corrélation spanwise as a naturalside effect of breaking up the corrélation of trans-versely passing seawater. In addition, it has been theconventional wisdom that changes in attack angle ofenvironmental current to a fixed fairing would bothlimit the effectiveness in vortex shedding and subjectthe tail of the fairing to significant rotational loadsand increased drag. Thus, fairings in general andfixed fairings in particular hâve been thought inappli-cable to solve VIV problems for spar hulls.

The fairing shaped profile section can be fixedlyconnected to the hull, or be rotatable relative to thehull.

The invention will be described further in moredetail and by way of example with reference to theaccompanying drawings in which: 01111 8 - 4 - FIG. 1 is a side elevational view of a fairingshaped spar in accordance with one embodiment of theprésent invention; FIG. IA is a side elevational view of a fairingshaped spar in accordance with another embodiment ofthe présent invention; FIG. 2 is a cross sectional view of the fairing shaped spar of FIG. 1, taken at line 2 -2 of FIG. 1; FIS. 2A is a cross sectional view of the fairing shaped spar of FIG. IA, taken at line 2A- -2A of FIG. FIG. 3 is a cross sectional schematic illustrationof a fairing shaped spar under the influence of océancurrent; FIG. 4 is a cross sectional schematic illustrationof a fairing shaped spar under the influence of océancurrent; FIG. 5 is a cross sectional schematic illustrationof a fairing shaped spar under the influence of océancurrent; FIG. 6 is a side elevational view of a multiplefairing staggered System deployed about a marine riserof a spar structure; FIG. 7 is a cross sectional view along line 2-2 ofFIG. 6; FIG. 8 is a cross sectional view of the multiplefairing staggered System of FIG. 6, taken atline 3A-3A; FIG. 9 is a cross sectional view of the multiplefairing staggered System of FIG. 6, taken atline 3B-3B; FIG. 10 is a cross sectional view of a multiplefairing staggered System illustrating schematically anincreased optimum angle of attack for effective VIVsuppression; 011118 - 5 - FIG. 11 is a cross sectional view of an alternateembodiment of a staggered fairing System; and FIG. 12 is a graph plotting RMS transverseaccélération against Reynolds number for tests of acylinder without VIV suppression and the same cylinderwith a staggered fairing System subjected to currentsat various angles of attack. FIG. 1 illustrâtes the environment of the présentinvention, with a spar 10 having a deck 12 above océansurface 14. Spars are a broad class of floating, mooredoffshore structures which are résistant to heavemotions and présent an elongated, vertically orientedhull 22 which is buoyant at the top 21 and is ballastedat its base 23. 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 plurality of risers 16 extend from the deck tothe océan floor 18 at wells 20 to conduct well fluids.Deck 12 is supported at the top of spar hull 22. Thehull is elongated and vertically oriented with abuoyant top section and a ballasted lower section. Aplurality of mooring Unes 24 are connected to a spreadof anchors (not shown) set in the océan floor to holdspar 10 in place over wells 20. In other embodiments,the risers may act alone as tethers to form theanchoring system securing hull 22 in place whileproviding conduits for conducting produced oil and gas.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 this embodiment, risers 16 are arranged within amoonpool 26 along the interior periphery of hull 22.

See also FIG. 2. Further, a slot 28 in hull 22 011118 - 6 - provides an opportunity to pass risers 16 from anauxiliary drill and completion vessel (not shown) tothe moonpool within the structure. FIGS. 1 and 2 illustrate an embodiment of aproduction spar, but appropriately adapted sparconfigurations are suitable for drilling operations orfor combined drilling and production operations as wellin the development of offshore hydrocarbon reserves.

The elongated, usually cylindrical hull or caisson 22is susceptible to vortex induced vibration ("VIV") inthe presence of a passing current. These currents causevortexes to shed from the sides of the hull 22,inducing vibrations that can hinder normal drillingand/or production operations.

In order to reduce such vibrations a verticallyoriented fairing shaped profile section 30 is fixed onthe hull. In the embodiment of FIGS. 1 and 2, this isprovided by the shape of the outer wall of hull 22itself. The fairing shaped profile section need notnecessarily extend ail the way the surface, nornecessarily to the bottom of the hull. Further thefairing shaped profile may hâve multiple orientations.

Returning to the embodiment of FIGS. 1 and 2,fairing shaped profile section 30 first projects fromhull 22 some distance below océan surface 14. Thisprovides a horizontal surface at the top of theprojection that will serve to entrap mass in the formof the overlaying seawater when the hull is driven torise with a passing wave. The inertia of this massfurthers the heave résistance of the overall sparstructure 10.

Here also, the fixed fairing shaped profile 30 isprovided with a vertical slit or slot 28 which runssubstantially the length of the hull and allows passageof production risers 16 from an auxiliary drilling - 7 - 011118 vessel to well slots inside moonpool 26. In thisembodiment, sût 28 is adjacent the tip of the tail ina manner shielded from the current attack angle (seeFIG. 3) with minimal impact on this flow about hull 22.Other slot configurations may be selected whichcoordinate in contributing to the VIV suppression.Further, slot 28 may be temporary with structuralspanning members deployed except when risers are beingpassed or may be paired with exterior and interiorstructural spanning members to be sequentially removedand replaced in passing risers. FIG. 2 illustrâtes the important chord ”c" andthickness "t" dimensions of the fairing shaped profile,which ratio is preferably between about 1.5 and 1.20,or more preferably between about 1.20 and 1.10. In thisembodiment, fairing shaped profile 30 formed by hull 22is provided with a tail section 38 which is essentiallya plate extending the trailing edge of the fairingshaped profile section beyond the terminus 40 of theangled converging sides 42. Adding tail 38 extends thecord length c with a minimum of materials.

In FIGS. IA and 2A, fairing shaped profilesection 30 extends the vertical length of hull 22 ofspar 10 and the trailing edge of the fairing shapedprofile section ends at the terminus 40 of convergingsides 42. Further, mooring Unes 24 radiate sym-metrically about the central axis of hull 22 as opposedto the asymmetrical mooring shown in dotted outline at24'. No slot is provided in this embodiment forlaterally passing risers from outside the spar tomoonpool 26 through the vertically floating hull. Thissuggests the wells be drilled from within the moonpool,that wells 20 be pre-drilled and risers 16 installedfrom within the moonpool, or that more complicatedriser handling techniques be employed to bring the 011118 - 8 - risers inside. In the illustrated embodiment, adrilling facility 44 has been deployed an a ratherlarge spar structure. Note too that the drilling rigmay be off center, taking advantage of additionalbuoyancy in the fairing shaped profile section. Alter-natively, fairing shaped profile section 30 may beballasted to neutral buoyancy with, e.g., equipment,seawater or oil storage. Exterior import and/or exportrisers may be conventionally employed passing down thesides of the hull without significantly affecting VIVsuppression or drag réduction effects on the spar hullfrom the fairing profile.

Further, any détriments in an asymmetricalarranged mass, effective mass, or even buoyancy isminimized with a either a short fairing or an ultra-short fairing in combination with base ballast. "Shortfairings" are defined as having a chord to thicknessratio between about 1.50 and 1.20 and "ultra-shortfairings" are those between about 1.20 and 1.10. US patent No. 5,410,979 discloses fairings having achord to thickness ratio of between about 1.50 and1.25. FIGS. 3-6 illustrate the effects of océan currenton the fairing shaped profile section 30 of sparhull 22 and methods to utilize and/or respond to theseeffects. FIG. 3 illustrâtes genericly a probabilitygradient of historical current data shown with azimuthand magnitude with arrows 50. It is common for patternsof prevailing currents for given locations to hâvesignificant year round corrélation.

The fixed fairing shaped profile is oriented ondeployment to align with the prevailing currentgradient alpha and can be effective for short fairingsat angles of attack up to about 52.5 degrees on eitherside of the nominal current orientation. Further, 011118 - 9 - ultrashort fairings can expand this range significantlywithout losing effective VIV suppression and whileretaining net drag réduction at high Reynolds numbers.Brief periods somewhat outside of these ranges may betolerable if VIV problems response to the current isprimarily an issue of fatigue failure which itself is afunction of time, the majority of which will findspar 10 in an effective orientation. Alternatively, theorientation can be altered to rotate spar 10 to a neworientation by using playing out and taking inasymmetrical mooring Unes 24. FIG. 5 illustrâtes another possibility, parti-cularly with somewhat longer fairings, in which thespar is deliberately rotated out of alignment with thecurrent to "fly" the spar with the fairing shapedprofile section acting as a wing, aiding to bias (seearrows 51) the spar to an offset position where it isretained by current and mooring 24. This may be usefulto provide vertical access over additional wells for anauxiliary drilling vessel. FIG. 6 is illustrâtes several fairing arrays 110grouped as staggered fairing Systems secured to asubstantially cylindrical marine element 112, hereriser 112A, of a schematically illustrated productionmini-spar 114. Three fairing arrays are shown, denotedas staggered fairing Systems 110A, 110B and HOC, toillustrate a range of possible embodiments. The middlearray is formed from two fairings 108 arranged indifferent azimuthal orientations. Here fairings 108 ofstaggered fairing system 110B are mounted adjacent oreven as a single unit about riser 112A. Gaps may beleft along the marine element, both between the arraysof staggered faring Systems and between the individualfairings within an array. 011 1 1 8 - 10 - FIG. 7 is a top view of a single fairing 108secured about the marine element in a substantiallyfixed, non-rotative manner. It can be connecteddirectly to the riser, e.g. in a tight circumferentialfriction engagement, or indirectly e.g. connected tobuoyancy modules which are themselves connected to theriser. Some rotational slippage may be allowed in someembodiments provided not ail individual fairings arefree to rotate to effectively weather-vane about marineelement 112, or fairings are secured to one another tomaintain relative alignment even if the array rotâtes.Relative engagement of adjacent fairings 108, such asin staggered fairing system 110A, may provide directinterconnection of fairings endwise to ensure anappropriate spread of orientations. The fairing has aleading edge 116 generally directed toward a possiblecurrent direction. The leading edge of fairing 108follows the circular profile of marine element 112A,departing therefrom with two shaped sides 118 con-verging at trailing edge 120. The trailing edge may ormay not include a tail 122.

Short fairing éléments with a short chord tothickness ratio of about 1.5 to about 1.2 and ultra-short fairings with a chord to thickness ratio of about1.20 to 1.10 are particularly useful for combinationinto arrays of staggered fairing Systems. A thirdparameter illustrated in FIG. 7 is the orientation ofthe fairing. Currents and relative position betweenfairings are defined in angular relationship (a) from aline taken from the longitudinal axis of the cylindri-cal marine element to trailing edge 120 of the fairing. FIG.8 is a cross section of marine element 112immediately above staggered fairing system 110B whichemploys two fairings, an upper fairing 108A and a lowerfairing 108B, connected about riser 112A. In this 011113 11 illustration, the fairings are arrangée! 30 degrees oneither side of the nominal design current orientation,see current vector V. Thus there are 60 degrees betweenorientations of the respective upper and lowerfairings. This is consistent with a preferred spreadbetween adjacent fairing éléments of between 20 and60 degrees.

However, the current is not always aligned with thenominal design orientation. FIG. 9 is a similar crosssection, here taken through staggered faring SystemHOC illustrated in FIG. 1. Here current vector Vdeviates substantially from the nominal orientation.Fairing 108B is itself 75 degrees out of alignment withthe current. Acting alone, this would be out of therange of effective VIV suppression. However, fairing108B is but one component of the System and fairing108A is well within the range for angle of attack forwhich effective VIV suppression will be provided thecylindrical marine element 112. Within a range, this isa trade-off of some increases in drag from non-alignedfairings as other fairings in the array remain or enterinto more effective VIV suppression alignment.

As a System, it appears that very effective VIVsuppression is possible across at least 90 degrees ofpossible current variance with drag increases whichremain acceptable for many of the offshore applicationswhere VIV suppression is important. See FIG. 10 inwhich increased variance is denoted by areas 130 and132 over the nominal optimal variance 134 schematicallyillustrated for a single fairing.

Where drag is less critical, the System can bepushed to the effective limits of individual fairingswithin the array, with orientations that are repeatedsystematically down the riser. See FIG. 11, where thearray présents orientations across about a 120 degree 011118 - 12 - range between fairings within the array. Here thenominal orientation is met with the whole array withinan effective orientation, i.e., within 60 degrees forultrashort fairings 108A that are most mis-aligned.However, in this embodiment, the chord to thicknessratio increases as individual fairing éléments are lesseccentric to the nominal current orientation. Thus,fairing element 108C which is aligned with the nominalcurrent orientation is outside the limited "ultrashort"range. This places those fairings which are leastsusceptible to net drag increases and most forgiving toangle of attack on the periphery, while those that canbest provide a drag réduction but with limited angularresponse are more nearly aligned with the nominaldesign current. Dotted line 136 illustrâtes this aspectof this embodiment. Further, some locations may hâvesecondary as well as primary design nominal currentorientations, e.g., prévalent seasonal shifts. Again,the array may be constructed to optimally address theseprévalent currents as well as a range of déviantcurrent orientations. It should also be noted that morethan one staggered fairing System may be deployed on asingle marine element and that it may be useful to hâvethese disposed to different orientations. For example,a given location may be routinely subject to differentcurrents as a function of depth in the water column. Inthis circumstance, different prevailing currents couldbe optimally addressed with staggered fairing Systemsdeployed at various levels which are designed for theorientation, magnitude, and projected variance expectedalong the marine element. FIG. 12 is a graph plotting RMS transverseaccélération A(m/s2) against Reynolds number Re fortests on a staggered fairing System configured likethose illustrated in FIGS. 8, 9 and 10. This is a 01111 8 - 13 -

System of two fairings about a cylindrical marineelement oriented to plus or minus 30 degrees from thedesign nominal current orientation which is designatedas 0 degrees for FIG. 12. VIV excitement was measuredfor the staggered fairing System at five differentangles (a) of attack, from 0 to 90 degrees. The baseline (B) for a bare pipe test is also illustrated onthe graph. Significant VIV suppression is stillobserved for this staggered fairing system even at anangle of attack of a = 90 degrees.

Although the staggered fairing System has beendescribed for application on a marine riser, it can beapplied to a full range of other cylindrical marineéléments, including, but not limited to subseapipelines; drilling, import and export risers; tendonsfor tension leg platforms; legs for traditional fixedand for compilant platforms; cables and other mooringéléments for deepwater platforms; and notably to thehul.l structure of spar type structures.

Other modifications, changes and substitutions areintended in the foregoing disclosure and in someinstances some features of the invention will beemployed without a corresponding use of other features.

Claims

ΤΗ 0970 PCT 011118 - 14 -

1. A spar structure for offshore hydrocarbon recoveryoperations having a vertically oriented elongatedfloating hull with a buoyant upper section and aballasted lower section and an anchoring Systemconnecting the hull to the océan floor, the sparstructure being provided with a vertically orientedfairing shaped profile section connected to the hull.
2. The spar structure in accordance with claim 1wherein the fairing shaped profile section is a shortfairing having a chord to thickness ratio between about1.5 and 1.20.
3. The spar structure in accordance with claim 1wherein the fairing shaped profile section is an ultra-short fairing having a chord to thickness ratio betweenabout 1.20 and 1.10.
4. The spar structure in accordance with any one ofdaims 1-3 wherein the vertically oriented faringshaped profile section is formed by an outer wall ofthe hull.
5. The spar structure in accordance with claim 1wherein the vertically oriented fairing shaped profilesection comprises a vertically extending ballastchamber connected to the outside of the hull.
6. The spar structure in accordance with any one ofdaims 1-5 wherein the anchoring system comprises aplurality of latéral mooring Unes connected to spar,wherein at least one of the latéral mooring linesconnect to the sides of the vertically oriented,fairing shaped profile section in a manner asymmetricalto the axis of the spar hull whereby the orientation of 011118 - 15 - the fairing shaped profile may be aligned with shifting currents by rotating the spar hall.
7. A spar structure in accordance with any one ofdaims 1-6 wherein the top to the fairing shapedprofile section présents a substantially horizontalsurface below the surface of the water.
8. The spar structure of any one of daims 1-7,wherein the fairing shaped profile is fixedly connectedto the hull.
9. The spar structure of any one of daims 1-8 whereinthe fairing shaped profile section comprises aplurality of fairings, each fairing comprising: a leading edge substantially defined by thecircular profile of the hull; and a pair of shaped sides departing from the circularprofile of the hull and converging at a trailing edge;and connections between the hull and the fairingsthrough which the fairings are fixedly secured to thehull in a non-rotative manner, said connectionssecuring the fairings in an array of orientations alongthe axis of the hull.
10. The spar structure staggered fairing System inaccordance with daim 9, wherein the leading edge ofeach fairing is substantially defined by the circularprofile of the marine element for a distance along atleast 250 degrees.
11. The spar structure in accordance with daim 9,wherein each fairings is a short fairing with a chordto thickness ratio between about 1.5 and 1.2.
12. The spar structure in accordance with daim 9,wherein each fairings is an ultrashort fairing with achord to thickness ratio between about 1.20 and 1.10. 011118 - 16 -
13. The spar structure in accordance with any one ofdaims 9-12, wherein the fairings are oriented inmultiple tiers of repeating patterns.
14. The spar structure in accordance with any one ofdaims 9-13, wherein the angle between adjacentfairings is between 20 and 60 degrees. 5