NO842405L - DEVICE AND PROCEDURE FOR SUPPLYING A HYDROCARBON PRODUCTION SYSTEM ASSOCIATED WITH A SHIP - Google Patents

Description

The present invention relates to hydrocarbon production from offshore oil fields to a floating production plant on board a ship. More specifically, it relates to a method and a device to enable mooring of the ship and to enable normal production.

Existing tanker-based floating production facilities evolved from tanker mooring facilities. After an initial success with such simple systems, more complicated types were developed to expand the operational possibilities. In order to provide the background for the present invention, two fundamentally different types of plant will be described. The difference lies in the mooring method and in the riser that connects the wellheads on the seabed with the tanker.

One type of floating production facility comprises a buoy anchored to the seabed using a conventional anchor. The tanker is attached to the buoy so that it can drift freely around the buoy as conditions at sea change. The risers in this facility are flexible hoses.

The other type of floating production plant uses a single mooring leg or tower instead of lines or the like, and a rigid arm connects the tanker to the tower. The tanker can rotate freely around the tower. In this case, the tower acts as both a riser and a mooring device.

The present invention involves an improvement in relation to previously known methods and devices, in that a production facility has been arrived at that is highly mobile and relatively unaffected by the water depth.

According to one aspect of the invention, this is achieved by using a riser that is put under tension, since tension and compensation for movements is achieved by means of a hydraulic system. The riser is connected to a gimbal-suspended mast which has means for attaching additional lengths of the riser to this while the riser is mooring the ship.

A suction anchor with a high capacity and shaped like a truncated cone provides a large vertical holding capacity and a high capacity in terms of resisting moments.

According to another aspect of the invention, a system has been devised for providing passive motion compensation in the transition between a ship and a riser in a floating production facility or storage tanker, which system comprises a ship with filled forward tanks, a truss bridge mounted on the deck of the ship, pivotable relative thereto with its aft end, the forward end projecting from the bow of the ship, a riser attached to the forward end of the bridge, vertical posts or the like along the sides of the bridge, of sufficient height in relation to for the vertical movement of the bridge, a float device fixed under the bridge of the forward tanks, as well as a swivel for a production line in a universal joint mounted at the forward end of the bridge, for connection to a production riser.

According to a further aspect of the invention, this relates to a system for movement compensation for a tanker moored by means of a riser, and this system comprises a tilting arm connecting a riser to the tanker, as well as a weight attached to the end of the arm opposite to the riser, so that the rocker arm compensates for movement of the tanker in relation to the riser.

The invention will be explained in more detail below, with reference to the attached drawings.

Fig. 1, 2a and 2b schematically show a mooring system

with a single leg.

Fig. 3 schematically shows the main idea of the present one

invention.

Fig. 4 shows the direction of forces acting on the ship. Fig. 5 shows, seen from the side, the system for handling

riser and motion compensation.

Fig. 6 shows the procedure for placing a ladder pipe before this is attached to the wellhead. Fig. 7 shows transport of produced fluid from the mast. Fig. 8 shows in perspective a vessel that includes it

present invention.

Fig. 9 schematically shows a suction anchor with high capacity,

seen from the side.

Fig. 10, 11 and 12 show the system in fig. 5 side view. Fig. 13 shows an embodiment of a device according to

to the invention mounted in the hull of a vessel.

Fig. 14 shows the construction of the vessel shown in fig. 13.

Fig. 15 shows the hull part in fig. 14 seen from the side.

Fig. 16 shows another embodiment of the invention,

seen from the side.

Fig. 17 shows a section of the vessel shown in fig. 16, set

from the side.

Fig. 18 and 19 show embodiments of the system for

handling of risers, according to the invention. Fig. 20 schematically shows forces acting on the ship.

Fig. 21 is a similar figure to fig. 20.

Fig. 22 shows the change in forces when using it

present invention.

Fig. 23 and 24 show an embodiment of a device i

according to the invention, seen in perspective.

The present invention is closely related to the simple mooring leg, but knowledge of the differences will contribute to the understanding of the invention. A difference between line anchoring and the simple tower is that an anchoring line only works in one direction, so that many lines are required to provide forces in several directions. The main difference, however, lies in the anchoring on the seabed. The tower, which is rigid, causes a large vertical force against the seabed, while the anchor lines are based on the weight of heavy chains and provide a horizontal force in the seabed. At the surface, the principles are the same for both systems. The holding force comes from the horizontal component of the tension in the anchor line or tower T, as shown in fig. 1.

In the tower, the stretch occurs due to buoyancy, either in the top of the tower T or in the arm connection of the tanker.

The tower system is designed to suit the water depth and sea conditions at a particular location. In order to move the tower to a different location, modifications are therefore required to adapt to the new water depth. The system is also permanent in that disconnecting the tanker requires a specific disconnection operation. Similarly, the buoyancy arm device, although attached to the tanker in a hinged connection, is a permanent part of the tanker, making it difficult for the tanker to change position in difficult sea conditions. With regard to deep water, the tower system has limitations in terms of use. Because the system is based on the tower being tilted to provide holding power for the tanker (that is, a horizontal component of the stretch), the top of the tower swings downward as the tilt direction of the tower increases, as shown in fig. 2b. This vertical displacement is proportional to the water depth. In deep water, the arm arrangement Y either requires greater movement, or the buoyancy force must be increased to reduce the requirements for an inclined position for the tower. In any case, the whole system becomes large, which reduces its practical and economic usability.

Anchoring systems with liners, which are less permanent than systems with towers and booms, have similar limitations. Movements and chain sizes become impractical in difficult sea conditions and deep water.

The arm arrangement Y is common to most large plants. It is connected to the ship S by hinges H, along one edge. The arm arrangement is necessarily large, for the following reasons: Its length must be free for pitching and dove movements, and its width must be such that it enables direct connection to the bow or stern of the ship with the edge.

It is heavy, in order to be able to withstand large tensile and compressive forces and torsional loads caused by the wave effect.

In all cases, the arm assembly is only free to swing up and down. When the ship rolls, the structure must follow the ship, which puts a strain on the hinge pins and turns the relatively long arm assembly around the connection to the riser, tower or buoy. This is a serious load problem. Doving will also pull the entire arm assembly to the side, which complicates the force combination in the hinges.

The arm devices are therefore very robust and correspondingly heavy. Even the smallest, which are used in very moderate sea conditions, weigh 500 - 600 tonnes. The best-known device, the TAZERKA, has a weight of over 2,000 tons.

Buoy systems are forfeited if a limit of 150 meters depth is exceeded. Cones with associated arm devices are also mainly omitted when the depth exceeds 180 metres. The reasons are that in deeper water, increasing chain length must be used for the buoy, which becomes larger, receives greater wave load and damages the connection between the arm device and the buoy. For towers, the horizontal tow-ing and uprighting is critical, and the tower is bent by too hard-fetched treatment.

In systems that include a link in the middle of the tower, the conditions are better. However, no such system has yet been installed in deep water.

Other known systems with towers entail the same disadvantage, which limits the use of the systems to shallow waters.

Common to the known boom systems is that the unit consisting of the riser, swivel joint and manifold is at a distance from the ship. This means access problems to the riser itself. All these systems entail limitations, particularly with regard to access, and fulfill the needs only for the purely functional aspects of the mooring.

With the present invention, the aim is to improve as many of the functional and operational improvements as possible, and most of these improvements are achieved by means of a device for compensation of movements.

The purpose of the present invention is to avoid the limitations mentioned above, and to arrive at a floating production system which is highly mobile and relatively insensitive with respect to water depth, and involves cheap, passive systems for compensation of movements.

An embodiment of the invention is illustrated in fig. 3, which illustrates an embodiment of the invention. A riser R formed by several sections extends downward from a production vessel. The riser is lowered from the ship T as it is assembled, attached to a base part on the seabed and tightened by a hydraulic compensator C for movement on board the ship. The tanker T can then move away from its original position under the influence of wind, waves and currents, until the riser R is at a sufficient angle to stop the movement of the ship. In the same way as with the systems with towers and arm devices, the horizontal component of the tensile force in the riser provides the braking force against the ship, as shown in fig. 4.

A basic idea of the present invention is that the ship is anchored directly in the riser. The riser can be of a similar type to that already used in offshore production systems, with the exception that it has sufficient strength to withstand the mooring loads, and the riser contains production pipes.

The riser is put under tension by means of a passive arrangement of a hydraulic cylinder and an accumulator, in a similar way to compensators for the movement of risers, but with modifications to adapt to the mooring. By passive is meant that the system works automatically and without the supply of energy from the outside or control. The system for compensation of movements therefore acts as a fluid spring.

In relatively shallow water, the motion compensation cylinders have sufficient stroke to compensate not only for pitching and rolling motions of the ship, but also for the riser moving from a vertical position to its maximum tilt, of approximately 20°C. The hydraulic system is arranged in such a way that when the riser is vertical, the riser is exposed to the minimum necessary tensile force. When the riser forms its greatest angle, the cylinder for compensation of movements will provide the greatest tensile force required. This is achieved in a simple way by filling or emptying the hydraulic accumulator.

When the ship is exposed to increasing forces from wind, waves and currents, it moves away from its original position, and the riser takes an inclined position. As the angle increases, not only will the horizontal component of the tensile force in the riser become larger, but also the tensile force itself will become larger due to the hydraulic system. For offshore drilling systems, this non-constant tension is undesirable, but for a ship anchored by means of a riser, it is beneficial, as it enables the formation of a simple and reliable system.

In deeper water, the movement required to compensate for the vertical movement to the top of the riser when the riser changes angle is too great for the principle to be used in practice (as described for the known systems). In this case, the angular range of the riser is limited to a range close to the largest angle, that is from 10 to 20°. To achieve this, the system includes additional funds. These means make it possible for the nominal operating pressure to change significantly. When a storm is developing, the forces against the tanker will cause the riser to increase its angle. After several hours, the riser will approach its maximum angle. The system pressure is then changed to the second highest, which causes a large tensile force in the riser, and the angle of the riser will decrease to the smallest angle.

It is believed that only two or three pressure steps are necessary. Although this results in "active" control, its use will occur very rarely and the accuracy with respect to time will likely be in the order of hours rather than minutes or seconds. There will therefore be sufficient time for alternative actions should a failure occur in this active component.

When it comes to movement compensation, a hydraulic cylinder can be used. Most compensators for riser movements consist of a hydraulic cylinder that works via a cable and pulleys. This reduces the demands on the cylinder's impact movement. But the cable is a constant source of error, and requires a great deal of maintenance. According to the present invention, long cylinders are therefore used, in such a way that they are constantly under tension. The arrangement of the mast enables this, and means that bending problems associated with long hydraulic cylinders are avoided.

The above-mentioned movement compensation is for movement of the tanker in the vertical direction, i.e. pitching or dove. Other movements of the tanker must also be able to be recorded or isolated from the riser. Dove and movements due to rough seas will move the riser in a horizontal direction through the water, which causes relatively little resistance and thus is not a significant problem. Rolling will twist the riser, and therefore a pivot bearing is provided at the top of the riser. Stamping and rolling of the tanker will cause unacceptable bending stresses on the riser. In order to isolate the riser from such loads, the equipment for keeping the riser under tension and to compensate for movements is attached to a mast, which is mounted on a cardan joint, as shown in fig. 5. The universal joint provides flexibility between the angular movements of the tanker and the riser. In order for the mast to move together with the riser, the mast is extended a certain length below the cardan joint, and the extension acts as an arm that the riser pushes against to keep the mast aligned with the riser. A weight 50 is also placed at the end of this arm, to balance the mast around the cardan joint. When the mast thus forms an angle, the weight of the mast will not cause bending of the riser, either statically or dynamically.

When ground, the riser and mast will not change angle in relation to a fixed point, such as the seabed, but instead the tanker will move in the waves in relation to the riser. However, there will be an angular movement of the mast due to secondary forces, so it is necessary that the mass of the mast be kept to a minimum, with the center of gravity near the gimbal, to keep inertial forces to a minimum.

Another feature of the gimbal-suspended support mast for the riser is the use of the mast when connecting the riser underwater. A technique for connecting the riser underwater without guides and without divers provides flexibility and economic benefits for the system as a whole. It can e.g. a device for the lower end of the riser described in Canadian patent application 421,909 is used. With this or any other guide cone system, the lower end of the riser must be brought sufficiently close to the pipe on the seabed to be within reach of the cone. This can be done by using a jet at the lower end of the riser, or by moving the tanker on the surface. In the present invention, the mast for the riser is also used to move the riser, as shown in fig. 6. During the installation of the riser, the mast is controlled by hydraulic cylinders. By placing the mast at an angle, the riser leaves the mast at an angle that gradually changes, until the riser hangs vertically with its lower end. The result is that the lower end of the riser is displaced horizontally when the angle of the mast is changed. The process of controlling the guidance can be carried out manually using sonar and TV information. However, it is better to use a computer to process information about the position and to control the mast directly. This system is similar to a system for dynamic positioning of a ship, with the exception that the hydraulic cylinders for the mast are controlled instead of auxiliary propellers. If the tanker has auxiliary propellers, these, as well as the tanker's main drive, can be controlled for positioning the ship. After the riser is connected to a foundation on the seabed, the hydraulic cylinders for steering the mast are put out of service, and the mast is controlled by the riser.

One of the reasons why the riser is set out from the tanker is that this can be done quickly and easily to any length. Another reason is to make it possible to increase the length of the riser when used in deep water and at an angle. This possibility is only necessary during the first deployment of the riser and when it hangs down from the tanker. The means for compensation of movements and handling of the riser are arranged to perform this task, and to be able to be used in the event of total failure of the compensator.

When the riser is laid out, it is suspended in a suspension device while the pipe parts are joined. In existing drilling systems, the suspension device is located on the drilling floor in the rig, which is not compensated. Compensation is only used after the riser has been fully assembled and the suspension cables have been attached to the top of the riser. In the present invention, the suspension platform 6 is movement-compensated, so that the suspended riser is always suspension-compensated while it is being produced. The system for handling the riser is located on the suspension platform, and mainly consists of a hydraulic cylinder 7 which holds the next length of pipe while it is attached to the already formed part of the riser. After joining, the hydraulic cylinder 7 lowers the entire riser until the top of the new length of pipe is held in the suspension device. This process is repeated until the entire riser is formed. After the riser is connected underwater and the tanker drifts away from its original position, the hydraulic cylinder 7 lowers the top of the riser as the angle of the riser increases. In deep water, another length of riser must be added. Because the suspension platform is motion compensated and the riser handling cylinder can accommodate the full extension of the riser, this is treated in the same way as any other length of riser. When the tanker has drifted sufficiently to give the riser the correct mean angle, the cylinder stretches the riser upwards towards a stop. The force from the cylinder is higher than from the movement compensation cylinder, but is less than the maximum allowable tension in the riser. The riser is thus held firmly to the suspension platform, which is movement compensated. If the motion compensation system is blocked for some unforeseen reason, the cylinder for the riser will extend as soon as the tanker moves up a wave, and the tension in the riser overcomes the tension from the cylinder. The cylinder for the riser thus acts as a temporary movement compensator, and has its own accumulator circuit. In this way, there is a complete and independent motion compensator that can be used instantly, and that does not require any mechanism for connection or any control or monitoring.

When the riser is motion compensated in relation to the movements of the tanker, the top of the riser will move a large length in relation to the deck of the tanker. For systems intended for quiet areas, it is possible to use flexible hoses for transferring fluid between the top of the riser and the deck of the tanker.

For troubled areas, long metal pipes can be used that bend to an angle that is sufficiently small to lie within the elastic area of the metal, as shown in fig. 7. The pipes can be bundled together and supported so that they form a flexible pipe bundle, as described in Canadian patent application 421,909. The geometry is adapted to suit the movement of the mast in all directions. This fluid transfer arrangement reduces failure and maintenance problems associated with flexible tubing. A similar arrangement

can be used for the lower end of the riser.

Fig. 8-10 shows a floating production system which is connected to an underwater anchorage 1 for the riser via a riser 2 which is under tension, the upper end of the riser being a rotary coupling 3, while the lower end is a coupling 4 which is adapted to a conical end coupling 5. The rotary coupling 3 is mounted on the work platform 6, which hangs in hydraulic jacks 7, the cylinders of which are mounted on the fixed, external framework 8. The internal framework 9 can be moved vertically on guide rails 10 which are mounted on the framework 8. In order for the ship to be free to roll and pitch, the framework 8 is suspended in a gimbal frame, which has an inner and an outer gimbal ring, 11 and 12. The inner gimbal bearings transfer the load from the mast to the outer gimbal ring via bearing 13, while the outer cardan ring transfers the load via bearing 14, which is mounted on bearing blocks 15 attached to the stiffening ring 16 which surrounds the shaft 17.

The system 18 for forming the riser is located in front of the shaft, and consists of a foundation 19, an elevator 20 and a horizontal slide 21. The task of this system is to bring riser sections horizontally to the mast. The transfer to a vertical position is carried out using a lifting head 22 and associated hydraulic jacks 23, which form the system for handling the riser above the shaft.

After the ship has taken its position above the bottom anchorage 1, the riser sections are supplied, secured and lowered until the depth is almost reached. At this point, the jacks 7 are activated for movement compensation, and the last length of the riser is formed. The riser is thus located above the foundation, and the connection is complete. The ship then drifts away, riser lengths are added as needed, and motion compensation is performed. The positioning is complete when the ship is at such a distance from the foundation on the bottom that the angle is between 10 and 20°.

The rest of the equipment on the deck of the ship includes the treatment plant 24, flaring tower 25, pipelines 26 for the products, a collection house 2 7 for products and hydraulics, as well as a helicopter deck 28.

The whole assembly shown in fig. 10 and 12 are carried by a cardan joint, parts 11 and 12, which transmit the weight of the riser and the mast as well as dynamic loads to the ship's deck, through the bearing blocks 15.

The framework 8 in the mast is a truss, which is permanently mounted on the inner part 11 of the cardan joint. The two legs of the mast are joined at the other ends by means of a transverse frame 35, so that a rigid structure is formed. Guide rails 10 are attached to the inside of the mast, throughout its entire height. These rails form a guide for the inner framework 9, which can be moved freely up and down inside the mast 8.

Hydraulic cylinders 7 are also attached to the legs of the mast 8. The piston rod ends of the cylinders 7 are attached to the work platform 6, which, when hydraulically actuated, serves to move the entire inner framework 9 up or down. Thereby, the working platform 6 will displace the upper end of the riser 2 and the rotary bearing 3 which is attached to it. By appropriate activation of the cylinders 7, a mutual movement occurs between the ship and the riser, and tension is maintained in the riser, so that an effective anchoring force is achieved without excessive tension in the riser or the end connections.

The inner framework 9 is equipped with 2 elements 36 with wheels which can be moved along the guide rails 10. At the upper end of the frame, a series of hydraulic cylinders 23 project up from the transverse frame 3 7 to the inner framework, supported by an inclined support frame 38. These cylinders 23 constitute drive devices for lifting head 22, which pulls riser pipe parts up and into the space above the work platform 6, lowers the parts down through the shaft and generally holds and transports pipes inside the mast, including bringing the pipe parts together. The inner framework 9 with the working platform 6 is a separate unit in the mast, connected to the mast only indirectly via the elements 36 with wheels and the hydraulic jacks 23. When supplying and discharging the riser, the use of the platform 6 is coordinated with the system 18 for handling riser parts on deck.

The installation 18 for handling riser pipe parts shown in fig. 11 comprises a combination of an elevator 20 and a traverse system 21. Riser pipe parts are stored in the lower structure 19, and are led to the middle elevator plate-form 39 by means of inclined rails 40 arranged inside the structure 19. The individual pipe parts are led to the elevator -tor 20, which is raised and delivers the pipe parts to the open jaws 41 in the traverse crane 42.

The hydraulic system for movement compensation is equipped with failsafe. The two main hydraulic drives 78 consist of groups 43 instead of large diameter units. A push head 44 transfers the forces from each unit in the group. The normal operating pressure is 105 kg/cm 2 , but should one or more groups fail, the platform 6 will remain fully supported and movement compensated. This is achieved by two-part supply of the hydraulic pressure, to give pressure to the diagonally opposite pairs of cylinders. This is the worst possible case, as half the lifting capacity is gone. If the primary hydraulic system fails, a secondary (passive)

system perform the task, as described earlier.

A passive hydraulic steering system has previously been mentioned as preferred. However, an active control system can also be used. The control system may be controlled by a computer, and may consist of a control center for a hydraulic circuit, means for monitoring the stretch and angle of the riser and a logical system for handling the riser. An alarm system can be arranged to warn of overload, and to warn of failure of the hydraulic and essential equipment. There is an automatic system for storing and transferring cargo.

Fig. 12 shows the mast 8 when it forms an angle of 20°. The size of the working platform 6 and the second pair of compensating cylinders 7 is shown. An important feature of the system is that the platform 6 is used to store a few extra riser parts, which are guided into place in the riser string, with all handling taking place while connected to the bottom anchor 1. The degree of automation in the system and the degree of control with compensation for vertical movements enable production to take place under minimal supervision.

During anchoring of the tanker, the riser will cause a very large vertical load on the bottom anchorage. For production systems that include a leg and a rigid arm, heavy foundations attached with piles have been used. These must necessarily be very large. Although a heavy foundation can be used in the present invention, it is an advantage in terms of transport and placement to use a lighter anchorage. Fig. 3 shows a suction anchor of cylindrical type. This gives very high resistance to lateral movements such as bending moment, but in certain types of bottom material the ability to absorb vertical load may be small. Fig. 8a shows an alternative type of suction anchor. This is a plate-type anchor where the weight of the bottom material above the anchor resists the vertical tension. This principle is the basis of "Hydropin" patented by the National Engineering Laboratory in the U.K. But this type of anchor does not provide the necessary vertical resistance for mooring the tanker via the riser, and can only be installed in bottom materials that can be fluidized.

The present invention therefore uses a rotating cutting device and a suction anchor plate, as well as an open mast for connecting the riser on the bottom. In the upper part of the mast, large plates are attached which provide lateral resistance in the bottom material. These plates not only provide the ability to absorb lateral loads, but also, in combination with the suction anchor, resistance to bending. Fig. 9 shows the suction anchor device 29, which uses suction, flushing and mechanical cutting during assembly. The unit is designed to penetrate most bottom materials, including clay. By creating negative pressure under the lower cone 30, a force is created which causes the anchor device to move downwards. This movement is increased by water jets 31 with high pressure and mechanical cutting devices 32. After the device has reached the desired depth, the inner drive shaft 33 (if used) is left in place. Rotation is achieved by means of a hydraulic motor, driven by fluid supplied from the surface. The cone device 5, which is adapted to the riser and is mounted on the rotary coupling 34, is thereby ready for use. The rotary coupling ensures that no bending moment is transferred to the riser 2, and an angle of up to 35° is tolerated.

The embodiments of the riser system are fully described in Canadian Patent Application 421,909, and include the upper pivot 3, the riser coupling 45 and the lower riser coupling device 4. The purpose of describing the riser system in this specification is to emphasize the superior strength and fatigue properties, which both are directly relevant when it comes to anchoring using the riser.

In the following, fig. 13 - 19 are described in more detail.

As in the leg and arm systems, the horizontal component of the tensile force in the riser causes the braking force on the tanker when it is allowed to move away from its original position under the influence of external forces.

Buoyancy causes significant forces, which are considered free. Hydraulics will do the same, but are undesirably complicated and expensive.

Floats in the sea next to a ship are affected by forces from the waves. If they are attached to struts, arms, grid structures or other devices, they must move in the water, causing great stress on joints and so on. Using floats attached to the ship, outside the hull, is not an advantageous way of obtaining free forces for anchoring. When the ship e.g. rolls, the float must follow, often with the greatest movement. This causes problems with friction, reinforcement of rolling, unwanted stresses and so on.

Turret systems are an example of a float on the outside of the ship that must be held in a massive structure to withstand the effects of the environment.

All the buoy anchoring systems have the same problem, as previously mentioned. At greater depths and under more difficult conditions, the buoyancy must be increased. However, there is a limit, and if this limit is exceeded, the only way to make the system work is to build constructions, floats and storages that are very large,

unwieldy and expensive.

By placing devices within the boundaries of the ship, according to an embodiment of the present invention, clear advantages are achieved: there is no influence of forces from the waves or blows in a splash zone,

the floats roll, bump and perform other movements together with the ship,

an area with good access has been created,

operators can observe and monitor the behavior of the floats,

the buoyancy can be controlled directly by using compressed air to remove ballast from the floats,

the specific gravity of the surrounding medium can be changed to achieve optimal buoyancy and viscosity,

the movements of the floats are a fraction of the vertical movements of the ship,

the accelerations and velocities of the floats are also a fraction of the values for the ship,

the shape of the floats can be varied to a greater extent due to the better defined surroundings during use,

the floats are located inside the ship and do not need to be deployed, and

the floats can be used to provide forces when deploying the riser.

The invention also includes two embodiments of a system for handling the riser, and in both systems a box-like carriage with wheels that rolls on rails up and down along a compensator bridge structure is used. This is designed to store approximately 360 meters of riser, in sections of 15 metres, in a vertical position. After unloading, the trolley is pulled to the hinge end of the bridge and parked. The height of the carriage is within the beam structure of the bridge, with lateral guide rails on top to keep the carriage within the bridge.

In a first embodiment, the actual lifting mechanism for the crane is a winch device that uses a cable and several pulleys. The winch is mounted together with the crane. The power supply can be electric or hydraulic. The spindles that move the crane in relation to the carriage are synchronized in each axis. The speeds needed to follow the moving riser are assumed to be approximately 15 cm/sec. (maximum). The system for returning information for control is of a simple proportional/integral type which uses registration transducers on the gimbal joint to provide information about the position. For gripping and lifting, the conical guide on the lifting head is automatically aligned with the pipe part, due to a ball joint in the unit.

The second embodiment can be considered a small tower that forms part of the cardan joint. The lifting mechanism is cables and pulleys. The transfer of a pipe part from the lifting unit to a manipulator arm requires perfect control, which is achieved with the help of transducers on the gimbal. The arm is partly a robotic arm, and must be able to lift 15 - 20 tonnes. It must also have sufficient reach at this lifting capacity to be able to place the pipe parts in a safe manner in the trolley frame.

As shown in fig. 13 and 14, a floating production system 60 is connected to a bottom anchor 61 by means of a riser 62 which is under tension, and the upper end of the pipe is a pivot 63, while the lower end of the riser is a coupling device 64 which fits together with a conical part 65 on the foundation. The swivel joint 63 is mounted in a plate

66 which is held in a framework which forms the outer end of the truss bridge structure 67. The bridge 67 is hinged at its rear end by means of a hinge bearing 68 mounted on the deck. The entire bridge is laterally blocked by two vertical bars 69 which consist of two columns and associated side bars. When the ship moves up and down, these rods remove side loads near the gimbal. The sides of the bridge hold bearing plates with roller guides 70 which reduce friction as the bridge moves relative to the rods. The vertical posts and the associated side bars which connect the sides of the bridge project upwards to a sufficient height to enable the vertical movement of the bridge. These posts absorb lateral forces from the anchorage, and no lateral forces are transferred to the bridge and its flimsy construction. When the ship takes an oblique direction in relation to the waves and wind, it is forced to take a direction so that the waves and wind act directly against the building. A carriage with rollers on each side of the bridge rests against the posts and constitutes a mechanism that can be easily moved. The pin at the rear end of the bridge is loaded in one plane only, and is not subjected to. torsion or bending.

If the plate 66 is considered as a fixed point, it will be understood that the ship can freely move up and down in pitching movements, rolling movements and other movements due to the following mechanisms: the joint 66, the floats 71 and the bridge 67 and the pivot joint 63.

Bridge 67 has a low weight, and is a construction consisting of two side trusses with crossbars connecting them. The bridge 67 can be placed at any desired angle by removing ballast from the floats 71 (Figs. 14 and 15) and to achieve compensation for vertical movements during the initial deployment of the riser, two hydraulic cylinders 83 being connected to the truss sides, as as shown in fig. 15.

Fig. 14 shows the location of the inner floats 71, which are directly below the two sides of the bridge structure 67. The top of the riser 62 and the pivot joint 63 protrude from the joint 66, and there is also shown the rods 69, the side rods 72 and an upper cross bar 73. Space for storing riser parts in addition to the space in the normal system is arranged in a vertical shaft 82 through an opening in the deck, as shown in fig. 14 and 15.

The floats 71 are separated to reduce the force against the water, effects of viscosity and mass inertia, and they have a low profile to achieve maximum vertical movement. The floats 71 are necessarily large to provide the necessary buoyancy. When mounting the floats 71 on the bridge 67 using rigid arms 74, the stiffness and dimensions of the construction are optimal. The full buoyancy of the floats 71 is approximately 2.5 x 10^ kp, which, although it is a lot, is significantly less than e.g. the systems with anchoring legs and rigid arms. Fig. 15 shows the ship in section, to show the row of internal floats 71. In practice, rows of four longitudinal and four transverse floats, which are interconnected, will be used when the requirements for buoyancy are great. Furthermore, the depth of the rear float will be greater than that of the front cylinders, so that a wedge-shaped row is formed. The floats 71 are firmly connected to the bridge 67 by means of arms 74 which are straight, but these can also be curved to achieve minimal opening in the cover 75. A lowering box 76 which can provide up to approximately 213 cm of additional tank length is shown at anterior end of the tanks. A float 77 on the riser forms the lower end of a reinforced, upper riser section 78, which enables the ship to be disconnected from the riser if conditions arise which mean a danger to the ship and the riser when these are connected. Dashed lines indicate the system 79 for movement of the riser. The vertical movement compensation cylinders 83 are shown with raised piston rods. Fig. 17 shows the counterweight 20 which helps to balance the bridge and the floats and enables a certain reduction of the float size. Bridge stoppers 81 are shown, which prevent the bridge from hitting the cover plates and which form a blocking mechanism. The stoppers also ensure that the bridge cannot push the floats past the tank bottom. This figure, together with fig. 16, shows an embodiment with a continuous shaft in the ship.

Two main tanks are used. Up to 425 cm of movement can be achieved from the cardan joint, and the floats are kept within the ship's own tanks. By adding a 150 cm dropbox at the forward end of the tanks, further movement of the joint can be achieved. 425 cm is a normal requirement in the North Sea.

The ship's transverse bulkheads between the tanks must be removed and the opening reinforced around the perimeter. The longitudinal bulkheads can be left in place.

As the system for handling the riser is only necessary when deploying and removing the riser, it is essential how it can be brought into place and stored during use. By placing the system above the main bearing of the bridge structure, the weight of the system is transferred to the ship's deck, and not to the floats. The system takes the form of a mobile cart with a special lifting mechanism.

Fig. 18a and 18b show an embodiment of the system for

handling of risers. The carriage 30 holds a crane which can move along two axes horizontally and runs on rails 30 using a set of wheels 32. The movements and position of the carriage are determined by two pairs of screw spindles, which when driven cause the crane to determine the movement of the riser which is located below. A storage rack 33 for riser parts is also provided. Inside the carriage is a work platform 34 formed from a metal plate with ribs. A latch 35 secures the carriage to the rails when it is correctly aligned over the cardan joint 36. A simple

control device with return of information is arranged between the cardan joint and the motor mechanism for the screw spindles. A cable system 37 is arranged to pull the carriage along the bridge, as shown. The upper crane beam 38 and the carriages 39 with wheels are located between the front and rear rails 40 of the carriage. The middle winch drum 41 and lifting head 42 can move along the crane boom on a rail system 43. The lifting unit also comprises a ball joint 44 and a conical locking mechanism 45. Two pairs of screw spindles 46 are connected to the overlying crane boom 38, and the winch drum 41 driven by hydraulic or electric motors 47 which are completely synchronized. Also shown is a feedback circuit 48 for control.

By constantly guiding the movable riser, the lifting head is kept close by, so that connection can be made. The conical locking mechanism 45 compensates for lack of alignment caused by the angular changes of the riser. After the lifting head 42 has been brought down to the riser, the cone is brought down over the end and fixed, forming a positive lock. The riser can then be raised.

Fig. 19 shows a tower structure 50 mounted on a cardan joint, and a lifting block 51 with pulley is arranged at the top of the structure. The winch 52 is located on a foundation which is mounted on the carriage. The purpose of the lifting mechanism is only to secure and raise the riser, and it therefore has relatively little weight. The lifting head 53 with the internal locking mechanism is shown above the gimbal joint 54, where the riser part 55 protrudes. An arm system 56 with a gripping head 57 is located so that it can hold pipe parts and place them on the storage rack in the carriage. A feedback circuit 58 is shown.

After a pipe part is pulled up, the lifter stands still while the arm 56 grips the pipe part and moves it away from the lifter. The pipe part is stored on the stand on the trolley. The arm is controlled by means of the feedback circuit 58, based on the angular movements in the gimbal joint, and it can therefore follow the movable riser so that a pipe part can be attached without problems with regard to synchronization.

The embodiment in fig. 18 means that an upper crane can follow lateral movement and create a lifting connection by means of a conical device with internal stops. The embodiment in fig. 19, the system is mounted on the cardan joint, and the pipe can be pulled without problems, as the pipe part can be transferred to an arm which is partly a robot and which follows the movement.

The invention can be used in the following way:

a) Laying out the riser.

1. The ship arrives at the site, the riser 62 is brought down, with a riser part in the gimbal joint 66. 2. The barrier for the bridge is lifted, so that the bridge construction-. one can be moved. 3. Ballast is removed from the inner floats 71 to raise the bridge from the stoppers on the deck. The buoyancy is regulated so that the bridge is held by the floats. 4. The crane 30 on the trolley grips a pipe part, the lifter is moved to place the pipe part over the cardan joint 66.

The pipe section is lowered and connected to another pipe section.

5. The lifting device in the handling system lowers the lower part of the riser and two pipe sections. The barrier

is opened, and then secures the riser again.

6. This is repeated until the entire riser is laid out, with the exception of the last pipe section, before connection. When pipe parts are added the bridge is made to float, by

use of increased buoyancy from the floats.

7. Hydraulic cylinders in the main vertical movement compensation system are attached to the bridge and the cylinders are activated. The bridge is now under active management for compensation. As the entire weight of the bridge and riser is carried by the floats, the cylinders are not needed to provide forces beyond inertia, friction

and resistance.

8. The crane system in the carriage places the last pipe section and lowers as before, with the active (hydraulic) control system acting on the cylinders to fine-tune the process with compensation for movements. In this way, it is possible to achieve an almost perfect connection to the steering cone on the bottom,

also in rough seas.

9. Several pipe parts (approximately two) are added, as in step 8, so that the riser assumes its anchoring angle of approximately 20° (the ship's distance 110 m when the water depth is 300 m). The required tension in the riser is maintained throughout the operation using forces from the bridge and/or active hydraulics

steering.

10. After the mooring position is reached, the swivel is attached and wires are connected. The floats are emptied to provide the necessary buoyancy for the prevailing weather conditions and the ship's draft. The active, hydraulic control of the cylinders for the bridge ceases, and the cylinders are disconnected. The bridge, floats and riser are now fully connected and the system is in a state of passive compensation.

b) Removal of the riser in bad weather.

1. The crane on the trolley is moved from the storage position in the framework in the bridge to the cardan joint. 2. When using the lifting head, the pivot joint is removed and stored. 3. The lifting head is attached to and raises the riser, while maintaining the correct tension. (The suspension device releases the riser). The ship must move forward.

4. The first pipe section is disconnected and stored.

5. The operation continues until the riser is removed.

c) Dangerous situation, the riser is left behind.

The following situations may occur:

i) Exhalation under water.

ii) Failure of the riser control system.

iii) Extreme weather conditions or immediate need to

leave the place.

iv) Other reasons for disconnection where each system (the riser and the ship) performs best when disconnected

state.

v) Practice disconnection at appropriate times.

In such cases, the following procedure is suggested:

1. Production stops, the pivot is removed.

2. Ballast is removed from an already installed float for the riser, or a pending float is installed.

3. The main engines are ready for use.

4. The engines provide thrust aft. The bridge is lifted using the hydraulic cylinders. The locks in the cardan joint are released. 5. The riser, float and upper protective grid structure are disconnected and aligned vertically. The riser is under full tension, and is protected due to the small area in the water surface and the reinforced upper part. The ship can safely leave the site. 6. New connection can be made easily, as the upper end of the riser is above the water surface.

It will be understood that the invention can include the following features: The foundation for the riser can be laid out and placed on the seabed from the tanker (when a light foundation is used which is supplied with ballast in the form of concrete from the surface). An anchoring device can also be used which is fixed with piles or forms a suction anchor.

An embodiment with a continuous shaft for the riser, as shown in fig. 16, can be used in waters where there is ice. The only modification that must be carried out with the ship is the arrangement of the shaft.

A counterweight that helps to balance the bridge, the float, the riser and the lifting device is used if a water depth greater than 240 m is expected, as shown in fig. 17. A torque arm behind the pivot means that the float size can be reduced somewhat for a given condition. Too much weight leads to increased mass inertia, so a compromise is used.

Curved rods that connect the floats to the bridge lead to reduced openings in the tank cover and reduce the splash effect. Simple seals along the edges, e.g. of rubber, holds the bag.

Arms with variable geometry can be used between the floats and the bridge, with ends that are fixed with bolts, as the floats are moved along a curved path forward or aft, to counteract force variations due to increased mass in the floats and resistance.

In the following, reference is made to fig. 20 - 24.

With the present invention, the aim is to arrive at a method for passive movement compensation between the riser and the tanker which reduces secondary forces, and which is universal in its application. The secondary forces in question here are forces that act against buoyancy containers and inertial forces in the device. The purpose is to reduce the load changes in the riser in order to increase the service life. Some known devices use a pivotable boom, the riser being attached to one end and a counterweight being attached to the other end. Fig. 20 shows the method schematically. Vertical loads from the riser are balanced by the weight, and horizontal loads from the riser are transferred to the tanker via the swivel joint. The vertical movement of the tanker is recorded by the beam swinging. This is a known way of anchoring a tanker, and it requires modifications for the method to be practical.

The purpose of motion compensation is to avoid that the vertical movements of the tanker are transferred to the riser. The vertical movement of the tanker accelerates the counterweight, which causes an inertial force that changes the tension in the riser. The acceleration of the counterweight is not the same as the acceleration of the tanker at the turning point, as the acceleration of the ship is multiplied by a factor that depends on the arm ratio, as shown in fig. 21. If the pivot point is located midway between the riser and the counterweight, this factor is 2. This applies to any counterweight system where the counterweight is used to provide a vertical force. If e.g. the counterweight is hung in a cable which is led up around a pulley and down to the riser, the counterweight will move twice the distance in relation to the pulley, and will therefore have twice the acceleration (provided that the riser is at rest and that the pulley is moving). The counterweight, cable and pulley have been used for movement compensation for risers for drilling because it is a simple arrangement, but are no longer used because the changes in mass inertial forces are large. The present invention results in a significant reduction in the effects of mass inertia during movement compensation using a counterweight.

The load on the riser is proportional to the weight and geometry of the beam and pivot point. With the present invention, means have been found to change the geometry of the beam and the pivot point proportionally to the change in the inertial forces, that is to say that the pivot point is moved to compensate for the changes in the inertial forces. This is achieved by replacing the swivel joint with a cradle surface, the size and shape of the cradle surface being chosen so that the desired properties are achieved, as shown in fig. 22.

The movements of the tanker at the turning point will be approximately sinusoidal. When the counterweight is at its lowest point, its velocity is zero and the acceleration is at a maximum, increasing the downward force from the weight. For this reason, the pivot point must be close to the weight, to reduce the moment arm of the weight and increase the moment arm of the riser. In contrast, when the weight is at its highest position, the weight has zero velocity and maximum acceleration, but in the opposite direction, which reduces the downward force due to the weight, and in this case the pivot point must be close to the riser. These are the two extreme positions of the pivot point. Intermediate positions can be achieved, depending on the movement of the weight. If the movement is sinusoidal, a cradle surface designed as a circular arc will ensure the correct location of the pivot point during the entire movement.

The arrangement of a cradle surface as described above enables the pivot point to move, and carries the weight of the entire beam. However, the arrangement cannot transfer horizontal loads, which is the primary purpose of the anchoring system. An arrangement with a rack and pinion is therefore used, with the cradle surface forming the pinion and the support forming the pinion. To prevent slipping, the rolling surface of the cradle surface must coincide with the pitch circle of the toothing. A circular arc can be used for the cradle surface and a flat surface for the support. However, other forms can be used for one or both surfaces, depending on which properties are desirable. If the movement of the tanker at the actual turning point is not sinusoidal, but has some sort of step shape, the cradle surface can be adapted to this. In practice, the movements will change continuously, depending on random conditions in the sea and how the tanker reacts. But the variations in relation to the characteristics of the cradle surface are likely to be miniscule with respect to the loads on the riser.

Fig. 23 shows the floating production vessel anchored by means of the riser. Although the riser is shown in front of the hull of the tanker, the riser can also be set out through a continuous shaft in the ship. Fig. 24 shows details of the equipment shown in fig. 23. The riser 101 is attached to the support mast 102 by means of a bearing so that the riser is prevented from all movements except rotation. The tanker can thus turn around the riser without twisting it. The holding mast 102 for the riser is attached to the swing beam 103 for movement compensation by means of a cardan joint 104 which enables the holding mast for the riser to swing in all directions. The holding mast protrudes below the cardan joint, so that a counterweight can be used to ensure that the mast stands mainly vertically, and to reduce bending loads in the riser. At the lowest point of the holding mast 102, a guide 105 is used to keep the holding mast aligned with the riser. The cardan joint 104 is located at one end of a swing beam 103. At the other end of the swing beam is a counterweight in the form of a tank 106. The tank can be filled with water or another fluid to adjust the weight. The weight is sufficiently large to balance the equipment in addition to the tensile force in the riser. The swing beam 103 is carried by cradles 107 which are located above the deck, approximately midway in the compensation movement. This is to reduce the horizontal movement of the riser due to the gimbal end of the beam swinging in an arc. This feature is not essential, but is beneficial. The swing beam 103 is shown as a truss construction with the cradles at a large distance from each other. This not only enables a light construction to be used, but also means that lateral forces from the riser can be absorbed by the cradles. Horizontal movements, both forward and aft and from side to side, are counteracted by the cradles, using the teeth described above. When the beam swings, the curved cradle surfaces on the beam roll along the support surfaces. No sliding movement occurs, because the division in the two front teeth is adapted to each other. Movements due to lateral loads from the riser or lateral inertial forces from the counterweight cause different loads on the teeth on each side of the beam. The lateral forces are counteracted by end loads on the teeth or by means of other suitable pressure surfaces.

The riser pipe lengths are stored at the front end of the beam, in the equipment 108 for supplying and storing pipe parts. This equipment raises each pipe part to the mast 102, where the equipment 109 is used to connect the pipe joints and to lower the riser towards the bottom. When oil is produced through the riser, a pivot joint 110 with several openings at the top of the riser is used. Flexible hoses and pipes are used to transport the oil from the pivot to the treatment equipment on the tanker.

The connection of the riser to the foundation on the seabed is carried out in the same way as described in Canadian patent application 430,623. The tanker is placed over the foundation on the seabed. The mast 102 for the riser is placed in a vertical position by means of hydraulic cylinders. The equipment 108 for supplying and storing riser parts then leads a length of pipe towards the mast, until the end is directly below the equipment 109 for moving the riser. The equipment 109 has a winch and a running block of a similar type that is usually used for drill pipe and casing on floating drilling rigs, including a small movement hydraulic motion compensator. This compensator is usually only used while the riser is connected to the foundation.

The running block in the equipment 109 is attached to the end of the pipe part and raises it. The pipe part then swings from a horizontal position to a vertical position in the mast. The lower end of the pipe part is controlled by the equipment 108. With the pipe part in a vertical position, it is lowered onto an already mounted pipe part in the riser and connected to this. The equipment 109 then lowers the entire riser until the upper end of it reaches the platform at the cardan joint. Additional pipe parts are mounted in the same way.

When the correct length of riser is set out, the counterweight tank is filled with water, so that the beam swings and places the gimbal and mast near the highest position. The riser is lowered towards the foundation using the equipment. The final placement in the horizontal plane is done by moving the gimbal, which swings the riser at an angle, and the lower end of the riser will hang in another place. Vertical movement during this operation takes place partly with the help of the swing beam, but mainly with the help of the compensator in the equipment. After the riser is attached to the foundation, the progress of the tanker and the position hold are interrupted, and the counterweight tank is filled with water to give the correct tension in the riser. At this point, the actively controlled systems are out of action, and the tanker drifts with the waves, wind and currents until the riser reaches an equilibrium position.

Claims (15)

1. Procedure for mooring a floating production system, characterized in that a riser is used which is put under tension by means of a movement compensation system comprising a counterweight, mounted on the deck of the floating production system, using a cradle arrangement to reduce changes in the load on the riser caused by the mass inertia of the counterweight . 2. Method as stated in claim 1, characterized in that horizontal forces are transferred to a swinging beam using a device with a rack and a curved toothing, the division of the teeth being adapted to each other. 3. Device for mooring a floating production system, characterized in that the mooring takes place using a riser that is under tension, that the tension in the riser and movements are recorded by a hydraulic compensation system, and that a mast stored in a cardan joint connects the riser with the floating production system, and includes means to add pipe lengths to the riser while this is mooring the ship. 4. Device as specified in claim 3, characterized in that it comprises a holding and balancing arm which is attached to the mast, the riser aligns the mast with the riser, and that a counterweight balances the weight of the mast for static and dynamic balance. 5. Device as stated in claim 3, characterized in that it includes means for angular adjustment of the mast, so that the lower end of the riser is correctly placed for connecting the riser to a foundation on the seabed. 6. Suction anchor providing high vertical holding force and high resistance to bending moment, comprising an anchor plate, a rotatable cutting device on the lower end of the plate, a mast in the form of an open structure attached to and projecting upward from the foundation, and plate elements projecting upward and inwards from the foundation to the upper end of the mast. 7. Device for effecting passive motion compensation in the connection between a ship and a riser, the ship comprising a production system or being a tanker for the storage of oil, and comprising filled forward tanks, a bridge structure mounted on the deck of the ship, which bridge structure is pivoted in relation to the deck at the aft end of the structure, the forward end hanging outside the bow of the ship, that a riser is attached to the forward end of the bridge, that vertical rods project along the sides of the forward part of the bridge and have sufficient height to cover the vertical movement of the bridge, that a float device is suspended below the bridge in the filled tanks in the ship, and that a pivot for a production line is mounted in a gimbal suspension device at the forward end of the bridge, for connection to a riser. 8. Device as specified in claim 7, characterized in that the flotation device comprises separate, interconnected flotation tanks which are connected to the underside of the bridge by means of arms. 9. Device as stated in claim 8, characterized in that the depth of the rear floats in the tank in the ship is greater than that of the front floats, so that a wedge pattern is formed. 10. Device as stated in claim 7, characterized in that it comprises a float designed to leave the riser, which float forms the lower end of a reinforced, upper part of the riser. 11. Device as stated in claim 7, characterized in that it includes a system for moving pipe parts, which system is movable between a passive position at a distance from the cardan joint and the riser and an active position above the cardan joint, which system includes an upper crane that can move horizontally along two axes, a drive device and spindle screws for movement of the crane, a device for storing pipe parts, a locking device for attaching the carriage to the rails when aligned over the gimbal, a winch device and a lifting head comprising a conical locking mechanism for connection with the pipe parts. 12. Device as stated in claim 7, characterized in that the system for moving pipe parts comprises a storage rack for the pipe parts, a carriage device which has a set of rails, a tower structure which is mounted on a gimbal joint and has a lifting block with pulleys mounted on the carriage , a lifting head with a locking mechanism for securing and raising the riser, and an arm assembly and a gripper head for holding pipe parts for movement to the storage rack. 13. Device as specified in claim 7, characterized in that it comprises a counterweight on the bridge, behind the pivot point of this. 14. Device for movement compensation by means of a counterweight in an anchoring system for anchoring a tanker by means of a riser, comprising a swing beam for attaching the riser to the tanker, a counterweight attached to the end of the beam furthest from the riser, the swing beam comprising means for causing the support point of the beam to move to compensate for the accelerations of the tanker. 15. Device as stated in claim 14, characterized in that the counterweight comprises a fluid-filled tank.