CA1064274A

CA1064274A - Drilling structure for drilling in the ocean floor

Info

Publication number: CA1064274A
Application number: CA271,103A
Authority: CA
Inventors: Joseph F. Schirtzinger
Original assignee: Sea Log Corp
Current assignee: Sea Log Corp
Priority date: 1976-02-17
Filing date: 1977-02-04
Publication date: 1979-10-16
Also published as: JPS52142801A; GB1523567A; SE7701620L; JPS5943609B2; DK64877A; DE2706245A1; NO770515L

Abstract

Abstract A mounting structure for supporting a drilling platform on the ocean floor comprises an open precast vessel having a bottom wall and a side wall extending up from and around the perimeter of the bottom wall which is adapted to be recessed in the ocean floor at the drilling site. A monopod drilling structure having a base and a supporting column extending upwardly from the base to a drilling platform is positioned on top of the vessel with the bottom of the supporting column having a vertical passage opening through the bottom of the base into the vessel. Sealing means is provided between the top of the vessel and the bottom of the drilling structure which provides a water-tight seal, permitting the interior of the vessel to be pumped dry when the drilling structure is in place.

Description

DRILLING STRUCTURE FOR DRILLING
IN THE OCEAN FLOOR.

Field of the Invention -This invention relates to a subsea drilling structure for drilling in the ocean floor, and more particularly, to such structures comprising a monopod drilling platform.
Background of the Invention With the increased interest in offshore oil production in the icy waters of the artic, there has developed a need for more mobile drilling platforms which can be readily moved during severe ice conditions but which can operate in the presence of surface ice. A ~onopod platform with icebreaking capability has advantages over more conventional drilling platforms for this type of operation. While the monopod type lS platform can be operated as a semisubmersible, in shallower : waters, it is preferable to anchor the platform directly on ; the ocean bottom. The monopod structure with its flat-bottom lower hull, presents a problem when used for drilling and completing a plurality of development wells at close locations, since room must be provided for mounting the "christmas tree"
and other equipment on top of the well after it is drilled and before moving the platform to the next drilling ]ocation.
Summary of the Invention.
In accordance with the present invention there is provided a drilling structure for drilling in the ocean floor, comprising an open precast vessel having a bottom wall and a side wall extending up from and around the perimeter of the ; ~ bottom wall, the side wall terminating in a continuou~ edge at the top lying in a plane, the top edge of the wall forming an opening in the top of the vessel, a monopod drilling structure -r; ~ 'r ' ,: . .: ' 10f~4274 having a base and a supporting column extending upwardly from the base, the column having a vertical passage opening through the bottom of the base, the base having a flat bottom surface surrounding the opening of the vertical passage, the opening of the passage being substantially smaller than the opening in the top of the vessel, the vessel being adapted to be sub-merged in the ocean floor with the bottom of the platform base extending across the top edge of the vessel, and sealing means secured to the vessel adjacent the top edge of the vessel and frictionally engaging the bottom of the platform to form a water-tight intersection while permitting lateral movement of the platform relative to the vessel to position the vertical passage over any selected portion of the open vessel.
Such a structure may also include means anchoring the drilling structure to the vessel.
In such a structure the sealing means may also include means mounted in the vessel for compressing the seal against the bottom of the drilling structure.
The vessel may be provided in the bottom wall thereof with downwardly directed fluid jets adapted for connection to a source of fluid under pressure and usable to remove material of the ocean floor from beneath the said bottom wall.
The monopod drilling platform may have an upper deck, the column supporting the deak from the base, the vertical passage forming at the bottom a moon pool through which a drill bit and drill stem can be lowered from the deck to the ocean floor, and including means for releasably attaching the base to the top of the vessel.
The present invention is directed to structures of the kind specified which can be anchored to the ocean floor for .,

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completion of multiple production wells, In brief,s$ructures of the present invention utilize a precast cellar having a flat bottom wall and upstanding sidewalls, the cellar being open at the top. After dredging the ocean floor to provide a level area larger than the bottom of the drilling platform, the cellar structure is lowered by a surface vessel to the center of the levelled area. Downwardly directed fluid jets may be provided in the bottom wall of the cellar which are connected to a source of fluid under pressure; these jets can be used to remove material from beneath the bottom of the cellar structure, permitting it to bury itself in the ocean floor so as to be depressed below the level of support of the drilling platform on the ocean floor. Means is provided for joining the base of the monopod platform to the cellar structure and forming a water-tight seal between the cellar and the platform. The sealing means are secured to the vessel and frictionally engage the flat bottom surface of the platform with the opening in the vertical passage being substantially smaller than the opening in the ` top of the vessel. This arrangement permits the positioning of the platform relative to the vessel laterally while main-taining the sealed relationship between the platform and vessel.
The interior of the cellar can then be pumped dry so as to be accessible to workmen through the center of the monopod platform column.
Description of the Drawings For a more complete understanding of the invention, reference should be made to the accompanying drawings, wherein:
FIG. 1 is a side elevational view of the drilling platform in operative position on the ocean floor;
FIG. 2 is a cross-sectional view of the cellar structure;
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FIG. 3 is a top view of the cellar structure and the base of an associated monopod drillin~ platform FIGS. 4 - 9 illustrate the operating sequence in placing the cellar structure at the drill site;
FIG. 10 i5 a perspective view of the drill site on the ocean floor; and FIGS. 11 and 12 are cross-sectional views of alternative . arrangements for anchoring and sealing a monopod drilling platform to the ocean floor.
Detailed Description Referring to FIG. 1, the numeral 10 indicates generally a monopod type drilling platform having a lower submerged hull 12, a vertical supporting column 14, and upper hull 16. The upper hull 16 includes a drilling deck 18 on which is supported a conventional drilling derrick 20. The drilling takes place through an open shaft or moon pool 22 extending from the drilling deck 18 down through the bottom 13 of the lower hull 12.

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' ' ' , ll l 10~4274 1 ¦ The moon pool provid~s acccss to the subctrata by th~
drilling rig. The bottom 13 of the lower hull 12 is designed to rest on the ocean floor after proper site preparation, as l hereinafter described, having an emplaced ce;lar 24.
51 Referring to FIGS. 2 and 3, the cellar 24 includes a cylindrical sidewall 26 and flat bottom wall 28. The cellar l is preferably cast from concrete with relatively thick side-¦ walls terminating in a flat annular top surface 30. The bottom 13 of the lower hull 12 rests in part on the surface 0¦ 30, as shown in FIG. 3.
~ he cellar is cast with a network of high-pressure water lines, indicated generally at 32, in the bottom wall 28 of the cellar. The network of high-pressure water lines supply water under pressure to a plurality of water jets 34 which direct water downwardly beneath the bottom wall 28. As best seen in FIG. 3, the pipe network is preferably arranged in quadrants, with each quadrant having itsown input stab connector 36 extending vertically upwardly through the bottom wall 28 on the inside of the cellar 24. Each inlet connector is in turn connected to a series of radiating pipes, each of which in turn is connected to a pattern of iets. By connect-ing the inlets to a source of water under high pressure, the pattern of high-pressure jets on the bottom of the cellar can be used, as hereinafter described, to control the emplacement of the cellar on the ocean floor.
Referring to FIGS. 4 through 9, the steps required to prepare the drilling site for the monopod drilling platform -- and to emplace-the cellar is shown in detail. As shown in FIG. 4, a surfacc ship or barge is moved to the drilling site, preferably during the summer when the area is free of ice.
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, , 7' ' ' ' '' ll 1064274 1 Initially a pipeline is laid extending from the drill site ¦ to an oil production collection terminal (not shown). The ¦ end of the pipeline is submerged below the mudline to a ¦ depth corresponding to the desired depth of the emplaced 5 ¦ cellar structure. The pipe is submerged in the bottom of the ocean by dredging or otherwise excavating a trench. The l balance of the pipeline need not be submerged to the same ¦ depth as the end of the pipeline.
l Once the pipeline is in place, a dredging operation is performed from the ahip 40, as shown in FIG. 5. The dredge removes rocks and large debris to a depth, for example, below any ice scorèd trenches in the ocean floor. The dredge is then used to level an area substantially greater than the area of the bottom of the drilling platform; for example, an area 300 ft. square is typical.
Once the site is prepared, the drilling cellar 24, with a temporary top cover to keep out water from the inside of the cellar so that it will float, is towed to the drill site.
~ With the drilling cellar positioned over the drill site, 2 the cover is removed from the cellar and high-pressure water lines are connected to the stab connectors 36. The cellar is then flooded to cause it to sink, the cellàr being suspended by a cable from a crane on the surface vessel. Water under high pressure is pumped through the water jets beneath the 2 cellar, the jets displacing mud and sand immediately beneath the cellar, permitting the cellar to bury itself below the mud line. Gages for sensing the attitude of the cellar as it is lowered are attached to the cellar with signal lines going to the surface vessel, so that the attitude of the cellar can be

3 continuously monitorcd. By controlling the water delivered J -Y
, 1 to the respective quandrants of the ~et sy~tem the emplaced cellar can be mainta~ned level.
As shown in FIG. 8, once the cellar 24 i~ emplaced, ; with the top at the level of the dredged area; any mud or debris inside the cellar is pumped out and the dredged area is smoothed out around the outside of the cellar so that the top of the cellar is flush with the smoothed area on which the bottom of the monopod driiling structure is later rested.
A pipeline is then coupled into the production pipe msnifola within the cellar, the manifold being indicated at 40 in FIG.
3.
As shown by the perspective view of FIG. 10, the completed drill site provides a depressed area which is sufficiently lower than the bottom of the ice-scored trenches to be relatively free from potential damage by surface ice.
The cellar 24 is emplaced in the center of the recessed area and is connected to a pipeline going to a gathering point.
~he side is now ready for development whenever a monopod drilling structure of the type described in connection with FIG. 1 can be moved on location. The top 30 of the cellar sidewall 26 is preferably provided with sonor or other type of transponders, such as indicated at 42, which can be used to locate the cellar from the surface and can be used to guide the monopod drilling structure into position over the drill 2 site. After the platform is positioned on the top of the celhar, as shown in FIG. 2, the lateral position of the plat-form can be adjusted relative to the cellar by means of a `
cable 44 extending down through the moon pool 22. The end of the cable 44 is attachcd to any one of a plurality of lugs 46 - 30 in the inside wall of the cellar 24. By applying tension to ': . ' . ,. ~
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'-' : , ' , . ~ -, ' -1 the cable the platform can be ~hifted in the manner described in detail in the above-identified patent. The positioning of the platform is described in detail'in d.s. Patent 3,871,184.
The transponders transmit a signal back to the receiver on the platform on receiving a signal from the transmitter.
Referring to FIG. 11, there is shown an arrangement by which the bottom 13 of the monopod platform can be sealed to the cellar 24 so that the interior of the cellar can be pumped dry and opened to atmosphere through the moon pool of the 0 platform. After the cellar 24 is imbedded in the ocean floor in the manner described above, cement 50 is squeezed between the outside of the cellar and the surrounding formation to anchor the cellar securely in place. The formation is then drilled into through the bottom of the cellar and the surface casing 52 is set and cemented in place, as indicated at 54.
The upper end of the surface casing opens into the interior of ; the cellar.
The upper edge of the cellar is provided with a com-pressible seal 56 made of rubber or other suitable compressible material. The top of the cellar is provided with a recess 57 around the top edge which retains 'the annular seal 56. The seal is compressed by the weight of the drilling platform, the bottom 13 pressing against the top of the seal 56. To insure that the platform remains securely in position and to compress the 'seal 56 to withstand the large hydrostatic pressures involved, a plurality of turnbuckles 58 are provided, the lower end of the turnbuckles being secured to the cellar by hooking into anchor plates 60 integrally formed with the cast cellar 24.
The upper end of the turnbuckles are hooked into anchor plates ~ 62 recesscd in the bottom 13 of the drilling platform. The '. ~ , '' . -.. ., , - . .

l ¦ turnbuckles 58 are t~ghtened to cinch the platform aqainst the ¦ seal 56.
: ¦ An alternative arrangement is shown in FIG. 12 in which 1 a seal 66 is in the form of an annular ring of sealing material 51 resting on top of annular metal ring 68. The seal 66 engages the bottom 13 of the drilling platform and the inside of the sidewalls of the cellar 24. A plurality of hydraulic jacks 70 resting on the bottom of the cellar 24 engage the annular metal ring 68, compressing the seal 66 between the ~ing 68 and the bottom 13 of the drilling platforni to form a seal. In the . arrangement of FIG. 12, the seal adjusts readily for misalign-: ment between the cellar 24 after it is implanted in the bottom of the drilling platform after it is brought to rest on the : ocean floor.
It will be seen that both the arrangementsof FIG. ll .~ and FIG. 12 permit the moon pool.and cellar to be used as a caisson, permitting access to wellheads located in the cellar j.
without reguiring the workers to operate in diving gear. ~ :

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Claims

I claim:

1. A drilling structure for drilling in the ocean floor, comprising an open precast vessel having a bottom wall and a side wall extending up from and around the perimeter of the bottom wall, the side wall terminating in a continuous edge at the top lying in a plane, the top edge of the wall forming an opening in the top of the vessel, a monopod drilling structure having a base and a supporting column extending upwardly from the base, the column having a vertical passage opening through the bottom of the base, the base having a flat bottom surface surrounding the opening of the vertical passage, the opening of the passage being substantially smaller than the opening in the top of the vessel, the vessel being adapted to be sub-merged in the ocean floor with the bottom of the platform base extending across the top edge of the vessel, and sealing means secured to the vessel adjacent the top edge of the vessel and frictionally engaging the bottom of the platform to form a water-tight intersection while permitting lateral movement of the platform relative to the vessel to position the vertical passage over any selected portion of the open vessel.

2. A structure as claimed in claim 1, also including means anchoring the drilling structure to the vessel.

3. A structure as claimed in claim 1, wherein the sealing means includes means mounted in the vessel for compressing the seal against the bottom of the drilling structure.

4. A structure as claimed in any one of claims 1 to 3, wherein the vessel is provided in the bottom wall thereof with downwardly directed fluid jets adapted for connection to a source of fluid under pressure and usable to remove material of the ocean floor from beneath the said bottom wall.

5. A structure as claimed in any one of claims 1 to 3, wherein the monopod drilling platform has an upper deck, the column supporting the deck from the base, the vertical passage forming at the bottom a moon pool through which a drill bit and drill stem can be lowered from the deck to the ocean floor, and including means for releasably attaching the base to the top of the vessel.