OA10940A

OA10940A - Lightweight offshore platform structure

Info

Publication number: OA10940A
Application number: OA9800241A
Authority: OA
Inventors: Jan Willem Van De Graaf
Original assignee: Shell Int Research
Priority date: 1996-06-17
Filing date: 1998-12-17
Publication date: 2003-02-26
Also published as: MY126376A; ID17087A; WO1997048858A1; NO985900L; NO985900D0; DK0910708T3; EP0910708A1; EP0910708B1; NO316126B1

Abstract

An offshore platform structure comprising a framework (1) supporting a platform deck (18) is provided, the framework (1) having at least one leg (5). The structure further includes conductor means for conducting wellbore equipment from the platform deck into at least one wellbore extending into the earth formation below the platform structure. Said conductor means includes at least one conductor conduit (22) extending through said at least one leg (5).

Description

010940

LIGHTWEIGHT OFFSHORE PLATFORM STRUCTURE

The présent invention relates to an offshore platformstructure comprising a framework supporting a platformdeck, the framework having at least on leg, and conductormeans for conducting wellbore equipment from the platformdeck into at least one wellbore extending into the earthformation below the platform structure.

An offshore platform structure of this type isdisclosed in GB-B-2000211. The platform structure isformed of a framework with a leg at each platform corner,the legs being fixed to the seabed by piles extendingthrough the legs. Conductor conduits extend from the topof the platform structure through a number of conductorguides into the seabed. Each conductor conduit consistsof a straight, slanted, lower part and a curved upperpart having a vertically extending upper end. Eachconductor conduit is installed by first installing astraight, slanted, conductor, cutting off and removing anupper part of the straight conductor and replacing thisupper conductor part by a curved conductor part which iswelded to the remaining lower straight conductor part.

It is an object of the invention to provide animproved offshore platform structure which is subjectedto reduced environmental loading and has a reducedweight.

It is a further object of the invention to provide aplatform structure having conductor means which areinstalled in a less complicated manner.

In accordance with the invention there is provided anoffshore platform structure comprising a frameworksupporting a platform deck, the framework having at leastone leg, and conductor means for conducting wellbore 010940 equipment from the platform deck into at least onewellbore extending into the earth formation below theplatform structure, wherein said conductor means includesat least one conductor conduit extending through said atleast one leg.

By locating the conductor conduit within the leg itis achieved that the platform structure is subjected toreduced wave- and current forces when compared to theconventional structure, so that the structure can belighter than the conventional structure. The conductorconduit can be installed, for example, by driving theconductor conduit through the leg.

Preferably the platform structure comprises aplurality of said legs and a plurality of said conductorconduits, each conductor conduit extending through acorresponding leg.

To achieve adéquate fixation of the conductorconduits, suitably the outer diameter of each conductorconduit and the inner diameter of the corresponding legare selected so that the longitudinal axes of theconductor conduit and the corresponding leg substantiallycoïncide.

Adéquate platform stability is achieved if eachconductor conduit which extends through a correspondingleg provides stability to the platform structure on theseabed by forming an anchoring pile.

Enhanced platform stability is achieved by groutingeach conductor conduit inside the corresponding leg.

The invention will be described hereinafter in moredetail with reference to the appended drawing, in which

Fig. 1 shows schematically a plan view of an embodi-ment of the offshore platform structure according to theinvention.

The platform structure shown in Fig. 1 includes aframework 1 with four tubular legs of which only two 010940 legs 5, 7 are shown, the legs being interconnected by anumber of horizontal and sloping braces 11, lia of whichthe braces lia are indicated by dotted lines as these arelocated at the side of the framework opposite the plane 5 of the drawing. Each leg has a curved upper part 12 with substantially constant curvature, and a substantiallystraight lower part 14. The upper end 16 of the curvedupper part 14 of each leg extends substantially verti-cally, and the lower part 14 of each leg extends out·- 10 wardly in downward direction at a selected inclination angle a. A platform deck 18 is arranged on top of the frame-work 1, the platform deck 18 being provided with a skidbeam arrangement 20 for allowing access of wellbore 15 equipment to a selected one of a number of conductor conduits described hereinafter.

The platform structure is provided with four con-ductor conduits of which only the two front conductorconduits 22, 24 are shown in dotted lines, each conductor 20 conduit extending through one of said legs. Thus, in

Fig. 1 conductor conduit 22 extends through leg 5 andconductor conduit 24 extends through leg 7. One of thepurposes of the conductor conduits 22, 24 is to conductwellbore equipment into wellbores below the platform 25 structure. To enable conventional drilling rigs to be positioned above ail four conductor conduits, the widthof the framework 1 at its upper end is restricted to adimension which is equal to or less than the reach of thedrill rigs considered for use. Further, to enable the 30 wellbore equipment to be inserted into the conductor conduits substantially vertically, the longitudinal axesof the legs at their upper ends 16 extend vertically. Thelegs are curved outwardly in downward direction so thatthe width of the framework base is sufficient to achieve 35 adéquate platform stability. The curvature of the legs is 010940 4 selected such that conventional wellbore equipment canpass through the legs without spécial adaptation. Theconductor conduits 22, 24 hâve been driven from the topof the platform structure through the respective legs to 5 a selected depth z into the seabed, for which purpose conventional pile driving techniques can be used. Toincrease platform stiffness, the conductor conduits hâvebeen fixed in the legs by inserting grout between eachconductor conduit and the corresponding leg. The con- 10 ductor conduits 22, 24 therefore hâve the twofold func- tion of conducting wellbore equipment into wellboresextending from the conductor conduits into the earthformation, and of providing a structural load bearingcapacity in analogy to anchoring piles driven into the 15 seabed. Furthermore, the conductor conduits provide increased strength to the legs, especially when theconductor conduits are grouted inside the legs.

Optionally, additional skirt piles (not shown) can beprovided which extend through skirt pile guides 25 into 20 the foundation so as to increase the résistance against latéral forces acting on the structure.

Since the conductor conduits 22, 24 extend throughthe legs 5, 7 of the framework, the conductor conduitshâve substantially the same shape as the legs, i.e. each 25 conductor conduit 22, 24 has a curved upper part with substantially constant curvature, and a substantiallystraight iower part. The upper end 27, 28 of the curvedupper part of each conductor conduit 22, 24 extendssubstantially vertically, and the Iower part 29, 30 of 30 each conductor conduit 22, 24 extends at the inclination angle et in outward direction relative to the verticallongitudinal axis of the platform structure. Conventionalblowout preventors (BOP's) and /or wellheads 31, 32 arelocated on top of the upper ends 27, 28 of the respective 35 conductor conduits 22, 24. - 5 - 010940

In a first mode of normal operation of the platformstructure of Fig. 1 BOP's 31, 32 are located on top ofthe upper ends 27, 28 of the respective conductor con-duits 22, 24. A drilling rig (not shown) is positioned on 5 the skid beam arrangement 20 vertically above one of the BOP's, for example BOP 31. A drill string (not shown)extends from the drilling rig through the BOP 31 andthrough the conductor conduit 22 into a wellboreextending from the conductor conduit 22 into the earth 10 formation. The drill string is rotated so as to deepen the wellbore and to finally reach a hyd^^oarbon con-taining zone in the earth formation below the platformstructure. Since the upper end 27 of the conductorconduit 22 extends vertically there is no need to apply a 15 slanted drilling rig and a conventional type rig can be used.

When drilling of the wellbore through conductorconduit 22 is finalised, the drilling rig is repositionedusing the skid beam arrangement 20 in order to place the 20 rig vertically above BOP 32. The drill string is then inserted into the conductor conduit 24 via BOP 32 andinto another wellbore extending from the conductorconduit 24 into the earth formation. The drill string isrotated so as to deepen the other wellbore and to finally 25 reach the hydrocarbon containing zone. This procedure is repeated for the other two conductor conduits.

In this manner four wellbores are drilled from theplatform structure, which wellbores extend in outwardlydeviating directions due to the outwardly extending lower 30 parts 29, 30 of the conductor conduits 22, 24. At the same time the conductor conduits 22, 24 form anchoringpiles which provide stability to the platform structureon the seabed 26.

In a second mode of operation of the platform 35 structure of Fig. 1, reference signs 31, 32 refer to 6 010940 wellheads which are provided on top of the upper ends 27,28 of the respective co'nductor conduits 22, 24. A pro-duction tubing (not shown) extends from each wellhead 31,32, through the respective conductor conduit 22, 24, intothe respective wellbore extending downward from theconductor conduit 22, 24. Hydrocarbon fluid is producedvia the production tubing's to suitable productionequipment (not shown) which can be of conventional type.

Instead of the framework having four legs, theframework can hâve any other suitable number of legs andcorresponding conductor conduits, for example three orfive legs/conductor conduits.

Instead of positioning the drilling rig on the skidbeam arrangement, the wellbores can be drilled using aseparate jack-up rig having a cantilever deck onto whichthe drilling rig is arranged. The jack-up rig ispositioned adjacent the platform structure in a mannerthat the cantilever deck and the drilling rig are locatedabove the upper ends of the respective conductor con-duits. The wellbores are then drilled from the jack-uprig. In this mode of operation the skid beam arrangementcan optionally be présent on the platform structure.

Claims

010940 ΑΜΕΝΡΕΡ CLAIMS

1. An offshore platform structure comprising a framework(1) supporting a platform deck (18), the framework havingat least one leg (5,7), and conductor means for conducting wellbore equipment from the platform deck intoat least one wellbore extending into the earth formationbelow the platform structure, wherein said conductormeans includes at least one conductor conduit (22,24)extending through said at least one leg, characterized inthat the outer diameter of each conductor conduit (22,24)and the inner diameter of the corresponding leg (5,7) areselected so that the longitudinal axes of the conductorconduit and the corresponding leg substantially coincide.
2. The platform structure of claim 1, comprising aplurality of said legs and a plurality of said conductorconduits (22,24), each conductor conduit extendingthrough a corresponding leg (5,7).
3. The platform structure of claim 1 or 2, wherein eachleg (5,7) is of tubular shape and extends from theplatform deck (18) to the seabed.
4. The platform structure of any of daims 1-3, whereinthe upper end of each leg (5,7) extends substantiallyvertically and the lower end of the leg extends outwardlyin downward direction at a selected inclination angle.
5. The platform of claim 4, wherein an upper end part ofeach leg has a selected curvature, and wherein a lowerend part of the leg is substantially straight.
6. The platform structure of any of daims 1-5, whereineach conductor conduit (22,24) is grouted inside the cor-responding leg (5,7). 010940 - 8 - Ί. The platform structure of any of daims 1-6, furthercomprising a skid beam (20) arrangement provided at saidplatform deck (18) for allowing access of the wellboreequipment to a selected one of the conductor conduits 5 (22,24).
8. The platform structure of any of daims 1-7, whereinsaid wellbore equipment is selected from a drill stringand a hydrocarbon production tubing string.
9. The platform structure of any of daims 1-8, wherein 10 each conductor conduit (22,24) extending through a corresponding leg (5,7) provides stability to theplatform structure on the seabed by forming an anchoringpile.
10. The platform structure of any of daims 1-9, wherein 15 the structure includes at least one skirt pile connected to the framework (1) and extending into the seabed.