Abstract
Stress reducing system and associated method for reducing stresses at a desired position in an offshore production or drilling system, the offshore production or drilling system comprising: a seabed structure, a floating structure and a riser (24) extending there between, the riser being tensioned, the riser (24) comprising at least a first part (45) and a second part (46), which second part (46) is connected to the first part (45) via a flexible connection (20) allowing an axial, angular and/or rotational movement between the first and second parts (45, 46), said stress reducing system comprises:a first sensor (41) for real-time monitoring of stresses at the desired position, positioned at or close to the desired position (20),an actuating system (42) arranged at the flexible connection (20, the actuating system (42) being connected to said first and second parts (45, 46), and wherein the actuating system (45, 46) is configured to apply a force to the first or second part (45, 46) when the first and second parts (45, 46) are moved out of a neutral position,a control system (40) adapted to receive monitoring data from the first sensor (41), wherein the control system (40) is connected to the actuating system (42) and is able of providing instruction signals to the actuating system (42), wherein the control system (40), based on said monitoring data from the first sensor (41), is able to calculate a real-time set of data for control of the applied force of the actuating system (42) and instructing the actuating system (42) to act accordingly, such as to reduce the stress at said desired position.
Claims
1. A stress reducing system for reducing stresses at a desired position in an offshore production or drilling system, the offshore production or drilling system comprising a seabed structure, a floating structure and a riser extending between the seabed structure and the floating structure, the riser being tensioned and comprising at least a first part and a second part which is connected to the first part via a first flexible connection which is configured to allow axial, angular and/or rotational movement between the first and second parts, said stress reducing system comprising: a first sensor which is configured to provide real-time monitoring of stresses at the desired position, the first sensor being positioned at or close to the desired position; an actuating system which is arranged at the flexible connection, the actuating system being connected to said first and second parts and being configured to apply a force to at least one of the first and second parts when the first and second parts are moved out of a neutral position relative to each other; a control system which is adapted to receive monitoring data from the first sensor, wherein the control system being connected to the actuating system and being configured to apply instruction signals to the actuating system; wherein the control system is configured to calculate, based on said monitoring data from the first sensor, a real-time set of data for control of the applied force of the actuating system and instruct the actuating system to act accordingly so as to reduce stress at said desired position.
2. The system according to claim 1, wherein the stress results in bending moments at said desired position, wherein the flexible connection is a flexible joint allowing angular displacement of the first part relative to the second part, wherein the first sensor is configured to provide real-time monitoring of bending moments at the desired position, wherein the actuating system is configured to apply the force is in the same direction as the movement of the first part relative to the second part out of neutral position, and wherein the control system is configured to calculate the real-time set of data for control of the applied force of the actuating system in order to provide a reduced bending moment at said desired position.
3. The system according to claim 2, further comprising: a second sensor for which is configured to provide real-time monitoring of a bending angle at the flexible joint; and a third sensor which is configured to provide real-time monitoring of tension in the riser; wherein the control system is configured to calculate the real-time set of data for control of the applied force of the actuating system based on monitoring data from the first sensor, the second sensor and the third sensor.
4. The system according to claim 2, wherein the first sensor comprises a sensor system which is configured to provide input in relation to at least one of the magnitude, direction and orientation of the bending moment.
5. The system according to claim 2, wherein the actuating system is arranged around a circumference of the flexible joint.
6. The system according to claim 5, wherein the actuating system comprises a set of hydraulic actuators, each hydraulic actuator comprising a cylinder which includes a cylinder barrel and a through-going piston rod, the through-going piston rod having a fixed piston separating an inner volume of the cylinder barrel into a first volume and a second volume.
7. The system according to claim 6, wherein the piston rods of the hydraulic cylinders extend substantially in the same direction as a longitudinal axis of the riser.
8. The system according to claim 6, wherein the first volume in one cylinder is connected to one of the first volume or the second volume in another cylinder and/or the second volume in one cylinder is connected to one of the first volume or the second volume in another cylinder.
9. The system according to claim 1, wherein the desired position is located at a wellhead, at a distance below an upper end of the wellhead, at a connection between the wellhead and a X-mas tree which is mounted to the wellhead, at a lower marine riser package (LMRP), at a blow out preventer, or at a riser joint in a lower half of the riser.
10. The system according to claim 9, wherein the first sensor is positioned at a distance from the desired position.
11. The system according to claim 1, wherein the desired position is located in an upper half of the riser.
12. The system according to claim 1, further comprising means for monitoring readings of one or more of the following additional parameters: an angle of different riser components, a temperature of different riser components, a tension of different riser components versus an inner pressure of a fluid in the riser, a torsion of different riser components versus an inner pressure of a fluid in the riser, a pressure experienced at different riser components, a tension in the riser versus effects of waves and/or currents on the riser, a tension in the riser versus a tension applied from a tension system holding the riser; wherein the control system calculates the real-time data set taking into account the monitoring readings from said one or more additional parameters.
13. The system according to claim 1, further comprising: a second flexible connection which is either (a) positioned between the first part and the second part and is configured to allow the first and second parts to be angularly displaced relative to each other, or (b) positioned between the second part and a third part of the riser and is configured to allow the second and third parts to be angularly displaced relative to each; wherein the first sensor is configured to provide real-time monitoring of bending moments at the desired position; a second actuating system which is arranged at the second flexible connection, the second actuating system being connected to said second and third parts and being configured to apply a force to at least one of the second and third parts when the second and third parts are moved out of a neutral position relative to each other; wherein the second actuating system is configured to apply the force in the same direction as the movement of the second part relative to the third part out of neutral position; wherein the control system is adapted to receive monitoring data from the first sensor, the control system being connected to the second actuating system and being configured to provide instruction signals to the second actuating system; wherein the control system is configured to calculate, based on said monitoring data from the first sensor, a real-time set of data for control of the applied force of the second actuating system to provide a reduced bending moment at said desired position and instruct the second actuating system to act accordingly.
14. The system according to claim 13, wherein the first flexible connection allows an axial and/or rotational movement between the first and second parts.
15. The system according to claim 13, further comprising: a third flexible connection which is configured to allow axial, angular and/or rotational movement between the first and second parts; a second sensor which is configured to provide real-time monitoring of stresses at the desired position, the second sensor being positioned at or close to the desired position; a third actuating system which is arranged at the third flexible connection, the third actuating system being connected to said first and second parts and being configured to apply a force to at least one of the first and second part when the first and second parts are moved out of a neutral position relative to each other; wherein the control system is adapted to receive monitoring data from at least one of the first sensor, the second sensor, and a third sensor, the control system being connected to the actuating system and being configured to provide instruction signals to the third actuating system; wherein the control system is configured to calculate, based on said monitoring data from the first, second and/or third sensors, a real-time set of data for control of the applied force of the third actuating system and instruct the third actuating system to act accordingly so as to reduce the stress at said desired position.
16. The system according to claim 2, wherein the system is adapted to reduce bending moments at a second desired position along the riser.
17. The system according to claim 1, wherein the first flexible connection comprises a dynamic seal which allows the first part and the second part to move axially relative to each other, and wherein the actuating system is arranged above a BOP in the riser and is configured to apply a force in an axial direction on at least one of the first and second parts when the first and second parts are moved out of an axially neutral position relative to each other.
18. A method of reducing stress at a desired position in an offshore production or drilling system, the offshore production or drilling system comprising a seabed structure, a floating structure, and a riser extending between the seabed structure and the floating structure, the riser being tensioned and comprising at least a first part and a second part which is connected to the first part via a flexible connection which is configured to allow axial, angular and/or rotational movement between the first and second parts and, the method comprising: providing a stress reducing system comprising an actuating system arranged at the flexible connection, the actuating system being connected to said first and second parts and being configured to apply a force to at least one of the first and second parts when the first and second parts are moved out of a neutral position relative to each other; monitoring in real time stresses at or close to the desired position using a first sensor; operating a control system to calculate a real-time set of data based on monitoring data from the first sensor and controlling the actuating system accordingly; and regulating the applied force of the actuating system to provide a force which reduces the stress at said desired position.
19. The method according to claim 18, wherein the stress results in bending moments, wherein the flexible connection is a flexible joint which is configured to allow angular displacement of the first part relative the second part, wherein the first sensor provides real-time monitoring of bending moments at the desired position, wherein the force is applied in the same direction as the movement of the first part relative to the second part out of neutral position, and wherein the control system calculates the real-time set of data for control of the applied force of the actuating system to provide a reduced bending moment at said desired position along the riser.
20. The method according to claim 18, further comprising reducing stress at a second desired position by using a second actuating system and the same or an additional control system.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0076] FIG. 1 discloses a prior art riser system;
[0077] FIG. 2 is shown a simplified sketch of a part of the riser system as depicted in FIG. 1;
[0078] FIG. 3 is a diagram of an example of a specific equation, showing the curve of the bending moments M.sub.wh as a result of varying the flex joint stiffness k.sub.;
[0079] FIG. 4 shows a plot of bending moment (kNm) on the Y-axis vs. deviation angle (degree) on the X-axis for the invention (ANew invention) compared to prior art, i.e. existing solutions (BExisting solutions).
[0080] FIG. 5 shows a setup of an offshore system and an example of the positioning of the invention in such a system in a first embodiment of the invention;
[0081] FIG. 6A-C shows an aspect of the invention in different views;
[0082] FIG. 7 shows a hydraulic flow diagram for the actuating system according to the present invention;
[0083] FIG. 8 shows a setup having four cylinders (only two is shown);
[0084] FIG. 9A shows example of experienced bending moments at the subsea wellhead when the actuating system is applying too little force when the floating structure is drifting in the right hand direction on the drawing;
[0085] FIG. 9B shows example of experienced bending moments at the subsea wellhead when the actuating system is applying too much force when the floating structure is drifting in the right hand direction on the drawing;
[0086] FIG. 9C shows example of experienced bending moments at the subsea wellhead when the actuating system is applying ideal force when the floating structure is drifting in the right hand direction on the drawing;
[0087] FIG. 10 shows a second embodiment of the invention for reducing axial or compression stresses in a desired position;
[0088] FIG. 11 shows a third embodiment of the invention for reducing torsion in a desired position;
DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT
[0089] In FIG. 1 there is shown a prior art riser system for use in well completions and workover operations. A well 10 has been drilled from the seabed 12 into the earth and completed in the normal manner, capped with a wellhead and subsea Christmas tree 14. A BOP or lower riser package (LRP) 16 is locked onto the Christmas tree 14. An emergency disconnect package (EDP) 18 is locked to the LRP. Above the EDP there is normally arranged a stress joint that will handle bending moments in the riser. The stress joint may be in the form of a bending restrictor. At the lower end of the riser there is also a safety joint or weak link 22. The riser 24 itself consists of a number of pipes that are screwed or otherwise locked together to form a pipe string as is well known in the art. At the top of the riser there is a telescopic joint 26. In the drawing the telescopic joint is shown in its collapsed position. The riser 24 is held in tension using a tensioner system 28 in the normal manner. A surface flow tree is attached to the top of the riser and held in tension using the heave compensator (not shown). The vessel has a cellar deck 32 and a drill floor 34. All operations are conducted on the drill floor.
[0090] In FIG. 2 is shown a simplified sketch of a part of the riser system as depicted in FIG. 1 showing the bending moment at wellhead 14. A flex joint 20 is mounted between the riser 24 and wellhead 14. The flex joint is typically located at a height H from the wellhead 14 datum to the flex joint axis. The riser can also be said to comprise two parts (see FIG. 5, first riser part 45 and second riser part 46) joined at the flex joint 20. As can be seen from FIG. 2, when tension is applied to the riser, an upward force F.sub.R acts on the wellhead 14. When the riser 24 is at an angle this force will split into a vertical and a horizontal component. As will be understood, when the riser 24 is vertical, the horizontal component is zero but as the angle increases, the horizontal component will also increase. The horizontal component will result in a bending moment generated at the wellhead 14, as represented by the formula
M.sub.WH=F.sub.R,hH+k.sub.
where [0091] H: Height from wellhead datum to flex joint axis [0092] : Global flex joint angle [0093] F.sub.R: Riser tension at flex joint axis [0094] F.sub.R,h: Horizontal component of F.sub.R [0095] k.sub.: Rotational flex joint stiffness
[0096] FIG. 3 is a diagram of one solution to the above equation, showing the curve of the bending moments M.sub.wh as a result of varying the flex joint stiffness k.sub.. This shows that even when the flex joint stiffness k.sub. is zero, which is an idealized joint with no friction or stiffness, there is still bending moment M.sub.wh acting on the wellhead 14, as can bee seen as the graph crosses the Y-axis in a distance from the X-axis. The bending moments on the wellhead 14 will as indicated with the graph also with an increasing flex joint stiffness have an increasing value. The diagram also shows that the least moment on the wellhead 14 is achieved if the stiffness in the flexible joint between two parts of the riser 24 is negative. This theoretical considerations show that if it could be possible to design a flex joint with a negative stiffness, the result will be an arrangement giving the least moment forces acting on the wellhead. There is a range of negative stiffness values for the flex joint stiffness k.sub. which gives this desired effect on the wellhead. One can see this in the Figure in that the graph has a dip close to a zero value for the bending moment at the wellhead, M.sub.wh, for a negative value of the joint stiffness k.sub.. One should here also notice that with a negative flex joint stiffness k.sub. which has a larger negative value, there will again be an increasing bending moment at the wellhead, as indicated in the graph. The challenge is to change the locking stiffness of a joint between two parts of a riser from positive to negative. This will reduce the overall dynamical/static bending moment on the wellhead during subsea operations.
[0097] FIG. 4 shows a plot of bending moment (kNm) on the Y-axis vs. deviation angle (degree) on the X-axis for the invention (A=New invention, hereinafter denoted A) compared to prior art solutions (B=Existing solutions, hereinafter denoted B). Experiments using the prior art solutions, have proven that the influence of friction is significant and essential. If there had been no friction in the system and the tension in the riser had been constant, it would have been possible to reduce the bending moment at the wellhead by approximately 80%. The rest value (approximately 20%) is due to hysteresis effect in the flex joint rubber. However, due to friction in the bearings and other parts, the efficiency is significant reduced. The friction depends on the force from the hydraulic cylinders and real tests prove the calculated reductions. FIG. 4 shows the influence of the increased friction as function of increased force in the cylinders. The 45 degrees elliptical curve (B) has a width caused by the rubber hysteresis in the flex joint rubber. When the cylinder pressure in the hydraulic cylinders in the actuating system increases, the friction in bearings make an additional hysteresis effect and the width of the elliptical curve increases. This reduces the efficiency in the bending moment at the desired position dramatically. As can be seen from the plot of (B), the amplitude of the bending moment is in the magnitude of +/130 kNm. However, by using the present invention (illustrated by curve A) it is clear that the amplitude of the bending moment is significantly reduced to about +/10 kNm. Thus, the present invention is much closer to the ideal situation, i.e. a situation where the elliptical curve cycles on the X-axis with as small value as possible on the Y-axis, resulting in minimal bending moments in the desired position.
[0098] Thus, as is apparent from FIG. 4, the system according to the present invention compensates for all friction that may occur, i.e. variable friction and or hysteresis effect(s) in different parts making up the system. The friction is e.g. dependent on the force from the hydraulic cylinders. Furthermore, the system also compensates for the hysteresis effects from the rubber in the flex joint connection as well as it will compensate for variations in the riser tension and friction in bearings (e.g. in bearings in the hydraulic cylinders). By using the system, the bending moment at the desired position, e.g. at the wellhead, is reduced by up to 99%. The system further compensates for pipe-in-pipe effect, riser tensions variations and dependent on the hydraulic damping and the regulating speed it is also compensated for vortex-induced vibrations (VIV).
[0099] FIG. 5 shows a setup of an offshore system and an example of the positioning of the invention in a riser system in a first embodiment of the invention. Many of the features are similar to the features discussed in relation to the prior art of FIG. 1. However, FIG. 5 further discloses a system of reducing bending moment in one or more desired positions along a riser according to the invention. The riser 24 is tensioned by the tensioner system 28 and is connected to a floating structure and a seabed structure 11, 14 and comprises a first riser part 45 and a second riser part 46, which first and second riser parts 45, 46 are connected by a flexible joint 20 allowing the first riser part and the second riser part 45, 46 to be angular displaced relative each other. The flexible joint 20 is any flexible joint or connection which allows two parts to be angularly displaced relative each other while still being connected. The system comprises a first sensor 41 for real-time monitoring of bending moments at the desired position, the first sensor 41 is positioned at or close to the desired position. In the disclosed embodiment, the desired position is at the wellhead 14 and the first sensor 41 is arranged at the wellhead 14. The system further comprises an actuating system 42 arranged at the flexible connection 20. The actuating system 42 being connected to said first and second parts 45, 46 and is configured to apply a force to the first or second riser parts 45, 46 when the first and second riser parts 45, 46 are moved out of a neutral position, which force is applied in the same direction as the movement out of neutral position. A control system 40 is adapted to receive monitoring data from the first sensor 41. Such monitoring data may be transferred through first connection line 43, or may also be transferred wirelessly. The control system 40 is connected to the actuating system 42 via a second connection line 44 and is able of providing instruction signals to the actuating system 42. Furthermore, the control system 40 is configured to, based on said monitoring data from the first sensor 41, calculate a real-time set of data for control of the applied force of the actuating system 42 by instructing the actuating system 42 to act accordingly, such as to provide a reduced bending moment at said desired position along the riser 24, i.e. the wellhead 14 in the disclosed embodiment. It shall thus be noted that the position of the first sensor 41 may be in a distance from the desired position (the wellhead 14 in the disclosed embodiment), e.g. the desired position is a position in a distance below or above the upper end of the wellhead and the first sensor 41 is positioned at a connection between the X-mas tree and an intervention stack. The system is further disclosed comprising a second sensor 47 for real-time monitoring of a bending angle at the flexible connection 20 and a third sensor 48 for real-time monitoring of tension in the riser 24. The control system 40 may then calculate the real-time set of data for control of the applied force of the actuating system 42 based on monitoring data from the first sensor 41, the second sensor 47 and the third sensor 48. The actuating system 42 may be connected to a hydraulic fluid reservoir, accumulator and pump (details of which is disclosed in FIG. 7), which may provide for supply of additional fluid under pressure to the hydraulic cylinders in the actuating system 42.
[0100] FIG. 6A-C shows an aspect of the invention in different views, wherein FIG. 6A shows an overview of the hydraulic cylinders in the actuating system 42 around a flexible joint 20, FIG. 6B shows a side view of FIG. 6A, and FIG. 6C shows a plan view from the side where the flexible joint and actuating system has been cut in the axial direction showing some details of the hydraulic cylinders and the flexible joint. The cylinders are double-acting hydraulically cylinders with same pressure area at both sides. Furthermore, in the disclosed embodiment, it is disclosed 8 spherical bearings 90 with fixing brackets for bolting to the flexible joint 20. The joint 20 is connected to a first riser part and a second riser part 45, 46 (the riser parts are not disclosed in this Figure).
[0101] FIG. 7 shows a hydraulic flow diagram for the actuating system 42 according to the present invention. The flow diagram discloses a closed circuit. The directions of the flow in each of the lines are shown by the arrows in each line. In the Figure, four hydraulic cylinders 70, 71, 72, 77 are disclosed, each having a piston 73 inside, the piston 73 separating the chamber in the hydraulic cylinder 70, 71, 72, 77 in a first volume (above the piston 73) and a second volume (below the piston 73). The hydraulic system comprises a hydraulic fluid reservoir 49 comprising hydraulic oil, the hydraulic fluid reservoir 49 is connected to a hydraulic pump 50 via line 81. The hydraulic pump 50 is connected to an accumulator 51 via line 81 and is configured to pressurize fluid from the hydraulic fluid reservoir 49 into the accumulator 51.
[0102] The accumulator 51 comprises pressurized hydraulic fluid and a gas, such as nitrogen (N2). The accumulator 51 is connected to two pressure regulating valves 53 via line 83 which line branches off in lines 83 and 83, where each of the pressure regulating valves 53 are connected to a first volume (i.e. an upper chamber) 70, 71 in one hydraulic cylinder 70, 71 and a second volume 70, 71 in another hydraulic cylinder 70, 71. The first volume 70, 71 is separated from the second volume 70, 71 within each hydraulic cylinder by a piston 73.
[0103] The hydraulics functions such that if the control system 40 (not shown in FIG. 7) calculates that the flexible joint is moving out of neutral position, an additional force is added by supplying pressure from the hydraulic fluid reservoir 49 to dedicated first and second volumes of the hydraulic cylinders dependent on the direction of the force. The amount of pressurized fluid may be adjusted by opening, closing or choking the pressure regulating valves 53 based on instructions from the control system. For example, if hydraulic cylinder 70 and hydraulic cylinder 71 is arranged on opposite sides of the flexible joint 20, then pressurized fluid may be added to the first volume 70 of hydraulic cylinder 70 and to the second volume 71 of the hydraulic cylinder 71 via line 80. The remaining hydraulic cylinders are configured in a similar manner in relation to the other hydraulic cylinders 72, 77. However, when the control system 40 calculates that no additional force is required, fluids may flow back from the respective pressurized first 70, 71 and/or second volumes 70, 71 to the pressure regulating valves 53 and further via line 82 back into the hydraulic fluid reservoir 49. The process is then continuously repeated based on the forces acting on the flexible joint 20.
[0104] If desired, it is obvious that line 80 may further be connected to any of the remaining first or second volumes of the hydraulic cylinders 72 and/or 77.
[0105] However, it is preferable that if two hydraulic cylinders 70, 71 72, 77 are arranged on opposite sides of a flexible joint, the two first and second volumes are connected as described above such as to achieve an increased force.
[0106] There may be arranged a fail safe open on/off valves 54 between opposite hydraulic cylinders 70, 71 and 72, 77, respectively.
[0107] Although it has been described and disclosed that a first volume of one hydraulic cylinder is connected to a second volume in another cylinder, it is clear that the first volume of one cylinder may be connected to a first volume of a second cylinder. Furthermore, a second volume in one cylinder may be connected to a first volume or a second volume in another cylinder.
[0108] FIG. 8 shows a setup having four cylinders (only two is shown in the large drawing) providing forces to the joint 20. Between two shoulders 60, 62, which shoulders 60, 62 are connected to the first and second riser parts 45, 46 respectively, there are arranged a number of hydraulic cylinders 70, 71, 72, 77 having pistons 73. The connection between the shoulders 60, 62 can be via spherical bearings 90 with fixing brackets for bolting to the flexible joint 20. The piston 73 is connected to a through-going piston rod 74 which through-going piston rod 74 in its two ends is attached to the respective shoulders 60, 62. The piston 73 is fixed relative the through-going piston rod 74. The piston 73 is reciprocally movable in the cylinder 72 thus limiting the cylinder into a first and second chamber. Each chamber is connected to a fluid line 80 and 80 for supplying fluid under pressure to one or the other chamber, for thereby regulating the force from the actuating system 42, i.e. the hydraulic cylinders 70, 71, 72, 77 on the flexible joint 20. The fluid lines are connected to an accumulator (see details in FIG. 6) and the flow to the different chambers of the different cylinders in the cylinder arrangement is controlled by a control system 40. The control system 40 receives monitoring data from the first sensor 41 through first connection line 43 and provides instruction signals to the hydraulic cylinders 70, 71, 72, 77 dependent on a calculated real-time set of data. The system also includes sensors for measuring the global riser angle as well as pressure and temperature transmitters as is common in control systems. The arrangement function such that the angle size and direction is measured, as well as the bending moment in the desired position measured by the first sensor 41, and when the riser starts bending the control system 40 will direct pressurized fluid into the chamber above the pistons in the different hydraulic cylinders 70, 71, 72, 77 to force an increase of the bending angle in the flexible joint 20.
[0109] The piston and cylinders are preferably attached to the shoulders with flexible joints to avoid excessive bending.
[0110] The system is shown having four cylinders equally disposed with 90 degree intervals around the flexible joint 20 but the number may be any that will achieve the desired result. In the actuating system 42, the first volume in one hydraulic cylinder 70, 71, 72, 77 may be connected to a first volume or a second volume in another hydraulic cylinder 70, 71, 72, 77 and/or a second volume in one cylinder is connected to a first volume or a second volume in another cylinder. This provides for a potential additional force to the joint 20.
[0111] As indicated in the figure there may be connection line 79 between the hydraulic cylinders 72 and the internal bore 54 of the riser through the joint. By this one may pressure compensate the system for the pressure within the riser and thereby have the possibility of regulating the systems and the forces from this arrangement on the riser parts, independent of the pressure within the internal bore 54.
[0112] FIG. 9A shows example of experienced bending moments at the subsea wellhead when the actuating system is applying too little force when the floating structure is drifting in the right hand direction on the drawing. In the illustrations in FIGS. 9A-C, the desired position is at the wellhead 14. The situation in FIG. 9A is as follows: the actuating system 42 is not providing sufficient force acting in the same direction as the movement out of neutral position of the joint 20, thus both the first part 45 and the second part 46 will try to compensate for the movement out of the neutral position resulting in a bending moment which will propagate downwardly towards the wellhead 14. The direction of the bending moment experienced by the wellhead 14 is indicated by the arrow M.sub.WH9A.
[0113] FIG. 9B shows example of experienced bending moments at the subsea wellhead when the actuating system is applying too much force when the floating structure is drifting in the right hand direction on the drawing. The situation in FIG. 9B is as follows: the actuating system 42 is providing too much force acting in the same direction as the movement out of neutral position of the joint 20, thus both the first part 45 and the second part 46 will overcompensate for the movement out of the neutral position resulting in a bending moment which will propagate downwardly towards the wellhead 14. The direction of the bending moment experienced by the wellhead 14 is indicated by the arrow M.sub.WH9B, which bending moment will act in the opposite direction than in FIG. 9A.
[0114] FIG. 9C shows example of experienced bending moments at the subsea wellhead when the actuating system is applying ideal force when the floating structure is drifting in the right hand direction on the drawing. In this Figure the bending moment M.sub.WH is constant. A constant bending moment will normally not result in fatigue, thus the aim of the first embodiment of the invention is to provide a system which operates as indicated in FIG. 9C where M.sub.WH is constant and any bending moments resulting from movement in the floating structure, is compensated for .In such ideal conditions, the moment transferred from the flexible connection 20
[0115] FIG. 10 shows a second embodiment of the invention for reducing stresses in a desired position. In the second embodiment, where the stress is compression or torsion, it is provided a system wherein the flexible connection 20 comprises a dynamic seal 100 allowing the first part 45 and the second part 46 axial movement relative each other. The actuating system 42 being arranged above a BOP 101 in the riser 24 and is configured to apply a force on the first or second part in the axial direction (direction indicated by arrow A) when the first and second parts 45, 46 are moved out of an axially neutral position. The actuating system 42 is disclosed as a cylinder arrangement similar to the actuating system described in relation to FIGS. 5, 6A-C and 7 above describing the first embodiment, may be used in this second embodiment as well and is incorporated in the second embodiment.
[0116] Typically, the neutral position in all aspects corresponds to the position where the riser is tensioned, e.g. by tensioning system, and the desired position experiences zero forces or stress or constant forces or constant stress. In theory, this should be enough, and under ideal conditions the desired position would not experience any forces or stress resulting in fatigue challenges over time. However, it has proven that the desired position experiences forces or stresses which are not constant, i.e. the forces or stresses will fluctuate or variate around the neutral position and result in fatigue over time. Thus, the invention minimizes and or reduces the forces or stresses experienced at the desired position. This is achieved by for example a short section of flexible connection 20, i.e. axially flexible connection 20 such as a telescopically controlled pipe 20, above the BOP 101 in the riser 24. The telescopically controlled pipe 20 comprising the actuating system 42. In order to reduce or eliminate the stress forces in the desired position on a seabed structure 11, 14, e.g. wellhead 14 on top a well 11. In FIG. 10, a first sensor 41 is provided, which first sensor 41 is configured to monitor stresses at the desired position and provide real-time monitoring of stresses at the desired position. The actuating system 42 is arranged at the flexible connection 20, and is configured to apply a force to the first or second part when the first and second parts 45, 46 are moved out of a neutral position. The control system 40 is adapted to receive monitoring data from the first sensor 41 wirelessly or via a first connection line 43. The control system 40 is connected to the actuating system 42 and is able of providing instruction signals to the actuating system 42 wirelessly or via a second connection line 44 and is able of providing instruction signals to the actuating system 42. The control system 40 is able to, based on said monitoring data from the first sensor 41, calculate a real-time set of data for control of the applied force of the actuating system 42 and instructing the actuating system 42 to act accordingly, such as to reduce the stress at said desired position either by compensating for tension or compression relative the neutral position by operating the actuating system 42. In order to reduce, minimize, or even eliminate, varying axial stresses (e.g. compression stress or drag stress) experienced at the desired position, the force applied by the actuating system 42 is thus controlled by axial monitoring of stresses by the first sensor 41 at the desired position. The hydraulics functions such that if the control system 40 calculates that the flexible connection 20 is moving out of neutral position, an additional force is added by supplying pressure from the hydraulic fluid reservoir 49 to dedicated first and second volumes of the hydraulic cylinders in the actuating system 42 dependent on the direction of the force or stress (as described in relation to the first embodiment in FIGS. 5, 6A-C and 7).
[0117] FIG. 11 shows a third embodiment of the invention for reducing torsion in a desired position. In this embodiment, it is provided a system wherein the actuation system 42 comprises a plurality of inclined cylinders 42 connected to the first part 45 and the second part 46 of the riser 24. The inclined cylinders 42 are arranged such that, upon torsion in one direction of the first part 45 relative the second part 46 (or relative between a second part and third part etc.), the plurality of cylinders 42 may reduce any torsional forces in the desired position, in FIG. 11 exemplified as the wellhead 14 by providing a force in an opposite direction of the torsional forces between the first and second part 45, 46 (or relative between a second part and third part etc.). If seen from the side, the inclined cylinders 42 may be arranged outside the flexible connection 20, with a center axis of the cylinders angled relative a center axis of the first part 45 extending through the second part 46 via the flexible connection such that the cylinders 42 are arranged tangentially around the flexible connection 20. Thus, the arrangement of the cylinders 42 are very similar to the setup of cylinders as in the first embodiment in FIGS. 7 and 8, and the actuating system 42 is disclosed as a cylinder arrangement similar to the actuating system described in relation to FIGS. 5, 6A-C and 7 above describing the first embodiment, may be used in this third embodiment as well and is incorporated in the third embodiment.
[0118] Typically, the neutral position in all aspects corresponds to the position where the riser is tensioned, e.g. by tensioning system, and the desired position experiences zero torsional forces or constant torsional forces. In theory, this should be enough, and under ideal conditions the desired position would not experience any torsional forces resulting in fatigue challenges over time. However, it has proven that the desired position experiences torsional forces which are not constant, i.e. the torsional forces fluctuates or variate around the neutral position and result in fatigue over time. Thus, the invention minimizes and or reduces the torsional forces or stresses experienced at the desired position. In order to reduce or eliminate the torsional forces in the desired position on a seabed structure 11, 14, e.g. wellhead 14 on top a well 11, the actuating system comprises a set of inclined cylinders 42, where each of the cylinders are connected both to the first part 45 and the second part 46. In FIG. 11, a first sensor 41 is provided, which first sensor 41 is configured to monitor torsional forces at the desired position and provide real-time monitoring of torsional forces at the desired position. The actuating system 42 is arranged at the flexible connection 20, and is configured to apply a force to the first or second part when the first and second parts 45, 46 are moved out of a neutral position. The control system 40 is adapted to receive monitoring data from the first sensor 41 wirelessly or via a first connection line 43. The control system 40 is connected to the actuating system 42 and is able of providing instruction signals to the actuating system 42 wirelessly or via a second connection line 44 and is able of providing instruction signals to the actuating system 42. The control system 40 is able to, based on said monitoring data from the first sensor 41, calculate a real-time set of data for control of the applied force of the actuating system 42 and instructing the actuating system 42 to act accordingly, such as to reduce the torsional force at said desired position either by compensating experienced torsional forces relative the neutral position by operating the actuating system 42. In order to reduce, minimize, or even eliminate, varying torsional forces experienced at the desired position, the force applied by the actuating system 42 is thus controlled by torsional monitoring of forces by the first sensor 41 at the desired position. The hydraulics functions such that if the control system 40 calculates that the flexible connection 20 is moving out of neutral position, an additional force is added by supplying pressure from the hydraulic fluid reservoir 49 to dedicated first and second volumes of the hydraulic cylinders in the actuating system 42 dependent on the direction of the force or stress (as described in relation to the first embodiment in FIGS. 5, 6A-C and 7). For example, if the first part 45 is twisted or wrenched in one direction, the hydraulics functions such that when the control system 40 calculates this, i.e. the flexible connection 20 is moving out of neutral position, a compensating force is added in an opposite direction of the wrench- or twist movement, by supplying pressure from the hydraulic fluid reservoir 49 to dedicated first and second volumes of the hydraulic cylinders in the actuating system 42, thereby reducing the torsional forces experienced at the desired position. The invention is now explained with reference to a non-limiting embodiment. However, a skilled person will understand that there may be made alternations and modifications to the embodiments that are within the scope of the invention as defined in the attached claims. It is clear that other types of actuating systems may also be used, such as Electrical, Hydraulic, Electro-Hydraulic, Magnetic or a combination of these.
