AU693550B2

AU693550B2 - A device for oil production at great depths at sea

Info

Publication number: AU693550B2
Application number: AU23760/95A
Authority: AU
Inventors: Havard Gulbrandsen; Stig Botker Rasmussen
Original assignee: Kvaerner Engineering AS
Current assignee: Kvaerner Oil and Gas AS
Priority date: 1994-04-15
Filing date: 1995-04-11
Publication date: 1998-07-02
Anticipated expiration: 2015-04-11
Also published as: NO941387D0; GB2302556A; GB2302556B; DK172680B1; BR9507456A; NO309079B1; WO1995028316A1; DK114796A; NO941387L; GB9621414D0; AU2376095A

Description

1 A DEVICE FOR OIL PRODUCTION AT GREAT DEPTHS AT SEA The invention concerns a device for oil production at sea.

There are previously known devices for use in connection with oil production which are composed of a combination of a semisubmersible platform and a relatively small tension leg platform. Such a structure is described, in US Patent No. 4,913,238. In this platform structure which is described in this patent a tension leg platform is terminated in a flotation unit which may either be located below the surface of the sea or above the surface, in an area which lies between the legs of the semisubmersible platform. A riser connection is then established between the tension leg platform's flotation unit or the upper part of the semisubmersible platform. In the case where the flotation unit is located below the surface of the water, a watertight barrier is provided to ensure water-free access to the upper part of the tension leg platform.

In US Patent No. 4,966,495 there is further described a floating drilling and production unit in which there is provided an upper part of a tension leg platform.

In this design too the top of the tension leg platform is located at surface level. In US Patent No. 4,702,321 there is described a drilling and production platform with a deep-floating flotation unit from which tension stays extend down to the sea bed 20 and where the floating part is secured by means of a radiating system of anchors.

None of these known structures, however, is intended for use at great depths and under rough weather conditions, nor will it be possible for the structure to be used under such conditions. A tension leg platform of a conventional type which extends over a water depth of 800 to 1000 m or S 25 more and under rough weather conditions will necessitate the use of a very heavy and very expensive tension leg system. It has further been found that this type of platform has its limitations. One of the difficulties, which is also partially encountered at normal water depths of 3-500 m, is problems in connection with the oscillations to which the tension stays are exposed, including natural frequencies. These will lead to fatigue loads on the stay and are the result, amongst other things, of movements in the water such as wave motion. These phenomena and fatigue loads will be reinforced at depths C:%WINVVMWUEUSSAIBRADOOEDUMmSOPA=760-5.DO I I I I 2 of the said magnitude of 800 to 1000 m under rough weather conditions and entail a limitation in the use of the tension leg platform under such conditions.

The use of flexible riser pipes will also be restricted at such great depths due to technical conditions which arise from the great hydrostatic pressure, amongst other factors. In any case, flexible risers suitable for this depth will be extremely expensive.

It is also referred to EP-A-0256177 in which a spar buoy construction having production and oil storage facilities is described. This is a single shaft construction which is anchored with a tension mooring or by catenary lines to the sea floor, with risers or drilling pipes respectively being guided along the center line of the structure from the sea floor to the deck through a flotation unit, whereby the risers are extending upwardly to the level of the structure. This spar buoy construction is a stiff construction, floating at sea level. Even if it is provided with an elongated submerged hull construction it will be influenced by rough weather conditions and will thus not fulfil the above mentioned requirements.

According to the invention there is provided an offshore platform for use in oil production or drilling at sea, said platform is anchored by a plurality of lines to the sea floor, with pipes extending from the sea floor to a deck of the platform via a flotation unit and are terminated in a manifold, a flexible pipe system extends 20 between the manifold and the deck of the platform, the pipe system being dimensioned to permit vertical movement between the deck and the flotation unit, wherein the platform includes a number of shafts which are connected to each other by means of cross elements which together with the shafts provide a :trusswork, and that the flotation unit includes a center element which is movable up and down and located in use in a position below a wave motion zone in the sea.

The design according to the invention permits the use of a tension leg structure even at such great depths, the movements of the tension leg structure and its oscillation frequency being controlled in such a manner that these are substantially suppressed. This is achieved by installing a tension leg C:%WNVVORDWELISSA BP--DWODELEIZSOPA=160-95.MC structure's part or flotation unit so far below the surface of the water that the entire tension leg structure is affected to as small an extent as possible by movements and disturbances in the water, these movements normally occurring in the uppermost layer. The flotation unit is tensioned either by means of the risers alone or in the structure with separate tension stays. The floating structure is equipped with a casing or the like which may be connected to a frame construction, for instance a trusswork which extends down to the level of the tensioned flotation unit, and which forms a guiding or sliding securing of the flotation unit. This results in the substantial stabilization of the tension leg structure in its vertical position, which enables tensioning by means of risers or by means of a simplified tension leg system, the loads on the tension leg unit thereby being reduced to a minimum. At the same time rigid risers can be employed which can extend upwardly through the flotation unit and right up to above the surface of the water until they are an a level with the deck of the floating structure. This means that the wellhead-manifolds which are placed at the top of the rigid risers will be located above the water surface, thus enabling coupling, uncoupling and other manoeuvres to be performed in. a. dry environment. A flexible riser part is then passed from the manifold over to the actual 20 platform with connections to the other production units. This transition could also be effected by means of rigid steel tubes which are articulated in special couplings.

The risers are arranged in this manner in an opening in the deck and will be S" easily accessible. At the same time this cavity in the deck can be used for installing a drilling rig, thus permitting drill pipes to be passed down through the tensioned part, and enabling the structure to be used both for drilling and "for production.

By designing the floating structure's lower part as a frame, for instance as a trusswork, a rigid and reliable protection and guidance of the tension leg 30 structure's flotation unit are obtained, while at the same time the rigid risers, projecting up from the flotation unit to the deck, are well protected, thus permitting them to stand or rise up freely. If so desired, these can be enveloped by a pipe member in order to provide further protection. It is not a condition that the lower part of the structure should be arranged as a trusswork. However, it is necessary to arrange a guidance system for the -r~crv:r ALuri ~IyI~IVLJC C~-Y flotation unit. This can be done by means of a trusswork system, amongst other solutions.

This deep-draught structure can also be employed for mounting a standard catenary mooring for t& floating structure, thus enabling the mooring point to be advantageously located in Its lower part. A catenary mooring of this kind is also an advantageous, inexpensive solution. In the event of wave movements and in strong wind the floating structure could drift out to the side, thus bending the tension stays about an angle of, e.g. 5-7.5 At a depth of e.g. 1400 m the consequence of this will be that the flotation unit will be pulled vertically downwards in its guide about a length of between 7 and 14 m. This displacement will only occur in one direction and there will be no problem in compensating for it by means of the flexible risers which have been extended from the wellhead- manifold at the top of the riser device. Thus it has surprisingly been shown that with a structural design according to the invention the tension leg structure part can be so well controlled that a simplified variant of this kind can also be used at very great depths, the loads which otherwise act on the tension stays being avoided, while at the same time achieving a dry and reliable transfer of the risers up to wellhead trees above the surface and with a flexible transition to 20 the actual production plant.

A further advantage of the design of the structure is the opportunity it offers to reduce the distance between the risers at deck level. The necessary distance between the risers, e.g. for 1400 m, will normally be 5 m or more in order to avoid collision. For 300-500 m this spacing will normally be 2.5 to 3 m.

0 By arranging the risers through a flotation unit which is well submerged, it will therefore be possible to maintain, e.g. a 5 m spacing between the risers at the bottom of the flotation unit, while the distance between the risers at .deck level can be less, e.g. 2.5 m. This allows a substantial saving to be made "on the deck, since the structural design of the deck becomes simpler with the consequent savings in steel weight and costs.

In this case such a splay of the risers will normally be in the range of 1-2 degrees. This will not entail any major problems for the risers or the drilling zA L operation, but will provide substantial benefits for the arrangement of the Qlli manifold area and related deck solution.

1 Ir -1 The invention will now be described in more detail by means of an embodiment which is illustrated schematically in the drawing, in which: fig. 1 is a side view of a device according to the invention used in a platform structure, and fig. 2 is a perspective view illustrating the upper part of this device according to the invention.

The drawings illustrate a device according to the invention, and it is as an embodiment shown a semisubmersible platform structure which is composed of a lower tension leg platform part 1 and an upper semisubmersible platform structure which is generally indicated by 2. The platform part, i.e. the deck is indicated by 9, while the shaft part or lower part is indicated by 8. The lower part is designed here as a framework, but can, e.g. in the case of a concrete structure, also be in a different form. The tension leg platform is attached to the sea floor and the risers which extend up to the platform part's flotation unit 5 extend from a riser frame 3 on the sea bed. The risers and possibly the tension stays are generally and jointly indicated by reference numeral 4. The.

tension leg platform's flotation unit 5 is mounted so far below the surface of the water that it is affected to the least possible extent by movements in the water. Above the flotation unit the risers extend further as rigid.elements 6 up to above the surface and are terminated there in a Christmas tree device 7. If so desired, the Christmas tree device can also be combined with a manifold.

From the Christmas tree device flexible risers lead to a connection point on the actual production platform deck 9. The length and the arrangement of the flexible risers are provided in such a length and in such a manner that they will be capable of absorbing displacements of the flotation unit in relation to the floating platform. Above the cavity in the platform deck 9 where the risers are terminated, drilling facilities 10 can be provided to enable drilling to be performed down through the platform to the sea bed. This is only illustrated in a purely schematic manner by indicating a drilling tower The floating platform is secured by means of a catenary mooring line which is generally indicated by 12.

In fig. 2 the upper part of the device according to the invention is illustrated Sin more detail. Here the floating platform structure 2 can be seen with the platform deck 9 and the shafts or the submerged structural part which i comprises vertical columns 14 with cross elements 15 and 16. These elements 14, 15 and 16 together form a trusswork part which, apart from providing the necessary buoyancy, is intended to protect the upper part of the tension leg platform and the upwardly projecting riser pipes 6. The submerged part of the platform is made so long that it projects down to the still water level where the flotation unit 5 is located. Tlhe flotation unit 5 is preferably provided in a guiding casing 17 which causes the flotation unit to be able to perform only vertical movements. It has been shown that under the influence of wind and weather the platform will essentially only move out to the side and this means that the flotation unit 5 will substantially move in a downward direction, and thereby will not be forced upward to a height above the structural level obtained with a purely vertical positioning of the tension leg platform. This means that the rigid risers 6 can be continued up to the level of the platform deck 9 where there can be provided wellhead Christmas trees, and possibly manifolds for further transmission of the products. Here in this dry location there are connected flexible risers which lead to production facilities on the declk 9. These structural parts are not illustrated in more detail. The catenary mooring line of the semisubmersible platform can be mounted parallel to the platform leg 14 in the upper part, with the result that the catenary line starts from an anchor point 13 which can be located at a brace in the trusswork.

The illustrated design with a trusswork lower structure on the semisubmersible platform results in the pattern of movement for the related tension leg platform part being controlled to such an extent that it is possible to have a simplified tensioning of the structure, which is simultaneously kept as still as possible, thus keeping material fatigue and all the problems which that entails to a minimum. Thus there is provided by simple means a production and drilling platform which can be used both at normal depths, but above all is also intended for use at great depths such as 1000-2000 m, and where the platform can also have space for storing oil products. The device can, of course, also be employed at water depths less than 600 m.

Even if the invention has been described above with reference to a semisubmersible platform structure, it is clear that the inventive idea also may be used for other types of vessels, for instance ships. Also such ctures are regarded as being within the scope of the invention.

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Claims

2. A platform according to claim 1, wherein the manifold is provided on a supporting frame which is mounted in an opening in the deck.
3. A platform according to claim 1 or claim 2, wherein the flotation unit, the pipes, the pipe system and trusswork are designed to permit a drill pipe from a drilling tower to be passed there through. S4. A platform according to any one of the preceding claims, wherein the pipes extend between 600 and 2000 m.
5. A platform according to any one of the preceding claims, wherein the pipes 25 adjacent the lines are located at a greater distance from each other than the pipe system adjacent the deck.
6. A platform for use in oil production or drilling substantially as herein before -:i3 described with reference to the illustrations. i30 DATED: 24 March 1998 PHILLIPS ORMONDE FITZPATRICK Attorneys for: SKVAERNER ENGINEERING A.S. C'%W NWOOEU SSABRNODELEEOPAl23O-9S.DDO I