WELLBORE RELATED ACTIVITIES.

The invention relates to an offshore drilling vessel and installation for performing subsea wellbore related activities.

In the field some offshore drilling vessels are in use that are provided with a drilling installation that is known under the tradename RamRig, originally developed by the company Maritime Hydraulics AS in Norway.

For example, US6095501 and US6094910 disclose an offshore drilling vessel with a RamRig drilling installation having tower that is positioned on the floating hull over or adjacent to the moonpool and with a main hoisting device. This main hoisting device comprises:

- a travelling top drive carrier adapted to support a top drive and a drilling tubulars string extending along a firing line through said moonpool;

- a top sheave assembly that is guided vertically relative to an elevated portion of the tower and comprises carrier suspension cable sheaves,

- vertically oriented hydraulic piston-and-cylinder type lift devices, each having a cylinder body and a piston rod, said lift devices each having one of the cylinder and the piston rod thereof fixed in relation to the hull and the other one of the cylinder and the piston rod supporting the top sheave assembly thereon so as to be vertically mobile relative to the tower,

- carrier suspension cables, each carrier suspension cable having one end thereof secured to the travelling top drive carrier and extending over a respective sheave of the top sheave assembly to another end of the carrier suspension cable, said other end being connected to an anchor that is fixed in relation to the hull,

- a hydraulic circuit connected to said lift devices and including a hydraulic pump, said hydraulic circuit being configured to lift the travelling top drive carrier by extension of the one or more lift devices.

In the operational RamRig installations, a set of hydraulic pumps in the hydraulic circuit provides a desired hydraulic liquid flow to the lift devices to lift the travelling top drive carrier and a drilling tubulars string when connected to the top drive carrier. In some practical embodiments, the lift devices are single-acting hydraulic cylinders. Other embodiments are double acting. The lift devices each have a lift chamber delimited by the piston so that supply of hydraulic liquid to the chamber causes a lift motion of the travelling top drive carrier. The heave compensation of the travelling top drive carrier is achieved by means of these lift devices. The hydraulic circuit includes a pressurized gas buffer connected via a piston accumulator to the hydraulic circuit. In embodiments, active heave compensation is obtained by control of the hydraulic pumps on the basis of a heave sensor, which results in controlled extension and retraction of the lift devices. Passive heave compensation is done on the basis of the pressurized gas buffer as is known in the art. Effectively the piston rods of the lift devices then rest on a gas spring. Hoisting and lowering is possible by means of control of the pumps.

In the known vessels having a RamRig installation, as also shown in US6095501 and in US6094910, a drill floor is stationary arranged above the moonpool, with a slip device being mounted on the drill floor as is common in the field. The slip device is configured to suspend a tubulars string in the firing line, e.g. when tripping in or tripping out a drill string from a subsea wellbore, or during an actual drilling process.

In WO2021/165143 an offshore drilling vessel for performing subsea wellbore related activities is discussed, wherein a vessel having RamRig installation is further provided with a vertically mobile working deck, which is vertically movable with respect to the tower and to the hull along the firing line within a motion range including a heave compensation motion range. The working deck is provided with a slip device that is configured to suspend a tubulars string in said firing line. The vertically mobile working deck is suspended from at least one working deck suspension cable, which working deck suspension cable is reeved independent from the one or more lift devices, so that the vertical position and vertical motion of the working deck are independent of the operation of said one of more lift devices.

In WO2021/165143 an integrated heave compensation system is provided that is configured to establish, in operation thereof, a heave compensated motion of the vertically mobile working deck relative to the tower and to the hull within said heave compensation motion range and a synchronous heave compensated motion of the travelling top drive carrier in order to obtain synchronous heave compensated motions of the vertically mobile working deck and the travelling top drive carrier. This system comprises a common heave compensation device that acts on both the one or more carrier suspension cables and the one or more working deck suspension cables so that when a load formed by a tubulars string initially suspended from the travelling top drive carrier is transferred to the slip device or vice versa, the load on the common heave compensation device remains substantially the same. For example, tripping a drill string into a wellbore can now be performed in full heave compensation mode, avoiding undue vertical motion of the drill string in the wellbore, e.g. thereby avoiding undue pressure variations in the wellbore which may occur when the slip device would not be heave compensated.

The present invention aims to provide a vessel with a RamRig installation and having a vertically mobile working deck wherein the synchronous heave compensation functionality is present in a structurally attractive manner.

The present invention provides an offshore drilling vessel for performing subsea wellbore related activities according to the preamble of claim 1 , which is based on US6094910, and is characterized in that the vessel further comprises a vertically mobile working deck, which is vertically movable with respect to the tower and to the hull along the firing line within a motion range including a heave compensation motion range, wherein the working deck is provided with a slip device that is configured to suspend a tubulars string in said firing line, wherein the vertically mobile working deck is supported by at least one hydraulic piston-and- cylinder type working deck motion device that is distinct from the vertically oriented hydraulic piston-and-cylinder type lift device, wherein the cylinder type working deck motion device has a cylinder body and a piston rod with a piston, wherein one of the cylinder and the piston rod thereof is fixed in relation to the hull and the other one of the cylinder and the piston rod supports the vertically mobile working deck, wherein the cylinder type working deck motion device has a hydraulic liquid filled first chamber such that supply of hydraulic liquid to the lift chamber causes a lifting of the vertically mobile working deck, and wherein the cylinder type working deck motion device has a hydraulic liquid filled second chamber separated from the first chamber by the piston, wherein the hydraulic circuit connects the second chamber of the cylinder type working deck motion device to the lift chamber of the lift device and to the hydraulic pump, wherein the hydraulic liquid filled chamber of the piston accumulator is connected to the first chamber of the cylinder type working deck motion device. Due to the inventive structure, an integrated heave compensation system is provided that is configured to establish, in operation thereof, a heave compensated motion of the vertically mobile working deck relative to the tower and to the hull within said heave compensation motion range and a synchronous heave compensated motion of the travelling top drive carrier in order to obtain synchronous heave compensated motions of the vertically mobile working deck and the travelling top drive carrier.

Due to the inventive structure, when a load formed by a tubulars string initially suspended from the travelling top drive carrier is transferred to the slip device or vice versa, as discussed in WO2021/165143, the load on the heave compensation system remains substantially the same.

For example, tripping a drill string into a wellbore can now be performed in full heave compensation mode, avoiding undue vertical motion of the drill string in the wellbore, e.g. thereby avoiding undue pressure variations in the wellbore which may occur when the slip device would not be heave compensated.

In practical embodiments, the hydraulic circuit further comprises an arrangement of one or more valves to control operation of the drilling installation. For example, one or more valves are arranged and configured to selectively close the connection between the cylinder type working deck motion device and the pump, so that the lift chamber of the lift device is connected to the pump and not to the heave compensation assembly with the piston accumulator and gas buffer. The lift device can then be extended, yet without heave compensation afforded by this heave compensation assembly. For example, one or more valves are arranged and configured to selectively close the connection between lift device and the pump, e.g. allowing to solely control the motion of the working deck. For example, the working deck can so be lowered to a stationary resting position relative to the hull.

For example, the working deck is movable into a stationary resting position relative to the hull. For example, in said stationary resting position, the mobile working deck is flush with an adjacent deck of the vessel. For example, locking means are provided that are configured to lock the mobile working deck relative to the hull in the stationary resting position. For example, mobile pins or other mobile lock members are provided to lock the mobile working deck in this position.

For example, the working deck rests directly on top of one or more vertically orientated cylinder type working deck motion devices, that are connected to the hull. For example, one or more vertically orientated cylinder type working deck motion devices are arranged in part within the moonpool.

In embodiments, the cylinder type working deck motion device is a double-ended hydraulic cylinder, wherein the piston rod extends through both ends of the cylinder body so as to create equal cross-sections of the first and second chambers.

In embodiments, the piston accumulator is configured to provide active heave compensation. Herein the accumulator has an active heave compensation drive of the piston separating the gas filled chamber and the liquid filled chamber. For example, the active heave compensation drive comprises an extension of the piston accumulator that defines a double acting hydraulic drive cylinder of which the piston rod is connected to the piston separating the gas filled chamber and the liquid filled chamber. A piston motion sensor may be provided to control operation of the active heave compensation drive.

Preferably, the carrier suspension cables, each having one end thereof secured to the travelling top drive carrier and extending over a respective sheave of the top sheave assembly to another end of the carrier suspension cable, said other end being connected to one of an anchor that is fixed in relation to the hull. In another embodiment, said other end is connected to a winch.

It is noted that, in embodiments, the working deck may be rather small, e.g. just big enough to support the slip device thereon. In other, more preferred, embodiments, the working deck simulates a drill floor, e.g. accessible for drilling personnel, e.g. with an iron roughneck device and/or other well center related equipment arranged thereon or arrangeable thereon.

In an embodiment, the vessel is provided with a heave motion compensating racker system comprising at least one racker device that is adapted to move a tubulars section, e.g. a drill pipe section, between a tubulars storage rack - that is mounted on the hull or tower vessel and subjected to heave motion - and a position wherein the tubulars section is in said firing line and between the vertically mobile working deck and the travelling top drive carrier, wherein said racker device comprises multiple racker assemblies, each of said racker assemblies having a motion arm and a gripper member at an end of said motion arm, said gripper member being adapted to grip a tubulars section, wherein said racker device further comprises an associated heave motion synchronization system configured to bring, in operation thereof, said racker assemblies in a heave compensation mode with respect to the tower so that a tubulars section that has been retrieved from the storage rack by means of said racker assemblies of said racker device is brought into a vertical motion that is synchronous with the heave motion of the mobile working deck and of the slip device provided on said mobile working deck. For example, a heave motion compensating racker system as disclosed in WO2015/133895 is provided for.

The present invention also relates to a drilling installation comprising a tower, main hoisting device, and mobile working deck as described herein, configured for installation on the floating hull of a drilling vessel, e.g. in view of retrofitting existing drilling vessels.

The present invention also relates to a method for performing a subsea wellbore related activity, wherein use is made of a vessel according to the invention. For example, the activity is one of tripping in or tripping out a tubulars string.

The invention will now be discussed with reference to the drawings. In the drawings:

- figs. 1a,b show an offshore drilling vessel provided with a drilling installation according to the prior art,

- figs. 2a, b show the prior art offshore drilling vessel of figures 1a, b more schematically,

- fig. 3 schematically shows the drilling installation according to the prior art including the hydraulic circuit and heave compensation system,

- fig. 4 schematically shows an example of the drilling installation according to the invention.

With reference to figures 1 a, b, and the more schematic figures 2a, b, as well as figure 3 first a prior art offshore drilling vessel for performing subsea wellbore related activities will be discussed. This vessel is equipped with a prior art RamRig drilling installation.

The vessel 1 has a floating hull 2 subjected to heave motion during drilling, here a mono-hull, comprising a moonpool 3. In another embodiment, for example, the vessel is a semisubmersible 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 figures 1a, b and 2a, b show a cross-section of the vessel 1 across the moonpool 3.

A drilling tower 10 is positioned on the hull at or near the moonpool 3, so as to perform wellbore related operation along a firing line 4 through the moonpool 3. The vessel is equipped with a drilling tower 10 at or near the moonpool. Here the tower 10 is a gantry structure over the moonpool having two legs 11 , 12 on opposite sides of the moonpool 3 and a gantry top over the moonpool 3. The tower 10 is embodied as a latticework.

As shown in, for example, WO2017/192046, the tower could also be embodied as a singular structure along a side of the moonpool. In the WO2017/192046 this embodiment is provided as a latticework tower.

In another embodiment, the tower is a singular mast having a closed outer wall and having a top and a base. The base of the mast is secured to the hull.

In the figures 1a,b and 2a, b it is shown that a drill floor 15 is stationary arranged above the moonpool 3.

A slip device 20 is mounted on the drill floor 15 as is common in the field. The slip device 20 is configured to suspend a tubulars string in the firing line 4, e.g. when tripping in or tripping out a drill string from a subsea wellbore, or during an actual drilling process.

The figures 1a,b and 2a, b show a main hoisting device 30 comprising a travelling top drive carrier 35 that is adapted to support a top drive 36 and a drilling tubulars string 100, e.g. via an elevator that is suspended from the top drive 36 or from the carrier 35 directly. The string 100 extends along the firing line 4 through the moonpool 3.

The carrier 35 is guided vertically along the tower 10 over one or more vertical guide rails 14, 15, e.g. one guide rail along each leg 11 , 12 of the gantry type tower.

The figures 1a,b and 2a, b show a top sheave assembly 40 that is guided vertically, e.g. on the same guide rails 14,15, relative to an elevated portion of the tower 10. The assembly 40 comprises one or more carrier suspension cable sheaves 41 .

The figures 1a,b and 2a, b show vertically oriented hydraulic piston-and-cylinder type lift devices 50, each having a cylinder body 51 and a piston rod 52. These lift devices 50 each having one of the cylinder and the piston rod thereof fixed in relation to the hull 2 and the other one of the cylinder and the piston rod supports the top sheave assembly 40 thereon so as to be vertically mobile relative to the tower. The figures 1a, b and 2a, b show carrier suspension cables 55, each carrier suspension cable having one end 56 thereof secured to the travelling top drive carrier 35. The cables 55 each extend over a respective sheave 41 of the top sheave assembly 40 to another end 57 of the carrier suspension cable. This other end 57 is connected to an anchor 60 that is fixed in relation to the hull, e.g. embodied as an equalizing anchor as described in US6095501.

As is known in the art, and for example as described in any of US6095501 , US6094910, and/or WO2017/192046, a hydraulic circuit 80 is connected to the lift devices 50.

The figures 1a, 2a show the lift devices 50 when fully retracted, and the figures 1b, 2b show the lift devices in extended position thereof.

As is known in the art, and for example as described in any of US6095501 , US6094910, and/or WO2017/192046, the lift devices 50 can be operated in passive or active heave compensation mode by suitable operation of the circuit 80, and thereby the assembly 40 is heave compensated, which results in the carrier 35 being heave compensated.

This circuit 80 includes a hydraulic pump 81 , e.g. multiple pumps in parallel.

The hydraulic circuit 80 includes a hydraulic line 82 connecting the hydraulic pump 81 to the lift chamber 50a of the lift device 50 so as to lift the travelling top drive carrier 35 by extension of the lift device 50.

The hydraulic circuit further includes a pressurized gas buffer 90 and an associated piston accumulator 95 to provide heave compensation of the travelling top drive carrier 35.

The piston accumulator 95 has a gas filled chamber 95a that is connected to the pressurized gas buffer 90 and hydraulic liquid filled chamber 95b that is separated from the gas filled chamber 95a by a piston 96.

The hydraulic line 82 connects to the hydraulic liquid filled chamber 95b.

As illustrated, one or more valves 101 , 102, 103 may be provided in the hydraulic circuit. For example, valves 101 , 102 control the connection of the lift device(s) 50 to the pump 81 , and valve 103 control the connection of the piston accumulator 90 to the line 82.

The figure 3 illustrates an embodiment of the piston accumulator 95 which allows for active heave compensation, when desired. Herein the accumulator has an active heave compensation drive 98 of the piston 96 that separates the gas filled chamber 95a and the liquid filled chamber 95b. It is illustrated that the active heave compensation drive comprises an extension of the piston accumulator 90 that defines a double acting hydraulic drive cylinder 99 of which the piston rod is connected to the piston 96 separating the gas filled chamber and the liquid filled chamber. A piston motion sensor may be provided to control operation of the active heave compensation drive.

Figure 4 illustrates an example of the invention. Components already discussed with reference to the illustrated prior art embodiment, are denoted with the same reference numeral.

Figure 4 illustrates that a vertically mobile working deck 70 is provided, which is vertically movable with respect to the tower 10 and to the hull 2 along the firing line within a motion range including a heave compensation motion range.

The working deck 70 is provided with the slip device 20 that is configured to suspend a tubulars string 100 in the firing line.

The vertically mobile working deck 70 is supported by at least one hydraulic piston-and- cylinder type working deck motion device 110 that is distinct from the vertically oriented hydraulic piston-and-cylinder type lift devices 50.

Each cylinder type working deck motion device 110 has a cylinder body 111 and a piston rod 112 with a piston, wherein the cylinder is fixed in relation to the hull 2 and the piston rod 112 supports the vertically mobile working deck 70.

The cylinder type working deck motion device 110 has a hydraulic liquid filled first chamber 110a such that supply of hydraulic liquid to the lift chamber causes a lifting of the vertically mobile working deck 70.

The cylinder type working deck motion device has a hydraulic liquid filled second chamber 110b separated from the first chamber 110a by the piston 113.

The hydraulic circuit connects the second chamber 110b of the cylinder type working deck motion device 110 to the lift chamber 50a of the lift device 50 and to the hydraulic pump 82. The hydraulic liquid filled chamber 95b of the piston accumulator 95 is connected to the first chamber 110a of the cylinder type working deck motion devices 110.

It is illustrated, as preferred, that the cylinder 110 is embodied as a so-called double ended hydraulic cylinder. Herein, the piston rod 112 of the cylinder type working deck motion device 110 extends through both ends of the cylinder body 111 so as to create equal cross-sections of the chambers 110a, 110b.

An integrated heave compensation system is provided that is configured to establish, in operation thereof, a heave compensated motion of the vertically mobile working deck 70 provided with slip device 20 within a heave compensation motion range and a synchronous heave compensated motion of the travelling top drive carrier 35 in order to obtain synchronous heave compensated motions of the vertically mobile working deck 70 and the travelling top drive carrier 35.

When a load formed by a tubulars string 100 initially suspended from the travelling top drive carrier 35 is transferred to the slip device 20 on the working deck 70 or vice versa, the load on the heave compensation accumulator 90 remains substantially the same.

For example, tripping a drill string 100 into a wellbore can now be performed in full heave compensation mode, avoiding undue vertical motion of the drill string in the wellbore, e.g. thereby avoiding undue pressure variations in the wellbore which may occur when the slip device 20 would not be heave compensated, e.g. as the slip device 20 is fixed in relation to the hull 2 of the vessel.

It is illustrated that a valve 105 may be provided to allow for selective disconnection of the motion devices 110, and thus the discussed heave compensation system, from the pump 82. This e.g. allows for operation of the lift devices 82 without heave compensation.

It is illustrated that a valve 106 may be provided to control the connection of the accumulator 95 to the motion devices 110.

Whilst the figure 4 shows that the working deck 70 rests directly on top of vertically arranged motion devices 110, other arrangements for supporting the deck 70 by means of one or more motion devices 110 can be envisaged as well. For example, some form of transmission may be present between the one or more motion devices 110 and the working deck 70, e.g. including one or more cables, chains, etc. In embodiments, the working deck 70 is hanging below the one or more motion devices 110.