GB2204107A - Turret systems for the mooring of vessels at sea - Google Patents

Abstract

A turret system for mooring crude oil processing and storage vessels incorporates a plurality of flexible hoses extending between an upper coupling member carried by the vessel and a lower coupling member carried by the turret, such that as rotation of the vessel takes place about the turret, under the influence of wind and/or tides etc, the hoses take up a helical configuration, the coupling members being biased apart so that the hoses are caused to straighten on rotation of the vessel back to its original position.

Description

Turret Systems for the Mooring of Vessels at Sea This invention relates to Turret Systems of the kind used for the mooring of vessels, such as tankers and/or crude oil processing and storage ships which are moored at sea thus avoiding the need for connections to under sea pipelines. Typically such a vessel, hereinafter referred to as a processing/storage vessel, will be moored at a point immediately above or adjacent to an oil field and will be used as a collection and early processing facility for the crude oil extracted from the field. After preliminary processing the crude oil will be stored in the tanks of the vessel and will be removed at suitable intervals by one or more shuttle tankers which will convey the crude either directly to a refinery or to a suitably located transhipment point.In addition, as will be known to those versed in'the--art, it-is~netessary,-~1n a-typical-4roducing field, to pass a number of fluids in a downward direction-from the vessel to the sea bed and in some cases onwards down to the producing level.

Typically such fluids could consist of injection water, injection gas, chemical dosing fluids, kill fluid and any other fluids which may be applied from time to time to ensure the maintenance of an economic flow rate from the well in question.

In use the processing/storage vessel will be moored in the open sea and hence will be directly subjected to the local environmental conditions. Thus it is desirable for the vessel to be moored in such a way that it can rotate bodily in plan in response to the various force vectors which are produced by wind, waves, sub surface currents etc. whilst the turret, which is positioned at a point significantly forward of the transverse mid line of the vessel, is secured against rotation by being moored to the sea bed. At the same time means need to be provided for enabling fluids to be fed to or from the vessel through the non-rotating turret. A variety of methods by which this end can be achieved have been reported in the literature.

Many of the known forms of turret structure require a relatively large diameter housing of cylindrical form to be inserted into the structure of the vessel. In this way a form of moonpool is produced in the vessel into which the turret structure is inserted as a separate element able to rotate relative to the hull of the vessel.

Heretofore such turret structures have been relatively large in diameter. This in turn has led to the necessity for special consideration to be given to the ship structural implications involved in the provision of so large a diameter of casing (for example 20 metres dia.) passing through the vessel. In certain cases this large diameter has presented so great a structural problem that it has not been possible to convert an existing tanker vessel to accommodate the turret. In such cases therefore it has been necessary to design a tanker vessel specially for the purpose. This solution while entirely feasible from a technical standpoint has tended to eliminate or reduce the valuable cost saving which is possible when an existing tanker (possibly several years old) is utilised as the starting point for a system as-described above.

One object of the present invention is to provide a turret which can be constructed with a relatively modest diameter at the point where it passes vertically through the main hull construction of the vessel, so that major structural problems are avoided, and virtually any existing tanker vessels of appropriate size may be adapted by the provision of a suitable vertical cylindrical casing for the reception of a turret as previous described.

Another object of the invention is to provide a relatively simple means of enabling fluids to be transferred between the turret and vessel and which avoids the need to employ multi-path swivels making use of large diameter elastomeric seals.

According to one aspect of the invention, in a turret system of the kind referred to for an oil processing/storage vessel, the turret incorporates, for transferring fluids upwards or downwards to or from the vessel, a plurality of flexible hoses extending between an upper coupling member carried by the vessel and a lower coupling member carried by the turret, the hoses being disposed around the axis of relative rotation of the vessel and turret, and the coupling members being biased away from each other so that in a neutral position of the vessel and turret the hoses are held substantially straight but so that, as relative rotation of the vessel and turret takes place, the hoses are caused to take up a helical configuration with the coupling members moving towards each other against the biasing force, the latter causing the hoses to straighten again on return of the vessel and turret towards the relative neutral position. Each hose is preferably terminated by a swivel at one end and by a fixed flange at the other end to prevent axial twisting.

Such an arrangement not only avoids the need for multi-path swivels for the passage of fluids between the turret and the vessel, with the attendant difficulties of forming satisfactory fluid seals, but requires only a relatively small diameter section of turret to house the hoses. Accordingly it is possible to make the section of turret which passes through the vessel hull of reduced diameter, for example less than 10 metres.

typically a reduced-diameter-turret section of a turret system in accordance with the invention could be of the order of 6 metres such a reduction in turret diameter readily being achieved while at the same time providing all of the necessary facilities needed to allow the rotating vessel to perform its requisite functions as a crude oil recovery, processing and tankage facility.

As is known to those versed in the art the degree of rotation of the vessel relative to the turret will depend on the actual location of the vessel and the particular combination of environmental factors to be found at that location. Investigations have shown that in the vast majority of offshore oil producing locations a total angular movement of vessel relative to turret of j1800 is enough to accommodate both immediate and seasonal changes in the direction of the vector representing the resolution of all of the contributing vectors associated with environmental forces. For a small minority of locations additional angular accommodation of the order of +360 may be necessary.

The length of the hoses, and the relative movement of the coupling members can readily be selected to permit relative rotation to take place from 0" to +1800 and from 0" to -180 respectively.

However the arrangement may be designed to accommodate i3600 or more should particular local circumstances demand an angular acconnodation figure in excess of the j1800.

Preferably a telescopic structure is connected between the upper and lower coupling members and carries buffer means against which the hoses rest when they are twisted into a helical configuration.

According to another aspect of the invention, in a reduced diameter turret system of the kind referred to for a processing/storage vessel, the turret accommodates a manifold assembly comprising a plurality of hollow toroidal manifold sections disposed coaxially one above the other, a plurality of pipes spaced around the common axis of the manifold sections, and connected to the manifold sections through valve means which permits each pipe to be connected to more than one-said manifold section,-the pipes having at their lower ends connectors enabling their to be coupled to flexible risers for transferring fluids to or from wellhead assemblies and/or to subsea manifolds, as the case may be, and each manifold section being connected to pipe means for transferring fluid to and from the vessel through a coupling means which permits a degree of relative rotation to take place between the manifold assembly and the vessel.

The spaced pipes and valves are conveniently arranged so that any pipe may be connected to any one of the manifold sections.

Such an arrangement enables any one of the risers to be diverted into any one of the various manifold sections of the manifold assenbly, and hence via another connection to the vessel through said coupling means, the latter conveniently incorporating a twisting hose assembly as described above. The arrangement of the sections of the manifold assembly and the associated valves which serve to join the sections together to form a complete assembly is conveniently such that when the valves are placed in a certain attitude a through passage of full bore size is provided from top to bottom of the manifold assembly without cross connections into any of the manifold sections.In this way a passage is created which enables pigging to be performed down to the wellhead assembly or template as the case may be, and indeed in some circumstances to the producing level of the well. This pigging facility is available for the servicing of any one riser without the need to interrupt the flow (upwards or downwards) from the remaining risers. It will be understood that in an assembly of this nature the upward rising flow is that of crude oil with or without a content of dissolved gas as the case may be at the oil field in question. The design of the manifold assembly is such that by suitable operation of the valves joining the various sections together it is possible to deflect any one of the downward flowing fluids to any one of the flexible risers connected ultimately to the welihead assemblies or sub-sea manifold unit as the case may be.

In summary the design and layout of the manifold assembly is such that by the operation of the joining valves it is possible to arrange for the pigging of any one of the flow lines while at the same time making it possible for an upward or downward fluid flow to be acconinodated-.relative-to-any-riser-and also temporarily to take any riser-out of service-for pigging and such like operations.-- - One embodiment of the invention will now be described, by way of example with reference to Figures 1 to 10 of the accompanying drawings, in which Figures 1 and 2 represent a plan view and a sectional elevation of a turret system embodying the invention, Figures 3 to 6 represent plan sections along the lines AA, BB, CC and DD respectively of Figure 2, Figure 7 represents an enlarged sectional elevation illustating the manifold assembly shown in Figure 2 in more detail, Figure 8 represents a plan section along the line EE of Figure 7, Figure 9 represents a diagrammatic representation of the manifold assembly, and Figure 10 represents a sectional elevation of a suitable pull-in coupling .for enabling connections to be made between the manifold system and flexible risers.

Considering Figs. 1 and 2 the turret assembly 1 consists of two main elements, the upper section 2 being of relatively large diameter while the lower section 3 is of smaller diameter (typically about 6 metres) and passes through a cylindrical casing 4 of comparable diameter built into the vessel 5. The turret unit is supported on a series of rollers 6 which are spring mounted at 7 via brackets 8 which in turn are bolted to a suitable surface 9 provided on the underside of the larger section 2 of the turret. The support rollers 6 are carried on a specially reinforced section of the main deck of the vessel 5, this reinforcement being illustrated at 10.

Likewise the plan location of the turret is determined at the upper end by a further series of rollers 11 the axes of which are vertical.

This latter group of rollers is divided into two categories, the majority being freely rotating while a smaller number are associated with geared driving units. These driving assemblies serve to ensure that the plan location of the turret remains on a relatively fixed bearing when the vessel in rotating in plan has a tendency to rotate the turrent due to frictional forces between the two. At the lower end of the turret assembly a pocket is provided at 12 to accomodate a series of fairleads 13 (typically 8 or 9 off)-each of which serves to deflect a mooring chain from its near vertical path 14 to its catenary formation on the way down to its location on the sea bed.

The start of such a catenary is illustrated at 15. From Fig. 2 it can be seen that a complete mooring chain system consists of a storage compartment 16, a hydraulically operated rotary stopper 17, a deflection pulley 18, a conventional stopper 19, a hawse pipe 20 and the previously introduced fairlead 13. In practice 8 or 9 of such assemblies would be arranged with their respective hawse pipes immediately inboard of the inner face of the smaller section of the turret 3. This layout is illustrated at Figs. 5 and 6.

It will be understood that the mooring chains, when loaded, will produce a significant horizontal component of force which is resolved into the turret structure by the fairlead 13 which is carried therefrom. In normal circumstances the total horizontal component applied to the lower part of the small diameter section of the turret will consist of horizontal components stemming from 3 or 4 of the mooring chains and hence 3 or 4 of the fairleads. This results in a bending moment being applied to the smaller diameter section 3 of the turret. To resist this bending moment a reaction ring 21 is attached at the lower end of the structural casing 4 and likewise a concentric ring 22 is attached at the lower end of the small diameter section of the turret 3. Under normal conditions a gap 23 exists between the concentric rings 21 and 22.However when the previously described horizontal components of the mooring chain loads occur, the deflection of the lower section of the turret 3, due to the previously described bending moment, causes the gap 23 to close locally, thus limiting the stress in the main turret tube 24 to a predetermined value. With regard to the pocket 12 provided in the underside of the vessel 5, it will be noted that the plate work is suitably built up at 25 to ensure that the integrity of the double bottom construction of the vessel is maintained if the tank adopted for the location of the turret is of double bottom construction.

The layout of the mooring chains and their associated components is such that the remaining elements of the system must be accommodated inboard of the ring of hawse pipes 20. The most important additional element is-the--assembly-of~fAexible risers riser which pass up from sea bed level through the tubular guide member 26 and terminate on the underside of the manifold assembly 27 which, in this particular arrangement, comprises four hollow toroidal manifold sections 59 (Fig. 7) disposed coaxially one above the other. For this purpose a series of pull-in devices 28 is provided on the underside of the lowermost hollow toroidal section of the manifold assembly 27.A series of rigid pipe connections such as 29 is provided between each individual manifold section and fixed piping leading to the lower end of a respective one of a plurality of flexible hoses 46 of a hose twisting assembly 30 which will be described below. Each section of the manifold assembly 27 is separated from the next by means of a two-position full bore valve unit such as 31. A diverter pulley 32 is placed at the upper end of the small diameter section of the turret. This is provided, primarily to lead a hoisting rope on to a winch conveniently positioned on one of the decks in the larger diameter section 2 of the turret. The hoisting rope is used for handling risers 33 and umbilicals during initial installation.

The larger diameter section 2 of the turret 1 is divided into decks and compartments for the accommodation of the various articles of equipment which are needed to complete the assembly and is surmounted by a pipe bridge 34. This is attached to the main deck of the vessel at 35 and 36 and serves to support the structure 37 which carries the upper elements of the hose twisting assembly 30 and also to support the fixed pipe runs such as 38 which eventually pass across the pipe bridge 34 on their way to the facilities provided in the vessel 5 for the accommodation and processing of the upwardly rising crude oil and the pressurising and transfer to well level of the specialised fluids previously described.

The connection between the fixed pipes such as 38 and the twisting hose assembly 30 takes the form of a series of radially disposed "U" shaped flexible pipe bends such as 39 between the pipes 38 and connections 52 carried by a crosshead 44 and to which the upper ends of the flexible hoses 46 are connected. The outer flange such as 40 of the "U" shaped bend 39 is attached to the steel work 41 which is in turn connected to the pipe-bridge 34-at-42.- rThe location in plan of the hose loops 39 relative to the turret centreline 53 is indicated in Fig. 3 which is a plan on the section "A"-"A" of Figure 2. The inner flange 43 of the "U" shaped loop 39 is attached to the crosshead 44.This unit is located by means of one or more keys such as 45 which serve to prevent the crosshead member 44 from rotating in plan but at the same time give it freedom to rise and fall in elevation. This facility is provided to allow for the reduction in effective vertical height of the hoses 46 which are the principal element of the hose twisting assembly. In order to sustain a suitable level of tension in the hoses 46, the crosshead 44 is tensioned by means of an annular weight 46A which is connected to the crosshead by a series of wire ropes such as 47. These pass upwards via various deflecting pulleys such as 48 and 49 and are eventually connected to the crosshead assembly at points such as 50.In this way the array of hoses such as 46 is held at a controlled level of tension while the twisting action resulting from the rotation of the vessel 5 relative to the anchorages 15 and hence the seabed takes place. During this twisting action the crosshead 44 is caused to descend to an extent determined by the apparent reduction in effective length of the hoses 46 due to the fact that in the twisted condition each hose is describing a helix instead of the straight line illustrated in Fig.2. A typical cycle of operation starting from a datum represented by 0" would be for rotation in plan of the vessel to take place to +1800 during the course of which the crosshead 44 would descend to its lowermost position this motion resulting in a rolling action by the "U" shaped flexible hose connections such as 39.After this the cycle continues by rotation from +1800 back to 0 and thence to -1800. In this way a typical total rotation of 3600 or one revolution is catered for. However these figures can be significantly adjusted to allow for variations in environmental conditions from one site to another. It is expected that a total 360" plan rotation will be adequate to meet the requirements of the vast majority of potential application areas.

Each hose 46 of the twisting array is terminated at the lower end by means of a fixed flange connection 51 and at the upper end by means of a swivel connection~52 of~known design and construction. The incorporation of the swivel ensures that as the twisting action takes place each hose 46 is isolated from direct twisting about its own "vertical" axis. That is to say in the fully twisted condition the hose 46 is lying along a helical path between its fixed flange 51 and its respective swivel 52 without itself having to accommodate an axial twist corresponding to the plan rotation of the vessel 5 relative to the seabed which may have taken place. In an alternative construction the swivel connections may be provided at the lower ends of the flexible hoses 46, with fixed connections at their upper ends.

It will be understood by those versed in the art that when the plan rotation of the vessel relative to the seabed takes place there will be a tendency for the hoses to assume the most direct possible path between their terminating connections such as 51 and 52 which by this time have been displaced in plan relative to each other. It will also be understood that when relative rotation exceeds 900 there would be a tendency for a single hose such as 46 to take up a position passing through or near to the centre line 53 of the group.

It will be apparent that where a group of hoses is involved when all are twisted there would be a tendency for all hoses to meet in a tight bunch on the centreline of the assembly with the possibility of resulting damage to the outer layers of the hoses. This tendency is eliminated by an assembly 54 which is rigidly attached to the turret structure at one end 55 and is located relative to the crosshead 44 at the other end. This sub assembly takes the form of a flanged mounting 56, which is attached in turn to a telescopic section 57 of the control assembly 54. This layout has the effect of locating the control assembly accurately relative to the vertical centreline of the array, but at the same time allows the crosshead 44 to rise and/or fall freely as it takes up variations in the effective height of the hose array.The control assembly 54 is provided throughout its length with a series of buffer members in the form of freely rotating rollers such as 58, the purpose of which is to prevent contact between the various hoses at the "pinch point" as previously described. The rollers 58 may be produced from any convenient material, one possibility being an arrangement incorporating a rubber facing against which the hoses rest as theçtwisting action takes place. Fig. 4 which is a section taken at "B" "B" of Fig. 2 illustrates the plan positions respectively of the flexible hoses 46, the rollers 58 and the counter weight 46A.

Figs. 7 to 10 illustrate the manifold assembly 27 in more detail. Considering first Fig. 7, the assembly 27 consists of a series of manifold sections 59 in the form of torus pipes. Each manifold section 59 is connected to its neighbour by means of a series of full bore three way valves such as 60. The number of valves 60 arranged around the circle represented by the centreline diameter of the manifold section corresponds to the number of risers which it is intended to accommodate. Thus for example if ten risers are to be terminated then ten valves 60 would be positioned between each manifold section. In the case quoted (i.e. ten risers) ten riser pull-in units 61 are accommodated on the lower flanges 62 of the lowermost manifold section unit.Each riser pull-in unit is bolted to its respective flange 62 which in turn communicates with a pipe 63 which passes straight through the manifold section without being in any way connected to the interior bore of that section.

This pipe 63 is terminated at its upper end by means of a further flange 64 to which is attached its corresponding three way valve 60.

The side branch of the valve 60 such as 65 is connected to a bend which in turn is welded at a point such as 66 to its corresponding manifold section which does, in turn, communicate with the interior of the section. Hence if it is intended to pass the flow from any given riser to the interior of any given manifold section the corresponding valve or valves 60 is/are appropriately set and the required flow duly takes place. Thus it will be seen that in the arrangement disclosed, any riser flow may be deflected into any manifold section or conversely in cases where the fluid flow is downwards towards the wellhead assembly the flow from any manifold section may be deflected to any riser.Each manifold section has associated with it an input or output pipe such as 67 which in turn is connected by suitable rigid piping to the hose twisting assembly disclosed in the earlier part of this specification.

It is also necessary for provision to be made for the termination of umbilical cables and an example is shown at 68. The umbilical itself is terminated by a suitable "plug" 69 the corresponding "socket" being shown at 70. This unit is connected to the underside of the lowermost manifold section by means of a blank flange 71. The flange though attached to the torus does not communicate with its interior and hence is to be regarded as a structural support for the socket unit 70. A conduit or pipe 72 is used to convey the electrical and/or small bore hydraulic connections to a corresponding flexible assembly based on a hose and which in turn forms part of the hose twisting assembly earlier described.

that is to say the hoses of the hose twisting assembly are made up of a combination of hoses conveying fluids to or from the wellhead fittings together with an appropriate number of additional hoses which convey the aforesaid multiple electrical connections and/or small bore hydraulic connections.

Fig. 8 is a section taken on the line EE of Fig. 7 in which the various elements of the manifold system can be identified from the key numbers previously introduced, the positions of the ten pipe/umbilical connections 29 being denoted by 29.1----29.10. This Figure also shows the hawse pipes 20 through which the mooring chains 15 pass on their way downwards first to their respective deflection fairleads 13 and secondly to their respective anchorages on the sea bed. Fig. 2 illustrates one such chain 15 as it leaves its fairlead on its way to the sea bed.

Fig. 9 illustrates in diagrammatic form the layout and arrangement of the manifold system 59 previously described. It will be seen that in this figure four manifold sections 59.1--59.4 and ten risers/umbilicals are provided but it will be understood that any reasonable number of manifold sections and risers/umbilicals can be provided in a system conforming to the principles of the invention.

It will also be understood that the practical limitations in the number of risers/umbilicals which can be accommodated in any given case is determined by the effective circumference of the manifold sections having regard to the necessity for the various penetrations which need to be designed along established pressure vessel lines.

That is to say nozzles of various types and/or reinforcing pads must be positioned in or on the manifold section in accordance with the appropriate pressure vessel rules. Thus in a given case if it proves impracticable to accommodate the required number of riser entry points it is merely necessary to increase the effective diameter of the manifold sections until sufficient space on the circumference has been created. Under limiting conditions where the maximum possible number of risers must be accommodated in the smallest possible diameter the wall thickness of the manifold sections may be increased to the point where additional reinforcement is unnecessary (intrinsic compensation). However, even under these circumstances the necessary ligament dimensions between adjacent penetrations must of course be correctly observed.

Fig. 10 illustrates a pull-in device 99 for the termination of the flexible risers 33 and is designed to provide a piggable riser termination. The device is automatically latched in upon insertion of the riser termination and can be remotely operated for release.

Alternatively the riser pull-ins may be released by a local control provided at a platform positioned inside or near to the manifold assembly.

The riser termination consists of a nozzle 73 which is attached to the upper side of a swivel 74 of known design. On the lower face of the swivel is attached the flange 75 which terminates the flexible riser proper. The nozzle is provided at its upper end with a reduced diameter section 76 arranged to co-operate with '0' ring seals 77 which are in turn carried in a seal carrier 78 which is screwed in to the main nozzle tube 79. An additional seal 80 is provided between the seal carrier 78 and the nozzle tube 79 to remove a leak path which would otherwise exist along the thread line.

Attached to the main nozzle tube 79 are a series of support lugs 81.

Conveniently these may be three or four in number. Pivotally attached to the support lugs 81 are a corresponding number of latches such as 82. These are designed to co-operate with a conical section 83 which forms part of the main body 73. The latches 82 are actuated by a corresponding number of small hydraulic or pneumatic cylinders such as 84. These actuators 84 are of known design and are internally spring loaded by springs 85 so positioned that the piston/piston rod assembly 86 is tending to be thrust in a downward direction. Thus it will be seen that the springs 85 are tending to sustain the latches 82 in a position such that they continue to maintain in position the main body 73 by means of the conical section 83. When it is desired to release the riser all of the cylinders 84 are supplied with pressurised working fluid to the internal volumes such as 87. This pressurised fluid may be supplied simultaneously to all of the actuators 84 by means of individual valves and pipe connections from a pressurised fluid source. Alternatively all of the actuators 84 may be connected together via a small ringmain 88.

In either event the. application of the pressurised working fluid causes the piston/piston rod assembly 86 to rise compressing the springs 85. Since the piston/piston rod assembly 86 is pivotally attached to its respective latch 82 through a pivot pin 89 this action causes the latch plates 82 to swing outwards thus releasing the riser termination main body 73 at the conical abutment 83 and hence the flexible riser assembly is in turn released and able to fall away from the main nozzle tube 79. It will be noted that the upper end of the reduced diameter section of the body is profiled at point 90 in the form of a cone and that likewise the outer circumference 91 is similarly formed.These features are provided respectively to ensure smooth passage of the pig through the bore of the main nozzle tube 79 and into the bore of the riser termination main body 73 and to assist in the entry of the main body 73 into the nozzle tube 79 when the pull-in is being completed.

The main body 73 is provided with at least two lugs 92 which are for use with the pull-in facilities represented in Fig. 10 by the chain dotted lines 93.

Reverting to Fig. 7 the pull-in arrangements are illustrated at 94. The two lifting cables 93 of Fig. 10 are indicated. In this case a lifting bridle 95 is shown in use, this item being specifically intended for the handling of the umbilical cable plug 69. Each lifting assembly consists of a bracket 96 arranged approximately radially to the lowermost manifold section 59.1 of Fig. 9, one bracket being provided per riser or umbilical termination point. The bracket is arranged to support pulleys 97 which in turn deflect the lifting ropes 93 to a convenient attitude 98 from which they can once again be led upwards via additional pulleys (not shown) to the central lifting point 32. Alternatively any other convenient means may be adopted for the tensioning of the pull-in ropes 93.

Claims (14)

1. A turret system of the kind referred to for a processing/storage vessel wherein the turret incorporates, for transferring fluids upwards or downwards to or from the vessel, a plurality of flexible hoses extending between an upper coupling member carried by the vessel and a lower coupling member carried by the turret, the hoses being disposed around the axis of relative rotation of the vessel and turret, and the coupling members being biased away from each other so that in a neutral position of the vessel and turret the hoses are held substantially straight but so that, as relative rotation of the vessel and turret takes place, the hoses are caused to take up a helical configuration with the coupling members moving towards each other against the biasing force, the latter causing the hoses to straighten again on return of the vessel and turret towards the relative neutral position.

2. A turret system according to Claim 1 wherein each hose is terminated by a swivel at one end and by a fixed flange at the other end to prevent axial twisting.

3. A turret system according to Claim 1 or 2 wherein the diameter of the section of turret which passes through the vessel hull is less than 10 metres.

4. A turret system according to Claim 3 wherein the diameter of the section of turret which passes through the vessel hull is of the order of 6 metres.

5. -A turret system according to any preceding Claim wherein the length of the hoses and the relative movement of the coupling members is such as to permit relative rotation of the vessel and turret to take place between at least -1800 and +1800 from the neutral position.

6. A turret system of the kind referred to for a processing/storage vessel wherein the turret accommodates a manifold assembly comprising a plurality of hollow toroidal manifold sections disposed coaxially one above the other, a plurality of pipes spaced around the common axis of the manifold sections, and connected to the manifold sections through valve means which permits each pipe to be connected to more than one said manifold section, the pipes having at their lower ends connectors enabling them to be coupled to flexible risers for transferring fluids to or from well head assemblies and/or to subsea manifolds, as the case may be, and each manifold section being connected to pipe means for transferring fluid to and from the vessel through a coupling means which permits a degree of relative rotation to take place between the manifold assembly and the vessel.

7. A turret system according to Claim 6 wherein the spaced pipes and valves are arranged so that any pipe may be connected to any one of the manifold sections.

8. A turret system according to Claim 6 or 7 wherein the turret incorporates, for transferring fluids upwards or downwards to or from the vessel, a plurality of flexible hoses extending between an upper coupling member carried by the vessel and a lower coupling member carried by the turret, the hoses being disposed around the axis of relative rotation of the vessel and turret, and the coupling members being biased away from each other so that in a neutral position of the vessel and turret the hoses are held substantially straight but so that, as relative rotation of the vessel and turret takes place, the hoses are caused to take up a helical configuration with the coupling members moving towards each other against the biasing force, the latter causing the hoses to straighten again on return of the vessel and turret towards the relative neutral position.

9. A turret system according to Claim 6, 7 or 8 wherein the arrangement of the sections of the manifold assembly and the associated valves which serve to join the sections together to form a complete assembly is such that by an appropriate setting of the valves at least one through passage of full bore size is provided from top to bottom of the manifold assembly without cross connections into any of the manifold sections.

10. A turret system according to Claim 1 wherein the upper coupling member is biased upwards by means of at least one rope which passes over a pulley located above the coupling member and is connected to a weight at its opposite end.

11. A turret system according to any one of Claims 1 to 5 or 10 incorporating a telescopic structure connected between the upper and lower coupling members and carrying buffer means against which the hoses rest when they are twisted into a helical configuration.

12. A turret system according to Claim 11 wherein the buffer means comprises a series of freely rotating rollers.

13. A turret system of the kind referred to substantially as shown in and as hereinbefore described with reference to Figures 1 to 10 of the accompanying drawings.

14. An oil processing/storage vessel incorporating a turret according to any preceding Claim.