GB2133446A - Offshore installation - Google Patents

Abstract

An offshore installation comprising a base 4 located on the seabed, a buoyant riser 3 flexibly connected to the base, and a separator plant 2 which is an integral part of the riser structure. The separator plant comprises cyclone separators of the type conventionally used onshore for gas separation. Such separators are relatively small and create an artificial gravitational field that is less affected by motion. The use of cyclone separators is more effective on a moving support. Cleaning of separated water is achieved by tilted-plate separators whereby the relatively small quantity of oil remaining is removed by a gravitational process that is enhanced by the large surface of the tilted plates. <IMAGE>

Description

SPECIFICATION Offshore installation The present invention relates to an offshore installation comprising a base located on the seabed, and a riser flexibly connected to the base.

With the general increase of design and development activity within the sphere of offshore marginal field development, brought about by the need to produce economically from oil or gas fields with small reserves, a variety of systems have been proposed. The majority of these systems employ the use of floating production facilities (FPF) in one guise or another, the most common being the converted tanker hooked-up to a buoyant riser. Briefly, in FPF systems crude oil or gas is piped along the seabed to a riser structure, the bottom of the structure being anchored to the seabed and the top which incorporates a buoy being free to move within a limited area. A combination of rigid pipes and flexible hoses connects the ends of the riser structure.

Permanently moored to the buoy is a dedicated production vessel, generally a tanker which has been converted for production service by having extensive process plant and accommodation modules installed upon its upper decks. The crude is transferred to this vessel via pipes, hoses and fluid swivels, the latter enabling the moored vessel to weather-vane around the buoy structure. The processed cargo is discharged into a shuttle tanker which can be moored alongside, the shuttle and production vessel weather-vaning together for the duration of the pumping operation. Once loaded, the shuttle tanker can cast-off and depart, the process plant continuing its operation and filling large storage tanks in the vessel. This stored cargo is subsequently discharged into the shuttle tanker upon its return.

In some instances, where environmental conditions allow, the processed crude may be pumped from the FPF down dedicated pipes on the buoy structure and along subsea export lines to onshore terminals, thus obviating the use of the shuttle tanker.

When considering whether or not to exploit a known field, four basic factors must be considered. Firstly, the price to be obtained for the product produced; secondly, the capital cost of the venture; thirdly, the operating cost of the field; and fourtly, the productivity of the field, that is at what rate can the field produce given downtime due to weather, maintenance etc. The first factor is beyond the control of the field operator but the other factors are all in part dependent upon the system selected for exploiting the field. An improved system design can convert a field from being sub-marginal to profitable, whether it be a relatively large field in deep water subject to violent storms, or a relatively small field in shallow, calm water.

Discounting the capital and operating costs of subsea wells, flowlines, manifolds, and the like to which this proposal does not relate, the known FPF systems described above employ two basic capital items, that is the buoyant riser, and the vessel (usually a converted tanker) equipped with the process plant. A primary component of the process plant is the equipment used for separating the oil, gas and water components of the crude product. Known floating production systems comprise conventional separator vessels which may be horizontal or vertical drums, and which relay on gravity for their effectiveness.

Substantial savings would result if the surface vessel could be reduced in size or complexity or dispensed with altogether. However, as some form of separator plant is almost invariably required, and the motor of the surface vessel can adversely affect the performance of the separators, it has generally been thought necessary to provide a surface vessel of substantial size.

According to the present invention there is provided an offshore installation comprising a base located on the seabed, a buoyant riser flexibly connected to the base, and a separator plant which is an integral part of the riser structure.

Preferably the separator plant comprises cyclone separators of the type conventionally used onshore for gas separation. Such separators are relatively small and create an artificial gravitational field that is less affected by motion.

The use of cyclone separators is more effective on a moving support, saves space and reduces the part of the duty required from conventional separators, allowing a more effective oil/water separation from them. Cleaning of separated water can be achieved by tilted-plate separators whereby the relatively small quantity of oil remaining is removed by a gravitational process that is enhanced by the large surface of the tilted plates, with a consequent small volume requirement.

By transferring all or part of the separation plant of a production facility to the riser structure which is subjected to only limited horizontal and effectively no vertical movement, cyclone separators can be used without the risk of their operation being frequently disrupted by movement of the structure in bad weather.

The riser structure can also include storage areas for separated products and may incorporate a gas flare stack on a surface buoy. Depending on the requirements of any particular field, the structure can be operated independently or in association with a relatively small support vessel.

Further features of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1 to 6 illustrate four possible installations according to the invention, Figs. 4 and 6 being plan views of the installations of Figs. 3 and 5 respectively; Figs. 7 to 10 illustrate in greater detail the structure of the upper section of the installation of Fig. 1, Figs. 8 and 10 being sections of the lines 8-8 and 1 0-10 respectively; Fig. 11 is a schematic process flow diagram for the installation of Figs. 7 to 10; and Figs. 12 and 13, Figs. 14 and 15 and Figs. 16 and 1 7 are respectively side elevation and plan view of three alternative buoy structures.

Referring to Fig. 1, this shows the basic deepwater riser structure without integral storage capacity. Whilst this particular arrangement is equipped to operate in say 1 40 metres of water, many of its features are common to shallow water configurations. The system consists of a surface buoy 1 incorporating a separator section 2 in which the necessary separation plant is contained, a riser shaft 3 and a base 4 attached by gravity and/or piles to the seabed 5.

The buoy 1 provides the overall restoring force to the system and must be stable under all static and dynamic conditions.

The tubular riser shaft 3 connects the buoy via upper and lower universal joints 6, 7 to the base 5. The riser shaft 3 has a buoyancy tank 8 at its upper end the purpose of which is to bestow a condition of approximately neutral buoyancy on the shaft, thus minimising the maximum static angle of the upper universal joint 6.

The lower universal joint 7 allows the overall system to deflect from the vertical and develop the required restoring force necessary for equilibrium, whilst the upper universal joint 6 provides an articulation between the buoy and riser to reduce the bending moment in each.

Use of an upper universal joint results in a significant reduction in the structural weight as compared with a rigid riser with only one lower joint.

The riser structure is anchored to the sea floor by a ballasted and/or piled foundation base structure 4. This base structure serves as the lateral restraint and takes up all dynamic vertical loadings due to wave action. The static uplift load from the buoy is withstood by the weight of the base.

Critical areas for fatigue damage occur were there are large changes in diameter such as between the buoy and the upper universal joint.

To avoid abrupt changes in structural stiffness, conical reducing sections 9 are employed.

The dynamics of the whole system are tuned so that the natural frequency of motion is significantly above the range of wave periods likely to be found at the installation site.

To make the structure more transparent to wave action, the upper part of the buoy is reduced in diameter from its very top to a distance below the surface.

In the interest of reliability rigid pipework is used whenever possible, with substantial flexible jumper hoses 10, 11 fitted in order to bypass the universal joints. Alternatively the fluid path can run through the articulation by way of rubber torsion seals designed to last the lifetime of the system so that change-out of the flexible jumper hoses is eiiminated. In any event, the riser pipelines can be made suitable for pigging operations if required. Only one riser pipeline is shown to avoid over complication of the drawing.

Referring now to Fig. 2, this shows a similar arrangement to that of Fig. 1 but with the addition of a storage tank 12 integral with the riser. Thus the slender riser shaft 3 of Fig. 1 and its buoyancy tank 8 are replaced by a storage riser 12 of greater diameter and a buoyancy tank 1 3.

The function of the storage riser 12 is to enable up to 30,000 bbls of processed crude to be stored, which equates to approximately 24 hours continuous production for a 30,000 bbls per day field. This is achieved by pumping in the oil at the top of the storage riser in order to displace the seawater below which previously occupied the space. The difference in up-thrust caused by the different specific weights of water and oil is automatically compensated by adjusting the amount of air in the segregated buoyancy tank 1 3 attached to the upper portion of the riser 1 2, resulting in its approximate neutral buoyancy.

Whenever it is required, the oil can be retrieved by simply pumping in more seawater at the lower end of the storage riser. The oil and water can be separated by a substance made to act as a membrane, the specific gravity of which lies between that of oil and seawater.

Figs. 3 and 4 show a design approach suitable for the environmental conditions found closer inshore in depths of say 40 m of water. In this shallow water system the buoy 1 and its separation plant section 2 are constructed in a more compact form whilst being identical in principle and operation to those already described with reference to Figs. 1 and 2. The limits of compactability are such that this buoy must be coupled via one universal joint 14 directly to the base structure 4, though in shallower, calmer seas this will have no detrimental effect on performance.

The necessary stability and restoring forces for all static and dynamic conditions are provided by the large diameter annular buoyancy tank 1 5 that forms an integral part of the buoy steelwork about 10 m below the surface. Once again conical transition sections 1 6 are utilised in order to minimise acute changes in section and associated stiffness which could eventually lead to fatigue cracking.

The ballasted and/or piled foundation base 4 serves as a lateral restraint and resists, in addition to the static up-lift of the buoy, all dynamicvertical loading due to wave action.

The beneficial gain in performance by making the system more transparent to wave forces is achieved in a similar manner to that of the deep water system i.e. by reducing the diameter of the very top portion of the buoy and continuing this feature down approximately 6 m below the surface.

Figs. 5 and 6 show the same arrangement as that of Figs. 3 and 4 except for the base which is constructed with an integral processed crude storage capacity. This base is fabricated from six large pipes 1 7 of for example 7 m diameter which act as six interconnected tanks with enough capacity for say 30,000 bbls.

Oil can be pumped into the top of each of these compartmentalised tanks in turn, displacing the seawater which hitherto filled the space. By pumping seawater back into the bottom of each tank the oil can be emptied out. The formation of oil/water emulsions can be minimised by careful design and the use of a substance to act as a flexible membrane between the oil and water.

The up-thrust caused by the different specific gravities of water and oil will not affect, to any descernible extent, the total reaction of the base, provided the design is such that the weight of the steelwork involved is far greater than the buoyancy generated by the combined effects of the buoy 1 and all six tanks 17 full of processed crude.

Careful design of the tank compartment divisions and the relative fluid interaction ensures that the gravitational effects of the base fabrication are always even and symmetrical, thereby obviating any possible turning moments.

Whilst the weight alone is great enough to resist the total static up-lift, lateral restraint could be augmented by the use of piling (not shown).

Figs. 7 to 11 illustrate in some detail the basic buoy and separator plant. The design of this equipment is in fact moderately complex, and therefore so as to avoid confusion only the essential elements are shown. Facilities such as test separation, backwash, and the like have all been omitted for the sake of clarity, as it is the intention here only to illustrate the important theory.

The buoy casing 1 8 is circular in section and, though this is not shown, is of double-skin construction with watertight bulkheads to provide security against collision damage.

Several of the inner void spaces are subdivided into baffled compartments which may be individually flooded in order to make any required adjustments to overall buoyancy, natural frequency, etc.

The buoy contains the separation plant and all necessary ladders, lighting, firefighting and communication systems.

For ease of access, inspection and maintenance all fundamental equipment contained within the structure is located in a 'shirtsleeve' environment. However all major items such as valves, controls, electrics, etc. are of intrinsically safe, submersible design so that they will remain fully operational in the even of a leak.

As regards the separation plant, crude oil piped from the subsea wells can be treated in several ways or simply diverted to storage (or elsewhere) without being processed.

A table of four alternative systems is shown in Fig. 11 (top right hand corner) listing the respective positions of the main flow control valves. Before expanding further the information concerned with the above it should be observed that the first and third of these systems require an FPF to be in attendance whilst the second and fourth systems do not.

Referring in particular to Figs. 7 and 11, the separator section of the buoy comprises a first separator stage 1 9 indicating three cyclone separators 20 each capable of handling 10,000 barrels per day, a second intermediate pressure separator 21, a third low pressure separator 22, and a water tank 23. A pipe 24 is connected to subsea wells, a pipe 25 serves for the export of crude or separated product, a pipe 26 serves as a water outlet, and pipes 27 and 28 serve respectively for conveying product to and from a floating production facility. Main flow control valves FCV1 to FCV5 can be controlled in accordance with the table of Fig. 11 to select any one of four operating system modes.

In the first system, which will be used when a high degree of separation is required, or when the crude is difficult to handle, oil is piped from subsea wellhead chokes up the riser into the buoy where it branches to supply the three identical high pressure cyclone type separators 20. From there it subsequently passes through the large intermediate pressure separator 21 and the low pressure separator 22. At this point a substantial amount of the gas and water will have been removed so that a comparatively clean product can be pumped under its own pressure to a suitable FPF moored close by. This FPF can be relatively small and simple as a considerable amount of work will have already been performed on the crude in the buoy separators.

After further refining aboard the FPF the oil can either be pumped back via the buoy to a subsea export line or to storage. Alternatively it may be loaded directly from the FPF to a shuttle tanker depending upon environmental conditions.

In the second system (wells, separators, export) which if the crude is of a suitable quality will be almost certainly the most cost effective, all the separation is carried out within the buoy and no other process equipment is needed.

The initial part of the process is the same as before but when the product leaves the L.P.

separator 22 it is piped directly into the subsea export line or possibly to storage. In the event of there being no export line the processed oil could be piped aboard a shuttle tanker.

The third system (wells, FPF, export) will only be used when the separator plant is shut-down for reasons of maintenance etc. Here the riser structure simply functions as a loading buoy for the crude brought up the riser pipeline from the subsea wells.

The crude bypasses the separator plant and runs directly to the FPF. From the FPF the processed oil can be pumped back via the buoy to storage or subsea export lines. It may also be possible to load processed oil from the FPF to an awaiting shuttle tanker.

The fourth system (well, export) may be regarded as an emergency back-up.

The control valves are set to divert the crude from the wells along subsea lines to some other form of plant or storage away from the site location.

The operation of the separators will now be described in more detail with reference to Fig. 11.

As mentioned above, Fig. 11 only shows the essential elements of the three-stage separation process. Facilities such as test separators backwash, pigging, etc. have all been omitted for clarity.

Wel' fluids flow under their own pressure from subsea wellhead chokes up the riser pipeline 24 into the buoy. There the flow passes through a manifold into the three first-stage H.P. cyclone type separators 20, each capable of a throughput of 10,000 b/d, giving a total capacity of 30,000 b/d. These H.P. cyclone separators have a limited 'turn-down' ability so if the flow of well fluids was, say, 1 8,000 b/d instead of 30,000 b/d, then the controls are automatically programmed to shut down one H.P. separator, thus ieaving the other two to run at 10,000 b/d and 8,000 b/d respectively. This results in greater efficiency than having all three run at a 'turn-down' rate of 6,000 b/d each.

The cyclonic action of this type of separator causes the heavier emulsions of oil and water to separate out from the lighter gas. The free gas subsequently passes out of the separator to be burned by the flare or used as fuel if an FPF is in attendance. The emulsion that is left in the firststage unit, whilst still fairly well mixed, tends to be oil-rich at the top and water-rich at the bottom.

Thus, the 'oily' element is drawn from the upper portion of the unit into the second-stage l.P.

separator 21, and the 'watery' element from the base goes to the water settling tank 23 below.

The important factor in the subsequent I.P. and L.P. separation is one of time. The distinct advantage of the described arrangement is that the second and third-stage separators have, by the nature of their volume, very long residence times, i.e. there is a considerable period in which the oil/water emulsions can separate thoroughly.

In the design shown, the residence time is about 30 minutes. The geometry of the second and third-stage separators 21, 22 is such that they are low in height compared with their relatively large diameter. This results in a large surface area but a short vertical bubble path. For obvious reasons this feature enhances performance still further.

Partially separated fluid, now with little gas, enters the second-stage l.P. separator 21, approximately at the dead center of the vessel.

After approximately 30 minutes the oil has floated upwards where it is drawn off to the third-stage L.P separator. The water, however, has sunk to the bottom where it is drained into the same water settling tank 23 as previously mentioned, whilst any gas can be led to the flare.

The third-stage L.P. separator 22 has basically an identical process to the second-stage, but the resulting oil content is much more pure.

This separated and purified oil can then be pumped under its own pressure to a tanker or to storage, or if required, onto further processing elsewhere, whilst the water settling tank is suitably discharged.

It should be observed that, to avoid any undue fluid surge within the plant due to buoy motion characteristics, the separators will need to be carefully baffled, though surge problems will not be so acute as they would be in an FPF operating in the same environment, as the buoy mounting results in only limited horizontal movement and virtually no vertical movement.

Referring now to Figs. 1 2 to 17, three alternative arrangements of the upper buoy structure are shown. The arrangement of Figs. 1 2 and 13 is mechanically the simplest and is shown on Figs. 1 to 4 though either of the other two arrangements could have been substituted. The choice of upper buoy structure is dependent on environmental conditions and whether or not other vessels will be in attendance, and if so, of what type and how moored.

In the arrangement of Figs. 12 and 13, no part of the structure revolves around the vertical axis.

This inability to rotate precludes the buoy from use as a conventional mooring for an FPF or shuttle tanker.

Should an FPF or shuttle tanker be required, then the vessel will have to be modified by the addition of several heavy anchors and two or three azimuth thrusters in order to enable it to be positioned in the seaway at a fixed location, with the ability to turn, or partially turn, into oncoming winds and waves. There cannot be, however, any direct mooring via hawsers to this uncomplicated, non-rotating type of buoy.

In order to pump hydrocarbon products to and from a dynamically positioned FPF or shuttle tanker a flexible hose bundle is suspended in a compliant, catenary loop from an articulated hose arm mounted on the side of the buoy about 30 m below the surface (see Fig. 1) through the water to, for example, a deck-mounted connection on the bow of the FPF. (This is the only buoy arrangement using the catenary loop/hose arm system). The catenary hose bundle is designed to withstand all forces imparted by the ambient currents together with the heavy loads generated by the FPF and the varying tensions caused by the relative movements of the buoy.

All gases separated by the process plant are piped up the inside of the buoy to a flare stack 29 standing some 15 m above the surface of the sea.

At approximately 5 m above and 5 m below the surface there is a tubular fender 30 built around the buoy like a bird cage, the purpose of which is to protect the main structure from accidental collision damage caused by another vessel. The upper buoy structure is fully equipped with navigational aids, access platforms, etc.

The arrangement of figs. 14 and 15 follows the general design principle found on CALM (Catenary Anchor Leg Mooring) buoys. All the technology is proven and basically centres on the idea that a tanker (or FPF) can be moored by hawsers 31 to a turntable or revolving mooring platform 32 attached by heavy duty, sealed rolier bearings 33 to the top of the buoy. This turntable is free to rotate continuously through 3600, the tension in the hawser dictating its orientation at any given instant i.e. the turntable will always lie in the same relative position to the vessel as the latter weather-vanes around the buoy. The turntable or mooring platform is fitted with mooring equipment, floating-hose connections, navigational aids, counterbalance weights, flare stack etc.

The centre of the buoy houses a multi-channel fluid swivel assembly 34 (not shown in detail) essential for the transfer of fluid products between the rotating and non-rotating parts of the buoy. It should be noted that the swivel is mounted on its own independent bearings and is isolated from all mooring forces. The swivel is located in 'shirtsleeve' conditions so it can be readily maintained and its seals quickly replaced.

The design of this swivel is such that it can be fully pigged along with the rest of the pipework.

Fluid product transfer between tanker (or FPF) and the turntable is through suitable floating hoses 35 the effect of which is balanced by counter-balance 36.

The arrangement of Figs. 1 6 and 17 is a compromise between the simplicity of the arrangement of Figs. 12 and 13 and the more expensive and sophisticated arrangement of Figs.

1 4 and 1 5. The main advantage of this system is the obviation of the fluid swivel, the penalty being that the attached vessel has limited weathervaning capability.

Essentially the process pipework is run up the inside of the buoy, the gas being diverted via pipe 37 to a fixed flare stack 38 some 1 5 m above the surface of the sea. The remaining pipes 39 are directed via watertight bulkhead fittings through the outer shell of the buoy just below the water line where they are connected to flexible hoses.

These hoses wrap a little way round the buoy circumference and lie approximately in the horizontal plane. When all the flexible hoses are well clear of their adjacent connections they are grouped to form one flexible hose bundle 40 suitably equipped with a strain member. This bundle is then coiled around the outer diameter of the buoy for several full turns.

The upper buoy structure is fitted with a revolving guide-roller cage/mooring platform 41.

The purpose of this device is twofold. Firstly to provide adequate fendering against accidental collision damage and secondly to act as a coiler/uncoiler for the hose bundle.

The principle relies on a vessel moored to the buoy via a central revolving hawser eye 42. It can be seen from the drawing that all mooring forces are transmitted directly to the main buoy structure and not to the isolated guide-roller cage/mooring platform. Because of this the bearing arrangement supporting the cage can be very simple and unsophisticated amounting to no more than a series of bogies (not unlike those used on cranes) travelling on a circular track. The hawser 43 is made to run from the eye to the vessel through a 'goalpost' fairlead 44 which constrains the rotation of the cage i.e. the cage is always made to lie in the same relative position to the vessel as the latter weather-vanes around the buoy.

As the vessel moves in one direction the processing guide-roller cage strips-off the hose bundle from the buoy shell whilst in the other direction it winds-on the hose bundle. As this movement takes place the hose bundle is protected from damage by having the hose guiderollers 45 designed such that no scuffing takes place.

The actual way this system would operate in practice would be to have a small standby vessel tow the hawser, and consequently the cage, around the buoy until there were two full turns (7200) of the hose engaged. Then the shuttle tanker (or FPF) would pick-up the other end of the hawser and make fast. This tanker (or FPF) could then weather-vane two turns (7200) around the buoy in one direction before completely strippingoff the hose. Alternatively the tanker (or FPF) could weather-vane two turns (7200) in the other direction thus winding-on the hose to give four full turns (14400) around the buoy.

In this way the tanker (or FPF) can weathervane to and fro up to a maximum of 14400 in any given direction. In the very unlikely event that prevailing conditions dictate continual weathervaning in only one direction, the standby vessel must assist by ensuring that the hawser, having been released by the tanker (or FPF), on reaching the coiling limit, is picked-up and is towed to reorientate the cage and hose bundle so that the tanker (or FPF) can re-attach and continue as before.

It is necessary, as with the arrangement of Figs. 14 and 1 5, for the hose bundle to be buoyant in order for it to float on the surface between the buoy and the main vessel. It should also be long enough to accommodate the coiling requirement without ever becoming taut.

Claims (Filed on 9/12/83) 1. An offshore installation comprising a base located on the seabed, a buoyant riser flexibly connected to the base, and a separator plant which is an integral part of the riser structure.

2. An offshore installation according to claim 1, wherein the separator plant comprises cyclone separators of the type conventionally used onshore for gas separation.

3. An offshore installation according to claim 2, comprising tilted-plate separators for removing by a gravitational process the relatively small quantity of oil remaining in water separated out by the cyclone separators.

4. An offshore installation according to claim 1, 2 or 3, wherein the riser structure includes storage areas for storing separated products.

5. An offshore installation according to any

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. bearings 33 to the top of the buoy. This turntable is free to rotate continuously through 3600, the tension in the hawser dictating its orientation at any given instant i.e. the turntable will always lie in the same relative position to the vessel as the latter weather-vanes around the buoy. The turntable or mooring platform is fitted with mooring equipment, floating-hose connections, navigational aids, counterbalance weights, flare stack etc. The centre of the buoy houses a multi-channel fluid swivel assembly 34 (not shown in detail) essential for the transfer of fluid products between the rotating and non-rotating parts of the buoy. It should be noted that the swivel is mounted on its own independent bearings and is isolated from all mooring forces. The swivel is located in 'shirtsleeve' conditions so it can be readily maintained and its seals quickly replaced. The design of this swivel is such that it can be fully pigged along with the rest of the pipework. Fluid product transfer between tanker (or FPF) and the turntable is through suitable floating hoses 35 the effect of which is balanced by counter-balance 36. The arrangement of Figs. 1 6 and 17 is a compromise between the simplicity of the arrangement of Figs. 12 and 13 and the more expensive and sophisticated arrangement of Figs. 1 4 and 1 5. The main advantage of this system is the obviation of the fluid swivel, the penalty being that the attached vessel has limited weathervaning capability. Essentially the process pipework is run up the inside of the buoy, the gas being diverted via pipe 37 to a fixed flare stack 38 some 1 5 m above the surface of the sea. The remaining pipes 39 are directed via watertight bulkhead fittings through the outer shell of the buoy just below the water line where they are connected to flexible hoses. These hoses wrap a little way round the buoy circumference and lie approximately in the horizontal plane. When all the flexible hoses are well clear of their adjacent connections they are grouped to form one flexible hose bundle 40 suitably equipped with a strain member. This bundle is then coiled around the outer diameter of the buoy for several full turns. The upper buoy structure is fitted with a revolving guide-roller cage/mooring platform 41. The purpose of this device is twofold. Firstly to provide adequate fendering against accidental collision damage and secondly to act as a coiler/uncoiler for the hose bundle. The principle relies on a vessel moored to the buoy via a central revolving hawser eye 42. It can be seen from the drawing that all mooring forces are transmitted directly to the main buoy structure and not to the isolated guide-roller cage/mooring platform. Because of this the bearing arrangement supporting the cage can be very simple and unsophisticated amounting to no more than a series of bogies (not unlike those used on cranes) travelling on a circular track. The hawser 43 is made to run from the eye to the vessel through a 'goalpost' fairlead 44 which constrains the rotation of the cage i.e. the cage is always made to lie in the same relative position to the vessel as the latter weather-vanes around the buoy. As the vessel moves in one direction the processing guide-roller cage strips-off the hose bundle from the buoy shell whilst in the other direction it winds-on the hose bundle. As this movement takes place the hose bundle is protected from damage by having the hose guiderollers 45 designed such that no scuffing takes place. The actual way this system would operate in practice would be to have a small standby vessel tow the hawser, and consequently the cage, around the buoy until there were two full turns (7200) of the hose engaged. Then the shuttle tanker (or FPF) would pick-up the other end of the hawser and make fast. This tanker (or FPF) could then weather-vane two turns (7200) around the buoy in one direction before completely strippingoff the hose. Alternatively the tanker (or FPF) could weather-vane two turns (7200) in the other direction thus winding-on the hose to give four full turns (14400) around the buoy. In this way the tanker (or FPF) can weathervane to and fro up to a maximum of 14400 in any given direction. In the very unlikely event that prevailing conditions dictate continual weathervaning in only one direction, the standby vessel must assist by ensuring that the hawser, having been released by the tanker (or FPF), on reaching the coiling limit, is picked-up and is towed to reorientate the cage and hose bundle so that the tanker (or FPF) can re-attach and continue as before. It is necessary, as with the arrangement of Figs. 14 and 1 5, for the hose bundle to be buoyant in order for it to float on the surface between the buoy and the main vessel. It should also be long enough to accommodate the coiling requirement without ever becoming taut. Claims (Filed on 9/12/83)

1. An offshore installation comprising a base located on the seabed, a buoyant riser flexibly connected to the base, and a separator plant which is an integral part of the riser structure.

2. An offshore installation according to claim 1, wherein the separator plant comprises cyclone separators of the type conventionally used onshore for gas separation.

3. An offshore installation according to claim 2, comprising tilted-plate separators for removing by a gravitational process the relatively small quantity of oil remaining in water separated out by the cyclone separators.

4. An offshore installation according to claim 1, 2 or 3, wherein the riser structure includes storage areas for storing separated products.

5. An offshore installation according to any

preceding claim, comprising a gas flare stack on a surface buoy.

6. An offshore installation substantially as hereinbefore described with reference to the accompanying drawings.