A DEEPWATER FLOATING DRILLING AND PRODUCTION FACILITY INCORPORATING FLEXIBLE PRODUCTION RISERS

Abstract: A method of using a floating platform (12) in deep (1000-3000 feet) water as a drilling/workover and production platform. By interconnecting the subsea wellheads of at least one template (16) to the platform using flexible production risers (26) connected through a reverse loop, the platform may be adjusted on its moorings to position the platform directly over a second template (18) for subsequent drilling or workover. This eliminates the need for separate drilling platforms and workboats. Further, wells of separate templates can be serviced separately and the entire field production need not be completely curtailed during such a workover.

Description: The present invention relates to a method of utilizing a floating platform in deep water as a combination drilling/workover and production platform. More particularly, the present invention is directed to the use of a floating platform (preferably, a semisubmersible) in 1000 feet (305m) to 3000 feet (915m) of water, which is interconnected to completed wells of a template by flexible risers using a reverse loop configuration, by adjusting the mooring of the platform to position it directly over a template to permit additional drilling or workover without curtailing production from wells of other templates. As the search for offshore oil moves into deeper water, the costs associated with producing that oil also increase. In fact, costs typically increase at a faster rate than the linear increase in water depth. This is especially true for conventional fixed platforms. These increasing costs require an increase in the size of proven oil reserves in order to justify proceeding beyond the initial exploration phase. Costs associated with operating an offshore drilling platform typically can exceed $100,000 per day. Unless ways can be developed to reduce drilling costs and accelerate return by offering early production, an increasing number of offshore fields with marginal reserves will not be produced. This problem has been exacerbated by the recent slide in oil prices. With reference to GB-A-2182083 the applicant has voluntarily limited the scope of the present invention in the United Kingdom and has submitted separate claims for that country. A method of performing drilling or workover operations in relation to and production from a plurality of well templates according to the precharacterising portion of claim 1 is known from US 3 327 780. The invention provides a method of performing drilling or workover operations in relation to and production from a plurality of well templates from a floating platform in deep water, the method involving moving the floating platform between a plurality of said well templates, and producing from one or more completed wells of a first well template interconnected to the platform by means of one or more risers whilst simultaneously performing drilling or workover of one or more wells of a second well template, characterised by said one or more risers being looped or otherwise having sufficient flexibility to remain so connected whilst said platform is moved between templates, said movement taking place without the need to interrupt production therefrom and by tightening and slackening mooring lines. A first prefered method according to the invention further comprises positioning said platform directly above the first one of said templates by tightening and slackening the mooring lines of said platform, drilling one or more wells through said first template, completing said well(s), and drilling one or more wells in said second template without interrupting production from said first template. In a second prefered method according to the invention the platform is used simultaneously as a production and a workover platform, said method further comprising curtailing production from said second template, conducting a workover of the well(s) of said second template without interrupting production from the well(s) of said first template, and restoring the well(s) of said second template to production. The method of the present invention provides reduction in costs by eliminating the need for a separate drilling platform. Further, this method takes advantage of the lower costs associated with floating platforms and enables early initial production from a first well template while additional drilling is performed on a second or subsequent template. In addition, the same floating platform is used for workovers to reduce maintenance costs without interrupting production from wells of other templates. Preferably, the floating platform can be deballasted for maintenance without curtailing production. While the use of flexible risers to interconnect subsea wells to floating production platforms is becoming conventional, the present invention incorporates the "benefit" of the mobility of the platform resulting from mooring in deep water (normally considered a drawback), to enable the platform to perform the additional drilling and workover functions. When moored in water on the order of from 1000 to 3000 feet (305 to 915 meters) deep, there is sufficient flexibility in the moorings to permit the position of the floating platform to be adjusted to lie directly above any of several well templates positioned on the ocean floor for drilling or workover. Templates may be spaced apart by 300 feet (91.5m) without causing the riser to deviate from vertical by more than 17° (in 1000 ft water depth, 6° in 3000 ft), an angle easily accomodated by flexible risers. By configuring the flexible risers in a reverse loop, even these relatively small angles will be substantially reduced. This combined flexibility in moorings and production riser capability also permits the floating platform to be deballasted to permit certain portions that normally occupy subsurface positions to extend above the surface for inspection and maintenance, without curtailing production. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, wherein: The Figure is a schematic side elevational view of a floating production platform utilizing a method in accordance with the invention, the drilling/workover position being shown in solid line, the normal production position being shown in phantom. The method in accordance with the invention is adaptable for usage with any floating production facility. It is preferred that a semisubmersible platform 12 be utilized in this method. A plurality of mooring lines 14 (typically eight or more, only two have been shown to simplify the drawing) secure platform 12 in position above templates 16 and 18 by virtue of attachments 20 to the ocean floor. Flexible production riser(s) 26 interconnect template 16 to platform 12. A rigid production riser section 28 traverses the zone (typically 20 to 30 feet) that is subjected to wave battering to extend riser life. The flexible riser 26 is depicted in the Figure in a reverse loop configuration that enhances the flexibility of the system. The use of a flexible riser obviates the need for motion compensation devices typically required with rigid risers and a floating production platform. The reverse loop configuration increases the amount of both lateral and vertical movement of which the platform is capable. The utilization of the flexibility afforded in the moorings in 1000 to 3000 feet water depths (normally considered a problem to be overcome), with the flexible risers affords a platform that can be utilized simultaneously as a production facility and a drilling/workover rig. The flexible riser 26 may be a single conduit carrying the consolidated flow from a manifold located in proximity to the template 16 if subsea wellheads are used; or the flexible riser may be a bundle of conduits carrying production from the individual wells to wellheads located on the platform 12. Platform 12 is ghosted into its normal position generally centered above its well templates 16 and 18. By adjusting the mooring lines 14, that is, tightening some while slackening others, the position of the platform may be adjusted to bring it directly above one of the well templates 18 as shown in the Figure. This enables drilling or workover equipment to be lowered into a well bore through a vertically positioned casing 22 from derrick 24. This mooring adjustment might be accomplished, for example, by coiling and uncoiling the mooring lines to and from winding drums (not shown). While it is preferred that simultaneous production and drilling/workover not be accomplished on a single template (for safety reasons), there is no reason a platform could not be used to drill/workover one template while simultaneously producing from another, particularly if production is to a location outboard of the drilling derrick axis. By adjusting the moorings and continuing production from other templates as described, several economic benefits are obtained: a) only a portion of production need be curtailed during workover and, b) a separate workover unit (with the aforementioned cost of $100,000 per day) is not needed. The flotation columns of platform 12 can be deballasted in calm weather to enable portions of the platform normally occupying subsurface positions to be raised above the level of the water for inspection and maintenance, as necessary. The flexibility of the mooring system and production risers enables this maintenance to occur without disengaging the risers or curtailing production. Further, should it be necessary to deballast from the operational to the survival mode for a hundred year storm or the like, no adjustment or disconnection of the production risers is necessary to accomodate the change of platform position. Although in describing the method of the present invention, lateral displacement of the platform in only one plane has been discussed, it will of course be understood that displacement in the second plane (into and out of the plane of the Figure) is also possible. Accordingly, the present method can accomodate four (or more) separate well templates with associated satellite wells. If four templates are used, it is preferred that they be generally positioned at the corners of a square. Such a combination drilling/workover and production platform could accomodate on the order of 32 wells with a combined production of about 80,000 barrels of oil per day. Various changes, modifications and alternatives will become apparent to one of ordinary skill in the art following a reading of the foregoing specification. It is intended that all such changes, modifications and alternatives as come within the scope of the appended claims be considered part of this invention.

Claims: Claims for the following Contracting States : DE, FR, IT, NL A method of performing drilling or workover operations in relation to and production from a plurality of well templates (16,18) from a floating platform (12) in deep water, the method involving moving the floating platform (12) between a plurality of said well templates (16,18), and producing from one or more completed wells of a first well template (16) interconnected to the platform (12) by means of one or more risers (26) whilst simultaneously performing drilling or workover of one or more wells of a second well template (18), characterised by said one or more risers (26) being looped or otherwise having sufficient flexibility to remain so connected whilst said platform (12) is moved between templates (16,18), said movement taking place without the need to interrupt production therefrom and by tightening and slackening mooring lines (14). A method according to claim 1 further comprising positioning said platform (12) directly above the first one of said templates (16) by tightening and slackening the mooring lines (14) of said platform (12) drilling one or more wells through laid first template (16), completing said well(s), and drilling one or more wells in said second template (18), without interrupting production from said first template (16). The method of Claim 1 or 2 wherein said flexible riser is looped. The method of Claim 3 wherein a reverse loop arrangement is used to interconnect the flexible riser(s) (26) to the completed well(s) from said first template (16). The method of any preceding claim further comprising the step of deballasting the floating platform (12) during periods of calm weather to permit certain portions of the platform normally occupying subsurface positions to extend above the surface for inspection and maintenance, as required, without interrupting production. The method of any preceding claim further comprising the step of inserting one or more rigid riser(s) (28) of sufficient length to traverse a zone impacted by waves between said platform and said flexible riser(s) (26). The method of any preceding claim further comprising the step of interconnecting the completed well(s) from the second template (18) to the floating platform (12) using one or more flexible riser(s) (26). The method of Claim 7 wherein said flexible riser(s) is/are looped. The method of Claim 8 wherein said flexible riser(s) (26) are interconnected to the completed well(s) of said second template (18) using a reverse loop arrangement. The method of Claim 7, 8 or 9 further comprising the step of inserting one or more rigid riser(s) (28) of sufficient length to traverse a zone impacted by waves between said floating platform (12) and the flexible riser(s) (26) of the completed well(s) of said second template (18). A method according to claim 7, 8, 9 or 10 wherein the platform (12) is used simultaneously as a production and a workover platform, said method further comprising curtailing production from said second template (18), conducting a workover of the well(s) of said second template (18) without interrupting production from the well(s) of said first template (16), and restoring the well(s) of said second template (18) to production. The method of Claim 11 further comprising the steps of tightening and slackening the mooring lines (14) of said platform (12) so as to position said platform (12) directly above said first template (16) curtailing production on the well(s) of said first template (16) conducting a workover of the well(s) of said first template (16) without interrupting production from the well(s) of said second template (18), and restoring the well(s) of said first template (16) to production. The method of Claim 10 or 11 further comprising the steps of deballasting said semisubmersible platform (12) during periods of calm weather to permit portions of the platform (12) normally occupying subsurface positions to extend above the surface for inspection and maintenance, as required, without interrupting production. A method as claimed in any preceding claim wherein the platform (12) is moored in water of 305 to 915 metres (1000 to 3000 feet) deep. Claims for the following Contracting State : GB A method of performing drilling or workover operations in relation to and production from a plurality of well templates (16,18) from a floating platform (12) in deep water, the method involving moving the floating platform (12) between a plurality of said well templates (16,18), and producing from one or more completed wells of a first well template (16) interconnected to the platform (12) by means of one or more risers (26) whilst simultaneously performing drilling or workover of one or more wells of a second well template (18), characterised by said one or more risers (26) being looped so as to have sufficient flexibility to remain so connected whilst said platform (12) is moved between templates (16,18), said movement taking place without the need to interrupt production therefrom and by tightening and slackening mooring lines (14). A method according to claim 1 further comprising positioning said platform (12) directly above the first one of said templates (16) by tightening and slackening the mooring lines (14) of said platform (12), drilling one or more wells through said first template (16), completing said well(s), and drilling one or more wells in said second template (18) without interrupting production from said first template (16). The method of Claim 2 wherein a reverse loop arrangement is used to interconnect the flexible riser(s) (26) to the completed well(s) from said first template (16). The method of any preceding claim further comprising the step of deballasting the floating platform (12) during periods of calm weather to permit certain portions of the platform normally occupying subsurface positions to extend above the surface for inspection and maintenance, as required, without interrupting production. The method of any preceding claim further comprising the step of inserting one or more rigid riser(s) (28) of sufficient length to traverse a zone impacted by waves between said platform and said flexible riser(s) (26). The method of any preceding claim further comprising the step of interconnecting the completed well(s) from the second template (18) to the floating platform (12) using one or more looped flexible riser(s) (26). The method of Claim 6 wherein said flexible riser(s) (26) are interconnected to the completed well(s) of said second template (18) using a reverse loop arrangement. The method of Claim 6 or 7 further comprising the step of inserting one or more rigid riser(s) (28) of sufficient length to traverse a zone impacted by waves between said floating platform (12) and the flexible riser(s) (26) of the completed well(s) of said second template (18). A method according to claim 6, 7 or 8, wherein the platform (12) is used simultaneously as a production and a workover platform, said method further comprising curtailing production from said second template (18), conducting a workover of the well(s) of said second template (18) without interrupting production from the well(s) of said first template (16), and restoring the well(s) of said second template (18) to production. The method of Claim 9 further comprising the steps of tightening and slackening the mooring lines (14) of said platform (12) so as to position said platform (12) directly above said first template (16), curtailing production on the well(s) of said first template (16), conducting a workover of the well(s) of said first template (16) without interrupting production from the well(s) of said second template (18), and restoring the well(s) of said first template (16) to production. The method of Claim 8 or 9 further comprising the steps of deballasting said semisubmersible platform (12) during periods of calm weather to permit portions of the platform (12) normally occupying subsurface positions to extend above the surface for inspection and maintenance, as required, without interrupting production. A method as claimed in any preceding claim wherein the platform (12) is moored in water of 305 to 915 metres (1000 to 3000 feet) deep.

Inventors: HUNTER, ANDREW F.
Applicants: CONOCO INC.
Application year: 1987
Main classification: E21B7
IPC codes: E21B7, E21B43, B63B35