GB2153747A - An offshore production vessel - Google Patents

Abstract

The riser (18) from the well-head, together with the associated tensioning devices (21, 23) is mounted at a bridge (19), which is displaceable along horizontal guides (20) out from and into, respectively, the operating range of a conventional derrick (15). With the bridge (19) in an alternative location, other operations can be performed by the derrick (15), for instance template well workover, drilling or recompletion while the production continues undisturbed, by way of the offset production riser (18) which is connected to hydrocarbon processing facilities in the vessel by way of flexible pipes (24). <IMAGE>

Description

SPECIFICATION An offshore production vessel Offshore oil production can be managed by equipment of different kinds. When the depth of the water exceeds a certain value it is difficult to use plants resting upon the bottom of the sea. Instead various types of vessels positioned above the oil well are used.

Usually one vessel is used for prospecting and drilling, as well as for arranging the necessary wellhead at the sea bottom. A production vessel is then brought to the proper position and connected to the wellhead, whereupon production starts. Very often a number of wells are located close by each other, and the stationary equipment at the bottom will then be formed as a manifold attachable to a number of risers.

A group of risers can be formed in various ways.

Usually the production vessel does not have any capacity for storing oil. There is a processing plant, which separates gas and water from the oil. This is pumped to a tank on the sea bottom, or directly to a waiting tanker.

Centrally in the riser groups there is a return flow pipe for the oil produced, which is, somewhat deceptively, called "export riser". Around the latter a number of "production risers" are grouped, as well as one or more "service risers". The latter can i.a. be used to supply pressure fluid to various servomotors at the manifold, or for injecting water or gas into wells.

A central "export riser" is usually of strong design, and carries the "production" and "service" risers grouped around itself.

When the production has run for some time it may be necessary, or desirable, to perform some work at one of the wells - for instance to clear, or to extend a hole, to recomplete the template or the manifold, or to check the pressure, or the like.

Normally the group of risers is served by the derrick of the vessel, and the location of the group will then be determined by the position of the derrick. Any work requiring handling of equipment by the derrick makes it necessary to disconnect the groups of risers if the vessel does not have a double set of derricks.

Due to unavoidable heavy movements of the vessel - caused by waves - the group of risers is suspended in tensioning devices, which ensure that the risers will not slacken, or be subjected to high tensioning forces. These tensioning devices make the location of the group of risers important, and normally the centerline of the group is aligned with the centerline of the derrick.

The object of the present invention is to provide an arrangement in which the mounting of the group of risers will be independent of the position of the derrick, and makes possible alternate operation by, or at the latter, without any need to interrupt the production of oil.

The invention will further make it possible, for instance to drill a new hole from a production vessel, without interruption of the oil production.

The invention thus refers to an offshore vessel adapted to be positioned above a production wellhead at the bottom of the sea and to maintain contact therewith by way of at least one riser, which in a manner known per se is biased by tensioning devices, said vessel furthermore having a derrick for performing service operations. The invention is characterised in that the riser with the associated tensioning devices is supported by a bridge, which is displaceable along horizontal guides out from, and into, respectively, the operating area beneath the derrick.

The operating range of the derrick preferably includes a so called moon-pool, which is extended in the direction of the guides and comprises a narrow portion below the derrick and a broader portion below a alternative position of the bridge.

The invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically shows a portion of a semi-submersible offshore vessel, Figure 2 shows a section through the moonpool, perpendicularly to the view in Figure 1, Figure 3 is a plan view of the moon-pool and the surrounding parts of the deck, and Figure 4 schematically shows a perspective view of the tensioning devices.

Figure 1 schematically shows a portion of a semi-submersible offshore vessel, comprising an operating platform 10, which is subdivided by tween-decks 11, to provide accomodation for the crew, for workshops, stores and the like. The platform is carried by columns 12 from underwater pontoons 13. Those are in a conventional manner subdivided into tanks and provided with pumps for transferring ballast water, so the draft of the vessel can be changed. In Figure 1 the operating water level is indicated by 14. During a voyage the ballast tanks are emptied, so the decks of the pontoons 13 will be raised about to water level.

The vessel may be provided with a propulsion machinery (not shown), preferably including azimuth propellers, which may be used for positioning the vessel above a selected well.

The vessel may be held in position by a number of anchors, or by so called dynamic positioning.

The vessel is provided with a derrick - of conventional type 15 only the lower portion thereof being shown in the drawings. The derrick is mounted upon a superstructure 16.

A well, a so called moon-pool 17 is provided in the platform 10. Through this well work may be performed from the derrick 15, divers and equipment may be sent down, and so forth.

Within the moon-pool a riser group 18 from the connection at the bottom of the sea is also received. This group comprises a central "export riser" 18a, as well as a number of individual risers, production risers 18b and/or service risers.

The central, "export" riser 18a is a strong structure, and carries the surrounding production and service risers. The group is suspended in tensioning devices, which according to the invention are mounted at a bridge 19, displaceable along horizontal guides 20 above the moon-pool to be positioned either below the derrick 15, or outside the operating area of the latter.

The design of a type of tensioning device is schematically illustrated in Figure 4, which shows the bridge 19 and parts of the guides 20.

The bridge 19 carries four co-operating tensioning devices, which by means of wires 21 are connected to an upper riser assembly 22 for the riser group 18.

Each wire 21 is attached to the piston rod of a piston operating in a pressure vessel 23. These vessels may contain gas or oil, which elastically take care of changes in the load upon the wires 21, including possible slacking and tensioning.

The design is very schematically illustrated in order to indicate the function, - in practical embodiments there are multiple pulley blocks permitting a considerable length of wire, and the pressure vessels may be connected to a source of pressure fluid (not shown). This source will, activated by sensors, supply pressure fluid to either side of the pistons in the various pressure vessels 23 to take care of sudden changes in the load.

The guides 20 are shown as simple channel beams, and a practical embodiment will include means facilitating the movement, as well as for locking the bridge in the selected position.

The essential feature is that the movable bridge makes it possible to displace the group 18 of risers away from the derrick 15 without the tensioning devices 21, 23 being detached, and without the production being interrupted. The individual production risers 18b are connected to the hydrocarbon processing facility within the vessel by means of flexible pipes 24. These have sufficient length to permit the bridge 19 being displaced along the guides, without any need for disconnection. The "export" riser 18a is likewise connected to the processing facility by means of a flexible hose 29.

Figure 2 shows the bridge 19 with the riser group 18 suspended in the wires 21 of the tensioning devices directly below the center line 25 of the derrick 15, which is the normal operating position.

The center line of the vessel is denoted by 31 (Figure 1).

In Figure 1 the bridge 19, together with the suspended riser group 18 is moved to an alternative position, outside the operating area of the derrick, to a position at the opposite side of the vessel center line 31.

Within the operating area of the derrick there are now mounted two guide lines 26 attached to telescoping beams 27, projecting above the moon-pool 17. The lower ends of the guide lines 26 are in the usual manner connected to the component at the bottom of the sea, where work is to be performed and equipment will have to be sent down, or removed.

Drilling and several other operations may make it desirable to apply tensioning devices to the extra equipment, and a substitute bridge 28, provided with tensioning devices is mounted in the guides 20.

Signals to the equipment, for instance valves at the manifold at the bottom of the sea are in a conventional manner transferred by way of an "umbilical" line 30.

The moon-pool 17 has an elongate shape, with a narrow portion 17a in the operating area of the derrick 15. The other portion 17b of the moon-pool, which is mainly used during an alternative position of the bridge 19 is broader than the first-mentioned portion 17a. Here equipment will have to be sent down and retrieved, respectively, and it is desirable that the conduits and lines do not reach contact with the walls of the moon-pool.

The embodiment described above and shown in the drawings is an example of the invention only, the details of which may be varied in many ways within the scope of the appended claims. The size and the shape of the vessel may vary, the essential feature being the sidewardly displaceable bridge, which is equipped in such a manner that the production can continue without the derrick being blocked in the riser group, which temporarily can be shifted away from the derrick.

The expression "well-head" is here used to cover any single or multiple connecting means at the bottom of the sea.

Claims (3)

1. An offshore vessel adapted to be positioned above a production well-head at the bottom of the sea and to maintain contact therewith by way of at least one riser, which in a manner known per se is biased by tensioning devices, said vessel furthermore having a derrick for performing service operations, characterised in that the riser with the associated tensioning devices is supported by a bridge, which is displaceable along horizontal guides out from, and into, respectively, the operating area beneath the derrick.

2. An offshore vessel as claimed in claim 1, characterised in that the operating range of the derrick includes a so called moon-pool, which is extended in the direction of the guides and comprises a narrow portion below the derrick and a broader portion below a alternatiye position of the bridge.

3. An offshore vessel substantially as hereinbefore described with reference to and as shown in the accompanying drawings.