GB2319548A - An apparatus for use on a vessel for working on subsea wells - Google Patents

Abstract

An apparatus for use on a vessel for the maintenance of subsea completed wells and subsea systems for production of hydrocarbons, characterized in that it comprises a well workover rig 8 which is arranged to be mounted at a side of the vessel so that it projects at least partly outside the vessel side, and a guide means 43 arranged beneath the rig 8 for guiding of workover equipment and the like through the splash zone at the vessel side near the surface of the water.

Description

A vessel for use in the production of hydrocarbons from offshore fields, and an apparatus for use on such a vessel The present invention relates to a vessel for use in the production of hydrocarbons from offshore fields, comprising a downwardly open receiving space for receiving a submerged bottom-anchored buoy arranged for transferring hydrocarbons to the vessel from a number of production wells at the sea bed, and allowing turning of the vessel about the buoy. Further, the invention relates to an apparatus for use on a vessel for the maintenance of subsea completed vessels and subsea systems for the production of hydrocarbons.

A vessel of the above-mentioned type for example is shown and described in the international patent application No.

PCT/N093/00079 (WO 93/24731) (corresponds to Norwegian patent application No. 922043). This known vessel is based on cooperation with a submerged bottom-anchored buoy comprising a swivel unit with several pipe courses, and which further is of the type which is shown and described in the international patent application No. PCT/N092/00056 (WO 93/11033) (corresponds to Norwegian patent application No. 923817).

In connection with the production of hydrocarbons from wells at the sea bed it is necessary at intervals to undertake overhaul and/or control of the different wells. According to conventional practice such overhaul and control operations are carried out from a special vessel which is provided with a centrally arranged moonpool extending trough the vessel hull, and over which there is arranged a workover rig which is adapted to cooperate with different types of equipment for carrying out the necessary operations. As mentioned, the moonpool is arranged centrally in the vessel, with a view to achieving the least possible relative movements and optimum stability.

It is an object of the invention to provide a vessel of the introductorily stated type giving vertical access to the wells to be able to carry out cost-efficient easy well interventions from the production vessel itself.

A further object of the invention is to provide an apparatus for use on such a vessel.

The above-mentioned objects are achieved with a vessel of the introductorily stated type which, according to the invention, is characterized in that it comprises a well workover rig mounted at a side of the vessel so that it projects at least partly outside the side of the vessel, and that the rig and the receiving space have such a mutual distance in relation to the field arrangements at the sea bed that the rig, when turning the vessel about the buoy, can be placed over a selected well, so that maintenance work or the like on a well can be carried out at the same time as the vessel is coupled to the buoy.

An advantageous embodiment of the vessel is characterized in that the rig comprises a vertical framework tower having inner legs which are supported on the deck of the vessel, and outer legs which are supported by a carrier frame projecting from the vessel side and having a central opening for passage of pipe lengths and well maintenance equipment.

According to the invention there is also provided an apparatus of the introductorily stated type which, according to the invention, is characterized in that it comprises a well workover rig which is arranged to be mounted at a side of the vessel so as to project at least partly outside the vessel side, and a guiding means arranged below the rig for guiding of workover equipment and the like through the splash zone at the vessel side near the water surface.

By means of the arrangements according to the invention there is provided a vessel which, in a very cost-efficient manner, gives vertical access to the wells at the sea bed, so that well workover operations can be carried out simultaneously with on-going production from other wells. This is achieved by combining such elements as field arrangement and vessel position in relation to well locations, so that the production vessel by means of a dynamic positioning system can manoeuvre the hind part of the vessel into position above the wells. With the present invention one has also overcome existing prejudices against positioning a well workover rig at a place which is more exposed to hull and wave movements than a moonpool which is centrally placed within a vessel, and this is a presupposition for achieving the advantages of the arrangement according to the invention Easy well interventions are here defined as wire or coil-tube operations for reservoir monitoring or reservoir maintenance, as well as execution of pumping jobs, such as removal of rust or other deposits in tubings.

The invention will be further described below in connection with an exemplary embodiment with reference to the drawings, wherein Fig. 1 shows a perspective view of a vessel according to the invention, and its apparatus for well workover operations; Fig. 2 shows a schematic side view of the vessel in Fig. 1 wherein oil is produced from a manifold at the sea bed during simultaneous execution of well service, Fig. 3 shows a somewhat enlarged schematic plan view of the rear portion of the vessel in Fig. 2; Fig. 4 shows an isometric perspective view of the well workover apparatus with associated equipment in the topical deck area of the vessel; and Fig. 5 shows an isometric perspective view of the workover apparatus wherein this is connected through a riser system with a well associated with a template, as shown in Figs.

1 and 2.

The vessel 1 shown in Figs. 1 and 2 may be a production vessel of a type similar to that disclosed in the aforementioned international patent application No. PCT/N093/00079. The vessel comprises a forward superstructure 2 which is shown to comprise i.a. a wheelhouse 3, a helicopter deck 4 and a service crane 5, and a rearward superstructure 6 which, i.a., may comprise equipment for unloading of oil to e.g. a shuttle tanker.

In front of the rear superstructure there is arranged a flare tower 7 for burning-off of gas and for handling of waste of oil and gas developed from the process plant or in connection with well workover operations. In front of the rear superstructure and the flare tower there is an allocated deck area which is equipped for well intervention purposes, and wherein the well workover rig 8 according to the invention is arranged, as further described in connection with Figs. 4 and 5. Further, there are shown a pair of lifting cranes 9 and 10 of which the rearward crane 9 is intended i.a. for lifting and manipulating of heavy modules and units used in connection with the well workover operations, and partly for lifting and manipulating of portable modules for processing of oil. A number of such modules 11 are arranged in a deck area between the cranes 9 and 10. The cranes may have a lifting capacity of approximately 50 tons.

As appears schematically from Fig. 2, at the forward end of the vessel 1 there is arranged a submerged, downwardly open receiving space 12 for receiving an underwater buoy 13, and a service shaft 14 extends between the receiving space and the deck 15 of the vessel. The topical buoy 13 will be arranged to cooperate with a swivel unit (not shown) arranged at the lower end of the shaft, as further shown and described in the abovementioned international application No. POT/N093/00079. As described in said application, the buoy is anchored to the sea bed by means of a suitable anchor system (not shown in the drawings), so that the buoy also constitutes an anchoring buoy for the vessel at the same time as it allows of turning of the vessel about a vertical axis through the buoy. Reference is here made to the above-mentioned application for a further description of the buoy arrangement.

In the case shown in Fig. 2, oil is produced from a number of oil wells (not further shown) which are associated with one or more templates 16, the wells through flowlines (not further shown) being connected to a manifold 17 which is in turn connected to the buoy 13 through flexible risers 18.

The vessel further is provided with a dynamic positioning system which, i.a., is used for manoeuvring of the hind part of the vessel into position above the well of interest.

As regards the main elements used in connection with the well workover arrangement according to the invention, these may be summarized as follows (some of the elements are mentioned above): * Standardized workover modules which, i.a., comprise a derrick, a pipe handling system, a trolley for moving heavy equipment into and out of the derrick, skid mounted mud tanks for well fluid and skid-mounted dredge pumps.

* A lifting crane for lifting of modules and heavy workover tools from/to a supply vessel, and for movement of heavy equipment around the deck near the derrick.

* A riser system for providing a conduit or channel from the top of the Christmas tree up to the derrick floor.

* Workover tools, shop containers and test frames which are especially allocated to the workover type.

* An underwater buoy (turret system) for connection of flowlines and umbilical.

Some of the above-mentioned elements are shown in the schematic plan view in Fig. 3 wherein the above-mentioned allocated deck area which is equipped for well intervention purposes, comprises the deck of the vessel between a rear control room 20 and a pipe rack 21 which is located irk the forward part of the deck area and is arranged adjacent to the workover rig 8.

This deck area may have an area of ca. 900 m2 and has a sufficient deck strength and prepared adaptation units (interfaces) for receiving the standardized workover modules and workover tool packages.

A pipe handling means 22 is arranged next to the pipe rack 21 and is used for transfer of pipe lengths from the pipe rack and into the rig tower (derrick) wherein a hoisting means enables connection of pipe lengths. The pipe handling means has a replaceable griping head to allow handling of different types of pipes, and is to be able to handle pipe lengths 23 having a length of ca. 15 m and a maximum weight of ca. 2000 kg. Cable equipment for operations into the well comprises a winch 24 having a cable or wire 25 running over a travelling block 26 arranged in the rig tower as shown in Fig. 4. The additional units which are schematically shown in Fig. 3, i.a. comprises a control unit 27, a shop 28 and a storage container 29.

The construction of the well workover rig 8 will be further described with reference to Figs. 4 and 5. As shown, the rig comprises a vertical tower 30 which is formed from framework walls and four corner legs, wherein a pair of inner legs 31 are supported on the deck of the vessel and a pair of outer legs 32 are supported by a cantilevered carrier frame 33 projecting from a side of the vessel. The carrier frame forms a floor or deck which essentially comprises a pair of girders 34 projecting from the vessel side and at their outer ends being connected by a cross beam 35. Thus, the beams define a central opening 36 in the frame for passage of pipe lengths and well maintenance equipment.

As shown, the carrier frame is supported against the vessel side by means of inclined braces 37, and it is further provided with a safety railing 38. The two parallel beams 34 support a pair of parallel rails 39 constituting extensions of rails 40 arranged on the deck of the vessel, for guiding of a carriage 41 for the transport of well workover equipment (exemplified by a block 42) to and from the carrier frame opening.

At the vessel side below the tower 30 there is arranged a guiding means consisting of a pair of parallel rails 43 extending vertically along the vessel side, for guiding and sideways support of workover equipment and the like through the splash zone at the vessel side near the water surface. This means is essential for achieving a secure guiding of the topical equipment through the splash zone.

The crane tower or derrick 30 is to have a lifting capacity of 150 tons and to be able to handle efficiently riser lengths of 15 m or other types of pipes having lengths of up to 21 m. In the tower there is arranged a heave compensator 44 which typically has a maximum stroke length of 8 m. The tower is equipped with two pneumatically or hydraulically driven winches.

A hoisting winch 45 in connection with the heave compensator may be either electrically or hydraulically driven.

The rig further is provided with a 4-wire tension system comprising four tension cylinders 46, as schematically shown. This system is to be able to maintain a constant tractive force in the riser with sufficient extra traction for riser loads determined by water depths, and the floor opening 36 in the derrick is to allow launching of modules of e.g. 5 x 5 meters.

A wire system including four guide wires 47 provides guiding down to the sea bed when launching equipment or workover tools.

As mentioned above, the arrangement comprises a riser system 50 which also comprises a lower riser package (LRP) 51 and a Christmas tree running tool 52, and which - in case of well workover operations - is mounted between the well in question and the workover rig. The riser system is shown in assembled condition in Fig. 5 and extends between the derrick 30 and the well, and is mounted at the top of the Christmas tree 53 at the topical well which is to be overhauled and is located beneath the template 16. The riser system comprises a load pipe length 54 which is the lowermost pipe length in the system, standard riser lengths 55 of 15 m, possible short riser units (not shown) for distance adaptation, a tension pipe length 56 to which the four tension wires 57 in the above-mentioned tension system are hooked, and an adapter pipe length 58, At the top of the adapter pipe length 58 there may be mounted a surface Christmas tree 59, and the uppermost unit may be a wire lubricator or a coil tube injector head, dependent on the well intervention type.

When mounting the riser system 50, there is used a remotely operated vehicle (ROV) 60 which in Fig. 5 is connected to a so-called A-frame 61 via an umbilical 62. The A-frame is used for launching and letting-down of the ROV vehicle. In connection with the mentioned ROV operations there is used a control housing 63.

The execution of a well intervention with cables or coil tubes is carried out in accordance with definite procedures.

The presuppositions are here that the modules forming part of the well workover rig are tested, and that the ROV is prepared for operations. Further, all running tools which are used in the well, are tested and prepared. For safety reasons the workover rig (at the stern of the vessel) is now located approximately 30 m laterally displaced from the- template.

The procedure for a well intervention may be summarized in the following steps: 1. A shutter in the template is opened by means of the ROV and a wire winch.

2. Guide wires are mounted to the four guide posts in the template by means of the ROV.

3. The Christmas tree jacket is pulled up by means of a wire or a drill pipe and is placed back on the trolley to the test deck.

4. The lower riser package with connected Christmas tree running tool is run into the tower opening.

5. A load pipe length is mounted on the Christmas tree running tool.

6. The lower riser package is run through the splash zone and prevented from relative movements in relation to the vessel side by means of the guide means.

7. Running towards the sea bed is continued by jointing riser lengths together and using short riser units at the top as required for distance adaptation.

8. A tension pipe length is connected and tension wires are mounted thereto.

9. An adapter pipe length is mounted and the riser assembly is hung off in a suspension means.

10. A surface Christmas tree is mounted at the top of the adapter pipe length, and the entire riser assembly is lifted up as high as possible by the hoisting means in the tower.

11. The vessel is moved by the dynamic positioning system so that the derrick is located over the well in the template on which it is worked.

12. A cable lubricator is mounted at the top of the surface Christmas tree by means of a cable tool in the lubrica tor, and the cable is inserted into the pulley arrange ment.

13. The cable operation is carried out in accordance with the work description.

In case of coil tube operations the cable lubricator is replaced by a coil tube injector at the top of the surface Christmas tree. The coil tube injector then is mounted in a framework which can carry the weight of the whole riser assembly including the lower riser package. Accordingly, the lubricator is hung off in a hook in the travelling block by way of a wire arrangement.

Claims (3)

1. An apparatus for use on a vessel for the maintenance of subsea completed wells and subsea systems for production of hydrocarbons, characterized in that it comprises a well workover rig which is arranged to be mounted at a side of the vessel so that it projects at least partly outside the vessel side, and a guide means arranged beneath the rig for guiding of workover equipment and the like through the splash zone at the vessel side near the surface of the water.

2. An apparatus according to claim 1, characterized in that the rig comprises a vertical framework tower having inner legs which are supported on the deck of the vessel, and outer legs which are supported by a carrier frame projecting from the vessel side and having a central opening for passage of pipe lengths and well maintenance equipment.

3. An apparatus according to claim 2, characterized in that the carrier frame comprises a pair of girders projecting from the vessel side and at their outer ends being connected by means of a cross beam , and that the girders support a pair of rails constituting extensions of parallel rails on the deck of the vessel , for guiding of a carriage for the transport of well workover equipment to and from the carrier frame opening 4 An apparatus according to any of the claims 1-3, characterized in that the guide means consists of a pair of parallel rails extending vertically along the vessel side.