US20100098498A1

US20100098498A1 - Anchor system for offshore dynamically positioned drilling platform

Info

Publication number: US20100098498A1
Application number: US12/288,111
Authority: US
Inventors: Gavin Humphreys
Original assignee: Stena Drilling Ltd
Current assignee: Stena Drilling Ltd
Priority date: 2008-10-16
Filing date: 2008-10-16
Publication date: 2010-04-22

Abstract

An anchor system may reduce stress on anchor cables when an offshore drilling platform weathervanes. The anchor system may hold the platform in a set position above the sea floor during drilling operations. The anchor system includes anchor cables that pass through a split bearing. Clamps may be connected to the bearing to hold the anchor cables in tension.

Description

BACKGROUND

This relates generally to offshore drilling operations.
Offshore drilling operations may be implemented with a variety of different platforms secured to the seabed floor. As used herein, an “offshore drilling platform” is a floating platform, whether towed, pulled, or self-propelled. It includes semi-submersible rigs and self-propelled vessels.
These offshore drilling platforms may be precisely positioned at a desired location so that the drilling equipment may be operated to precisely drill wells at desired locations. The platform may be maintained in position under dynamic positioning even in extreme seas.
These offshore drilling platforms are often deployed in extreme conditions which can be hard on equipment. Once drilling begins there is a maximum distance that the platform can move before the amount of stress on the drilling system is beyond a stress threshold that the drilling system can tolerate. Drill stems may bend or break allowing oil to contaminate the sea.
A dynamic positioning system can maintain the position of the platform to reduce the stress on the drilling equipment. When the dynamic positioning system fails, only a limited amount of time is available to disconnect from the drill stem to reduce the possibility of an environmental contamination as the platform drifts away from the drill site.
Anchors attached to the bow of a ship are not feasible with dynamic positioning because a bow anchor prevents a ship from weathervaning above a drill site. The anchor would have to be removed and reconnected every time the dynamic positioning system weathervanes the ship.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partial top plan view of an offshore drilling platform according to one embodiment of the present invention;

FIG. 2 is a partial cross-section taken generally along the line 2-2 in FIG. 1 in accordance with one embodiment of the present invention;

FIG. 3 is a partial bottom plan view in accordance with one embodiment of the present invention;

FIG. 4 is a partial cross-section of the platform shown in FIG. 1 attached to the sea floor in accordance with another embodiment of the present invention;

FIG. 5 is a perspective view of a platform anchored to the sea floor in accordance with one embodiment of the present invention;

FIG. 6 is a representation of a ship weathervaning in accordance with one embodiment of the present invention;

FIG. 7 is a schematic depiction of one embodiment of the present invention; and

FIG. 8 is a method of assembling an anchor system according to one embodiment.

DETAILED DESCRIPTION

An anchoring system can hold a drilling platform on the sea floor above a drill site at a location with greater efficiency and reliability, in some embodiments, than dynamic positioning systems alone. The anchoring system may include a bearing with anchor clamps mounted to the bearing for holding the anchor cables in tension. In one embodiment, a dynamic positioning system can weathervane the platform around a pivot point while rotating the bearing of the anchor cables so that they do not become crossed.
Dynamic positioning is a computer controlled system to automatically maintain a vessel's position and heading. Position reference sensors, for example global position sensors or sonar, combined with wind sensors, motion sensors and compasses, provide information to a computer relating to the vessel's position and the magnitude and direction of environmental forces affecting its position.
A computer program has a mathematical model of the vessel that includes information pertaining to the wind and current drag of the vessel and the location of the thrusters. This information, combined with the sensor information, allows the computer to calculate the required steering angle and thruster output for each thruster.
A drilling platform 100, shown in FIGS. 1, 2, and 3, includes a deck 160 with moon pool 155 through the deck 160 and the hull 162. The drilling platform 100 may be towed, pulled, or self-propelled and may be a ship or a drilling rig. The platform 100 may float entirely on the surface or it may be fully or partially submerged.
Above the moon pool 155, moving actuators 105 and 110 can retract to open the moon pool 155. In one embodiment, the actuators may include hydraulic rams with reciprocating pistons. In other embodiments, geared drives, driven by hydraulic or electric power may also be used. In some embodiments, the actuators 105 and 100, below the deck 160, retract into the sides of the moon pool 155. The retraction of the actuators 105 and 110 allows a drilling operator to pass drilling equipment down through the moon pool 155.
Attached to each of the actuators 105, 110 is a portion of a split bearing 115. The inner race 120 of the bearing 115 includes anchor cable guides 125 that allow an anchor cable 205 to pass through the inner race 120. As used herein, “cable” refers to wire rope, chains, or any other device used for connecting to an anchor. The inner race is split and can be retracted with the actuators 105 and 110 and the outer bearing 115. The anchor cables 205 can remain in the anchor cable guides 125 when the actuators 105 and 110 are retracted to reduce the time to retention the cables 205.
Each anchor cable 205 is connected to a cable tensioner 230 by passing the cable over a pulley 240. The cable tensioner may, for example, be an automatically controlled winch. The anchor cable 205 passes through the anchor cable clamps 130 and through the anchor cable guides 125. The anchor cable 205 extends from the anchor cable guide 125 through the moon pool 155 and around a pulley 225. The anchor cable 205 extends from the pulley 225 to the sea floor, where the cable is attached to an anchor. The pulleys 225 are attached to a lower bearing 215 by the pulley support bracket 220. The lower bearing 215 and the inner bearing 135 allow the tube 140 to rotate independently of the anchor cables 205 in some embodiments. In some embodiments, a stabilizing clamp (not shown) and an additional bearing (not shown) with anchor cable guides may be used add support for the tube 140. In some embodiments, the lower bearing, the inner bearing or both bearings can be removed so that the inner race 120, the tube 140 and the pulleys 225 are all rotated together.
When the anchor cables 205 are attached to the sea floor, the tensioner 230 applies tension to the cables 205 to hold the platform in a position in concert with the dynamic positioning system.
Anchor cable clamps 130 are attached over the inner race 120. The anchor cable clamps 130 fix the anchor cables 205 under tension. The tensioner reduces slack in the anchor cables between the anchor cable clamp 130 and the sea floor once tension is applied to the anchor cables.
Tension may be applied from tensioners 230 when the clamps 130 are released from the cable 205. The tensioners 230 may be located above or below the clamps 130. Then when the cables are tensioned, a clamping force may be radially applied by the clamps to the cables to fix them in tension.
In some embodiments, an inner bearing 135 is attached to the inner race 120. The inner bearing 135 allows a tube 140 to rotate independently of the inner race 120. The tube 140 can be used to access the sea floor through the hull and to support rockers 225 that guide the anchor cables below the platform 100.
When the tensioners tension the anchor cables, stress is applied to the outer bearing 115. The stress on the outer bearing could cause additional friction that could not be overcome causing the inner race to change its heading along with the platform when weathervaning. Weathervaning is the ability of the ship to rotate about the cables 205 in heavy seas. In some embodiments, a system can be used to mechanically assist the inner race to maintain a heading while the platform's heading is changed. The mechanical system for example may include gears and a motor.
The lower bearing 215 is connected to the pulley support brackets 220. The pulley support brackets hold the pulley 225 which guides the anchor cable 205 from the anchor cable guide, shown in FIGS. 1 and 2, to an anchor location on the sea floor.
In some embodiments, the anchor cables 205 can pass through pipes (not shown) between the anchor cable guides 125 and the lower pulley 225. The pipes may have the anchor cable 225 come out of the lower end of the pipes allowing the removal of the lower pulleys 225.
The position of the platform 100 can be adjusted by the propulsion devices 235 of the dynamic positioning system before or after the anchor cables are attached to the sea floor. For example, the dynamic positioning system can move the platform 100 to a location directly above each anchor location on the sea floor, allowing the anchors to be connected before centering the platform in position above the drill site. In another example the dynamic positioning system can move the platform into position above the drill site and then the anchor cables can be extended from the ship to the anchor sites on the sea floor.
The dynamic positioning system can use the propulsion devices 235 to position the platform above the drill site location while the anchor cables 205 are attached to the sea floor. The dynamic positioning system uses the propulsion devices to keep the platform at a designated position to reduce stress on the anchor cables 205.
The anchor cable clamps 130 can be applied to the anchor cable 205 to hold the cables in tension with the platform 100 at a specified location. The tension on the cable between the clamp 130 and the tensioner 230 can be released when the cable is clamped. The tension on the cable between the clamp 130 and the tensioner 230 can be released to allow the inner race 120 to rotate on the outer bearing 115 independently of the actuators 105 and 110 and the platform 100.
With the cable tension released at the tensioner 230, the platform can weathervane around the moon pool 155 without having to disconnect the anchor cables 205 and move the anchor cables 205 to a different tensioner 230 to retension the anchor cables 205. In some embodiments, the cables may be completely disconnected from the tensioner 230 to allow the inner race 220 to rotate 360 degrees around the axis 150 in relation to the platform 100, without anchor cables 205 crossing each other or the top of the tube 140. If the anchor cables 205 extend over the top of the tube 140, the anchor cable 205 may interfere with drilling equipment entering the tube 140. If the platform is ready to move to a new location the cables can be reconnected to a tensioner so that the anchor cables 205 can be retrieved from the sea. In some embodiments, the anchor cables 205 may remain attached to the sea floor after the platform is ready to leave the site and the platform disconnects from the anchor cables 205.
The actuators 105 and 110, the outer bearing 115, the race 120, and the anchor cable guides 125 can be retrofitted to a variety of platforms having a moon pool with minor modifications to the platform to reduce down time.
Referring to FIG. 7, in one embodiment, a positioning system 300 includes a dynamic positioning system 302 with a computer and appropriate software to control the positioning of the platform 100 in heavy seas. The dynamic positioning system 302 may be coupled to motive power sources 306, such as thrusters 235. In addition, the dynamic positioning system 302 may receive position information 310 from global positioning devices and other position systems. At the same time, wind and wave information 312 may be supplied to the dynamic positioning system 302.
The dynamic positioning system 302 communicates with the anchoring system 304. The anchoring system 304 includes a computer or controller and may receive position information 310 and wind/wave information 312 that is also supplied to the dynamic positioning system 302. The anchoring system 304 may provide outputs to the cable tensioners 230 to control the tension on cables 205. In addition, the anchoring system 304 may control motive power sources 308 to control the position of the actuators 105 and 110 and clamps 130 and bearing 115.
In another embodiment, the inner race 120 may be large enough to accommodate the tensioner 230. In this embodiment, the cable does not need to be removed from the tensioner 230 to prevent the cable 205 from crossing other cables or the top of the tube 140.
The dynamic positioning system can use a multitude of propulsion devices 235 to hold the platform 100 with its axis of rotation above a drill site on the sea floor. The dynamic positioning system can also use the propulsion devices 235 to weathervane the platform. If the platform is rotated, the mechanical system can also be controlled by the dynamic positioning system to rotate the inner race 120 in the opposite direction of platform rotation, maintaining the heading of the inner race 120 and the anchor cables 205 when the platform's heading is changing.
For example, if the dynamic positioning system uses the propulsion devices 235 to change the heading of the platform 100 from a heading of 0 degrees to a heading of 15 degrees, the inner race 120 can be rotated on the bearing 115 so that it remains at a heading of 0 degrees, preventing the anchor cables 205 from becoming crossed between the lower pulley 225 and the sea floor or between the lower pulley 225 and the anchor cable guides 125.
The dynamic positioning system can use the propulsion devices 235 to hold the platform in position above the sea floor without the addition of the anchor cables 205. The anchor cables 205 can hold the platform 100 in a position above the sea floor without assistance of the dynamic positioning system and the propulsion devices 235. A platform with these independent systems does not drift away from the drill site unless there is a failure to both the dynamic positioning system and the anchor cabling system.
Drilling may continue using only one system to reduce down time if one of the systems fail. Conditions such as extreme weather, sensitive environment, drilling regulations or insurance may cause drilling to be suspended if one of the systems has failed, however the crew of the platform would have additional time to remove drilling equipment and suspend drilling operations since the platform can be positioned by the other system. The additional time reduces the possibility of environmental contamination that may result if the platform drifts away from the drill site.
In some embodiments, the fuel required to operate the dynamic positioning system is reduced when the anchoring system is deployed. For example, the dynamic positioning system may not operate as frequently when an anchor system is also holding the platform at a location. The dynamic positioning system may operate to reduce the stress on the cables when there is stress on the anchor cables 205 above the limits of the anchor cables 205 and at other times the dynamic positioning system conserves fuel.
In some embodiments, the anchor receiver 415, shown in FIG. 4, is installed on the sea floor before the arrival of the platform 100. In alternative embodiments, the anchors do not need to have preinstalled receivers on the sea floor to provide a connection to the sea floor.
The anchor 420 can be attached to the anchor receiver 415 by use of a remotely operated vehicle (ROV) or by divers. The anchor cable 205 is held in tension between the anchor receiver 415 and the clamp 130. Since multiple anchor cables may be attached to the sea floor, the platform is held in position when there is tension on the cable between the anchor receiver 415 and the clamp 130.
In rougher seas, too much stress could be put on the cable 205, the clamp 130, the race 120, the bearing 115 or the tube 140, causing any of those components to buckle or fail. A cable tensioning rod 405 with a cable tensioning cylinder and piston 410 can be placed inline in the anchor cable 205 between the anchor receiver 415 and the clamp 205. The cable tensioning rod 405 can include valves that open and close, allowing the cable tensioning cylinder and piston 410 to move within the cable tensioning rod 405 to apply tension to the cable. The cable tensioning rod 405 allows the platform to move with tides and reduce stress to the cable that could cause the anchor cable 205, the clamp 130, the race 120, the bearing, 115 or the tube 140 to fail under stress.
Four anchor cables 205 may be in tension between the platform and the sea floor 505, as shown in FIGS. 5 and 6. The platform may weathervane from an initial heading, shown in FIG. 6, to a new heading as indicated at 100 a. The anchor cables 205 a, 205 b, 205 c, and 205 d may remain in position. The anchor cables 205 may not become crossed or over-stressed, in some embodiments, and may not encounter any change in heading of the platform 100. In some embodiments, more or less anchor cables may be used.
A wireline connection can be made to the anchor 420, anchor receiver 415, the cable tensioning rod 405 or cable-tensioning cylinder and piston 410 to disconnect them from the anchor cable 205 when the anchor cable 205 is retracted after drilling operations are complete.
Referring to FIG. 8, a method of assembling the anchor system includes mounting the split bearings in the moon pool, as indicated in block 350. Clamps may be secured to anchor cables, as indicated in block 352. The actuators may be connected between the split bearings and tubes in the moon pool, as indicated in block 354. A tensioner, such as a winch, may be connected to one end of the anchor cables, as indicated in block 356. Couplers to mount anchors may be coupled to the opposite end of the cables, as indicated in block 358.
The dynamic positioning system may be coupled to the appropriate operators and to the controller for the anchor system. Similar software may be programmed in a controller to use wind and wave information to actuate the anchor system. For example, wind and wave information may be utilized to clamp or release clamping pressure on the cables at the appropriate times. Likewise, the same information may be utilized to rotate an inner race of the split bearing relative to the outer race. Also, tension on the cables may be controlled as necessary.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate the numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this invention.

Claims

1. An offshore drilling platform comprising:

a dynamic positioning system; and

an anchoring system to couple tensioned cables to anchors, said platform being rotatable above said anchoring system.

2. The platform of claim 1 wherein the anchoring system includes a split bearing.

3. The platform of claim 2 further comprising retractable actuators coupled to said split bearing.

4. The platform of claim 2, said anchoring system further comprising a tube coupled to said bearing.

5. The platform of claim 4 further comprising pulleys coupled to said tube to guide the tensioned cables.

6. The platform of claim 1 further comprising a winch to adjust tension on the anchor cable.

7. The platform of claim 1 further comprising a motor to rotate the bearing to account for the rotation of the platform.

8. The platform of claim 1 wherein said platform is a drilling ship having a moon pool, said anchoring system mounted in said moon pool.

9. A method comprising:

tensioning an anchor cable coupled to an anchor on the sea floor, while an offshore drilling platform is being positioned by a dynamic positioning system.

10. The method of claim 9 further including clamping the anchor cable in a clamp.

11. The method of claim 10 including enabling said offshore drilling platform to rotate around said anchor cable.

12. The method of claim 11 including passing said cable through a moon pool.

13. The method of claim 12 including guiding said cable through said moon pool using a guide that is retractable relative to said moon pool.

14. The method of claim 11 including passing said cable through a split ring bearing and allowing said cable to rotate relative to said platform.

15. The method of claim 14 including passing a cable through each portion of said split ring bearing.

16. The method of claim 14 including driving the portion of said bearing including said cables to counteract motion of said platform.

17. An anchor system for an offshore drilling platform comprising:

a split bearing including outer and inner races, a pair of openings in an inner race to allow anchor cables to pass through the inner race; and

clamps to hold the anchor cables in tension.

18. The system of claim 17 further comprising a moveable actuator mounting the split bearing.

19. The system of claim 17 further comprising tensioners to adjust tension on the anchor cables.

20. The system of claim 17 further comprising a device for rotating the inner race to account for movement of the platform.

21. The system of claim 17 including a tube coupled to said inner race, said tube having one end connected to said inner race and an opposite end including guides to guide said anchor cables.

22. The system of claim 21 including a bearing between said guides and said tube.

23. The system of claim 17, said anchor system to receive information about movement of said platform and to use said information to drive said inner race relative to said outer race.

24. The system of claim 17 including a controller to operate said clamps and to receive information about wind and wave action affecting said platform.

25. The system of claim 17 including a dynamic positioning system.

26. A method comprising:

mounting a split bearing in a moon pool with openings in the bearing to pass anchor cables through; and

connecting clamps to the anchor cables to hold tension on the anchor cables.

27. The method of claim 26 including mounting retractable actuators to retract the bearing with respect to the moon pool.

28. The method of claim 26 including coupling said anchor cable to a tensioner.

29. The method of claim 28 including providing couplers on said cables to couple to anchors.

30. The method of claim 26 including coupling a controller system for said anchor cables and a dynamic positioning system.