GB2141470A - Offshore production systems - Google Patents

Abstract

A ship-shaped floating production system is moored via a tensioned riser (2), with associated motion compensation and riser pipe handling equipment. In one embodiment the attachment of the riser to the ship is through an hydraulic motion-compensation system including a gimballed mast (8). The pipe handling system (18) on the deck of the ship allows normal production to proceed, while ship motions are isolated from the riser, preventing excessive load transfer or unacceptable dynamic responses. In another embodiment a trussed bridge structure is pivotally mounted to the ship's deck at the aft end of the structure and the riser is attached to the fore end thereof. Float means are suspended below the bridge structure in flooded foretanks of the ship and the riser is connected to a production line swivel in a gimballed spider on the fore end of the bridge structure. In another embodiment the riser is attached to the ship by a rocking beam pivotally mounted on the ship, a rack and pinion arrangement being used to transmit horizontal loads. <IMAGE>

Description

SPECIFICATION Offshore Production Systems This invention relates to offshore production systems in which hydrocarbon production from offshore oil fields feeds a floating, ship-shaped production facility. In particular, it relates to the methods and apparatus to provide mooring of the vessel and to facilitate normal production in an integrated design.

Existing tanker-based floating production systems evolved from tanker mooring terminals.

After initial successes with these simple systems, more sophisticated types were developed to broaden the operational capabilities. For the purpose of putting the present invention into perspective, there are two fundamentally different types of systems. The difference is in the tanker mooring method and in the riser which connects the wellheads on the seabed to the tanker.

One type of floating production system consists of a buoy anchored to the seabed by a conventional catenary mooring spread. The tanker is attached to the buoy by a hawser and is free to swing around the bouy as the sea conditions change. The risers with this system are flexible hoses.

The other type of floating production system uses a single anchor leg or tower, instead of a catenary moor, and a rigid link or yoke connecting the tanker to the tower. Again the tanker is free to weathervane around the tower. In this case the tower acts as a riser as well as the mooring device.

The present invention improves upon the aforementioned methods by providing a tankerbased floating production system that is very mobile and relatively insensitive to water depth.

According to a first broad aspect, apparatus for mooring a large ship-shaped floating production system comprises a deployable tensioned riser, the riser tension and motion being accommodated by an hydraulic compensation system, a gimballed mast connecting the riser to the floating production system including means for adding additional lengths of riser while the riser is anchoring the ship.

In a further aspect, the invention provides a high capacity suction anchor, preferably of frustoconical configuration which provides high vertical holding capacity and high moment resisting capacity comprising a suction anchor plate, a rotatable cutter on the lower end of the plate, an open structural mast secured to and extending upwardly from the base, and web members extending upwardly and inwardly from the base to the upper end of the mast.

The invention also provides in another aspect a self-contained system for providing passive motion compensation at a ship-riser interface of a riser-moored floating production system or oil storage tanker, the system comprising a ship having flooded foretanks; a trussed bridge structure mounted on the deck of the ship, the bridge structure being pivotally mounted to the deck at the aft end of the structure and having its fore end overhanging the bow of the ship; a riser attached to the fore end of the bridge; vertical stanchions straddling the sides of the forebridge and being of sufficient heights to cover the vertical motion of the bridge; float means suspended below the bridge structure in the flooded foretanks of the ship; and a production line swivel in a gimballed spider mounted in the fore end of the bridge structure for connection to a production riser.

In a further broad aspect the invention provides a weight type motion compensation system for a riser moored tanker, the system comprising a rocking beam attaching a riser to the tanker, and a weight attached to the end of the beam remote from the riser, the rocking beam providing means whereby the beam support point moves to compensate for inertial accelerations of the tanker.

The invention also provides a method of mooring a ship-shaped floating production system by means of a deployable riser tensioned by a weight type motion compensation system mounted on the deck of the floating production system and using a rocker arrangement to reduce load fluctuation in the riser caused by the inertia of the weight.

Further features of the invention will emerge from a consideration of various embodiments thereof, which are illustrated by way of example in the accompanying drawings, in which: Figures 1, 2a and 2b are schematic views of an SALS single anchor leg system; Figure 3 is a schematic view illustrating the concept of the present invention; Figure 4 shows the direction of forces acting on the platform; Figure 5 is an elevation of a riser handling and motion compensation system; Figure 6 illustrates a method of positioning a riser prior to locking on to the well head; Figure 7 shows the production fluid offtake from the riser; Figure 8 is a perspective view of a craft incorporating the present invention; Figure 9 is a schematic elevation of'a high capacity suction anchor;; Figures 10, 11 and 12 are elevations of the apparatus shown in Figure 5; Figure 13 is an elevation of a bow mounted version of apparatus according to the invention: Figure 14 is a plan view of the bow of the craft shown in Figure 13; Figure 1 5 is an elevation of the bow section shown in Figure 14; Figure 1 6 is an elevation of another embodiment of the invention; Figure 17 is an elevation of a section of the craft shown in Figure 16; Figures 18 and 19 are separate embodiments of the riser handling system of the invention; Figure 20 is a diagrammatic elevation illustrating forces acting on the ship; Figure 21 is a view similar to Figure 20; Figure 22 illustrates changes in forces using the present invention; and Figures 23 and 24 are perspective views of the invention.

The present invention relates more to the single anchor leg, but a knowledge of the differences in the loading of the mooring system will help in the understanding of the invention.

One difference between the catenary moor and the single tower is that a catenary anchor line only acts in one direction, so many lines are required for multidirectional load capability. But the main difference is in the anchoring at the seabed. The tower, being rigid, puts a high vertical load into the seabed whereas the catenary moor relies on heavy chain weight and puts a horizontal load into the seabed. But at the surface, the principle is the same for both systems. The restraining force is provided by the horizontal component of the tension in the anchor line or tower T as shown in Figure 1.

Dealing now only with the tower, the tension is provided by buoyancy, either in the top of the tower T or in the yoke connection to the tanker.

The tower system is designed to suit the water depth and sea conditions of a specific site. Thus, to move the tower to a different location would require modifications to suit the new water depth.

The system is also permanent in that the release of the tanker requires a significant decommissioning operation. Similarly, the buoyant yoke assembly, although attached to the tanker by hinges, becomes a permanent part of the tanker, making it difficult for the tanker to move location in bad sea conditions. When considering deep water, the tower system has operation limitations. Because the system relies on the tower being at an angle to provide tanker restraint (i.e. a horizontal component of tension), the top of the tower swings downward as the angle of the tower increases as shown in Figure 2B. This vertical displacement is proportional to water depth. In deep water the yoke Y either requires greater movement or the buoyancy force must be increased to reduce the angular requirements of the tower.Either way, the whole system becomes larger, reducing its practical and economic viability.

Catenary anchor systems, although slightly less permanent than tower/yoke systems, have similar limitations. Movements and chain sizes become impractical in severe sea conditions and deep water.

The yoke Y is common to most of the larger facilities. It is coupled to the ship S with hinges H, on its beam girth line. The yoke is necessarily large for the following reasons: Its length provides heave and pitch freedom and its width must be such to allow direct mounting to the bow or stern of the ship at its girth line; It is heavy so as to be structurally capable of handling very large tensile, compressive, and torsional loads due to mooring and wave action.

In all cases, the yoke only has freedom to hinge up and down. Whenever the ship rolls, the structure must follow the ship, hence loading the hinge pins and twisting the relatively long yoke about the riser/tower/buoy connection. This is a serious load problem. Sway also "drags" the entire yoke to the side further complicating the force combination at the hinges.

Suffice to say that the yokes are extremely robust and correspondingly heavy. Even the smallest ones, used in quite moderate sea conditions, weigh 500-600 tons. The best known unit, TAZERKA, has a yoke weight of over 2000 tons.

Buoy systems "disappear" on crossing the 500 ft. depth boundary. Towers with associated yokes also lose favour at 600 ft depth. The reasons are that the deeper water means more chain length for the buoy: it gets bigger, catches more valve loading and ruins the yoke-buoy connection. For towers, towing it out horizontally and uprighting it is critical: too much bad treatment and it bends.

For the "SALM" systems, which introduce an articulation at the centre of the tower, there is an improvement. However, a system has not yet been installed in deep water.

The "SALS" system tends to stand out on its own, but again, it is presently bounded by the "tower" weakness which also limits the system to a specific, shallow water site.

One thing common to all these known yoke systems, is that the riser/swivel/manifold unit is remote. That means access problems to the riser itself. All these systems impose limitations Qn themselves, especially their access features, by answering only the strictly functional, mooring, problems. To say nothing of deployment.

The features of the present invention attempt to address as many of the functional and operational aspects as possible, most benefits being realized from the unique motion compensation arrangements.

The objective of the present invention is to overcome the above mentioned limitations of the art and to provide a tanker-based floating production system that is very mobile and relatively insensitive to water depth, featuring inexpensive, passive motion compensation systems.

Description of Figures 1-12 The objective of the present invention is to overcome the above limitations and provide a tanker-based floating production system that is very mobile and relatively insensitive to water depth. This objective is achieved by having a riser R that is made up from sections of riser and deployed from the production tanker as seen in Figure 3. The riser is lowered from the tanker T as it is made up, locked to a riser base on the seabed, and tensioned by a hydraulic motion compensator C on the tanker. The tanker T is then ailowed to move away from its original position under the action of wind, waves and current until the riser R is at a sufficient angle to stop the tanker movement. As in the tower and yoke systems, the horizontal component of the riser tension provides the restraining force on the tanker as shown in Figure 4.

The basis of the present invention is that the tanker is moored directly by the riser. The riser is similar to those already used as marine drilling risers, except that it has sufficient strength to take the mooring loads, and it contains the production tubing.

Tensioning of the riser is by a "passive" hydraulic cylinder and accumulator arrangement similar to drilling riser motion compensators, but with modifications to suit the mooring requirements. The passive designation means that the system is self-contained and operates without any external energy input or control. The motion compensation system, therefore, is acting as a fluid spring.

In shallower water the motion compensation cylinders will have sufficient stroke to cater for not only heave and pitch of the tanker but also for the riser moving from vertical to its maximum operating angle of about 20 degrees. The hydraulic system is arranged so that when the riser is vertical, the minimum tension necessary is applied to the riser. With the riser at its maximum angle, the motion compensation cylinder will be operating at the other end of its stroke and will provide the maximum tension necessary. This characteristic is achieved simply by the action of filling or emptying the hydraulic accumulator.

When the tanker is subjected to increasing forces from wind, waves and current, it moves away from its centre position and the riser inclines at an angle. As this angle increases, not only does the horizontal component of the riser tension become greater, but the tension itself becomes greater due to the hydraulic system. For marine drilling riser systems, this non-constant tension characteristic is undesirable, but for the risermoored tanker it is beneficial. This makes a simple reliable system achievable.

In deeper water, the stroke required'to make up for the vertical displacement of the top of the riser as the riser changes angle is too large to be practical (as described earlier for tower and yoke systems). In this case the riser operating angle is restricted to a range near to the high angle end, i.e. from 10 degrees to 20 degrees. To enable this to be accomplished, an additional feature is added to the system. This feature allows the nominal operating pressure to be changed in broad increments. Thus, as a storm builds up, the forces on the tanker will cause the riser to increase its angle. After several hours, the riser will begin to reach its maximum angle. The system pressure is then changed to the next higher increment, which puts a high tension into the riser, and the riser angle will move back to its minimum angle.It is anticipated that only two or three increments will be required. Although this is adding an "active" control, its use is very infrequent, and the timing of its use is probably a matter of hours, rather than minutes or seconds. Thus, there would be adequate time for alternative action if a failure should occur in this active component.

In discussing motion compensation, an hydraulic cylinder has been assumed. Most riser motion compensators consist of a hydraulic cylinder acting through a cable and sheave system. This reduces the cylinder stroke requirements. But the cable is a constant source of failures and is a high maintenance item. Thus, for the present invention, long cylinders are used directly and used so that they are always in tension. The arrangement of the mast makes this possible, and it avoids the buckling problems associated with long hydraulic cylinders.

The motion compensation discussed above is for motion of the tanker in a vertical direction, i.e.

heave. Other tanker motions must also be accommodated or isolated from the riser. Sway and surge of the tanker will move the riser in a horizontal direction through the water, which will provide relatively little resistance, and thus will not be a significant problem. Yaw of the tanker will twist the riser, so a swivel S is provided at the top of the riser. Pitch and roll of the tanker will induce unacceptable bending loads into the riser.

To isolate the riser from these loads, the riser tensioning and motion compensation equipment is attached to a mast, which is mounted on a gimbal as shown in Figure 5. The gimbal provides the flexibility between the angular movements of the tanker and the riser. In order for the mast to move with the riser, the mast is extended some distance below the gimbal where this extension acts as a lever that the riser pushes against to keep the mast in alignment with the riser. A weight 50 is also placed at the end of this lever in order to balance the mast about the gimbal. Thus, when the mast is at an angle, its overhanging weight will not induce bending into the riser, either static or dynamic.

Normally, the riser and mast will not be moving angularly relative to a fixed point such as the seabed, but instead the tanker will move in the waves about the riser. However, there will be angular movement of the mast due to secondary forces so it is necessary that the mass of the mast is kept to a minimum and near the gimbal in order to keep inertial loading to a minimum.

A secondary feature of the gimballed riser support mast is its use during lock-on of the riser subsea. A guidelineless and diverless riser subsea lock-on technique gives operational flexibility and economic advantages to the overall system. It is expected that the guidelineless lower riser package described in our copending application No. 8404269 will be used. With this or any other guidecone system, the base of the riser must be brought close enough to the seabed mandrel so that it is within the catchment area of the cone.

This can be done using a jet at the base of the riser, or by moving the tanker at the surface. The present invention also uses the gimballed riser mast to move the riser as seen in Figure 6. During the riser deployment stage, the mast is controlled by hydraulic cylinders. By placing the mast at an angle, the riser leaves the mast at an angle which gradually changes until at the bottom of the riser it is hanging vertically. The net result is that the bottom of the riser is displaced horizontally when the angle of the riser mast is changed. The process of controlling the guidance can be handled manually using sonar and TV information.

But is would be more satisfactory to use a computer to assess the positional information and control the riser mast directly. The system would be similar to a ship's dynamic positioning system, except that instead of controlling thrusters, the mast hydraulic cylinders would be controlled. If the tanker has thrusters, then these, as well as the tanker main propulsion, could also be controlled to give some ship positioning. After the riser is locked to the riser base on the seabed the hydraulic cylinders for the control of the riser mast are deactivated and the mast is guided by the riser.

One of the reasons for deploying the riser from the tanker is that it can be made up quickly and easily to any length. Another reason is to enable the riser length to be increased when it is used in deep water and at an angle. This ability is only required during the initial running of the riser and the hanging off of the tanker. The motion compensation and riser handling is arranged to accomplish this task and also to embody a backup for a total compensator failure.

When the riser is being run it is suspended from a spider or other holding device while the next joint or length of riser is being added. In existing riser drilling systems the spider is located on the drill floor of the rig, which is not compensated. Compensation is only used after the riser is completely made up and the final suspension cables attached to the top of the riser.

In the present invention the spider platform 6 is motion-compensated so that the suspended riser is always motion-compensated while it is being made up. The riser handling system is located on the spider platform; it consists essentially of a hydraulic cylinder 7 that holds the next length of riser while it is being attached to the already made-up riser. After the connection is made, the hydraulic cylinder 7 lowers the complete riser until the top of the new length of riser is held in the spider. This process is repeated until the full length of riser is made up. After the riser is attached subsea and the tanker drifts away from its original location, the riser handling hydraulic cylinder 7 lets the top of the riser descend as the riser angle increases. In deep water another length of riser will need to be added.Because the spider platform is motion compensated and the riser handling cylinder can take full riser tension, this is handled in the same way as any other new length of riser attachment. When the tanker has drifted sufficiently to give the riser its correct mean angle, the riser handling cylinder tensions the riser upward against a stop. The force from the handling cylinder is higher than the motion compensation cylinder, but below the maximum riser tension rating. Thus, the riser is held rigidly to the spider platform which is motioncompensated. If, for any unforeseen reason, the motion-compensation system should jam, or lock up, the riser handling cylinder will extend as soon as the tanker moves upward on a wave, and the riser tension overcomes the tension in the cylinder.The riser handling cylinder thus acts as a temporary motion compensator, it having its own accumulator circuit. In this way a completely independent motion compensator is available as a backup instantaneously, which requires no mechanism to engage or any control or monitoring input whatsoever.

With the riser being motion-compensated relative to the motion of the tanker, the top of the riser will travel a large distance relative to the deck of the tanker. For systems designed for less hostile areas it is possible that flexible hoses can be used for fluid transfer between the top of the riser and the tanker deck. For severe environments it is proposed to use long solid metal tubing that flexes through an angle that is small enough to allow flexure within the elastic range of the metal as illustrated in Figure 7. The tubing can be bundled and supported to form a multi-tube flex unit as proposed in our copending application No. 8404269. The geometry is arranged to suit the movement of the mast in all directions. This provision of fluid transfer will reduce the failure and maintenance problems associated with flexible hoses. A similar arrangement is proposed for the riser base.

Combined System Referring to Figures 8-10 a floating production system is connected to a subsea riser base anchor 1 by a tensioned riser 2, the upper termination of which is a multiple-pass swivel 3, the lower termination being a connector assembly 4 which mates with a conical riser base termination 5. The swivel 3 is mounted on the working platform 6, which in turn is suspended from hydraulic jacks 7, the cylinders of which are mounted on the fixed external framework 8. The internal framework 9 runs vertically in guide rails 10, which are mounted on the mast superstructure 8. To permit the ship freedom in the rolling and pitching axes, the mast superstructure 8 is supported by a gimbal frame, having inner and outer gimbal rings, items 11 and 1 2 respectively. The inner gimbal bearings transmit the mast loads to the outer gimbal ring by bearings 13, while the outer gimbal ring transfers its loads by bearings 14, which seat on bearing blocks 15, secured to the stiffening ring 1 6 which surrounds the moon pool 1 7.

The riser handling system 1 8 is located forward of the moon pool area and consists of a self-storing structural base 19, a riser elevator 20, and a horizontal traverse slide 21. The duty of the handling system is to present riser sections to the mast horizontally. The transition to the vertical is accomplished by using the lifting head 22 and associated hydraulic jacks 23, which form the vertical riser handling system over the moon pool.

Once the ship establishes its position over the riser base anchor 1, riser pipe sections are handled, made up, and lowered until the depth is almost reached. At this point, the motion compensation jacks 7 are energized and the final distance made up with sufficient riser pipe. The riser is then located over the riser base and the connection completed. The ship then drifts to an offset position, riser pipe added as required, motion compensation applied throughout. A position is accomplished where the ship has an offset from the riser base such that the offset angle is between ten and twenty degrees.

The remaining deck-mounted equipment on the ship includes the process plant 24, flare stacks 25, port and starboard, product pipeline 26, product and hydraulic manifold house 27, and helideck 28.

Motion Compensated Riser Handling Mast The entire assembly shown in Figures 10 and 12 is carried on a gimbal, items 11 and 12, which transfer the riser and mast deadweights and dynamic loads to the ship's deck, through bearing blocks 1 5.

The mast superstructure 8 is a lattice-braced open frame, which is rigidly fixed to the inner gimbal 11. Both legs of the mast are joined at their upper ends by a crosspiece frame 35, forming a rigid structure. Guide rails 10 are secured to the inner faces of the mast, running the full height. These rails provide guidance for the internal framework 9, which is free to ascend and descend within the confines of the mast 8.

Also secured to the mast legs 8 are hydraulic cylinders 7. The rod ends of cylinders 7 are attached to the working platform 6, which, once energized hydraulically, will serve to move the entire internal framework 9 up or down. By so doing, the working platform 6 will effectively displace the top end of the riser 2 and the attached multiple-pass swivel 3. By stroking cylinders 7 appropriately, the relative motion of ship and riser can be accommodated, tension maintained in the riser, and an efficient mooring tether achieved without undue stresses in the riser or end connections.

The internal framework 9 is equipped with four wheeled shoes 36 which run in the guide rails 10.

At the upper end of the frame, a bank of hydraulic cylinders 23 extends from the internal framework crosspiece 37, suitably supported by a tapered stanchion frame 38. These cylinders 23 form the drive for the lifting head 22, which draws riser joints up into the space above the working platform 6, lowers them down through the moon pool, and generally handles pipe within the mast, including stabbing in of riser joints. The internal framework 9 with its working platform 6 is a separate entity in the mast, connected to the mast legs only indirectly by the wheeled shoes 36 and by hydraulic jacks 23. In the riserfeed and removal operations, the working platform 6 sequencing is coordinated with the deck-mounted riser handling system 18.

The riser handling installation 18 shown in Figure 11 has a combined elevator 20 and traverse system 21. Riser joints are stored within the structural base 19, these being fed toward the central elevator gallery 39 by tilting rails 40 arranged within the base 19. Individual riser joints are fed onto the elevator 20, which ascends and presents the joint to the open jaws 41 in the traverse gantry 42.

The hydraulic system for motion compensation has fail-safe capability. The two main hydraulic rams 78 are composed of ram clusters 43 rather than single, large diameter units. A thrust head 44 combines the ram efforts from each unit in the cluster. Normal operating pressure is 1 500 psi; but, should one or more clusters fail, the platform 6 remains fully supported and motion compensated. This is achieved by duplexing the hydraulic supply pressure, providing pressure to the available diagonally-opposed cylinder pair.

This is a worst-case condition, where effectively half the hydraulic lift capacity is lost. Should the primary hydraulic system be lost, a secondary (passive) system will assume the duty as described earlier.

A passive hydraulic control system was described earlier as the preferred method.

However, an active control system could also be used. The control system would be computer controlled and would consist of a hydraulic circuit control centre, a riser tension and deflection angle monitor, and a riser handling logic system. An alarm system would be provided for excessive loading conditions, and for hydraulic and critical equipment failures. Load-shedding and secondary system load transfer is arranged automatically.

Figure 12 shows the riser mast 8 tilted at a typical mooring angle of twenty degrees. The extent of the working platform 6 and the other pair of heave compensation cylinders 7 are clearly seen. A significant feature of the system is that platform 6 is used to store a few additional riser joints, which are manipulated into position in the riser string, all the handling taking place while connected to the subsea riser base anchor 1. The level of automation in the handling system, and the degree of heave compensation control, allows production to proceed under minimal supervision.

Riser Base Anchor System The riser, while mooring the tanker, places a very high vertical load on the seabed anchor. For tower and yoke production systems piled gravity bases have been used. These, of necessity, have to be very large. Although a gravity base can be used with the present invention, there are advantages in terms of transportation and commissioning in having a lighter anchor. Figure 3 shows a cylindrical type suction anchor. This has a very good side and moment resistance, but in some soils it could have low vertical load capability. Figure 8a shows an alternative type suction anchor. It is a plate type anchor where the weight of soil on top of the anchor resists the vertical pull. This principle is the basis for the "Hydropin" patented by the National Engineering Laboratory in the U.K.But this type of anchor does not possess the vertical rigidity required for mooring the tanker through the riser, and can only be installed in soils that can be fluidized.

The present invention, therefore, provides a rotatable cutter to a basic suction anchor plate, plus an open structural mast for the seabed riser connection. At the top portion of the mast, large webs are attached that provide lateral resistance in the soil. These webs not only provide side load capability, but also, in combination with the suction base, provide moment resistance. Figure 9 shows the suction anchor device 29, which utilizes suction, jetting, and mechanical cutting in its installation. The unit is designed to penetrate most seabed soils, including clay. By applying reduced pressure below the lower cone 30, a driving force is established which causes the anchor device to move down. This motion is augmented by high-pressure water jets 31 and optional rotating mechanical cutters 32.Once the device has reached the desired depth, the internal driving shaft 33 (if used) is abandoned in place.

Rotation is provided by a hydraulic motor, powered by fluid supplied from the surface. The riser mating cone assembly 5 mounted on the swivel joint 34 is then ready for service. The swivel joint ensures that no bending is induced in the riser 2, and an offset angle of up to thirty-five degrees is tolerated.

The Riser System The embodiments of this system are fully described in our earlier application No. 8404269 and includes the upper riser swivel 3, riser connector joint 45, and lower riser connector package 4. Inclusion of the riser system in this disclosure is to emphasize its superior strength and fatigue characteristics, both directly relevant to riser mooring.

Description of Figures 1 3-1 9 As in the tower and yoke systems, the horizontal component of the riser tension provides the restraining force on the tanker when it is allowed to move away from its original position under the action of the elements.

Flotation provides substantial forces, which are considered "free". Hydraulics will do the same, but with unwanted complexity and expense.

Floats in the sea beside a ship pick up waveinduced forces. If they are attached to push rods, levers, cage structures or other devices, they invariably have to move around in the water, inducing high loads in the linkages, etc. Basically, having floats attached to the ship, external to the hull, is not an intelligent way of finding free forces for mooring. Whenever the ship rolls, for example, so must the float, often at its worst extension.

This causes problems of friction, roll amplification, unwanted structural loads, etc.

The SALS system is a prime example of a float external to the ship which must be held in a massive structure just to survive its demanding environment.

All the buoy mooring systems have the same problem, as mentioned previously. As depths and sea states get more demanding, the buoyancy must be increased. However, a definite limit is reached; if this limit is ignored, the only way to make the system work is to make structures, floats and bearings very large, clumsy and expensive.

By putting devices within the confines of the ship in accordance with one embodiment of the present invention some clear advantages are observed: not influenced by wave induced forces, or splash zone pounding; floats roll, pitch, yaw, sway and surge with the ship; it is a controlled environment with good access; operators can observe and monitor float behaviour, conditions; buoyancy can be controlled directly by using compressed air to de-ballast the floats; the S.G. of the surrounding medium can be altered to derive optimum buoyancy, viscosity; travel of the float or heave is a fraction of the ship's heave; float accelerations and velocities (heave) are also a fraction of the ship's values; float shapes can be more innovative due to the better defined operating environment; the float is totally self-contained within the ship and needs no deployment steps whatsoever; and the float can be used to provide base forces during riser deployment.

The invention also includes two embodiments of a riser handling system. Both embodiments utilize a box-like, wheeled carriage which runs on rails up and down a compensator bridge structure. It is designed to store approximately 360 m of riser pipe, all in 15.25 m joints, in the vertical position. Once unloaded, it is winched to the hinge end of the bridge and parked. The carriage is contained within the truss structure of the bridge, with lateral guide rails at the top to secure the carriage within the bridge.

In a first embodiment, the actual lifting mechanism of the overhead crane is a winch assembly, using cable and multiple sheaves. The mounting of the winch must be integral with the overhead crane. Power supply may be electric or hydraulic. The leadscrews which move the crane relative to the carriage are synchronised in each axis. Response velocities to follow the moving riser are expected to be about 1 5 cm/sec (maximum). The control feedback system is a simple proportional/integral type which uses pickup transducers on the gimbal for position information. For the actual latching/lifting sequence, the conical guide on the lifting head is self-aligning to the riser joint due to a balljoint in the unit.

The second embodiment may be considered as a miniature derrick which forms part of the gimbal. The lifting mechanism is typically cables and sheaves. The handover of a riser joint from the lifter unit to a manipulator arm requires a perfect phasing control, again derived from transducers on the gimbal. The arm is semirobotic and must be capable of handling 1 5-20 tonnes.

It must also have sufficient reach at this load capacity to store the joint safely in the carriage rack.

As shown in Figures 13 and 14, a floating production system 60 is connected to a subsea riser base anchor 61 by a tension riser 62, the upper termination of which is a multiple pass swivel 63, the lower terminal end of the riser being a connector assembly 64 which mates with a conical riser base termination 65. The swivel 63 is mounted in a gimballed spider 66 which in turn is held in a framework that forms the fore end of the trussed bridge structure 67. The bridge 67 is pivoted at its aft end by a deck-mounted hinge bearing 68. The entire bridge is constrained laterally by two vertical stanchions 69 which consist of two columns and associated lateral bracing. As the ship heaves up and down, these stanchions remove lateral loading near the gimbal. The bridge sides carry bearing pads with roller guides 70 which reduce friction as the bridge moves relative to the stanchions.The vertical posts and associated side bracing that straddle the sides of the forebridge extend upwards to a sufficient height to cover the vertical motion of the bridge. These posts absorb lateral forces which arise from mooring upsets; no lateral forces are transmitted into the bridge and hence its modest structure. Whenever the ship takes an upset angle of instance to the weather, it is forced to return-weather vaning perfectly from the bow.

A roller carriage on each side of the bridge engages the posts providing an easy-running mechanism. The pin on the aft bridge is loaded in one plane only (tension induced shear) with no torsion or lateral bending permitted.

Taking the gimbal 66 as the "fixed point" it will be appreciated that the ship is free to heave, pitch, roll, yaw, surge and sway by virtue of the following uncoupling mechanisms: the gimbal 66 which uncouples roll, sway, surge and basic pitch; the floats 71 and bridge 67 which uncouples heave and implied pitch heave; and the swivel 63 which uncouples yaw.

The bridge 67 is of light weight, transparent structure consisting of a double sided truss with cross bracing to complete a box section. The bridge 67 can be set at any desired ang!e of inclination by de-ballasting the floats 71 (Figures 14 and 15) and to provide a heave compensation ability on initial riser deployment, twin hydraulic dylinders or compensating rams 83 are latched to the truss sides as shown in Figure 1 5.

Figure 14 shows the location of the internal floats 71 which are directly below the two sides of the bridge structure 67. The top of the riser 62 and swivel 63 are seen emerging from the gimballed spider 66, the stanchions 69, lateral braces72 and top cross head 73 are also illustrated. The riser storage capacity, in excess of the normal handling system, is arranged in a vertical shaft 82 through a deck cutout as shown in Figures 14 and 1 5.

The floats 71 are separated to reduce drag, viscous effects and added virtual mass inertia while kept low in profile to achieve maximum vertical traverse. The floats 71 are necessarily large to meet the buoyancy requirement. By mounting the floats 71 to the bridge 67 with rigid links 74, the structural rigidity and dimensions of the truss are optimized. Full buoyancy of the floats 71 is approximately 2.5 x 106 kg. which, though high, is several orders less than the SALS system for example.

Figure 1 5 is a cut away drawing to reveal the array of internal floats 71. In practice, an integrated matrix array of four longitudinal and four transverse floats, fully interlocked, would be used for the high sea state buoyancy requirements. Furthermore, the aft float depths would be greater than the fore cylinders, hence producing a wedge-shaped array. The floats 71 are rigidly fixed to the bridge 67 by links 74 which are straight but may be curved suitably to achieve minimal tank cover 75 penetration. A coffer dam 76 which can provide up to 2 m additional shiptank head is shown at the fore end of the tanks. A riser abandonment float 77 forms the lower end of a reinforced upper riser section 78 which allows the ship to uncouple from the riser if conditions come about which places the ship/riser in jeopardy.An outline of the riser handling system 79 is indicated in phantom line, depicting the riser deployment/withdrawal mode. The active heave compensation rams 83 are shown in an extended position.

Figure 1 7 shows the counter weight 20 which helps to balance the dead weight of the entire bridge/float assembly and permits a slight reduction of actual float size. Bridge stops 81 are shown, these preventing the assembly from slapping the deck plating in transit and providing a sea-lock mechanism. They also ensure that the bridge cannot depress the float beyond the ship tank bottom. This Figure together with Figure 16 is a moon pool version of this embodiment of the present invention.

Two main tanks are utilised in the ship structure. Up to 43 m design traverse from the gimbal can be attached and the floats are kept within the ships own tanks. By adding 1.5 m coffer dam around the tanks at the fore end, extra traverse can be achieved at the gimbal.

Fortythree metres is a typical North Sea requirement.

The ship's transverse bulkhead between tanks must be removed and the opening reinforced at the periphery. The longitudinal bulkheads are left in place.

Riser Handling System Since the handling system is required only when deploying and retrieving the riser, moving it into position and storing it during operation is a major feature. By setting the handling system above the bridge structure main bearing, its own dead weight is transferred to the ship's deck, not the float array, The system takes the form of a mobile carriage with a specialized lifter mechanism.

Figures 1 8a, 1 8b shows one embodiment of the riser handling system. The carriage 30 mounts an overhead crane which is free to move in two axes, horizontally and runs on rails 31 using a set of trolley wheels 32. Its motion and position is determined by a double pair of lead screws which, when driven, cause the crane to track the motion of the riser directly beneath. A storage rack 33 for riser pipe is also provided. Within the carriage is a ribbed metal plate working platform 34. Latches 35 secure the carriage to the rails when properly aligned over the gimbal 36. A simple feed back control feature is incorporated between the gimbal and the lead screw motor mechanisms. A cable system 37 is provided for hauling the carriage along the bridge as illustrated. The overhead crane beam 38 and wheeled trolleys 39 traverse the fore and aft carriage rails 40.The central winch drum 41 and lifting head 42 traverse the crane beam on a rail system. A ball joint 44 and conical latching mechanism 45 complete the lifter unit. Two pairs of leadscrews 46 engage with the overhead crane beam 38 and winch drum 41 driven by hydraulic/electric motors 47 which are fully synchronised. The feedback control loop 48 is also illustrated.

By constantly tracking the moving riser, the lifting head is kept in close proximity, thus a connection can be made. The conical latching mechanism 45 compensates for the final misalignment caused by the riser's angular gyrations. Once the lifting head 42 is brought down to the riser, the cone engages over the end, seats, and then accomplishes a positive latch. The riser can then be lifted.

Referring to Figure 19, a gimbal mounted derrick structure 50 is illustrated with a sheavetype crown block 51 at the apex of the structure.

The lifting winch 52 is set on a foundation, mounted to the carriage. The purpose of the lifting mechanism is only to secure and lift the riser, hence its relatively light weight. The lifting head 53 with internal latching mechanism is shown above the gimbal 54 where the riser joint 55 protrudes upwardly. A manipulator arm system 56 with a gripping head 57 is located such that it can secure joints of riser pipe and place them in the carriage storage rack. The feedback control loop is illustrated at 58.

Once a joint is pulled, the lifter stands idie while the manipulator arm 56 secures the joint and pulls it clear of the lifter. This joint is then stored within the rack on the carriage. As the manipulator arm is controlled by the feedback control loop 58, based on gimbal angular movement, it therefore "tracks" the moving riser so that it can attach and pull a joint without time phasing problems.

The embodiment of Figure 18 causes an overhead crane to track lateral motion and establish a lifting connection by a conical device with internal latches. The embodiment of Figure 1 9 has the system mounted on the gimbal, pulling the pipe with no tracking problems then transferring the pull joint "on the move" to a semi-robotic manipulator arm which follows the motion.

Principle of Operation, Typical Sequence a) Deploying Riser, Start-Up 1. The ship arrives on station, lowers the riser package 62 and one riser joint in the gimbal 66.

2. Sea locks opened-bridge structure free.

3. Internal floats 71 de-ballasted to lift bridge off deck stops. Tune buoyancy to float bridge.

4. Carriage crane 30 picks up one riser joint; lifter is traversed to place joint over gimbal 66.

Joint lowered and connection made to waiting joint.

5. Lifting device in handling system lowers the lower riser package and two joints. Spider opens, then re-secures riser.

6. Repeat until full riser deployed, minus last joint, before latch-on. As riser is added, the bridge is floated as before, using more buoyancy force from the floats.

7. Hydraulic rams in main heave-compensation system latched to the bridge; rams are energised.

Bridge is now under active compensation control.

Since all the bridge and riser weight is carried by the floats, the rams are not required to provide forces other than inertia, friction and drag breakout values.

8. Carriage crane system places final joint and lowers as before, with active (hydraulic) control now applied to its rams to "fine tune" the overall heave compensation process. This way, a near perfect latching operation to the riser base guide cone should be possible, in elevated sea states (5-6).

9. Further joints (about two) are added, as in step 8., to allow the ship to take up its mooring angle of approximately 200 (offset 364 ft. in 1000 ft. water depth). Necessary riser tension is maintained throughout operation by bridge force and/or active hydraulic control.

1 0. Once mooring position is reached, swivel is attached and flowlines are connected. Floats are blown out to the required buoyancy for the specific weather condition and ship draft. Active hydraulic control of the bridge rams is terminated rams are unlatched. Bridge, floats and riser are now fully inter-connected and the system is in its passive compensation mode.

b) Tripping out Riser-Bad Weather 1. Carriage crane is moved from its stowed position within bridge framework to gimbal station.

2. Using lifting head, swivel is removed and stored.

3. Using lifting head, attach to and lift riser, maintaining appropriate tension. (Spider releases riser, then re-sets). Ship must move forward.

4. First joint is disconnected and stored.

5. Operation is continued until riser tripped out.

c) Jeopardy Situation-Riser Abandonment Consider any of the following: i) Subsea blowout ii) Riser handling system failure iii) Extreme weather conditions or immediate need for abandoning location iv) Other grounds for upper release provision where either system (riser, ship) can better survive only if separated v) A routine separation due its convenience In this regard, the following procedure is suggested: 1. Shut in production. Remove swivel. Arm gimbal release latches.

2. De-ballast ready-installed riser abandonment float OR install awaiting float.

3. Standby main engines, zero revolutions.

4. Reverse thrust from engines. Bridge lifted sharply upward with active hydraulic rams.

Gimbal latches released.

5. Riser, float and upper protective cage structure will separate and self-right to the vertical. The riser is fully tensioned; the small waterplane area and reinforced upper section would assure survival. Ship can abandon location safely.

6. Re-connection is straightforward since the riser upper attachment point is above the surface.

Additional features of the invention listed below will be appreciated.

The riser base could be deployed and set on the sea bed from the tanker (assuming lightweight base which is ballasted by pumped concrete from the surface). Pile or suction anchor devices are also feasible.

A moonpool version of the system as shown in Figure 1 6 is feasible for ice-infested waters. The only significant variation is the ship modification necessary in a moonpool design.

A counterweight which helps to balance out the bridge/float/riser/lifter weights is used if water depths exceeding 250 m are expected as seen in Figure 17. Adding moment arm aft of the pivot permits the float sizes to be reduced slightly for a given sea state. Too much weight incurs a penalty of inertia, so a compromise is used.

Curved struts linking the floats to the bridge structure would ensure minimal tank cover penetration and splash effects. Simple cuff seals, rubber, contain the liquid.

Variable geometry linkages between floats and bridge, where the ends are pin-jointed and an inclined or curved track displaces the float array forward or aft to counteract remaining force variation due to float added mass and drag.

Description of Figures 20-24 The present invention seeks to provide an "inert" or passive method of motion compensation between the riser and the tanker that minimizes secondary forces, and is universal in its application. The secondary forces referred to here are drag forces on buoyancy cans and inertia of the apparatus. The objective is to reduce that load fluctuations in the riser in order to increase the fatigue life. Some known devices use a pivoting beam whereby the riser is attached at one end and a counterweight is attached to the other end. Figure 20 shows the method diagrammatically. Vertical loads from the riser are thus balanced by the weight, and horizontal loads from the riser are transmitted to the tanker via the pivot. The vertical motion of the tanker is accommodated by the beam pivoting.Although this is a classical mechanism its use in mooring a tanker requires modifications in order to make it practical.

The purpose of the motion compensation is to uncouple the vertical motion of the tanker from the riser. The vertical motion of the tanker accelerates the counter balance weight resulting in an inertia load, directly changing the riser tension. The acceleration of the weight is not just the acceleration of the tanker at the pivot point but is factored up to the lever arm, Figure 21.

Thus if the pivot is equidistant between the riser and weight, a factor of 2 applies. This result is inherent to any weight system where the weight is used to apply an upward vertical force. For instance if the weight were hung on a cable that passed up over a sheave and down to the riser, the weight would travel twice the distance relative to the sheave and thus have twice the acceleration (assuming that the riser remains stationary and the sheave moves). This weight, cable and sheave arrangement has been used in the past for motion compensation of drilling risers because it is so simple, but is no longer used because of the high inertia load fluctuations. The present invention significantly reduces the inertia effects of weight type motion compensation.

The load in the riser is proportional to the weight and the beam/pivot geometry. The present invention provides a means of changing the beam/pivot geometry in proportion to the change of inertia, i.e. the pivot point is moved to compensate for the change of inertia load. This is accomplished by substituting the pivot with a rocking surface with the size and shape of the rocker being chosen to suit the characteristics required, Figure 22.

The motional of the tanker at the pivot point will be approximately sinusoidal. When the weight is at the lowest point its velocity will be zero and its acceleration will be at a maximum, increasing the downward force due to the weight.

For this condition the pivot point needs to be near the weight to reduce the moment arm for the weight and increase the moment arm for the riser.

Conversely, when the weight is at its highest position the weight again has zero velocity and maximum acceleration but in the opposite direction, decreasing the downward force due to the weight. Thus for this case the pivot needs to be near the riser. These are the two extreme positions for the pivot point. Intermediate positions can be derived based on the motion of the weight. If the motion is sinusoidal then a rocker based on an arc of a circle provides the correct location of the pivot point throughout the range.

The rocker arrangement described above allows the pivot point to move and also supports the weight of the complete rocking beam. But it cannot transmit any horizontal load-which is the primary objective of the mooring system. A rack and pinion gear arrangement is therefore used whereby the rocker is the pinion and the support is the rack. In order to prevent any relative slippage the rolling surface of the rocker must be coincident with the pitch circle diameter of the gear geometry. For simplicity a circular arc has been used for the rocker and a flat surface for the support. However, any shape could be used for either, depending on the characteristics required.

If the motion of the tanker at the effective pivot point is not sinusoidal but some type of step function this can be accommodated by changing the rocker shape. In practice the motion characteristics will continually change depending on the randomness of the sea condition and the response of the tanker. But the variations from the characteristics built into the rocker will probably be minimal from the riser fatigue loading viewpoint.

System Description Figure 23 shows the floating production vessel being moored by the riser. Although the arrangement shows the riser being deployed over the bow of the tanker it could also be deployed through a moonpool. A detail of the mooring and motion compensation equipment is shown in Figure 24. The Riser 101 is attached to the riser support mast 102 by a thrust bearing whereby the riser is restrained from moving in all degrees of freedom except in rotation. Thus the tanker can rotate around the riser without twisting the riser.

The riser support mast 102 is attached to the motion compensation rocking beam 103 by a gimbal 104 allowing the riser support mast to pivot in all directions. The riser support mast extends below the gimbal to enable a counterweight to be used to ensure that the mast stays nominally in a vertical position and reduce bending loads in the riser. At the lowest point of the riser support mast 102 a riser guide 105 is used to keep the riser support mast always aligned with the riser. The riser mast gimbal 104 is located at one end of a rocking beam 103. At the other end of the rocking beam is a weight in the form of a tank 106. The tank can be filled with water or other fluid to adjust the counter balance weight. The amount of weight required is enough to balance the equipment plus the riser tension load required.The rocking beam 103 sits on top of the rocking beam supports 107 which are located above the deck level at about half the height of the motion compensation stroke. This is to minimize the horizontal movement of the riser due to the gimbal end of the beam swinging through an arc. This feature is not critical to the overall function of the invention but is chosen as a helpful feature. The rocking beam 103 is shown as a space frame structure with the supports far apart. This not only allows a light structure to be used but allows riser side loads to be reacted easily at the supports. Horizontal loads, both fore and aft and side to side are reacted at the supports by the gear arrangement described earlier. As the beam rocks the curved surface on the beam rolls along to support surface.No sliding takes place because the pitch circle diameter of the gear teeth is coincident with the rolling/rocking surface. The movement produced by side loads of the riser or sideways inertia loads of the weight are reacted as differential loads on the gear teeth on each side of the beam. The actual side loads themselves are reacted as end load on the gear teeth or other suitable thrust surface.

The lengths of riser (called joints) are stored on the forward end of the beam in the riser loading and storage equipment 108. This equipment raises each piece of riser into the riser mast 102 where the riser handling equipment 109 is used to connect the riser joints together and lower it towards the seabed. When oil is being produced through the- riser a multi pass swivel 110 is used on the top of the riser. Flex hoses and piping are used to transport the oil from the swivel to the process equipment on the tanker.

Description of Operation The attachment of the riser to the riser base on the seabed is done in the same way as described above. The tanker is positioned over the riser base on the seabed. The riser mast 102 is located in a vertical position by hydraulic cylinders. The riser loading and storage equipment 108 then moves a length of riser towards the riser mast until the end is directly below the riser handling equipment 109. The riser handling equipment has a winch and travelling block arrangement similar to that normally used for handling drill pipe and casing on floating drill rigs, including a small stroke hydraulic motion compensator. This compensator is normally only used during the locking on of the riser to the riser base.

The travelling block of the riser handling equipment 109 locks onto the end of the riser and lifts it upwards. The riser then swings from a horizontal position to a vertical position in the riser mast. The lower end of the riser is guided by the riser loading equipment 108. With the riser joint (length of riser) in the vertical position it is lowered onto the lower riser package on an existing length of riser, and connected to it. The riser handling equipment 109 then lowers the complete riser assembly until the upper end of the riser reaches the support platform at the gimbal.

Further joints of riser are then added in the same way.

When the correct length of riser has been layed out the counter balance tank is filled with water so that the beam rocks and places the gimbal and riser mast near its highest position. The riser, with the last new joint of riser attached, is lowered towards the riser base by the riser handling equipment. Final positioning in a horizontal plane is done by moving the gimbal which will swing the riser over at an angle and the bottom of the riser will hang in a different location. Vertical motion combination during this operation is done grossly by the rocking beam but mainly by the handling equipment compensator. After the riser is locked to the riser base the tanker propulsion and station keeping system is shut down and the counterbalance tank filled with water to provide the correct riser tension. There are now no actively controlled systems working and the tanker drifts with the wave, wind and the current forces until the riser finds its equilibrium position.

Claims (14)

1. Apparatus for mooring a large ship-shaped floating production system by means of a deployable tensioned riser, the riser tension and motion being accommodated by an hydraulic compensation system; a gimballed mast connecting the riser to the floating production system including means for adding additional lengths of riser while the riser is anchoring the ship.

2. Apparatus according to claim 1 and including a guide and balance arm attached to the gimballed riser support mast whereby the riser aligns the support mast with the riser and a weight balances the overhanging weight of the mast for static and dynamic balance.

3. Apparatus according to claim 1 or 2 and including means for angling the gimballed riser support mast so that the lower end of the riser is correctly positioned for engagement with the riser base on the seabed.

4. A high capacity suction anchor which provides high vertical holding capacity and high moment resisting capacity comprising a suction anchor plate, a rotatable cutter on the lower end of the plate, an open structural mast secured to and extending upwardly from the base, and web members extending upwardly and inwardly from the base to the upper end of the mast.

5. A self-contained system for providing passive motion compensation at a ship-riser interface of a riser-moored floating production system or oil storage tanker, the system comprising: a ship having flooded foretanks; a trussed bridge structure mounted on the deck of the ship, the bridge structure being pivotally mounted to the deck at the aft end of the structure and having its fore end overhanging the bow of the ship; a riser attadhed to the fore end of the bridge; vertical stanchions straddling the sides of the forebridge and being of sufficient heights to cover the vertical motion of the bridge; float means suspended below the bridge structure in the flooded foretanks of the ship; and a production line swivel in a gimballed spider mounted in the fore end of the bridge structure for connection to a production riser.

6. A system according to claim 5 wherein the float means comprises separated, interconnected float tanks connected to the underside of the bridge structure by link arms.

7. A system according to claim 6 wherein the depth of the aftermost float in the tank of the ship is greater than the fore end floats thereby producing a wedge-shaped array.

8. A system according to claim 5, 6 or 7 including a riser abandonment float forming the lower end of a reinforced upper riser section.

9. A system according to any one of claims 5 to 8 and including a riser handling system moveable between an inoperative position remote from said gimbal and riser and an operative position over said gimbal, the handling system including an overhead crane free to move horizontally in two axes; motor means and leadscrews for moving the crane; storage means for riser pipe; latch means for securing the carriage to the rails when aligned over the gimbal; winch means and a lifting head including a conical latching mechanism for engaging the riser pipes.

10. A system according to any one of claims 5 to 8 and including a riser handling system comprising: a storage rack for the riser pipe; a carriage system having a set of rails; a gimbal mounted derrick structure having a sheave-type crown block mounted on the carriage; a lifting head with an inter.nal latching mechanism for securing and lifting the riser; and a manipulator arm system and gripping head for securing joints of riser pipes for movement into -the storage rack.

11. A system according to any one of claims 5 to 10 and including a counterweight on the bridge structure aft of the pivot point thereof.

12. A weight type motion compensation system for a riser moored tanker, the system comprising a rocking beam attaching a riser to the tanker and a weight attached to the end of the beam remote from the riser, the rocking beam providing means whereby the beam support point moves to compensate for inertial accelerations of the tanker.

1 3. A system according to claim 1 2 wherein the weight comprises a fluid-filled tank.

14. A method of mooring a ship-shaped floating production system by means of a deployable riser tensioned by a weight type motion compensation system mounted on the deck of the floating production system and using a rocker arrangement to reduce load fluctuation in the riser caused by the inertia of the weight.

1 5. A method according to claim 14 including the step of transmitting horizontal force on the rocking beam through the use of a rack and gear arrangement and wherein the pitch circle diameter of the gear teeth is coincident with the rolling surface of the rocker.

1 6. A floating production system and assemblies and components for use therein substantially as hereinbefore described with reference to the accompanying drawings.