WO2008010726A1

WO2008010726A1 - System and vessel hydrocarbon production and method for intervention on subsea equipment

Info

Publication number: WO2008010726A1
Application number: PCT/NO2007/000273
Authority: WO
Inventors: Atle Ingebrigtsen; Ole Gams Steine
Original assignee: Framo Engineering AS
Current assignee: Framo Engineering AS
Priority date: 2006-07-19
Filing date: 2007-07-19
Publication date: 2008-01-24
Anticipated expiration: 2009-01-19
Also published as: AU2007275960B2; AU2007275960A1; BRPI0714301A2; NO20063352L; EP2047063A1; US20100025043A1; NO326575B1

Abstract

A system for operation and service of at least one hydrocarbon-producing well (24) and of equipment (27, 35), which is disposed under water, for further transport of the well stream, where the system comprises - at least one pipeline (32), - at least one booster unit (27), - a vessel (10) comprising - an anchoring system about which the vessel can rotate or swivel, - means (60, 61, 62) for conducting an intervention on subsea equipment, - means for communication with and control of the at least one well (24) and other equipment (35), - means for injecting chemicals into the well streams (25, 32), - means (73) for transferring to and inserting pigs in the pipeline (32), - means (14, 15) for production of energy for operation of subsea equipment, - means (22) for transferring energy from the means (14, 15) for production of energy to the booster unit (27) and any other subsea equipment.

Description

System and vessel for hydrocarbon production and method for intervention on subsea equipment

The present patent application relates to a system for operation and service of hydrocarbon-producing wells, a vessel for use in operation and maintenance of hydrocarbon-producing wells and a method for intervention on subsea equipment which is in use in connection with production and further transport of a well stream.

The principle is known of producing oil and gas from a ship lying anchored/DP- positioned over a subsea field of hydrocarbons. Until now the main task of this ship has been to receive, process and store the well stream. The unprocessed well stream is processed and electric power and other energy required for running the process is generated on board the ship. In recent years systems have been developed where parts of the processing equipment are located on the seabed, for example multiphase pumps and water injection pumps. In this case, electric power is passed in a riser/umbilical system from the ship down to the bottom-mounted equipment. Where environmentally vulnerable areas are involved, it is desirable to reduce the environmental exposure to a minimum during the production of oil and gas. Concepts are therefore being considered which are based on transportation of unprocessed well stream in pipelines from the field to a land-based plant or processing plant on other adjacent platforms or floating platforms for further processing. The principal challenge with concepts of this type appears to be the actual pipe transport (flow assurance problems) and operation of pressure booster equipment such as multiphase pumps and wet gas compressors with associated electric power system. When the field and the land-based plant are at a great distance apart, offshore power generation seems to be the most attractive solution. From the prior art, in US 6, 2363,971 Bl a system is disclosed for subsea production of hydrocarbons where an intermediate platform is arranged between the seabed and the water surface and a floating platform on the surface. The well stream is taken up to the intermediate platform on which are mounted various pieces of equipment including pumping equipment and equipment for measuring volume flow. Gas from the produced well stream can be separated and taken up to the floating platform where it is used for production of electrical energy, while a riser connected to the intermediate platform transports the rest of the well stream to a processing destination near the hydrocarbon field.

In GB 2 292 760 A a production vessel is disclosed, the aft end of which is provided with a device for receiving a buoy, and round which the vessel can pivot, the vessel being anchored to the bottom. The risers transporting the well stream are connected to the buoy. At the other end of the vessel a rig is provided for use in maintenance of the production well. US 6,364,021 Bl discloses a system comprising a riser from the wellhead to a floating buoy on the ocean surface. The system further comprises a vessel which can go alongside the buoy and conduct predetermined maintenance operations.

The proposed system seeks to provide an integrated system for production of hydrocarbons from subsea wells which will have a number of advantages when compared with the prior art.

Since the hydrocarbons will not be processed on the vessel, the normal processing equipment will be superfluous, and this results in a number of advantages. Since the vessel will not be provided with processing equipment, the usual "rules and regulations" applying to production vessels are avoided, thereby making the vessel cheaper to build. When the vessel has no processing equipment on board, space is also made available for storing, for example, equipment for use in intervention on subsea equipment, spare parts, pigs and equipment for launching pigs, chemicals for injecting into the well stream, fuel for energy production, etc. The fact that the well stream is not taken up to the vessel for processing also reduces the environmental risk associated with production, an aspect of offshore production of hydrocarbons which will become increasingly important in the future. Another advantage of the system is the possibility it offers of simultaneous intervention and production, which may be important for maintaining flow and temperature in the transport pipe. An object of the invention, therefore, is to provide a system for offshore production of hydrocarbons where the risk of accidents resulting in environmental damage is reduced.

Another object of the invention is to provide a system which permits the combination of power generation and intervention on subsea installations. A further object of the present invention is to provide a system for offshore production of hydrocarbons with improved assurance of uninterrupted production of hydrocarbons, and particularly assurance that the transport system for the hydrocarbons is in continuous operation.

It is also an object of the invention to provide a system which results in simplified operation and maintenance of subsea equipment, such as well, wellhead, pressure booster equipment and other equipment disposed below the surface of the water.

This is achieved by means of a system according to claim 1, a vessel according to claim 16 and a method according to claim 22. Further preferred embodiments of the invention are indicated in the dependent claims. A system is provided for operation and service of at least one hydrocarbon- producing well, and of equipment for further transport of the well stream and any equipment required for treatment of the well stream. The equipment and the at least one well are disposed below a water surface, preferably on a seabed or an ocean floor. The system further comprises at least one pipeline for transport of the well stream to the desired destination, and at least one pressure booster unit for driving the well stream through the at least one pipeline, a vessel for operation and service of the at least one well and the equipment disposed below the water surface, where the vessel comprises means for controlling operating and service functions, together with means for conducting an intervention on equipment disposed below the water surface. The system further comprises means for production of energy for operation of the equipment disposed below the water surface, as well as means for transferring energy from the means for production of energy to at least the at least one pressure booster unit which drives the well stream of hydrocarbons through the at least one pipeline and means for communication with and control of the at least one well and the equipment disposed below the water surface. The system may also comprise two or more hydrocarbon-producing wells. In this case a pressure booster unit may be mounted at each well or the system may be provided with a pressure booster station where the well streams from the hydrocarbon-producing wells are gathered and pressurised. From the pressure booster station at least one pipeline is provided for further transport of the well streams to the desired destination. In a preferred embodiment the vessel is provided with an anchoring system comprising a mooring system, a dynamic positioning system or a combination of the two.

The anchoring system will advantageously be arranged in such a manner that the vessel can rotate about the anchor point on the vessel or swivel around the anchoring system when the vessel is anchored via one or more mooring lines to an anchored buoy. The buoy may be arranged in such a manner that it is lying on the surface of the water or it may be arranged so that it is located between the surface and the bottom. The vessel will then be able to pivot round the buoy. It will also be possible to fasten the vessel directly to an anchor on the seabed or ocean floor, in which case the vessel will be able to pivot round this point. When the vessel is anchored to a buoy, risers or umbilicals may be arranged in such a manner that they either extend from the vessel to the buoy and from there on to a swivel device at the anchor on the bottom, or they may extend directly from the vessel down to the swivel device at the anchor on the bottom. When one or more adjacent wells are developed, it will be advantageous to place essential equipment, which has to be disposed below the water surface, under or within the curve described by the vessel's outer point as the vessel rotates about or swivels around the anchoring system, i.e. the vessel rotates about an anchor point in the vessel or pivots around a buoy to which the vessel is anchored. This will make it easy to position the vessel when maintenance or repairs have to be carried out on equipment located below the surface of the water, and preferably on the seabed or ocean floor.

The vessel is advantageously arranged so that it can be disconnected from its anchoring system and riser and umbilical system for positioning over the intervention object on the seabed or ocean floor if this object is not located within the above-mentioned curve. This will also be advantageous in bad weather when it may be necessary to perform a rapid disconnection of the anchoring system and riser and umbilical system.

The means for production of energy for operation of equipment disposed below the water surface may either be mounted in its entirety on the vessel or in its entirety below the water surface. Parts of the means for energy production may possibly be mounted on the vessel, while the rest of the means are mounted below the water surface.

In order to provide energy, the system may be provided with means below the surface of the water, preferably on the seabed or ocean floor in connection with the well or the at least one pipeline, in order to remove a part of the gas in the well stream together with means for transporting the gas up to the vessel. The transport of gas up to the platform may be carried out by means of a separate pipeline or through an umbilical, which is also used for other purposes in connection with the operation of the well.

One of the advantages of not taking the well stream of hydrocarbons up to the vessel for processing is that the vessel is not encumbered with all the processing equipment with which the production vessels are normally equipped today. This absence of processing equipment for processing the well stream makes a great deal of space available on the vessel. The vessel is therefore provided with storage capacity for necessary contributory factors in connection with operation and maintenance of the system, such as chemicals for injecting into the well stream, pigs and equipment for launching and receiving pigs, spare parts and equipment for use in intervention on equipment below the water surface and possibly fuel required for energy production.

In order to ensure that the stream of hydrocarbons in the at least one pipeline does not come to a stop, the system may be provided with means for transferring chemicals from the vessel to the at least one pipeline. These are chemicals which are injected in order to prevent the formation of wax and hydrate in the pipes and they can be transferred from the vessel by means of a separate pipeline or through an umbilical which is also used for other purposes in connection with the operation of the well or wells. The system is also provided with necessary equipment such as valves for injecting the chemicals into the well stream in the pipeline. The system will also advantageously comprise means for launching and receiving pigs in the at least one pipeline and means for transferring pigs down from the vessel to the at least one pipeline or from the at least one pipeline to the vessel. These may be pigs for scraping and cleaning the at least one pipeline in order to avoid shutdown or they may be intelligent pigs that are passed through the at least one pipeline for inspection of the pipeline's integrity.

The system may also advantageously comprise heating means for heating the at least one pipeline as an additional preventive measure against stoppage of the production of hydrocarbons, as well as means for transferring energy from the vessel required for the heating means provided in connection with the at least one pipeline. The energy transferred from the vessel to the at least one pipeline will preferably be electrical energy, and the means for transferring the energy from the vessel to the at least one pipeline will therefore be an electric cable. This may be a separate cable or it may form a part of an umbilical. These heating means for the at least one pipeline may extend over a shorter or longer distance along the pipeline which transports the hydrocarbons to the desired destination where processing will take place. The heating means may also be arranged along the entire length of the pipeline.

In addition to the equipment already mentioned above, the system may also include additional equipment for treatment of the well stream, disposed below the surface of the water, and particularly on the seabed or ocean floor. This may include equipment for mixing, desanding and water separation.

Furthermore, it will be necessary to monitor the production of hydrocarbons and the rest of the system. The system will therefore advantageously include means for communicating production and control data from the at least one well and other equipment, which is disposed below the water surface, to the vessel. Similarly, the system will advantageously include means for transferring control signals from the vessel to the at least one well and other equipment disposed below the surface of the water. Such signals will normally be transmitted through an umbilical containing both electric cables for electrical signals and pipes for hydraulic or pneumatic signals.

A vessel is also provided for use in connection with operation and maintenance of a system for production of a well stream containing hydrocarbons from at least one well, where the at least one well and the equipment are disposed below a water surface, particularly on a seabed or an ocean floor. The vessel is provided with a system for anchoring, e.g. a mooring system, a dynamic positioning system or a combination of the two. Furthermore, the vessel may advantageously comprise means for production of energy for operation of equipment disposed below the water surface, and also possibly for heating at least one pipeline disposed below the water surface.

Moreover, the vessel may advantageously comprise means for transferring chemicals to at least one pipeline disposed below the water surface. The chemicals are added to the well stream in order to prevent hydrate formation and they can be transferred from the vessel to the at least one pipeline through a separate pipeline or through an umbilical which can also be used for other purposes in connection with the production of the hydrocarbons. Moreover, the vessel may advantageously comprise storage space for storing the chemicals which are to be injected into the pipeline as explained above.

Furthermore, the vessel may advantageously comprise means for carrying out an intervention on equipment disposed below the water surface. These means include tool systems which can be lowered through an opening in the vessel or from a lifting arrangement which is normally located at the after end of the vessel. The anchor point and location of the equipment for lowering and raising the tool systems will normally be located in different parts of the vessel so that the vessel covers a greater area when it rotates or pivots round the anchor. The tool system used in a given situation will be dependent on what kind of equipment is to be worked on and which operations have to be performed on the equipment.

Furthermore, the vessel may advantageously comprise storage space for storing tool systems and equipment used during intervention on equipment below the water surface and spare parts for said equipment below the water surface. In particular this will be parts for equipment which is critical for the production of hydrocarbons and for ensuring that the well stream can flow uninterruptedly through the at least one pipeline.

Furthermore, the vessel may advantageously comprise means for transferring pigs to a pipeline disposed below the water surface. A pig is transferred from the vessel to a subsea receiving station either in a tool which is lowered and connected to the at least one pipeline or in a flexible transfer riser which may be secured to the at least one pipeline's pig launching system which can be hoisted up to the vessel as required. The pig is installed in a controlled environment on the vessel and passed through the flexible riser in a controlled manner down to the pig receiving system. It is expedient to pass both sophisticated instrument pigs through the pipe in order to monitor the pipe's integrity and simpler scraper pigs to remove wax, sand, water, etc. that have been deposited in the pipe.

Furthermore, the vessel may advantageously comprise storage space for storing pigs and equipment for transferring pigs to the pipeline. The anchoring system may be arranged in such a manner that the vessel rotates about an anchor body on the vessel. Anchor lines extend from the anchor body for anchoring the anchor body and thereby also the vessel. Alternatively, the vessel may swivel around the anchoring system by the vessel being anchored via a mooring line to an anchored buoy. The vessel will then be able to pivot round the buoy. It will also be possible to fasten the vessel directly to an anchor on the seabed or ocean floor, in which case the vessel will be able to pivot round this point.

The vessel will advantageously be arranged in such a manner that it can be disconnected from the anchor as well as communication line(s), umbilical(s) and other devices connecting the vessel to the at least one well and the equipment disposed below the surface of the water. As mentioned above, this will be particularly advantageous where bad weather, possibly combined with a mishap or an accident, make it necessary for the said equipment to be disconnected from the vessel. The means for producing energy on board the vessel may consist of at least one generator set. This may typically comprise a diesel engine or a gas turbine driving an electric generator. In this case the vessel will advantageously also include storage space for necessary fuel such as diesel or gas for operation of the at least one generator set. The vessel may advantageously be provided with two or more generator sets so that the vessel has at least one spare set in case a set breaks down.

The means for producing energy for operation of the at least one well and equipment disposed below the water surface may also comprise a small processing plant. In this case an appropriate amount of gas will be removed from the well stream and transferred to the vessel for processing. The vessel will then also be provided with at least one generator set for production of energy from the processed gas. As in the above case, the vessel will advantageously be provided with two or more generator sets in case operational problems should arise with one of the sets.

The vessel is furthermore advantageously provided with means for transferring the produced energy from the vessel to subsea equipment and possibly the at least one pipeline. This may involve a separate electric cable or an electric cable which is a part of an umbilical. If the energy transferred is in the form of hydraulic or pneumatic energy, these means could be a separate pipeline or a pipeline which is a part of an umbilical.

As mentioned earlier, parts of or all the equipment for energy production may be provided below the water surface instead of on the vessel.

A method is also provided for intervention on equipment in use in connection with production of hydrocarbons from at least one well and equipment for further transport of the well stream, where said equipment is disposed below a water surface, particularly on a seabed or ocean floor. The intervention is conducted from a vessel which is specially arranged for operation and maintenance of equipment disposed below the water surface. The vessel is provided with a system for anchoring, e.g. a mooring system, a dynamic positioning system or a combination of the two, where the anchoring system is arranged in such a manner that the vessel can rotate about or swivel round the anchoring system. The vessel is further provided with means for conducting an intervention on one or more intervention objects disposed below the water surface and preferably on the seabed or ocean floor, and means for lowering intervention equipment and possibly spare parts down to the intervention object or intervention objects. The method for conducting the intervention consists of the following steps: o the operating and service vessel is positioned in such a manner that means for lowering intervention equipment and any spare parts are located substantially above the intervention object, o the operating and service vessel may be disconnected from its anchoring system as well as communication line(s), umbilical(s) and other devices connecting the vessel to one or more wells and equipment disposed below the water surface, thus enabling it to be positioned over the intervention object, o the intervention equipment and any spare parts are lowered down to the intervention object, o necessary service or repair is carried out on the intervention object, o if necessary, both service and repair are carried out on the intervention object, o the intervention equipment is lifted up to the operating and service vessel.

When the operating and service vessel is positioned for lowering necessary tool systems and any spare parts for carrying out an intervention, underwater currents, for example, may make it necessary to position the vessel slightly to the side instead of being located vertically directly above the intervention object. The term "substantially above" should therefore be understood to mean that the vessel may be located slightly to the side relative to a vertical line from the intervention object if so dictated by the existing conditions.

An embodiment of the invention will now be described with reference to the figures, in which: Figure 1 illustrates the system with some of its elements,

Figures 2-4 illustrate alternative methods of anchoring the vessel, Figure 5 illustrates an intervention situation on equipment located below the surface of the water,

Figure 6 illustrates an example of how equipment for launching pigs can be arranged. Figure 1 is a general schematic view including most of the system's main components. A vessel 10 or other floating structure is kept in position by means of an anchoring system where anchor lines 1 1 are attached to a lower part of the swivel system 13 mounted on the vessel 10, thus enabling the vessel to rotate about the anchoring system. At their other end the anchor lines are secured by the anchors 12. The vessel or floating structure 10 may also be anchored by alternative means, which will be explained below.

The ship is positioned in a suitable position relative to a subsea production system for oil and gas. The production system may be organised with one or more wells 24 with templates 34 with at least one integrated pressure booster unit 27 on each template or the well streams 25 may be collected in a central pressure booster station 35, where the pressure boosting is provided by at least one pressure booster unit 27, comprising one or more pumps or compressors. If necessary, several such pressure booster stations 35 may be provided. At the front edge of the at least one pressure booster unit 27, pre-processing means 26 may be provided for pre- processing the well stream and in the same way at the after edge of the at least one pressure booster unit 27, post-processing means 28 may be provided for postprocessing the well stream. These pre- and post-processing means may, for example, be mixers, desanding equipment, water separation equipment and chemical injection equipment. The bottom-mounted equipment communicates with the ship through one or more risers 20 or umbilicals 21.

The vessel 10 is provided with at least one suitable power generating unit 14 which produces electric power and delivers it through the vessel's swivel system 13 and on through an electrical umbilical 22 to a termination point 30. If necessary, several electrical umbilicals 22 may be installed. At least one power regulating unit 15 may either be mounted on the vessel in its entirety, or the whole or parts of this at least one unit 15 may be mounted underwater in connection with associated power consumers such as pumps and compressors.

For operation of the vessel's at least one power generating unit 14, two possible solutions are suggested. The vessel 10 may carry the necessary fuel, such as, e.g., diesel, stored in separate fuel tanks 16 on the vessel. These tanks 16 may be refilled from other ships through a conventional transfer system. Alternatively, a small portion of the process flow, removed from the gas phase, may be passed up to the vessel 10 through a suitable riser 23. A compact little processing plant 36 pre- processes this gas flow so that it can be used as fuel in the power generating unit 14. A processing plant of this kind may also be provided under water, preferably on the seabed. Parts of the processing plant may also be provided on the vessel while the rest is mounted under water. In order to ensure regularity and fuel during start-up and shutdown phases, it may also be expedient to have a compact little intermediate store 19 for gas. Alternatively, operating conditions of this kind may be met by supplying the power generating unit 14 with fuel from the ship's tanks 16. It may also be expedient to mount parts of or all the power generating equipment under water if public regulations or other legislation dictate that it is necessary.

In figures 2-4 various alternative methods are indicated for anchoring the vessel 10. In general the vessel or floating structure can be anchored by means of a mooring system or a dynamic positioning system or a combination of the two. In figure 2 the vessel is moored to a floating buoy 40 via a mooring line 47 which is connected to the vessel and the buoy by suitable connections 41, 42. The buoy is secured via at least one anchor line 48 to a swivel device 44 mounted on an anchor 45. One or more risers 49 and one or more umbilicals 46 pass from the ship down to the swivel 44 on the seabed and continue from there to subsea equipment (not shown in the figure). In figure 3 an anchoring method is illustrated where riser 50 and umbilicals 51 together with one or more anchor lines 52 extend directly from the vessel 10 down to a swivel device 44 connected to an anchor 45. The riser and umbilical system continues to, for example, a pressure booster station 35 or other subsea equipment. In figure 4 a vessel 10 is illustrated where riser 56 and umbilical(s) 57 together with one or more mooring lines 59 are connected to a buoy 55. The buoy is secured via one or more anchor lines 58 to a swivel device 44 connected to an anchor 45. Riser 56 and umbilical(s) 57 continue from the buoy 55 to the swivel 44 and from there on to subsea equipment such as a pressure booster station 35. In figures 2 and 4 the buoys 40, 55 are shown floating on the surface of the water. These buoys 40, 55 may also be arranged in a submerged position, located between the water surface and the seabed or ocean floor, thereby providing a simpler riser system. The vessel 10 may be positioned in such a manner that by pivoting the vessel to given positions it can be employed as an intervention platform for subsea-mounted equipment such as, for example, a pressure booster station 35, i.e. the at least one pressure booster unit, pre- and post-processing means 26, 28, etc. as illustrated in figure 5. Alternatively, the vessel 10 may be disconnected from its anchoring and riser system and positioned freely over the intervention object. When the vessel 10 is in position, relevant tool systems 62 (figure 5) are lowered in the conventional manner to subsea equipment 63, 64 from the vessel through either an opening 60 in the vessel or from a lifting arrangement 61 which will normally be positioned in the after end of the vessel 10. When the vessel 10 is positioned without disconnection, in many cases the intervention system may be arranged so that the vessel can continue to deliver electric power and other contributory factors to parts of the production system not affected by the ongoing, simultaneous intervention operation.

An important operation which can be conducted from the vessel 10 will be installation and operation of a subsea system for launching pigs. A pig is transferred from the vessel 10 to its subsea receiving station either in a tool 73 (figure 6) which is connected to the pipe system through a subsea station 71, or in a flexible pig transport pipe (not shown in the figure) which is secured to the pipe system's pig launching system, and is hoisted up to the vessel when required. The flexible pig transport pipe may be laid on the seabed or suspended under the vessel in a suitable manner when not in use. The pig is installed in a controlled fashion on the vessel and passed in a controlled fashion down through the riser to the pig receiving system. It may be expedient to pass both sophisticated instrument pigs through the pipe in order to monitor the pipe's integrity and simpler scraper pigs in order to remove wax, sand, water, etc. which have been deposited in the pipe. Equipment, pigs and any spare parts used in connection with pigging and other intervention on the subsea installations in the system may be stored in suitable storage spaces 18 (figure 1) on board the vessel 10.

In addition to power generation and monitoring functions, in most cases it will be desirable to add chemicals to the well stream in order to safeguard the integrity of the pipe and an efficient flow (corrosion, wax and/or hydrate inhibitors) in the pipe system 32 to shore 33 where further processing will take place. These chemicals will advantageously be able to be stored in separate tanks 17 (figure 1) on the vessel 10 and transferred to the subsea equipment through the umbilical system. By injecting the chemicals on the suction side of the pressure booster unit 27 (the pump(s)/compressor(s)), very good distribution of the fluid volume will be obtained, i.e. good mixing.

It may also be expedient, either under special conditions (for example at start-up), or on a permanent basis, to supply electric power through at least one heating means 31 (figure 1) for direct heating of the pipeline system, from the well or other equipment located on the seabed or ocean floor and some distance towards land 33 or possibly all the way to land 33. Such a system may also be employed together with pipe monitoring systems, thereby enabling the vessel 10 to regulate in an efficient manner processing equipment and injection equipment according to the state of the transport system (pipe). The vessel 10 will also be a platform for necessary monitoring and control systems for parts of, or preferably all subsea-mounted equipment. This may be operated from onshore or locally from the vessel.

Claims

1. A system for operation and service of at least one hydrocarbon-producing well (24) and of equipment (27, 35) for further transport of the well stream and any equipment (26, 28) for treatment of the well stream, where said equipment and at least one well are disposed below a water surface, particularly on a seabed or an ocean floor, which system comprises

- at least one pipeline (32) for transport of the well stream to the desired destination (33),

- at least one pressure booster unit (27) for driveling the well stream through the at least one pipeline (32), c h a r a c t e r i s e d i n that the system further comprises a vessel (10) for operation and service of the at least one well (24) and the equipment disposed below the water surface, which vessel comprises - an anchoring system comprising a mooring system, a dynamic positioning system or a combination of the two, and where the anchoring system is arranged in such a manner that the vessel can rotate about or swivel around the anchoring system, means (60, 61, 62) for conducting an intervention on equipment disposed below the water surface, - means for communication with and control of the at least one well

(24) and other equipment (35) disposed below the water surface, means (21 , 46, 51, 57) for transferring chemicals from the vessel (10) to the well streams (25) and the at least one pipeline (32) and means for injecting the chemicals into the well stream (25) and the well stream in the pipeline (32),

- means (73) for inserting pigs in the at least one pipeline (32) and means for transferring pigs from the vessel to the at least one pipeline (32), means (14, 15) for production of energy for operation of equipment disposed below the water surface, - means (22) for transferring energy from the means (14, 15) for production of energy to the at least one pressure booster unit (27) and any other subsea equipment.

2. A system according to claim 1, characterised in that at least one pressure booster unit (27) is mounted at each well (24).

3. A system according to claim 1, characterised in that the system comprises a pressure booster station (35) where the well streams (25) from the hydrocarbon-producing wells are collected and pressurised and from which at least one pipeline (32) is provided for further transport of the well streams (25) to the desired destination.

4. A system according to one of the claims 1-3, characterised in that equipment disposed below the water surface is preferably arranged below and within the curve described by the vessel's outer point as the vessel rotates about or swivels around the anchoring system.

5. A system according to one of the claims 1-4, characterised in that the means (14, 15) for production of energy for operation of equipment disposed below the water surface is either provided in its entirety on the vessel or in its entirety below the water surface.

6. A system according to one of the claims 1-4, characterised in that parts of the means (14, 15) for production of energy for operation of equipment disposed below the water surface is provided on the vessel (10), while the rest of the means (14, 15) are provided below the water surface.

7. A system according to one of the claims 1-6, characterised in that the system is provided with means for removing a part of the gas in the well stream as well as means (23) for transporting the gas to means (14, 15) for production of energy.

8. A system according to claim 7, characterised in that the means (14, 15) for producing the energy for operation of equipment and the at least one well (24) which are disposed below the water surface also comprise a processing plant (36) adapted for processing the gas removed from the well stream for operation of the power generating unit (14).

9. A system according to one of the claims 1-8, characterised in that the vessel (10) is provided with storage capacity (17, 18) for necessary contributing factors in connection with operation and maintenance of the system, such as chemicals for injecting into the well stream, pigs and equipment for launching and receiving pigs, spare parts and equipment for use in intervention on equipment below the water surface, and possibly fuel required for energy production.

10. A system according to one of the claims 1-1 1 , characterised in that the system comprises heating means (31) for heating the at least one pipeline (32), and means (22) for transferring necessary energy from the means (14, 15) for production of energy to the heating means (31).

1 1. A system according to one of the claims 1-10, characterised in that the system also comprises additional equipment (26, 28) for treatment of the well stream (25) disposed below the water surface, and particularly on the seabed or ocean floor, such as equipment for mixing, desanding and water separation.

12. A system according to one of the claims 1-11, characterised in that the system comprises means (21, 46, 51, 57) for communicating production and control data from the at least one well (24) and other equipment, which is disposed below the water surface, to the vessel (10).

13. A system according to one of the claims 1-12, characterised in that the system comprises means (21, 46, 51, 57) for transmitting control signals from the vessel (10) to the at least one well (24) and other equipment disposed below the water surface.

14. A vessel for operation and maintenance in connection with production of a well stream (25) containing hydrocarbons from at least one well (24) and maintenance of production equipment and equipment for further transport of the well stream, where the at least one well (24) and the equipment are disposed below the water surface, particularly on a seabed or an ocean floor, characterised in that the vessel further comprises: a system for anchoring comprising a mooring system or a dynamic positioning system or a combination of the two, which anchor is arranged in such a manner that the vessel (10) can rotate about or swivel around the said anchor, - means (21 , 46, 51, 57) for transferring chemicals to at least one pipeline

(32) disposed below the water surface, storage space (17) for storing chemicals that are to be injected into the pipeline (32), means (60, 61) for conducting an intervention on equipment disposed below the water surface, storage space (18) for storing equipment (62) for use in intervention on equipment below the water surface and spare parts for said equipment below the water surface, means for transferring pigs to a pipeline (32) disposed below the water surface, storage space (18) for storing pigs and equipment for transferring pigs to the pipeline.

15. A vessel according to claim 14, characterised in that the vessel (10) is arranged in such a manner that it can be disconnected from the anchor as well as communication line(s), umbilical(s) and other devices connecting the vessel to the at least one well and the equipment disposed below the water surface.

16. A vessel according to one of the claims 14-15, characterised in that the vessel comprises means (14, 15) for production of energy for operation of equipment disposed below the water surface, as well as any heating of at least one pipeline (32) disposed below the water surface.

17. A vessel according to claim 16, characterised in that a part of the means (14, 15) for production of energy is provided on the vessel, while the rest of the means (14, 15) are provided below the water surface.

18. A vessel according to one of the claims 14-17, characterised in that the means (14, 15) for producing energy comprise at least one generator set.

19. A vessel according to claim 18, characterised in that the vessel (10) comprises fuel tanks (16) for fuel required for operation of the at least one generator set.

20. A vessel according to one of the claims 14-19, characterised in that the vessel (10) comprises means (22) for transferring the produced energy from the vessel (10) to subsea equipment and possibly the at least one pipeline (32).

21. A method for intervention on equipment in use in connection with production of a well stream containing hydrocarbons from at least one well (24) and equipment (35) for further transport of the well stream (25), where said equipment is disposed below a water surface, particularly on a seabed or ocean floor, and where the intervention is conducted from a vessel (10) for operation and maintenance of equipment disposed below the water surface, where the vessel (10) is arranged in such a manner that it can rotate about or swivel round its anchoring system, and which vessel comprises means for conducting an intervention on subsea equipment (64) disposed below the water surface, and means (60, 61) for lowering a tool system (62) and any spare parts down to the subsea equipment (64), characterised in that

- the operating and service vessel (10) is positioned in such a manner that means for lowering a tool system (62) and any spare parts are located substantially over the subsea equipment (64), the operating and service vessel (10) may be disconnected from its anchoring system as well as communication line(s), umbilical(s) and other devices connecting the vessel to one or more wells (24) and equipment disposed below the water surface, thus enabling it to be positioned over the subsea equipment (64), the tool system (62) and any spare parts are lowered down to the subsea equipment (64),necessary service or repair is carried out on the subsea equipment (64), if necessary, both service and repair are carried out on the subsea equipment (64), the tool system (62) is lifted up to the operating and service vessel (10).