WO2015022477A1

WO2015022477A1 - Apparatus and method for offshore production of hydrocarbons

Info

Publication number: WO2015022477A1
Application number: PCT/GB2013/052169
Authority: WO
Inventors: Richard Selwa
Original assignee: Richard Selwa
Priority date: 2013-08-15
Filing date: 2013-08-15
Publication date: 2015-02-19

Abstract

An apparatus (1) for offshore production of hydrocarbon fluids comprises a buoyant structure (2) connectable to a subsea well (3) via a riser (4). The buoyant structure comprises a storage reservoir for storing hydrocarbon fluid delivered from the subsea well. The storage reservoir constitutes an integral part of the buoyant structure and is adapted to allow stabilisation of the fluid delivered from the well to take place in the storage reservoir. The buoyant structure comprises processing equipment for processing the fluid delivered from the well.

Description

APPARATUS AND METHOD FOR OFFSHORE PRODUCTION OF

HYDROCARBONS

Field of the Invention

The present invention relates to the field of offshore oil and gas production, and in particular to an apparatus and a method for producing, processing and storing hydrocarbons offshore.

Background of the Invention

Petroleum production offshore requires the placement of a structure, often referred to as an offshore platform, or oil platform in an offshore location. These structures include facilities to extract and process hydrocarbons and to store the stabilised products for delivery onshore. Since marine environment is a hostile environment for oil exploration and production, it is difficult to achieve a satisfactory balance between the costs of building such a structure and its production capabilities. Relatively, low cost structures have limited processing and storage capabilities, which ultimately results in additional costs related to delivery and processing onshore. Typical relatively low cost structures include Tension Leg Platforms (TLPs) which comprise buoyant structures taut to seabed so as to eliminate vertical and horizontal movement of the structure and thus to prevent heave and provide a stable marine structure. Typically at TLPs crude oil is subjected to basic processing steps, such as initial separation, with fluids transferred to a storage vessel for collection by a tanker for delivery onshore for further processing. Complex structures include spar platforms which are moored to seabed similar to TLPs, but without significant vertical tension because spars have inherent stability and do not require vertical tension to hold them in vertical orientation. Spar platforms are common for deepwater operations. They are aimed at large production quantities to justify the costs of offshore exploration and production and, accordingly, tend to be very large and complex structures. Large production quantities impose certain restrictions on the methods of processing the hydrocarbons. For example, production spars use pressure based stabilisation method for stabilising crude oil because of the necessity to manage large quantities of gas separated from the produced fluids. Complicated gas handling involves the need for the presence of personnel at the production facility. Therefore a spar includes a platform at surface on which most of the production facilities are concentrated. For both spars and TLPs, the need to man platforms results in a high operating cost.

The problem of cost justification results in that only high yield or large reservoirs become developed whereas a considerable number of relatively small reserves (fields with less than 10 million barrels or with a production rate of 500 to 5000 barrels per day) remain undeveloped.

In view of the above, it is an object of the present invention to obviate and mitigate the above drawbacks associated with capital and operating costs and to provide an apparatus for producing hydrocarbons offshore which is suitable for all stages of processing hydrocarbons without being complex and costly, capable of operating unmanned and economically acceptable for the development of relatively small reserves, end of field life or even as an early production system for larger fields where oil is discovered but not exploited.

Summary of the Invention

Accordingly, the present invention provides an apparatus for offshore production of hydrocarbon fluids, the apparatus comprising

a buoyant structure connectable to a subsea well via a riser,

the buoyant structure comprising a storage reservoir for storing hydrocarbon fluid delivered from the subsea well wherein the storage reservoir constitutes an integral part of the buoyant structure and is adapted to allow stabilisation of the fluid delivered from the well to take place in the storage reservoir; and wherein said buoyant structure comprises processing equipment for processing the fluid delivered from the well. Preferably, said buoyant structure comprises means to provide power, control and chemical and fluid injection to the subsea well to facilitate production.

Preferably, the processing equipment comprises at least a means for separation and, preferably, a means for stabilising the fluid.

Preferably, the means for separation inter alia includes one or more compartments in the storage reservoir where the stabilisation and separation take place. The means for separation is preferably configured to separate water from the fluid, to provide a degree of degassing, thereby functioning as a means for a preliminary stabilisation of the fluid, and, preferably, to reduce viscosity of heavy oil.

Preferably, the means for stabilising is adapted to carry out temperature based stabilisation. Preferably, the means for stabilising comprises means for heating the fluid, either in the storage reservoir or, in a specific case, in a separation compartment, so as to cause the egress of volatile components from a heavier fluid fraction thereby reducing the vapour pressure and rendering the fluid safe for transportation. The egress of volatile components includes the egress of hydrogen sulphide, carbon monoxide and carbon dioxide the presence of which in the fluid renders the fluid corrosive. Preferably, the means for heating the fluid is adapted to heat the fluid up to 80^QC.

The buoyant structure preferably comprises gas utilisation equipment for processing the gas extracted during separation and/or stabilisation for providing energy for use for the functioning of the apparatus. Ideally, the gas utilisation equipment comprises a means (e.g. gas combustion equipment) for converting gas energy into another form, such as heat, electricity, light or mechanical energy for use by the apparatus. Preferably, means for converting the gas energy into another form comprises at least, but not limited thereto, a heating system. The heating system is preferably arranged in cooperation with the means for stabilisation to heat the fluids being stabilised. Preferably, the means for converting gas energy into another form further comprises electricity generating equipment.

Preferably, the apparatus comprises an arrangement for regulating the rate of production of fluid from the well in accordance with the amount of gas emitted during the separation and/or stabilisation processes.

Preferably, the arrangement for regulating the rate of production of fluid from the well includes throttling means for suppressing the fluid production rate if the amount of gas produced during the separation and/or stabilisation processes exceeds gas processing capacity of the gas utilisation equipment.

Preferably, the gas utilisation equipment is adapted to convert the gas energy into other forms of energy such as, for example, heat or electricity for use for the functioning of the buoyant structure. Preferably, the capacity of the gas utilisation equipment is sufficient to satisfy energy requirements of the apparatus.

Preferably, the apparatus comprises energy back up means, such as fuel storage facility, the energy back up means providing additional fuel to meet the energy requirements of the apparatus in case the amount of the gas produced during the fluid processing is insufficient. Preferably, the buoyant structure is moored or tethered to seabed.

Preferably, the buoyant structure is configured so that it is suitable for being moored in shallow waters (less than approximately 100m deep) and tethered in deeper waters (greater than approximately 100m deep).

Accordingly, the apparatus preferably comprises a mooring system for use with the apparatus in shallow waters and/or a tethering system for use with the apparatus in deeper waters.

The buoyant structure is preferably over 50m in height and, preferably, between, 60m and 80m. Preferably, the buoyant structure comprises a submersible section and a topside section.

Preferably, the buoyant structure comprises a housing in which the processing equipment and the storage reservoir are accommodated. Preferably, the housing forms part of the submersible section. Preferably, the housing has a top region and a base region. Preferably, the storage reservoir is located within the base region of the housing.

Preferably, the housing comprises a hull. The hull is suitably shaped to accommodate the storage reservoir and the processing equipment. The hull is also suitably shaped to ensure buoyancy of the structure. The hull is preferably of a generally cylindrical configuration having a height and a diameter but, optionally, can be flat sided. Preferably, the height of the hull is greater than its diameter. In one embodiment, the hull has a diameter between 12m and 25m, e.g. 22m.

Preferably the housing comprises a number of decks of which one or more decks are preferably submersible (i.e. become or are capable of being submersed below water line when the buoyant structure is deployed), which house the storage reservoir and the processing equipment and which also form buoyancy chambers and, preferably, fuel (e.g. diesel) storage chambers. The buoyant structure can comprise three, four, five or six decks. A first, upper, deck may house subsea well and general control equipment, transformers, a power distribution system and work and storage area. A second, intermediate, deck may contain power generators, heat exchange systems and lube oil day tanks. A third, intermediate, deck may accommodate further generators. A fourth, lower, deck may house the processing equipment including process pumps and further generators. A fifth deck preferably contains the storage reservoir. The purpose of the decks is to safely control and manage the extraction of hydrocarbons. The structure is designed for safe access and egress for maintenance and is designed to float in the vertical position from shallow draft for simplified towing, installation and decommissioning. Preferably, the topside section of the buoyant structure comprises a mast section extending upwardly from the housing so that the mast section is at least partially located above the sea level. Preferably, the mast section comprises a landing deck. Preferably, the mast section comprises a number of sections which house access locations and further equipment. In one embodiment the mast section comprises two sections, an upper section and a lower section. The upper section preferably accommodates air intake and exhaust equipment, a vent tower, satellite controls, cold vent tower, and an optional helicopter deck. The lower section preferably accommodates a boat landing deck and access/egress shaft.

Within, the topside section comprises one or more and, preferably, three decks. One deck, preferably, a first deck houses the control room and navigation aids. Another deck, preferably, a second deck, houses Heating, Ventilating, and Air-Conditioning (HVAC) equipment and a back-up power engine. A further deck, preferably, a third deck, houses a maintenance hatch area. The buoyant structure preferably has a roof which can be accessed by a ladder to allow access for maintenance and removal of personnel by helicopter.

Preferably the storage reservoir has a storage capacity of up to 60,000 barrels. The buoyant structure is preferably connected to the well via the storage reservoir, preferably via the base of the storage reservoir. The riser preferably extends between the storage reservoir, preferably via the base of the storage reservoir, and the well.

Preferably, the processing equipment comprises equipment for initial heating of well fluids, removing solid components from the fluids and/or degassing the fluid prior to the delivery of the fluid into the storage reservoir and subjecting the fluid to separation and stabilisation. The apparatus of the invention is suitable for the production of hydrocarbon fluids from subsea wells located at depths from 50m to over 3000m.

Due to the relatively safe and uncomplicated principle of its operation compared to existing offshore platforms and the utilisation of the low pressure temperature based stabilisation, the apparatus of the invention can be operated unmanned, i.e., without requiring the presence of personnel during production. Personnel are only required to visit the facility to conduct scheduled

maintenance or in response to plant trips.

The buoyant structure is preferably provided in the form of a spar buoy having a centre of gravity below its centre of buoyancy to ensure stability of the buoyant structure, i.e. to prevent the buoyant structure from toppling, regardless whether it is moored, tethered or free floating.

The buoyant structure preferably comprises thermal insulation to help maintain the temperature required during the processing of the fluid and stable operation of plant.

Preferably, the apparatus comprises gas pressure relief system, such as, for example, cold venting and/or emergency flaring adapted to vent any excess gas. The storage reservoir preferably comprises an export line for offloading the processed fluid, e.g. to a transport vessel, or export pipeline.

Preferably the buoyant structure comprises all the processing equipment necessary for processing hydrocarbon fluids delivered from the well so that the fluids are ready for export onshore (via a transport vessel, e.g. a tanker or via pipeline to adjacent platforms) without further stabilisation being required to be performed at an onshore processing facility.

The temperature based stabilisation provided for by the apparatus of the invention causes less damage to the reservoir being developed compared to the typical pressure based stabilisation dictated by the financial considerations, renders the apparatus suitable for unmanned production, removes the requirement for complex and expensive plant and is safer; low pressure prolongs reservoir life and renders it economically stable in the long term.

The temperature based stabilisation renders the apparatus of the present invention suitable for the development of relatively small fields which would otherwise be regarded as economically unsuitable for development.

The temperature based stabilisation, limits the production of

hydrocarbon fluid by setting an upper limit on the amount of consumption of the gas produced during the processing of the fluid. All the gas is utilised either as fuel gas for engines to generate power or in boilers to heat the fluid during the stabilisation process in the storage reservoir. In the case of flaring during startup or emergencies, gas can also be consumed which raises the potential oil production limit. Temperature based stabilisation is well suited to lower rates of production, which is why to date it has been overshadowed in the offshore industry by the pressure based stabilisation required to maximise exploitation of larger capital assets. The temperature based stabilisation method requires managing production flow, by controlling the flow in accordance with the reservoir conditions.

Pressure based stabilisation requires management of large quantities of gas either as re-injection or export. Temperature based stabilisation removes the requirements of complicated gas handling. Its simplicity and safety allows high reliability operations to occur and is the fundamental key to unmanned operations.

In the apparatus of the present invention, with medium to higher Gas-Oil- Ratio (GOR) reservoirs, production of hydrocarbon fluid is throttled back, limited by the maximum gas consumed. With low GOR reservoirs, any shortfall of fuel is supplemented using diesel via dual-fuel or bi-fuel engines and heat boilers. The effect of this field management methodology is slower field exploitation with less damage to the reservoir and prolonging the lifespan of the field and increased proportion of extractable reserves. The apparatus of the invention is designed to have a broad hydrocarbon processing operating envelope, which in turn minimises any equipment changes when re-deploying from one field to another. The apparatus of the invention is also designed with a multiple redundancy of equipment and simplicity to produce a reliable long term production facility, which can operate safely and autonomously and with high availability of continuous operating plant.

The present invention is designed to produce stabilised crude suitable for tanker export with a Reid vapour pressure of 0.7Bara (70Kpa). In the apparatus of the invention, the base, solids and water (BS&W) specification for the produced crude is typically set at 0.5% (typical for tanker export). The apparatus typically cleans sand from fluids down to 0.2% ^w/_w-

In another aspect, the present invention provides a method of offshore processing of hydrocarbon fluid recovered from a subsea well, the method comprising

providing an apparatus in accordance with any aspect described above delivering the fluid from the well to the storage reservoir within the apparatus,

subjecting the fluid to separation and temperature based stabilisation processes within the apparatus; and storing the processed fluid in the storage reservoir for subsequent offloading for transportation onshore.

Preferably, the method further comprises the step of removing any solid components (e.g. sand) from the fluids and/or de-gassing the fluid in a separate vessel prior to subjecting the fluid to separation and/or stabilisation in the storage reservoir. The de-gassing and de-sanding vessel also serves as a slug catcher and a flare knock-out drum. Brief Description of the Drawings

The present invention will now be described by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic elevation of the apparatus of the invention tethered to seabed and connected to a subsea well via a riser; and

Figure 2 is a schematic elevation of the apparatus of the invention moored to seabed and connected to a subsea well via a riser;

Figure 3 is a schematic cross-sectional elevation of the buoyant structure part of the apparatus of Figures 1 and 2; and

Figure 4 is a schematic illustration of processing of fluids within a storage reservoir forming an integral part of apparatus such as that shown in Figs. 1 to 3.

Description of the Specific Embodiments

Referring initially to Figures 1 and 2 the apparatus of the invention is indicated generally with reference numeral 1. The apparatus comprises a buoyant structure 2 connected to a subsea well 3 via a riser 4. The buoyant structure 2 comprises a submersible section 21 and a topside section 22.

As shown in Figure 3, the submersible section comprises a housing 23 having a top region 23a and a base region 23b. A storage reservoir 25 for storing the hydrocarbon fluid delivered from the subsea well 3 is located within the base region 23b of the housing 23 and constitutes an integral part of the buoyant structure 2.

The buoyant structure 2 comprises processing equipment for processing the fluid delivered from the well 3. The processing equipment is accommodated within the housing 23. The processing equipment comprises at least a means for separation and a means for stabilising the fluid. The means for separation, among other components, includes one or more compartments 26 in the storage reservoir 25 where water is separated from the hydrocarbon fluid. The means for separation may be configured to provide a degree of degassing, thus functioning as an initial stabilising means.

The means for stabilising is adapted to carry out temperature based stabilisation and includes a heating system 40 (see Figure 4) for heating the fluid in the storage reservoir 25 up to 80^QC so as to cause the egress of volatile components from the heavier fluid fraction thereby reducing the vapour pressure and rendering the fluid safe for transportation. The heating process comprises two stages. Initially fluid arriving at housing 23b is directed to a degasser and a desander 31 for initial heating. Fluids are then directed to the storage reservoir 25 where the second stage of separation and stabilisation takes place. The egress of volatile components includes the egress of hydrogen sulphide, carbon monoxide and carbon dioxide whose presence in the fluid renders the fluid corrosive.

The buoyant structure 2, as shown in Figure 3 comprises gas utilisation equipment (engines and heaters for the tank) for processing the gas extracted during the processes of separation and stabilisation for providing energy for use for the functioning of the apparatus 1. The gas utilisation equipment may include at least one or more boilers 32 which may form part of the heating system used for the stabilisation and one or more generators for producing electricity for use for the various functions of the apparatus. As an alternative, conventional electric heaters using power generated by the engines can also be used.

Although not illustrated, the apparatus 1 comprises an arrangement for regulating the rate of production of fluid from the well 3 in accordance with the amount of gas emitted during the separation and/or stabilisation processes. The arrangement for regulating the rate of production of fluid from the well 3 includes throttling means for suppressing the fluid production rate if the amount of gas produced during the separation and/or stabilisation processes exceeds the gas processing capacity of the gas utilisation equipment. Although not shown in the drawings, the apparatus 1 comprises energy back up means which includes a fuel storage facility providing additional fuel to meet the energy requirements of the apparatus 1 should the amount of the gas produced during the fluid processing be insufficient.

The buoyant structure 2 is configured so that it is suitable for being moored in shallow waters (less than approximately 100m deep) (Figure 2) and tethered (Figure 1) in deep waters (greater than approximately 100m deep). In Figure 1 the apparatus 1 is shown tethered via a tethering system for use with the apparatus in deep waters. In Figure 1, the apparatus 1 is shown tethered to seabed via tethers 5 which prevent both vertical and horizontal displacement and provide for greater stability of the apparatus 1 in deep waters even through the apparatus 1 is configured to remain buoyant at all times, whether moored, tethered or free floating. In Figure 2, the apparatus 2 is shown moored to seabed via anchor lines 50 which allow a limited degree of horizontal and vertical displacement. This allows the unit to be moored for shallow water conditions.

The buoyant structure 2 is preferably over 50m in height and, preferably, between, 50m and 80m.

The housing 23 comprises a hull 28. The hull 28 is suitably shaped to accommodate the storage reservoir 25 and the processing equipment. The hull 28 is also suitably shaped to ensure buoyancy of the structure 2. For this purpose, the hull 28 has a generally cylindrical configuration having a height and a diameter and the height of the hull 28 is greater than its diameter. An optimal diameter for the hull 28 in the present embodiment is between 12m and 25m, e.g. 22m.

As shown in Figure 3, the housing 23 comprises a number of decks, which house the storage reservoir 25 and the processing equipment and which also form buoyancy chambers. In the present embodiment, the buoyant structure comprises four decks. A first, upper, deck 35 houses control equipment, power distribution system, switch gear, Electrical Submersible Pump (ESP) variable speed drives and work area. A second, intermediate, deck 34 contains generators. A third, intermediate, deck 33 accommodates further generators. A fourth, lower, deck 30 houses the processing equipment and the storage reservoir 25. Although not shown in the drawings, decks 30, 33, 34 and 35 have a blast proof segregation arrangement rated for explosion overpressure of 8 barg (800Kpa). The purpose of the decks 30, 33, 34, 35 is to safely control and manage the extraction of hydrocarbons. The buoyant structure 2 is designed for safe access and egress for maintenance with a central maintenance lift used for moving spare parts and equipment to and from the landing platform and decks. The buoyant structure 2 is designed to float in the vertical position from shallow draft for simplified towing, installation and decommissioning.

The topside section 22 of the buoyant structure 2 comprises a mast 24 extending upwardly from the housing 23 so that the mast 24 is at least partially located above the sea level. The mast 24 comprises two sections, an upper section 62 and a lower section 64. A first part 62a of the upper section 62 accommodates control room, wash and toilet facilities, air intake and exhaust equipment, a vent tower (external), satellite controls, cold vent tower, and an optional helicopter deck 60. A second part 62b of the upper section 62 contains the HVAC for the buoyant structure 2 and also a back-up emergency generator. The lower section 64 accommodates a boat landing deck 65 and access/egress shaft 67. Preferably the storage reservoir 25 has a storage capacity of up to 60,000 barrels.

The buoyant structure 2 is connected to the well 3 via the storage reservoir 25. The riser 4 extends between the storage reservoir 25 and the well 3.

Although not illustrated, the processing equipment comprises equipment for first stage de-sanding and/or de-gassing the fluid prior to the fluid being deposited in the storage reservoir 25 and subjected to further second stage separation and stabilisation.

The apparatus 1 is suitable for the production of hydrocarbon fluids from subsea wells located at depths from 50m to over 3000m. The apparatus 1 can be operated unmanned, i.e., without requiring the presence of personnel during production. Personnel are only required to visit the facility to conduct scheduled maintenance and production trips.

In the present embodiment, the buoyant structure 2 is provided in the form of a spar buoy having a centre of gravity below its centre of buoyancy to ensure stability of the buoyant structure 2, i.e. to prevent the buoyant structure from toppling, regardless whether it is moored, tethered or free floating.

Although not illustrated, the buoyant structure 2 comprises thermal insulation to help maintain the temperature required during the processing of the fluid. Although not illustrated, the apparatus 1 comprises gas pressure relief system, in the present embodiment, cold venting, adapted to vent any excess gas which cannot be processed by the gas utilisation equipment. Apparatus 1 also includes gas flare system which can be used during early field life to safely flare excess gas or in emergency cases.

The storage reservoir 25 is coupled to an export line 70 for offloading the processed fluid to a transport vessel 72.

The buoyant structure 2 comprises all the processing equipment necessary for processing hydrocarbon fluids delivered from the well 3 so that the fluids are ready for export either onshore (via a transport vessel, e.g. a tanker) or to a nearby host platform without further stabilisation being required to be performed at an onshore processing facility. The apparatus 1 is designed to have a broad hydrocarbon processing operating envelope, which in turn minimises any equipment changes when re-deploying from one field to another. The apparatus 1 is also designed with a multiple redundancy of equipment and simplicity to produce a reliable long term production facility, which can operate safely and autonomously and with high availability of continuous operating plant.

The apparatus 1 is designed to produce stabilised crude suitable for tanker export with a Reid vapour pressure of 0.7bara (70Kpa). In the apparatus of the invention, the base, solids and water (BS&W) specification for the produced crude is set typically at 0.5% (typical for tanker export). The apparatus typically cleans sands from fluids down to 0.2% ^w/_w.

Referring to the process diagram presented schematically in Fig. 4, a storage reservoir 425 receives a feed of fluid materials comprising gas, oil, water and sludge from flexible riser 44. These fluid materials are passed through a de- sander 431 and optionally via heater 40 which may be used to increase the temperature of the fluid materials when required. Some gas separation may occur in the de-sander 431. Lines introduced into the storage reservoir 425 at different levels in the storage reservoir allow selective removal of separated components of the feed from the flexible riser 44. The lines are positioned so as to be above or below an interface between dissimilar materials, for example above the gas/oil boundary to collect gas, below the gas / oil boundary but above the oil / water boundary to collect oil, and below the oil / water boundary to collect water. Sedimentary or settled sludge is collectable near the bottom of the storage reservoir. In Fig. 4, line 41 is located for removal of gas collected over the oil. Line 42 is located for removal of water. Line 43 is located for removal of oil. Line 45 is located for removal of sludge. Line 46 delivers de- sanded oil / water into the storage reservoir 425 for separation. Detection means may sense low oil level and signal for collection to be made or for feed to be interrupted until collection is possible. A low oil level is detectable by positioning of a sufficient number of sensors around the storage reservoir. This indicates that the storage reservoir is approaching capacity and requires off-loading or draw off for the next stage in production. The sensors may be used to register an alert e.g. by transmitting a signal to an operations control facility in order to call for off-loading of oil from the storage reservoir. Diversion to another storage reservoir is also an option.

Modifications and improvements are envisaged without departing from the scope of the present invention as defined in the appended claims.

Claims

CLAIMS:

1. An apparatus (1) for offshore production of hydrocarbon fluids, the apparatus comprising

a buoyant structure (2) connectable to a subsea well via a riser (4), the buoyant structure comprising a storage reservoir (25) for storing hydrocarbon fluid delivered from the subsea well

wherein the storage reservoir constitutes an integral part of the buoyant structure and is adapted to allow stabilisation of the fluid delivered from the well to take place in the storage reservoir; and

wherein said buoyant structure comprises processing equipment for processing the fluid delivered from the well.

2. An apparatus as claimed in Claim 1, wherein the processing equipment comprises at least a means for separation and a means for stabilising the fluid.

3. An apparatus as claimed in Claim 2 wherein the means for separation is configured to separate water from the fluid; and to provide a degree of degassing thereby functioning as a means for a preliminary stabilisation of the fluid.

4. An apparatus as claimed in Claim 2 or Claim 3, wherein the means for separation includes one or more compartments in the storage reservoir where the separation takes place.

5. An apparatus as claimed in any one of Claims 2 to 4, wherein, the means for stabilising is adapted to carry out temperature based stabilisation.

6. An apparatus as claimed in Claim 5, wherein the means for stabilising comprises means for heating the fluid so as to cause the egress of volatile components from a heavier fluid fraction thereby reducing the vapour pressure and rendering the fluid safe for transportation.

7. An apparatus as claimed in Claim 6, wherein the means for heating the fluid is adapted to heat the fluid up to 80^QC.

8. An apparatus as claimed in any one of Claims 5 to 7, wherein the buoyant structure comprises gas utilisation equipment for processing the gas extracted during separation and/or stabilisation for providing energy for use for the functioning of the apparatus.

9. An apparatus as claimed in Claim 8, wherein the gas utilisation equipment comprises a means for converting gas energy into another form for use by the apparatus.

10. An apparatus as claimed in Claim 9, wherein the means for converting the gas energy into another form comprises at least, a heating system arranged in cooperation with the means for stabilising to heat the fluids being stabilised.

11. An apparatus as claimed in any one of Claims 8 to 10, wherein the capacity of the gas utilisation equipment is sufficient to satisfy energy requirements of the apparatus.

12. An apparatus as claimed in any one of Claims 8 to 11, wherein the apparatus comprises energy back up means, including a fuel storage facility for providing additional fuel to meet the energy requirements of the apparatus in case the amount of the gas produced during the fluid processing is insufficient.

13. An apparatus as claimed in any one of Claims 5 to 12, wherein the apparatus comprises an arrangement for regulating the rate of production of fluid from the well in accordance with the amount of gas emitted during the separation and/or stabilisation processes.

14. An apparatus as claimed in Claim 13, wherein the arrangement for regulating the rate of production of fluid from the well includes throttling means for suppressing the fluid production rate if the amount of gas produced during the separation and/or stabilisation processes exceeds gas processing capacity of the gas utilisation equipment.

15. An apparatus as claimed in any preceding claim, wherein the buoyant structure comprises a submersible section and a topside section.

16. An apparatus as claimed in Claim 15, wherein the submersible section comprises a housing in which the processing equipment and the storage reservoir are accommodated, the housing having a top region and a base region, wherein the storage reservoir is located within the base region of the housing.

17. An apparatus as claimed in Claim 16, wherein the housing comprises a hull suitably shaped to accommodate the storage reservoir and the processing equipment, the hull being configured to ensure buoyancy of the structure.

18. An apparatus as claimed in Claim 16 or Claim 17, wherein the housing comprises a number of decks, which house the storage reservoir and the processing equipment and which also form buoyancy chambers, and, optionally, fuel storage chambers.

19. An apparatus as claimed in any one of Claims 15 to 18, wherein the topside section of the buoyant structure comprises a mast section extending upwardly from the housing so that the mast section is at least partially located above the sea level.

20. An apparatus as claimed in any preceding claim, wherein the storage reservoir has a storage capacity of up to 60,000 barrels.

21. An apparatus as claimed in any preceding claim, wherein the buoyant structure is connected to the well via the storage reservoir and wherein the riser extends between the storage reservoir and the well.

22. An apparatus as claimed in Claim 2 in combination with any other preceding claim, wherein the processing equipment comprises equipment for initial heating of well fluids, removing solid components from the fluid and/or de-gassing the fluid prior to the delivery of the fluid into the storage reservoir and subjecting the fluid to separation and stabilisation.

23. An apparatus as claimed in Claim 5 in combination with any other preceding claim, wherein the apparatus of the invention is suitable to operate unmanned.

24. An apparatus as claimed in any preceding claim, wherein the buoyant structure is provided in the form of a spar buoy having a centre of gravity below its centre of buoyancy to ensure stability of the buoyant structure, regardless whether it is moored, tethered or free floating.

25. An apparatus as claimed in any preceding claim, wherein the buoyant structure comprises thermal insulation to help maintain the temperature required during the processing of the fluid.

26. An apparatus as claimed in Claim 2 in combination with any other preceding claim, wherein the apparatus comprises gas pressure relief system adapted to vent any excess gas produced during separation and/or stabilisation and, optionally, a flare system for flaring during field start-up and for emergency response.

27. An apparatus as claimed in any preceding claim, wherein the buoyant structure comprises all the processing equipment necessary for processing hydrocarbon fluids delivered from the well so that the fluids are ready for export onshore without further stabilisation being required to be performed at an onshore processing facility.

28. An apparatus as claimed in any preceding claim, wherein the buoyant structure is configured so that it is suitable for being moored in shallow waters and tethered in deep waters.

29. An apparatus as claimed in any preceding claim, wherein the buoyant structure is between, 50m and 80m in height.

30. An apparatus as claimed in any preceding claim, wherein the buoyant structure comprises a means to provide power, control and chemical and fluid injection to the subsea well to facilitate production.

31. A method of offshore processing of hydrocarbon fluid recovered from a subsea well, the method comprising the steps of

providing an apparatus (1) comprising a buoyant structure (2) connectable to a subsea well (3) via a riser (4), the buoyant structure comprising a storage reservoir (25) for storing hydrocarbon fluid delivered from the subsea well

wherein said buoyant structure comprises processing equipment for processing the fluid delivered from the well;

delivering the fluid from the well to the storage reservoir within the apparatus; subjecting the fluid to separation and temperature based stabilisation processes within the apparatus; and storing the processed fluid in the storage reservoir for subsequent offloading for transportation onshore.

32. A method of Claim 31, comprising the step of removing solid components from the fluids and/or de-gassing the fluid prior to subjecting the fluid to separation and/or stabilisation.

33. An apparatus for offshore production of hydrocarbon fluids substantially as herein described with reference to and/or as shown in the accompanying drawings.

34. A method substantially as herein described with reference to the accompanying drawings.