CA1223486A - Riser moored floating production system - Google Patents
Riser moored floating production systemInfo
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- CA1223486A CA1223486A CA000430623A CA430623A CA1223486A CA 1223486 A CA1223486 A CA 1223486A CA 000430623 A CA000430623 A CA 000430623A CA 430623 A CA430623 A CA 430623A CA 1223486 A CA1223486 A CA 1223486A
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- riser
- mast
- tanker
- ship
- hydraulic
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Abstract
RISER MOORED FLOATING PRODUCTION SYSTEM
ABSTRACT
Apparatus for mooring a ship-shape floating production system uses a tensioned riser, with associated motion compensation and riser pipe handling equipment. The attachment of the riser to the ship, and the motion-compensation and pipe handling system 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. The riser system is specifically designed for high loading and fatigue endurance and, hence, suitable for mooring as well as production.
ABSTRACT
Apparatus for mooring a ship-shape floating production system uses a tensioned riser, with associated motion compensation and riser pipe handling equipment. The attachment of the riser to the ship, and the motion-compensation and pipe handling system 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. The riser system is specifically designed for high loading and fatigue endurance and, hence, suitable for mooring as well as production.
Description
~z234a~i FIELD OF THE INVENTION
This invention pertains to hydrocarbon production from offshore oil fields to a floating, ship-shape 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.
BACKGROUND OF THE INVENTION
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 hauser and is free to swing around the buoy 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 the riser as well as the mooring device.
~, 12~348~
The present invention improves upon the aforementioned methods by providing a tanker-based floating production system that is very mobile and relatively insensitive to water depth. This is accomplished by using a deployable tensioned riser, tension and motion compensation being achieved by an hydraulic system. The riser is connected to a gimballed mast that has provision for adding additional lengths of riser while the riser is anchoring the ship.
In accordance with another aspect, a high capacity suction anchor of frustoconical configuration provides high vertical holding capacity and high moment resisting capacity.
The invention is illustrated in the accompanying drawings in which:
Figures 1, 2a and 2b are schematic views of a 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 view of the riser handling and motion compensation system;
Figure 6 illustrates the 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 8A is a schematic elevation view of a craft moored to an anchor;
l~
~L2;~48~, Figure 9 is a schematic, elevation view of a high capacity suction anchor;
Figures 10, ll and 12 are elevation views of the apparatus shown in Figure 5; and Figure 10A is a schematic cross-sectional enlargement of the area designated 10A in Figure 10.
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 as shown in Figure l.
Dealing now only with the tower, the tension is provided by buoyancy, either in the top of the tower 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 lZ234l~6 the tanker to move location in bad sea conditions.
When considering deep water, the tower system has operational 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 Figures 2A and 2B. This vertical displacement is proportional to water depth. In deep water the yoke 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 becomeimpractical in severe sea conditions and deep water.
General Description 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 2 that is made up from sections of riser and deployed from the production tanker T as seen in Figure 3. The riser is lowered from the tanker as it is made up, locked to a riser base anchor 1 on the seabed, and tensioned by a hydraulic motion compensator on the tanker. The tanker is then allowed to move away from its original position under the action of wind, waves and current :~Z34~6 until the riser 2 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 2 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, (shown generally at 50 in Figures 3 and 4~ and 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. This motion compensation system 50, therefore, is acting as a fluid spring.
In shallower water the motion compensation cylinders 7 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 2 is vertical, the minimum tension necessary is applied to the riser 2. With the riser 2 at its maximum angle, the motion compensation cylinder ~7 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 T is ~2~34~3~
subjected to increasing forces from wind, waves and current, it moves away from its centre position and the riser 2 inclines at an angle. As this angle increases, not only does the horizontal component of the riser tension become gr~ater, 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 riser-moored 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 ias 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 -25 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 12234~36 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 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 tension-ing and motion compensation equipment is attached to a mast superstructure 8, which is mounted on a gimbal 52 (Figure 10) as shown in Figure 5. The gimbal 52 provides the flexibility between the angular movements of the tanker and the riser 2. In order for the mast 8 to move with the riser 2, the mast is extended some distance below the gimbal where this extension 54 acts ~Z~486 as a lever that the riser pushes against to keep the mast in alignment with the riser. A weight 56 is also placed at the end of this lever in order to balance the mast about the gimbal 52. 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 Canadian application no. 4~1,909 filed February 18, 1983 will be used. With this or any other guidecone system, the base of the riser 2 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 ~223486 gradually changes until at the bottom of the riser it is hanging vertically. The net result is that the bottom of the riser 2 is displaced horizontally when the angle of the riser mast 8 is changed. The process of controlling the guidance can be handled Manually using sonar and TV information. 8ut it 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 2.
One of the reasons for deploying the riser 2 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
This invention pertains to hydrocarbon production from offshore oil fields to a floating, ship-shape 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.
BACKGROUND OF THE INVENTION
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 hauser and is free to swing around the buoy 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 the riser as well as the mooring device.
~, 12~348~
The present invention improves upon the aforementioned methods by providing a tanker-based floating production system that is very mobile and relatively insensitive to water depth. This is accomplished by using a deployable tensioned riser, tension and motion compensation being achieved by an hydraulic system. The riser is connected to a gimballed mast that has provision for adding additional lengths of riser while the riser is anchoring the ship.
In accordance with another aspect, a high capacity suction anchor of frustoconical configuration provides high vertical holding capacity and high moment resisting capacity.
The invention is illustrated in the accompanying drawings in which:
Figures 1, 2a and 2b are schematic views of a 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 view of the riser handling and motion compensation system;
Figure 6 illustrates the 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 8A is a schematic elevation view of a craft moored to an anchor;
l~
~L2;~48~, Figure 9 is a schematic, elevation view of a high capacity suction anchor;
Figures 10, ll and 12 are elevation views of the apparatus shown in Figure 5; and Figure 10A is a schematic cross-sectional enlargement of the area designated 10A in Figure 10.
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 as shown in Figure l.
Dealing now only with the tower, the tension is provided by buoyancy, either in the top of the tower 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 lZ234l~6 the tanker to move location in bad sea conditions.
When considering deep water, the tower system has operational 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 Figures 2A and 2B. This vertical displacement is proportional to water depth. In deep water the yoke 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 becomeimpractical in severe sea conditions and deep water.
General Description 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 2 that is made up from sections of riser and deployed from the production tanker T as seen in Figure 3. The riser is lowered from the tanker as it is made up, locked to a riser base anchor 1 on the seabed, and tensioned by a hydraulic motion compensator on the tanker. The tanker is then allowed to move away from its original position under the action of wind, waves and current :~Z34~6 until the riser 2 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 2 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, (shown generally at 50 in Figures 3 and 4~ and 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. This motion compensation system 50, therefore, is acting as a fluid spring.
In shallower water the motion compensation cylinders 7 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 2 is vertical, the minimum tension necessary is applied to the riser 2. With the riser 2 at its maximum angle, the motion compensation cylinder ~7 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 T is ~2~34~3~
subjected to increasing forces from wind, waves and current, it moves away from its centre position and the riser 2 inclines at an angle. As this angle increases, not only does the horizontal component of the riser tension become gr~ater, 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 riser-moored 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 ias 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 -25 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 12234~36 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 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 tension-ing and motion compensation equipment is attached to a mast superstructure 8, which is mounted on a gimbal 52 (Figure 10) as shown in Figure 5. The gimbal 52 provides the flexibility between the angular movements of the tanker and the riser 2. In order for the mast 8 to move with the riser 2, the mast is extended some distance below the gimbal where this extension 54 acts ~Z~486 as a lever that the riser pushes against to keep the mast in alignment with the riser. A weight 56 is also placed at the end of this lever in order to balance the mast about the gimbal 52. 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 Canadian application no. 4~1,909 filed February 18, 1983 will be used. With this or any other guidecone system, the base of the riser 2 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 ~223486 gradually changes until at the bottom of the riser it is hanging vertically. The net result is that the bottom of the riser 2 is displaced horizontally when the angle of the riser mast 8 is changed. The process of controlling the guidance can be handled Manually using sonar and TV information. 8ut it 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 2.
One of the reasons for deploying the riser 2 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
2-5 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 ~, ~
~Z~39L86 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 risex. In the present invention the spider platform 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 58 cylinder that holds the next length of riser 60 while it is being attached to the already made-up riser.
After the connection is made, the hydraulic cylinder 58 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 58 lets the top of the riser descencl 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 i8 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 motion-compensated. 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 whatqoever.
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 62 (Figure 3 and 4) 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 enought 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 Canadian patent application serial no. 421,909 filed February 12, 1983. 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 hose~. A similar arrangement is proposed for the riser base.
Combined System Referring to Figures 8 and 8A a floating production system is connected to a subsea riser base anchor 1 ~1;223~
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
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 ~, ~
~Z~39L86 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 risex. In the present invention the spider platform 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 58 cylinder that holds the next length of riser 60 while it is being attached to the already made-up riser.
After the connection is made, the hydraulic cylinder 58 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 58 lets the top of the riser descencl 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 i8 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 motion-compensated. 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 whatqoever.
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 62 (Figure 3 and 4) 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 enought 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 Canadian patent application serial no. 421,909 filed February 12, 1983. 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 hose~. A similar arrangement is proposed for the riser base.
Combined System Referring to Figures 8 and 8A a floating production system is connected to a subsea riser base anchor 1 ~1;223~
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.
Tne 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 12 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 16 which surrounds the moon pool 17.
The riser handling system 18 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 ener~ized and the final distance made up ~Z~23~3~;
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 15.
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, ~;2Z~48~6 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 riser feed 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.
~Z234'~36 The hydraulic system for motion compensation has fail-safe capability. As shown in Figure lOA, the two main hydraulic rams 7 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 1500 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, andd 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 ~223486 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 2, 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 ~Z3~
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 Canadian patent application serial no.
421,909 filed February 18, 1983 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.
~;2234~36 While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.
The terms and expressions which have been employed in this specification are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
Tne 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 12 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 16 which surrounds the moon pool 17.
The riser handling system 18 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 ener~ized and the final distance made up ~Z~23~3~;
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 15.
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, ~;2Z~48~6 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 riser feed 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.
~Z234'~36 The hydraulic system for motion compensation has fail-safe capability. As shown in Figure lOA, the two main hydraulic rams 7 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 1500 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, andd 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 ~223486 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 2, 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 ~Z3~
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 Canadian patent application serial no.
421,909 filed February 18, 1983 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.
~;2234~36 While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.
The terms and expressions which have been employed in this specification are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
Claims
1. In a floating hydrocarbon production arrangement, apparatus for mooring a large ship-type production system to a subsea riser base anchor, comprising;
a tensioned riser having base anchor connecting means on the lower end thereof;
mast means on said floating production system connecting the upper end of the riser thereto, and supporting said riser;
gimball means mounting said mast means on said ship-type system;
hydraulic means associated with said mast means to provide motion compensation to said riser and mast means;
means for adding lengths to said riser while said riser is anchoring said system;
a guide and balance arm attached to said gimball mounted riser supporting mast whereby said riser aligns said mast means with said riser, and a weight for balancing over-hanging weight of the mast for static and dynamic balance; and means for deploying a tensioned riser so as to angle the gimball mounted support mast so that the lower end of the riser is correctly positioned for engagement with the base anchor on the sea bed.
a tensioned riser having base anchor connecting means on the lower end thereof;
mast means on said floating production system connecting the upper end of the riser thereto, and supporting said riser;
gimball means mounting said mast means on said ship-type system;
hydraulic means associated with said mast means to provide motion compensation to said riser and mast means;
means for adding lengths to said riser while said riser is anchoring said system;
a guide and balance arm attached to said gimball mounted riser supporting mast whereby said riser aligns said mast means with said riser, and a weight for balancing over-hanging weight of the mast for static and dynamic balance; and means for deploying a tensioned riser so as to angle the gimball mounted support mast so that the lower end of the riser is correctly positioned for engagement with the base anchor on the sea bed.
Priority Applications (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000430623A CA1223486A (en) | 1983-06-17 | 1983-06-17 | Riser moored floating production system |
| NO842405A NO842405L (en) | 1983-06-17 | 1984-06-15 | DEVICE AND PROCEDURE FOR SUPPLYING A HYDROCARBON PRODUCTION SYSTEM ASSOCIATED WITH A SHIP |
| JP12356584A JPS6085093A (en) | 1983-06-17 | 1984-06-15 | Mooring gear for floating petroleum production facility |
| GB08415468A GB2141470B (en) | 1983-06-17 | 1984-06-18 | Offshore production systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000430623A CA1223486A (en) | 1983-06-17 | 1983-06-17 | Riser moored floating production system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1223486A true CA1223486A (en) | 1987-06-30 |
Family
ID=4125501
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000430623A Expired CA1223486A (en) | 1983-06-17 | 1983-06-17 | Riser moored floating production system |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS6085093A (en) |
| CA (1) | CA1223486A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2020245120A1 (en) * | 2019-06-04 | 2020-12-10 | Dublin Offshore Consultants Ltd | A buoyant rotatable marine transducer |
| CN115743407A (en) * | 2022-11-24 | 2023-03-07 | 广东精铟海洋工程股份有限公司 | Mooring system with compensation function and control method thereof |
-
1983
- 1983-06-17 CA CA000430623A patent/CA1223486A/en not_active Expired
-
1984
- 1984-06-15 JP JP12356584A patent/JPS6085093A/en active Pending
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2020245120A1 (en) * | 2019-06-04 | 2020-12-10 | Dublin Offshore Consultants Ltd | A buoyant rotatable marine transducer |
| CN114072328A (en) * | 2019-06-04 | 2022-02-18 | 都柏林离岸顾问有限公司 | Floating rotatable ocean transducer |
| CN115743407A (en) * | 2022-11-24 | 2023-03-07 | 广东精铟海洋工程股份有限公司 | Mooring system with compensation function and control method thereof |
| CN115743407B (en) * | 2022-11-24 | 2023-07-21 | 广东精铟海洋工程股份有限公司 | Mooring system with compensation function and control method thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6085093A (en) | 1985-05-14 |
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