A SYSTEM AND A METHOD FOR CONTROLLING A MOTION COMPENSATED PILE GUIDE FOR A FLOATING VESSEL, AND A VESSEL

Abstract

A system for controlling a motion compensated pile guide for a floating vessel comprises a pile guide for guiding a monopile in its longitudinal direction during driving the monopile into a seabed, an actuator for moving the pile guide in horizontal direction with respect to a vessel to which the pile guide is mounted, a control unit for controlling the actuator, which control unit is configured for compensating motion of the vessel to which the pile guide is mounted so as to maintain the horizontal position of the pile guide during driving a monopile into a seabed, a first sensor for determining an inclination angle of a monopile with respect to the vertical during driving the monopile into a seabed, and a second sensor for determining magnitude and direction of an actual force of a monopile onto the pile guide during driving the monopile into a seabed. The control unit is configured to determine a desired force of the pile guide onto the monopile for minimizing the inclination angle when determined by the first sensor, and to control the actuator for moving the pile guide opposite to the direction of the actual force when the desired force is larger than the actual force and in the same direction as the actual force when the actual force is larger than the desired force.

Claims

1. A system for controlling a motion compensated pile guide for a floating vessel, comprising a pile guide for guiding a monopile in its longitudinal direction during driving the monopile into a seabed, an actuator for moving the pile guide in a horizontal direction with respect to the floating vessel to which the pile guide is mounted, a control unit for controlling the actuator, which said control unit is configured for compensating motion of the floating vessel to which the pile guide is mounted so as to maintain a horizontal position of the pile guide during driving of the monopile into the seabed, a first sensor for determining an inclination angle of the monopile with respect to vertical during driving the monopile into the seabed, a second sensor for determining a magnitude and direction of an actual force of the monopile onto the pile guide during driving the monopile into the seabed, wherein the control unit is configured to determine a desired force of the pile guide onto the monopile for minimizing the inclination angle when determined by the first sensor, and to control the actuator for moving the pile guide in a direction opposite to the direction of the actual force when the desired force is larger than the actual force and in a same direction as the direction of the actual force when the actual force is larger than the desired force.

2. The system according to claim 1, wherein the control unit is configured such that when the desired force exceeds a predetermined force level, the desired force is set at the predetermined force level.

3. The system according to claim 2, wherein the predetermined force level is proportional to a maximum power of thrusters of the floating vessel to which the pile guide is mounted.

4. The system according to claim 1, wherein the actuator comprises a set of hydraulic cylinders which are controllable by the control unit.

5. The system according to claim 4, wherein the second sensor comprises pressure sensors for measuring pressure in the hydraulic cylinders.

6. The system according to claim 1, wherein the desired force is determined by controlling the actuator to move the pile guide in a direction from an inclined orientation towards a vertical orientation of the monopile and determining an increase rate of the actual force in relation to a reduction of the inclination angle.

7. The system according to claim 1, wherein the desired force is determined based on a relationship between soil structure of the seabed into which the monopile is driven and a required force of the pile guide onto the monopile for moving the monopile from its inclined orientation towards a vertical orientation.

8. The system according to claim 1, wherein the control unit is provided with an algorithm for compensating motion of the floating vessel to which the pile guide is mounted, which algorithm uses the desired force as an input.

9. The system according to claim 1, wherein the system is provided with a hammer for driving the monopile into the seabed, and the first sensor is provided at the hammer.

10. The system according to claim 1, wherein the first sensor is an inclination sensor and the second sensor is a load sensor.

11. A method for controlling a motion compensated pile guide for a floating vessel during driving a monopile into a seabed, wherein the monopile is guided in its longitudinal direction by the pile guide and the pile guide is moved in a horizontal direction for compensating motion of the floating vessel to which the pile guide is mounted, said method comprising: determining an inclination angle of the monopile with respect to vertical and a magnitude and direction of an actual force of the monopile onto the pile guide during driving the monopile into the seabed, determining a desired force of the pile guide onto the monopile for minimizing the determined inclination angle, and moving the pile guide in a direction opposite to the direction of the actual force when the desired force is larger than the actual force and in a same direction as the direction of the actual force when the actual force is larger than the desired force.

12. The method according to claim 11, wherein when the desired force exceeds a predetermined force level the desired force is set at the predetermined force level.

13. The method according to claim 11, wherein the desired force is determined by moving the pile guide in a direction from an inclined orientation towards a vertical orientation of the monopile and determining an increase rate of the actual force in relation to a reduction of the inclination angle.

14. The method according to claim 11, wherein, the desired force is determined based on a relationship between soil structure of the seabed into which the monopile is driven and a required force of the pile guide onto the monopile for moving the monopile from its inclined orientation towards a vertical orientation.

15. A vessel, comprising the system according to claim 1.

16. The system according to claim 1, wherein the control unit is configured such that when the desired force exceeds a predetermined force level, the desired force is set at the predetermined force level, the predetermined force level is proportional to a maximum power of thrusters of the floating vessel to which the pile guide is mounted, and the actuator comprises a set of hydraulic cylinders which are controllable by the control unit.

17. The system according to claim 16, wherein the second sensor comprises pressure sensors for measuring pressure in the hydraulic cylinders.

18. The system according to claim 1, wherein the desired force is determined based on a relationship between soil structure of the seabed into which the monopile is driven and a required force of the pile guide onto the monopile for moving the monopile from its inclined orientation towards a vertical orientation, and the control unit is provided with an algorithm for compensating motion of the floating vessel to which the pile guide is mounted, which algorithm uses the desired force as an input.

19. The system according to claim 18, wherein the system is provided with a hammer for driving the monopile into the seabed, and the first sensor is provided at the hammer.

20. The system according to claim 19, wherein the first sensor is an inclination sensor and the second sensor is a load sensor.

Description

[0022] The invention will hereafter be elucidated with reference to the schematic drawings showing an embodiment of the invention by way of example.

[0023] FIG. 1 is a perspective view of a vessel including an embodiment of a system for controlling a motion compensated pile guide according to the invention.

[0024] FIG. 2 is an enlarged perspective view of a part of the vessel as shown in FIG. 1.

[0025] FIG. 3 is a top view of the motion compensated pile guide mounted to the vessel of FIG. 1.

[0026] FIG. 4 is a diagram which illustrates an embodiment of the system for controlling the motion compensated pile guide of FIG. 3.

[0027] FIG. 1 shows a floating installation vessel 1 for installing a monopile 2 in a seabed S. The installation vessel 1 is provided with a crane 3 for lifting the monopile 2. FIGS. 1-3 show a motion compensated pile guide 4 for guiding the monopile 2 in its longitudinal direction during driving the monopile 2 into the seabed S. The pile guide 4 is mounted to the vessel 1 and movable in horizontal direction with respect to the vessel 1 by an actuator in the form of hydraulic cylinders 5, i.e. in longitudinal and sideward direction or X and Y direction in order to compensate for vessel motions. The pile guide 4 can be opened and closed in a well-known manner in order to enclose the monopile 2. The function of the pile guide 4 is to install the monopile 2 vertically from the beginning of the operation until it is hammered deep enough such that it is supported by the soil to stand safely upright on its own.

[0028] Under operating conditions, the monopile 2 is transported horizontally by the vessel 1 from a production facility to an off-shore installation site. Before the monopile 2 is installed it is up-ended by means of the crane 3, after which the monopile 2 suspends vertically from the crane 3. Subsequently, the monopile 2 is lowered and placed onto the seabed S. Depending on the soil structure the monopile 2 may first penetrate the seabed S to a certain depth due to its own weight. Alternatively, the monopile 2 may be up-ended by means of an independent system or a pile-guide integrated upend system (not shown). The pile guide 4 is coupled to the monopile 2 and a pile driving hammer 6 is placed on top of the monopile 2 by the crane 3 in order to drive the monopile 2 into the seabed S. The pile guide 4 is controlled by the hydraulic cylinders 5 to maintain a vertical orientation of the monopile 2. Installing a monopile 2 from a floating vessel as such allows higher pay-loads and crane capacities than jack-up barges, but is challenging because of wave-induced vessel motions and strict monopile vertical installation tolerances.

[0029] The installation vessel 1 is provided with a system for controlling the motion compensated pile guide 4, which is illustrated by a diagram in FIG. 4. In addition to the pile guide 4 and the hydraulic cylinders 5 the system comprises a control unit 7 for controlling the hydraulic cylinders 5, an inclination sensor 8 for determining an inclination angle of the monopile 2 with respect to the vertical during driving the monopile 2 into the seabed S and pressure sensors 9 for measuring pressure in the hydraulic cylinders 5 in order to determine magnitude and direction of an actual force of the monopile 2 onto the pile guide 4 during driving the monopile 2 into the seabed S. In FIG. 4 the inclination sensor 8 is shown next to the monopile 2, but in an alternative embodiment it may be located at the pile driving hammer 6.

[0030] The control unit 7 is configured for compensating motion of the vessel 1 so as to maintain the horizontal position of the pile guide 4 during driving the monopile 2 into the seabed S. Such a control is based on conventional active vessel motion compensation that intends to position the pile guide 4 geostationary and is well-known. It tries to move the pile guide 4 exactly opposite of the vessel motion such that the pile guide 4 stands still in horizontal direction and the monopile 2 remains upright. Forces of the pile guide 4 acting on the vessel 1 may disturb positioning of the vessel 1. Therefore, the control unit 7 according to the present invention is also configured to determine a desired force of the pile guide 4 onto the monopile 2 for minimizing the inclination angle in the event that the inclination sensor 8 detects an inclination angle. A response of the control unit 7 to detecting an inclination angle may be that the hydraulic cylinders 5 are operated such that they move the pile guide 4 opposite to the direction of the actual force. However, this response only happens when the desired force is larger than the actual force. If the actual force is larger than the desired force the hydraulic cylinders 5 are operated such that they move in the same direction as the actual force. The latter conditional control step serves to prevent the compensation control for wave-induced motion from running out of its operating range, resulting in unstable motion control.

[0031] The desired force of the pile guide 4 onto the monopile 2 for minimizing the inclination angle can be determined in different ways. For example, the desired force is determined by operating the hydraulic cylinders 5 such that they move the pile guide 4 in a direction from an inclined orientation towards a vertical orientation of the monopile 2, whereas an increase of the actual force in relation to reduction of the inclination angle is determined. This can be achieved by means of a PID controller, which increases the pressure in the hydraulic cylinders 5 step-by-step in order to obtain a relationship between the reduction of inclination angle and force of the pile guide 4 onto the monopile 2. From this relationship the desired force to tilt the monopile 2 back to its vertical orientation is derived. The desired force is compared with the actual force of the monopile 2 onto the pile guide 4. As stated above, the hydraulic cylinders 5 are operated such that they move the pile guide 4 opposite to the direction of the actual force when the desired force is larger than the actual force and in the same direction as the actual force when the actual force is larger than the desired force. Under operating conditions the actual force may follow the set desired force by a time delay, for example a few seconds, because of dynamics and system inertia and the selected speed of control. It is noted that a time delay on the force may increase the operational window of the system/vessel. Vessels allow a certain amount of time, for example five to eight seconds, of higher forces on the pile guide than the force which thrusters of the vessel can deliver.

[0032] An alternative way for determining the desired force of the pile guide 4 onto the monopile 2 for minimizing the inclination angle is based on a relationship between soil structure of a seabed into which a monopile 2 is driven and required force of the pile guide 4 onto the monopile 2 for moving the monopile 2 from its inclined orientation towards a vertical orientation. Such a relationship may be derived from a model of the soil structure and a penetrating monopile 2. Alternatively, there may also be a relationship between blow count and penetration of the monopile 2 per blow, which can be used to derive fixation of the monopile 2 in the seabed S in order to define a profile for force control in order to bring the monopile 2 in vertical orientation, i.e. to determine the desired force.

[0033] The control unit 7 is configured such that when the desired force exceeds a predetermined force level, the desired force is set at the predetermined force level. The predetermined force level is proportional to the maximum power of azimuth thrusters 10 of the vessel 1. This prevents the system from attempting to push the monopile 2 towards a vertical orientation whereas required force exceeds the maximum force of the azimuth thrusters 10 of the vessel 1.

[0034] The invention is not limited to the embodiment shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents.