NL2023412B1 - Offshore drilling system, vessel and method - Google Patents

Abstract

The invention relates to an offshore drilling system for performing subsea wellbore related activities involving a riser extending between the vessel and a subsea wellbore. The offshore drilling system comprises a mobile working deck positioning system, for moving a mobile working deck along the firing line within a motion range including a heave compensation motion range. The mobile working deck positioning system comprises a positioning winch with an associated positioning cable a control device, which control device is adapted to control the speed of the positioning winch; and one or more sheaves, the sheaves guiding the positioning cable in a loop along the heave compensation trajectory. The positioning winch can pull the piston of the heave compensation cylinder in opposite directions along the heave compensation trajectory and, and thus position the mobile working deck using the mobile working deck support cylinder that is hydraulically connected to the heave compensation cylinder.

Description

P34167NLOO/MHR Title: Offshore drilling system, vessel and method The present invention relates to an offshore drilling system for performing subsea wellbore related activities, e.g. drilling a subsea wellbore, comprising a floating drilling vessel that is subjected to heave motion due to waves. The present invention also relates to a floating drilling vessel adapted for use in the system and to methods that are performed using the system.

In the art, e.g. as marketed by the present applicant, offshore drilling vessels are known that comprise: - a floating hull comprising a moonpool; - a drilling tower positioned on said hull at or near the moonpool; - a tubular string main hoisting device comprising: - a main hoisting winch and a main cable driven by said main hoisting winch; - a crown block; - a travelling block suspended from said crown block via said main cable, which travelling block is adapted to suspend a tubulars string, therefrom along a firing line extending through said moonpool; - a vertically mobile working deck positioned above the moonpool and vertically movable with respect to the drilling tower along the firing line within a motion range including a heave compensation motion range; - a mobile working deck support cylinder, which support cylinder is connected to the vessel and to the mobile working deck to vertically move the working deck relative to the vessel, within the motion range including the heave compensation motion range, - a heave compensation system configured to provide heave compensation of the travelling block as well as of the mobile working deck, the heave motion compensation system comprising: - a heave compensation cylinder, which heave compensation cylinder is connected to a gas buffer for providing the hoisting device with passive heave compensation; and

- a sheave head, comprising one or more sheaves for engaging the main cable of the main hoisting device, wherein the sheave head is supported by a piston of the heave compensation cylinder for movement along a heave compensation trajectory; wherein the mobile working deck support cylinder is hydraulically connected with the heave compensation cylinder of the heave compensation system, such that in operation the mobile working deck support cylinder moves synchronously with the heave compensation cylinder of the heave compensation system, and thus the mobile working deck moves synchronously with the travelling block.

The advantage of linking the support cylinder of the mobile working deck with the heave compensation cylinder of the tubular string main hoisting device is that, during heave compensation, the relative position of the mobile working deck and the crown block of the main hoisting device are synchronous. Thus, such a system allows for a more accurate and more efficient heave compensation system. Furthermore, while both the crown block and the mobile working deck are in synchronic heave compensation, the main hoisting winch can be used to position the crown block relative to the mobile working deck. For example WO2016/062812 discloses such a vessel. The disclosed system allows for a synchronous heave compensated motion of the working deck and the travelling block in a simple manner with high accuracy and reliability. By operation of the main hoisting winch of the hoisting device, the travelling block can be positioned independently from a position of the working deck. By virtue of a connection cable winch one can position the working deck independent from the travelling block position. Furthermore, by suitable control of the connector cable winch it may be possible to bring the working deck in a stationary position relative to the hull, e.g. in a lowermost parking position, possibly the working deck being locked in said parking position, whereas the travelling block may be continued to operate in heave compensation mode. To enable controlled positioning of the mobile working deck along motion range, in particular outside the heave compensation motion range, the connector cable is to be used. Also, it is known to provide such systems, more in particular the hydraulic compensator of such a system with an active actuator to obtain an active control of the heave compensation system. Instead of a passive control of the heave compensation system including for example a gas buffer, an active control is obtained by using the active actuator. The active control may contribute to a quicker responding and a more accurate heave compensation system.

For example, from WO2018/151593 it is known to provide the heave compensation cylinder of a heave compensation system with an adjustment system to contribute to a quicker responding and a more accurate heave compensation system. The adjustment system is configured to compensate, i.e. to improve the heave compensation provided by the cylinder. For example, by providing a pulling force on the cylinder, a delay in movement of the cylinder and/or a lack in amplitude of the cylinder can be reduced or even corrected. Typically, an adjusting winch used in such a system is small and agile, e.g. more responsive, compared to a hoisting winch. A smaller winch requires less power to run and allows for more accurate compensation due to the smaller inertia of the motor. Another benefit is that there is less wear and tear of the wire (e.g. no drum crushing) compared with an active winch system employing the hoisting winch for providing heave compensation.

It is an object of the invention to provide an alternative offshore drilling system, preferably provide an offshore drilling system in which one or more of the above mentioned drawbacks are eliminated or reduced. It is a further object of the invention to improve the practical use of equipment as addressed above, e.g. in view of drilling project efficiency, efforts of drilling personnel, etc. A further aim of the invention is to provide an accurate heave compensation system, preferably with an alternative, preferably a more efficient, control over the positioning of the mobile deck. The present invention provides an offshore drilling system for performing subsea wellbore related activities involving a riser extending between the vessel and a subsea wellbore according to claim 1. According to the invention, the offshore drilling system comprises: - a floating hull comprising a moonpool; - a drilling tower positioned on said hull at or near the moonpool; - a tubular string main hoisting device comprising: - a main hoisting winch and a main cable driven by said main hoisting winch; - a crown block;

- a travelling block suspended from said crown block via said main cable, which travelling block is adapted to suspend a tubulars string, therefrom along a firing line extending through said moonpool;

-a vertically mobile working deck positioned above the moonpool and vertically movable with respect to the drilling tower along the firing line within a motion range including a heave compensation motion range;

- a mobile working deck support cylinder, which support cylinder is connected to the vessel and to the mobile working deck to vertically move the working deck relative to the vessel, within the motion range including the heave compensation motion range, - a heave compensation system configured to provide heave compensation of the travelling block as well as of the mobile working deck, the heave motion compensation system comprising: - a heave compensation cylinder, which heave compensation cylinder is connected to a gas buffer for providing the hoisting device with passive heave compensation; and - a sheave head, comprising one or more sheaves for engaging the main cable of the main hoisting device, wherein the sheave head is supported by a piston of the heave compensation cylinder for movement along a heave compensation trajectory; wherein the mobile working deck support cylinder is hydraulically connected with the heave compensation cylinder of the heave compensation system, such that in operation the mobile working deck support cylinder moves synchronously with the heave compensation cylinder of the heave compensation system, and thus the mobile working deck moves synchronously with the travelling block; and - a mobile working deck dynamic positioning system, for moving the mobile working deck along the firing line (16) within the motion range including the heave compensation motion range (72), wherein the mobile working deck positioning system comprises: - a positioning winch with an associated positioning cable; - a control device, which control device is adapted to control the speed of the positioning winch; and - one or more sheaves, the sheaves guiding the positioning cable in a loop along the heave compensation trajectory,

wherein the positioning cable is connected to the piston of the heave compensation cylinder and/or the sheave head of the heave compensation cylinder, such that the positioning winch can pull the piston of the heave compensation cylinder in opposite directions along the heave compensation trajectory and, and thus position the mobile working deck with, i.e.

5 using, the mobile working deck support cylinder that is hydraulically connected to the heave compensation cylinder. According to the invention, the mobile working deck dynamic positioning system is configured to position the heave compensation cylinder at, and hold the heave compensation cylinder in, predetermined positions along the heave compensation trajectory, and thus to position the mobile working deck at, and hold the mobile working deck in, predetermined positions along the heave motion compensation range, preferably along the motion range of the mobile working deck.

Itis submitted that the mobile working deck dynamic positioning system controls the position of the mobile working deck via direct control over the heave compensation cylinder, and not via direct control over the mobile working deck support cylinder, or by using a connector cable.

The mobile working deck positioning system thus allows for the mobile working deck to be moved and positioned relative to the vessel The invention thus provides an alternative offshore drilling system, more in particular improves the practical use of equipment of the system, e.g. in view of drilling project efficiency, efforts of drilling personnel, etc.

Furthermore, the invention thus provides an accurate heave compensation system, and an alternative, more efficient, control over the positioning of the mobile deck.

In an embodiment, the control device of the mobile working deck positioning system is connected with, and configured to control, the main hoisting winch, to enable the control device to use the main hoisting system to position and/or move the travelling block while pulling the rod of the heave compensation cylinder, e.g. to keep the travelling block in a particular position relative to the vessel while moving the rod of the heave compensation system to move the mobile working deck relative to the vessel.

The mobile working deck positioning system is thus configured to also control the position of the travelling block, in particular to control the position of the travelling block relative to the mobile working deck, while moving the mobile working deck relative to the vessel Thus, in such an embodiment, the mobile working deck dynamic positioning system is configured to control the main hoisting winch to compensate for the movement of the heave compensation cylinder, such that the travelling block stays in a fixed positon relative to the vessel, while the mobile working deck dynamic positioning system moves the heave compensation cylinder, more in particular moves the sheave head of the heave compensation cylinder. In an embodiment, the dynamic positioning system is configured to adjust passive heave compensation of the travelling block as well as of the mobile working deck by increasing and/or lowering the speed at which the piston of the heave compensation cylinder moves along the heave compensation trajectory while the heave compensation system provides passive heave compensation. The dynamic positioning system is thus configured to tune the passive heave compensation provided by the heave compensation system, preferably provide more accurate heave compensation system. Furthermore, the dynamic positioning system can thus be used to switch between a condition in which a load supported by the mobile working deck or the travelling block is heave compensated to a condition in which the that load is not compensated for heave compensation, by adjusting the controlled heave compensation. For example, by gradually reducing the provided heave compensation, i.e. by slowing down the movement of the heave compensation cylinder, the system can switch between a condition in which a load supported by the mobile working deck or the travelling block is heave compensated to a condition in which the that load is not compensated for heave compensation.

In an embodiment, the dynamic positioning system is configured is adapted to register and/or predict heave, e.g. relative to the sea floor, and is configured to provide active heave compensation of the travelling block as well as of the mobile working deck by pulling the piston of the heave compensation cylinder in opposite directions along the heave compensation trajectory.

Thus the mobile working deck dynamic positioning system is configured to not only control the position of the mobile working deck, but also allows for providing the mobile working deck and the travelling block with heave compensation, in particular active heave compensation. Thus, according to the invention, the mobile working deck dynamic positioning system allows for example for lifting a riser relative to the sea floor using the mobile working deck supporting the riser, by lifting the mobile working deck relative to the vessel while providing the mobile working deck active heave compensation relative to the vessel.

Furthermore, the invention thus provides a dynamic control system that is configured to provide active heave compensation, via the heave compensation cylinder, while the travelling block and the mobile working deck are not supporting a riser, i.e. are not under a load. It is submitted that this requires a much more powerful winch compared to the prior art, in which the winch is only configured to tune the movement of the heave compensation cylinder.

In a further embodiment, the mobile working deck dynamic positioning system is configured to control the main hoisting winch to compensate for the movement of the heave compensation cylinder while the main hoist provides active heave compensation. Thus, the travelling block is heave compensated relative to the vessel using the main winch, while the mobile working deck dynamic positioning system moves the heave compensation cylinder, more in particular moves the sheave head of the heave compensation cylinder, to adjust the position of the mobile working deck relative to the vessel. Thus, the position of the mobile working deck can be adjusted without interfering with the active heave compensation provided for the travelling block by the main winch.

In an embodiment, the system is configured to block the hydraulic communication between the heave compensation cylinder and the mobile working deck support cylinder, e.g. by providing one or more blocking valves in a hydraulic circuit connecting the mobile working deck cylinder with the heave compensation cylinder, to enable the mobile working deck positioning system to move the travelling block only, e.g. to provide only the travelling block with heave compensation.

In an embodiment the system is provided with a vertically mobile working deck that is vertically mobile within a motion range including a lower stationary position, wherein the working deck is used as stationary drill floor deck with the slip joint unlocked, and the motion range further including a heave compensation motion range that lies higher than said lower stationary position. In this heave compensation motion range the working deck can perform heave compensation motion relative to the hull of the vessel. Advantageous embodiments of the system according to the invention and the method according to the invention are disclosed in the sub claims and in the description, in which the invention is further illustrated and elucidated on the basis of a number of exemplary embodiments, of which some are shown in the schematic drawing. In the figures, components corresponding in terms or construction and/or function are provided with the same last two digits of the reference numbers.

In the drawings: Fig. 1 shows schematically in vertical cross-section a drilling vessel of an offshore drilling system according to the invention; Fig. 2 shows a portion of the vessel with a drilling tower with a compensator cylinder therein and a mobile working deck; Fig. 3 shows in a perspective view a main hoisting device provided with a heave compensation system including two sheave compensators; Fig. 4 shows in a perspective view the heave motion system of Fig. 3, wherein a heave compensation cylinders is hydraulically connected to a mobile working deck support cylinder; Fig. 5 shows the heave motion system of Fig. 4 with a mobile deck dynamic positioning system according to the invention; Fig. 6 shows another exemplary embodiment of an offshore drilling system according to the invention; Fig. 7 shows the offshore drilling system of fig. 6 in a first working condition; Fig. 8 shows the offshore drilling system of fig. 6 in a second working condition; Fig. 9 shows the offshore drilling system of fig. 6 in a third working condition; and Fig. 10 shows the offshore drilling system of fig. 6 in a fourth working condition.

With reference to the drawings an example of an offshore drilling system for performing subsea wellbore related activities, e.g. drilling a subsea wellbore, according to the invention will be discussed.

As shown in Fig. 1 and Fig. 2, the system comprises a drilling vessel 1 having a floating hull 2 subjected to heave motion, the hull comprising a moonpool 5, here the moonpool having a fore portion 5a and an aft portion 5b.

As is preferred the vessel 1 is a mono-hull vessel with the moonpool extending through the design waterline of the vessel. In another embodiment, for example, the vessel is a semi- submersible vessel having submergible pontoons (possibly an annular pontoon) with columns thereon that support an above-waterline deck box structure. The moonpool may then be arranged in the deck box structure.

The vessel is equipped with a drilling tower 10 at or near the moonpool. In this example, as is preferred, the tower is a mast having a closed outer wall and having a top and a base. The base of the mast is secured to the hull 2. In this example the mast is mounted above the moonpool 5 with the base spanning the moonpool in transverse direction. In another embodiment the tower 10 can be embodied as a derrick, e.g. with a latticed derrick frame standing over the moonpool.

The vessel 1 is provided with a tubular string main hoisting device, the tubular string for example being a drill string 15. The main hoisting device is further illustrated in Fig. 3 and Fig. 4.

The main hoisting device comprises: - a main hoisting winch, here first and second winches 20, 21, and a main cable 22 that is connected to said winches 20, 21, - a crown block 23, here at the top end of the mast 10, and a travelling block 24 that is suspended from the crown block 23 in a multiple fall arrangement of the main cable 22. In fig. 8, the travelling block 24 is suspended in a 12-fall arrangement from the crown block 23. In the exemplary embodiment shown in fig. 3 one or more main cable sheaves connected to the travelling block 24 have an individual lower latching device 25 allowing to connect and disconnect the individual sheave to and from the travelling block 24. Preferably these one or more sheaves also have an upper latching device 26 allowing to latch the sheave to the crown block if the sheave is disconnected from the travelling block. This “splittable block” arrangement is known in the art. The travelling block 24 is adapted to suspend a tubular string, e.g. the drill string 15, therefrom along a firing line 16, here shown (as preferred) with an intermediate topdrive 18 that is supported by the travelling block 24 and that is adapted to provide a rotary drive for the drill string.

Fig. 5 shows in a schematic view, the main hoisting device which comprises a first main hoisting winch 20 and the second main hoisting winch 21, wherein the main cable 22 is connected at either end thereof to a respective one of the first and second main hoisting winches 20,21.

The vessel 1 is provided with a heave compensation system adapted to provide heave compensation of the travelling block 24. This heave compensation system comprises a main cable heave compensation sheave head, here two sheave heads 30,31, one each in the path between each of the main hoisting winches 20, 21 and the travelling block 24. These sheave heads 30, 31 are each connected to a passive and/or active heave motion compensator device, here including hydraulic heave cylinders, also called heave compensation cylinders 32 , 33, which are each connected to a respective sheave head 30,

31.

In the shown embodiment each heave compensation cylinder comprises a piston rod, the main cable heave compensation sheave head 30,31 being connected to said piston rod. For example the heave compensation cylinders 32, 33 each have a stroke between 5 and 15 meters, e.g. of 6 meters. As is preferred, the cylinders 32, 33 are mounted within the mast in vertical orientation. Figure 3 shows the lengthy cylinders 32, 33 including a fully extended position of the piston rods thereof which are preferably mounted vertically within the mast

10. As further shown in Fig. 5, as an example of a passive heave compensation system, each heave compensation cylinder 32, 33 is connected to a hydraulic/gas separator cylinder A, B, C, D, one chamber thereof being connected to a gas buffer as is known in the art. As shown in fig. 4, the heave compensation system is arranged to provide heave compensation of the travelling block 24 of the main hoisting device, but also to provide heave compensation of a mobile working deck 70. The mobile working deck 70 is positioned above the moonpool 5. The working deck 70 comprises equipment, like a slip tool 77, to carry out operational steps to a tubular string, in particular a drill string 15. The mobile working deck 70 is supported by two hydraulic cylinders 61, 62 which are positioned below the mobile working deck mobile working deck support cylinders. The deck support cylinders are each connected to the vessel 1 and the mobile working deck 70. The deck support cylinders comprises at least one double acting hydraulic cylinder. The hydraulic support cylinders 61, 62 are positioned opposite each other. The hydraulic support cylinders 61, 62 are positioned at opposite sides of the firing line 16. Here, the firing line 16 and the two hydraulic support cylinders 61, 62 are positioned in a common plane which is oriented in a vertical direction. Advantageously, the arrangement of the deck support cylinders 61, 62 contribute to the accessibility of the area below the working deck 70. As shown in fig. 4, the heave compensation cylinders 32, 33 and the deck support cylinders 60,61 are hydraulically interconnected by a hydraulic conduit 65, 66. The first hydraulic heave compensation cylinder 32 is hydraulically connected via the first hydraulic conduit 65 to the first hydraulic mobile deck support cylinder 61 and the second heave compensation cylinder 33 is hydraulically connected via the hydraulic conduit 66 to the second hydraulic mobile deck support cylinder 62. Advantageously, the hydraulic interconnection of the heave compensation cylinders and the deck support cylinders provides a synchronous compensated heave motion of both the travelling block 24 and the working deck 70.

The mobile working deck 70 is movable with respect to the vessel 1, in particular the drilling tower 10, along the firing line 16 within a motion range including a heave compensation motion range 72. The motion range is further illustrated and explained hereafter.

Fig. 5 shows an example of an offshore drilling system for performing subsea wellbore related activities involving a riser 19 extending between the vessel and a subsea wellbore, more in particular shows a mobile working deck dynamic positioning system according to the invention.

The offshore drilling system comprises the floating hull 1, the moonpool 5, the drilling tower 10 positioned on said hull at or near the moonpool 5, the tubular string main hoisting device, and the vertically mobile working deck 70.

The tubular string main hoisting device comprises a main hoisting winch, in the particular embodiment shown two hoisting winches 20, 21, and a main cable 22 driven by the main hoisting winches, a crown block 23, and a travelling block 24.

The travelling block 24 is suspended from the crown block 23 via the main cable 22, and is adapted to suspend a tubulars string 15 along the firing line 16. The firing line 16 extends through the moonpool 5.

The vertically mobile working deck 70 is positioned above the moonpool 5, and is vertically movable with respect to the drilling tower 10 along the firing line 16 within a motion range including a heave compensation motion range 72.

The mobile working deck 70 is supported by a support cylinder, in the embodiment shown by two support cylinders 60. The support cylinders 60 are each connected to the vessel and to the mobile working deck 70 to vertically move the working deck 70 relative to the vessel 1, within the motion range including the heave compensation motion range 72.

In the embodiment shown, the support cylinders 60 are located below the mobile working deck. It is submitted that in an alternative embodiment the mobile deck support cylinder, or support cylinders, may be located above the mobile working deck, supporting the mobile working deck form above.

The heave compensation system is configured to provide heave compensation of the travelling block 24 as well as of the mobile working deck 70. The heave motion compensation system comprises a heave compensation cylinder, in the embodiment shown two heave compensation cylinders 32, 33. The heave compensation cylinders 32, 33 are connected to a gas buffer for providing the tubular string main hoisting device with passive heave compensation.

Sheave heads 30, 31, comprising one or more sheaves for engaging the main cable 22 of the main hoisting device, are supported by a piston of the respective heave compensation cylinder 32, 33 for movement along a heave compensation trajectory.

The mobile working deck support cylinders 60 are hydraulically connected with the heave compensation cylinders 32, 33 of the heave compensation system, such that in operation the mobile working deck support cylinders 60 move synchronously with the heave compensation cylinders 32, 33 of the heave compensation system. Thus the mobile working deck 70 moves synchronously with the travelling block 24.

The mobile working deck 70 is movable with respect to the vessel 1, in particular the drilling tower 10, along the firing line 16 within a motion range including a heave compensation motion range 72. The motion range includes a heave compensation motion range 72 that lies higher than the lower stationary position 71 of the mobile working deck 70. In this heave compensation motion range the mobile working deck 70 can perform heave compensation motion relative to the hull of the vessel.

For example the heave compensation motion range is between 5 and 10 meters, e.g. 6 meters. For example the average height of the working deck in heave motion above the driller cabin deck 73 with cabin 74 of the vessel is about 10 meters.

According to the invention, the offshore drilling system comprises a mobile working deck dynamic positioning system for moving the mobile working 70 deck along the firing line 16 within the motion range including the heave compensation motion range 72. The mobile working deck positioning system comprises a positioning winch 101, 102 with an associated positioning cable 103, 104, a control device 107 and one or more sheaves 105,

106. In the particular embodiment shown, the heave compensation system is provided with two heave compensation cylinders 32, 33, and each heave compensation cylinder is connected with a positioning winch 101, 102. The positioning winches 101, 102 are each provided with an associated positioning cable 103, 104. In the embodiment shown, the mobile working deck positioning system further comprises two sheaves 105, 106, the sheaves guiding the positioning cable in a loop along the heave compensation trajectory. In the particular embodiment shown, the cables are each guided over two sheaves, and thus form a loop that extends along the heave compensation trajectory of the related heave compensation cylinder. On one side the looped positioning cable is connected with the winch, and on the other side to the heave compensated cylinder.

The control device 107 is adapted to control the speed of the positioning winches 101, 102. The positioning cables 103, 104 are connected to the piston of the heave compensation cylinders 31, 32, and can in addition or as an alternative be connected to the sheave heads of the heave compensation cylinders, such that the positioning winches 101, 102 can pull the piston of the heave compensation cylinders 32, 33 in opposite directions along the heave compensation trajectory and, and thus position the mobile working deck 70 with, i.e. using, the mobile working deck support cylinders 60 that are hydraulically connected to the heave compensation cylinders 32, 33.

In an embodiment, a trolley is provided, which trolley is coupled to the piston of the heave compensation cylinder, preferably to the sheave head supported by the piston of the heave compensation cylinder, via a connector device and is coupled to the adjusting winch via the adjusting wire. Such a trolley is thus connected to the positioning cable, and pulls the piston rod of the heave compensation cylinder.

In an embodiment, a trolley, i.e. a rail bound vehicle, is supported on a track adjacent the heave compensation trajectory of the sheave head of the heave compensation cylinder. In such an embodiment, the trolley track moveably supports the trolley, such that the trolley can move along the heave compensation trajectory while movement in a direction perpendicular to the trolley track is prevented. Thus, the main purpose of the trolley track is to keep the trolley adjacent the sheave head, preferably at a constant relative position, while the sheave head and trolley travel along the heave compensation trajectory. In heave compensation systems, a cylinder is typically connected to the hoisting wire, i.e. to the reeving of a drilling drawworks, using a sheave head. Employing a trolley on a track adjacent the heave compensation trajectory of the sheave head, and thus the outer end of the piston, for pulling the piston of the cylinder along the heave compensation trajectory, allows for integrating the adjusting system with prior art heave compensation systems. It is submitted that the configuration known from heave compensation adjusting systems known form the prior art, in particular from WO2016/062812 can be used.

In the particular embodiment shown, the heave compensation system is provided with two heave compensation cylinders, and each heave compensation cylinder is connected with a positioning winch. In an alternative embodiment, for example two compensation cylinders are connected to a single positioning winch. In another embodiment, the heave compensation system comprises a single heave compensation cylinder, which is connected to a single compensation winch. In yet another embodiment, the heave compensation system comprises a single heave compensation cylinder, which is connected to two compensation winches. Many configurations are possible to combine the heave compensation system with the mobile working deck positioning system.

In a preferred embodiment, the mobile working deck positioning system comprises a motion reference unit, or MRU, to provide the mobile working deck positioning system with information, for example information relating to the heave of the vessel.

Figure 6 shows another exemplary embodiment of an offshore drilling system according to the invention. The figure schematically depicts the draw works, i.e. main hoisting winches 20, 21, crown block 23 and travelling block 24. The figure further schematically depicts the heave compensation system, the mobile working deck 70, the moonpool 5 of the vessel and the mobile working deck positioning system. In contrast with the embodiment shown in figure 4, the drawworks are provided with a single heave compensation cylinder 31.

The main hoisting device of the drilling tower is fitted with the heave compensation cylinder 31, inidicated as the Passive Heave Compensating (PHC) cylinder. The cylinder is pressurized, via a medium separator, by a volume of pressurized nitrogen. The PHC is coupled with the mobile working deck dynamic positioning system, which can force the PHC cylinder rod/head/sheave actively up and down.

The mobile working deck is supported by hydraulic mobile working deck support cylinders, which in this embodiment are located above the mobile working deck and thus function as pull cylinders.

The bottom of the heave compensation cylinder and the mobile working deck support cylinders are coupled by a hydraulic line. With this hydraulic coupling, both the load in the travelling block and the load in the mobile working deck are supported by same pressurized nitrogen system. When the load is transferred from the travelling block to the mobile working deck, or vice versa, the total load supported by the nitrogen is not changing. No valves have to be opened or closed. It is a pure passive system.

The hydraulic connection between travelling block and mobile working deck automatically synchronizes the motion of the travelling block and the motion of the mobile working deck. The mobile working deck and travelling block can be kept stationary above the seabed by means of a wire connected to the RT ring for passive heave compensation. As an alternative, using active heave compensation, the positioning winch of the positioning system can be used to control the heave compensation cylinder.

Depending on the operation, the depicted system can be operated in the following conditions:

1. mobile working deck not heave compensated, and flush with the base structure, wherein the travelling block is provided with passive heave compensation by the heave compensation cylinder only, shown in figure 7;

2. mobile working deck not heave Compensated, and flush with base structure, wherein the travelling block is provided with active heave compensation, using the mobile working deck positioning system, shown in figure 8;

3. mobile working deck and travelling block are hydraulically coupled and provided with active heave compensation provided by the mobile working deck positioning system, wherein the control device is provided with heave information by a motion reference unit, shown in figure 9;

4. mobile working deck and travelling block are hydraulically coupled and are provided with active heave compensation by the mobile working deck positioning system, wherein the control device is provided with heave information by a cable connected to the RT-ting of the riser, shown in figure 9; Fig; 7 shows the offshore drilling system of fig. 6 in the first working condition.

In this mode the mobile working deck is flush with the base structure and not heave compensated. No heave compensated connections can be made.

The traveling block is passive compensated, i.e. supported by a nitrogen pressurized heave compensation cylinder acting as a spring. When the load in the tackle increases (caused by friction and/or accelerations) the cylinder moves retracts or extends. The stiffness and nominal force can be adjusted by the volume and pressure of the nitrogen. The positioning winch is idling with the heave compensation cylinder head.

When transferring the load to the mobile working deck, the heave compensation cylinder has to blocked by closing valves.

Fig; 8 shows the offshore drilling system of fig. 6 in a second working condition.

In this mode the mobile working deck is flush with the base structure and not heave compensated. No heave compensated connections can made.

The traveling block is actively compensated. The load is supported by a nitrogen pressurized heave compensation cylinder acting as a spring. A Motion Reference Unit measures the heave of the vessel and controls the positioning winch such that the heave compensation cylinder head is moved such that the travelling block remains constant above the seabed.

The nitrogen pressurized heave compensation cylinder carries 80-90% of the load and the positioning winch the remaining 10-20%. Therefore the positioning winch consumes only a fraction of the power of actively heave compensated drawworks (which carry the full load).

When transferring the load to the mobile working deck, the heave compensation cylinder has to blocked by closing valves.

The positioning winch can create instantly an additional pull up or down on the heave compensation cylinder head. With this instantly an additional pull or set down force of the travelling block can be created.

Fig; 9 shows the offshore drilling system of fig. 6 in a third working condition.

The traveling block and mobile working deck are actively compensated. The load is supported by a nitrogen pressurized heave compensation cylinder acting as a spring. A Motion Reference Unit (MRU) measures the heave of the vessel and controls the Positioning winch such that the heave compensation cylinder head is moved such that the travelling block and mobile working deck remains constant above the seabed.

The nitrogen pressurized heave compensation cylinder preferably carries about 80-90% of the load and the positioning winch the remaining 10-20%. Therefore the positioning winch consumes only a fraction of the power of actively heave compensated drawworks (which carry the full load).

Both the load in the travelling block and the load in the mobile working deck are supported by same pressurized nitrogen system. When the load is transferred from the travelling block to the mobile working deck, or vice versa, the total load supported by the nitrogen is not changing.

The positioning winch can create instantly an additional pull up or down on the heave compensation cylinder head. With this instantly an additional pull or set down force of the travelling block can be created. To make sure that the additional pull (or set down force) is lead to the pipe string (and not in to the riser), the mobile working deck and RT ring are not be connected by the steel cable, or the cable has to be slacked off.

Preferably, the system is sued to drill, trip drill pipe and trip casing in positioning winch mode, i.e. without a steel wire connected to RT ring, to be able to pull pipe free instantly and set weight on hangers etc. Fig; 10 shows the offshore drilling system of fig. 6 in a fourth working condition.

The traveling block and HCF are passively compensated. The load is supported by a nitrogen pressurized heave compensation cylinder acting as a spring. A steel cable connecting the mobile working deck to the RT ring (and therefore with the seabed) holds the travelling block and mobile working deck at a constant elevation above the seabed. The nitrogen pressurized PHC cylinder preferably carries about 110% of the load. The steel wire prevents the HCF and travelling block from moving upwards. Over speed detection on the cylinders will close valves preventing the floor to shoot upwards in case of steel wire failure. Both the load in the travelling block and the load in the mobile working deck are supported by same pressurized nitrogen system. When the load is transferred from the travelling block to the mobile working deck, or vice versa, the total load supported by the nitrogen is not changing. No valves have to be opened or closed. The positioning winch potentially can create instantly an additional pull up or down on the heave compensation cylinder head. With this instantly an additional pull or set down force of the travelling block can be created. However one cannot say whether this additional force is lead to the pipe string and/or to the riser. This is depended on the total stiffness of the wire/riser (water depth and wire/riser characteristics) and the pipe string (depending on depth, pipe characteristics etcetera).

Claims (7)

P34167NLOO / MHR CONCLUSIONS An offshore drilling system for performing underwater borehole related activities having a riser extending between the vessel and an underwater borehole, the offshore drilling system comprising: - a floating hull comprising a moonpool; - a derrick positioned on said hull at or near the moon pool; - a tubular string main hoist comprising: - a main hoist winch and a main rope driven by said main hoist winch; - a crown block; - a lifting block (traveling block) suspended from said crown block via said main cable, which lifting block is adapted to suspend a tubular string thereon, along a firing line extending through said moon pool; -a vertically mobile working deck positioned above the moon pool and movable vertically with respect to the derrick along the firing line within a range of motion including a swell compensation range of motion; - a mobile working deck support cylinder, which support cylinder is connected to the vessel and to the mobile working deck to move the working deck vertically relative to the vessel, within the range of motion comprising the swell compensation range; - a swell compensation system adapted to provide swell compensation for the lifting block as well as the working deck, the swell compensation system comprising: - a swell compensation cylinder, which swell compensation cylinder is connected to a gas buffer for the equipped with the hoist with passive swell compensation; and - a sheave head comprising one or more sheaves for engaging the main rope of the main hoist, the sheave head being supported by a piston of a swell compensation cylinder for movement along a swell compensation path; wherein the mobile working deck support cylinder is hydraulically connected to the swell compensation cylinder of the swell compensation system, such that in use, the mobile working deck support cylinder moves in synchronism with the swell compensation cylinder of the swell compensation system, thus moving the mobile working deck in synchronism with the lifting block; and - a mobile work deck dynamic positioning system, for moving the mobile work deck along the firing line within the range of motion comprising the swell compensation range of motion, the mobile work deck positioning system comprising: - a positioning winch with an associated positioning cable; - a control device, which control device is adapted to control the speed of the positioning winch; and - one or more sheaves, which sheaves guide the positioning cable in a loop along the swell compensation path; wherein the positioning cable is connected to the swell compensation cylinder piston and / or to the swell compensation cylinder disc head such that the positioning winch can pull the swell compensation cylinder piston in opposite directions along the swell compensation trajectory, and thus position the mobile work deck with the mobile work cover support cylinder connected to the heave compensation cylinder. The system of claim 1, wherein the mobile work deck positioning system control device is connected to, and adapted to control, the main hoist winch, to enable the control device to use the main hoist system for positioning and / or or moving the lift block while pulling the heave compensation cylinder piston, for example to maintain the lift block in a predetermined position relative to the vessel while moving the heave compensation system piston to move the mobile work deck relative to the vessel . The system of claim 1 or claim 2, wherein the dynamic positioning system is adapted to adjust passive swell compensation of the lifting block as well as of the mobile work deck by increasing and / or decreasing the speed at which the swell compensation cylinder piston moves along the swell compensation trajectory. while the heave compensation system provides passive heave compensation. System according to one or more of the preceding claims, wherein the dynamic positioning system is adapted to register and / or predict swell, for example with respect to the sea bed, and is arranged to provide active swell compensation for the lifting block as well as for the mobile work deck by pulling the plunger of the swell compensation cylinder in opposite directions along the swell compensation path. System according to one or more of the preceding claims, wherein the system is adapted to block the hydraulic communication between the swell compensation cylinder and the mobile working deck support cylinder, for example by providing one or more blocking valves in a hydraulic circuit that connects the mobile working deck. cylinder connects to the heave compensation cylinder, to enable the mobile work deck positioning system to move only the lifting block, for example, to provide only the lifting block with heave compensation. The system according to one or more of the preceding claims, wherein the heave compensation system further comprises: - a disk main path extending parallel to the heave compensation path; a trolley, which trolley is connected to the swell compensation cylinder piston, preferably with the disc head supported by the swell compensation cylinder piston, via a connecting device and coupled to the adapter winch via the adapter cable; and - a trolley track, which trolley track extends parallel to and near the swell compensation track. A method for performing borehole related work using a system according to any one or more of the preceding claims.