EP3755618A1

EP3755618A1 - Vessel and method for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea well bore

Info

Publication number: EP3755618A1
Application number: EP19716250.6A
Authority: EP
Inventors: Joop Roodenburg; Diederick Bernardus Wijning
Original assignee: Itrec BV
Current assignee: Huisman Equipment BV
Priority date: 2018-02-19
Filing date: 2019-02-14
Publication date: 2020-12-30
Anticipated expiration: 2039-02-14
Also published as: WO2019160414A1; EP3755618B1; CN214397139U

Abstract

The invention relates to a vessel (1) for performing subsea wellbore related activities such as workover activities, well maintenance, installing an object on a subsea well bore (2), wherein the vessel comprises a floating hull (3), a hoisting device (7), a mobile working deck (15) and a mobile deck compensator (18). According to the invention, the vessel is configured to enable the hoisting device to lower and lift objects in a firing line below the mobile working deck.

Description

Title: Vessel and method for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea well bore.

The present invention relates to a vessel for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea well bore, while the floating vessel is subjected to heave motion due to waves.

The present invention also relates to methods that are performed using the vessel according to the claimed invention.

In the art, e.g. as marketed by the present applicant, offshore drilling vessels are known that comprise:

- a floating hull having a deck surface and a moonpool with a firing line extending there through;

- a hoisting device, e.g. a crane or multipurpose tower, which hoisting device is

mounted on the floating hull and is configured for lowering and lifting an object along the firing line, said hoisting device comprising:

o a hoisting winch and hoist cable connected to said winch, wherein the hoisting device is configured to support the hoist cable along the firing line;

o a travelling block supported by the hoist cable for connecting with the object; and

o a heave compensation system adapted to provide heave compensation for the hoist cable to provide a heave compensated motion of the travelling block;

- a mobile working deck, the mobile working deck having a working deck surface, which working deck in a lowered position preferably covers the moonpool, and which working deck surface in the lowered position of the mobile working deck is level, preferably flush, with the deck surface.

Embodiments are known wherein the travelling block is configured to be connected with the mobile working deck to enable the hoisting device to lift the mobile working deck out of the lowered position, to move the mobile working deck between the lowered position and an elevated position, and to provide a heave compensated motion of the mobile working deck when lifted in the elevated position. Such a vessel is for example disclosed in WO

2013/169099. It is also known for the mobile deck to be supported by hydraulic cylinders for providing the mobile working deck with heave compensation. Such a vessel is for example disclosed in WO 2016/062812.

It is submitted that these prior art vessels are directed for performing drilling activities. The heave compensated deck is provided to provide a stationary working deck on top of a riser extending between the surface and a subsea well. The vessel is designed to build and remove these risers on top of a subsea well bore.

Typically, the working deck of such vessels is provided with a riser string suspension device that allows to suspend a top end of a string of riser sections from the working deck in the firing line. The hoisting device is configured to be coupled with the top end of the riser string, and to lift and lower the top end of the riser into and out of the riser string suspension device.

Once the riser string is completed, due to the heave compensation capability, the mobile working deck can be landed on top of the riser string, to provide workers access to the riser, for well bore related activities, e.g. well completion and well maintenance.

The riser string is used to lower and land an object, typically a subsea blow out preventer also referred to as BOP, towards an onto a well bore. Also, any tools for performing subsea wellbore related activities are subsequently lowered through the riser.

However, with well maintenance a riser is not always required. Building and removing a riser is time consuming. Furthermore, supporting a riser requires a large vessel. As an alternative, it is known to use a flexible riser, or not use a riser at all.

The aim of the invention is to provide a vessel that is able to provide a wide range of wellbore related activities, in particular to provide a vessel that is configured for installing an object on a subsea well bore without using a riser.

The invention therefore provides a vessel according to claim 1 , which is configured for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea well bore.

According to the claimed invention, the vessel comprises: - a floating hull, the floating hull having a deck surface and a moonpool with a firing line extending there through;

mounted on the floating hull and is configured for lowering and lifting an object along the firing line and via the moonpool towards and from a sea floor respectively, said hoisting device comprising:

- a mobile working deck, the mobile working deck having a working deck surface, which working deck in a lowered position preferably covers the moonpool, and which working deck surface in the lowered position of the mobile working deck is level (preferably flush) with the deck surface;

- a mobile deck compensator, which mobile deck compensator is connected between the hull of the vessel and the mobile working deck, wherein the mobile deck compensator is configured to lift the mobile working deck out of the lowered position, to move the mobile working deck between the lowered position and an elevated position, and to provide a heave compensated motion of the mobile working deck when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position;

- and preferably an object loading device, for introducing the object below the mobile working deck and in the firing line, such that the object can be coupled with the travelling block of the hoisting device, to enable the hoisting device to move the object along the firing line below the mobile working deck.

The hoisting device can be used for lifting and lowering objects along the firing line below the mobile working deck, and therefore for lowering and lifting an object via the moonpool towards and from a sea floor respectively. Possibly, the hoisting device can also be used for lifting and lowering objects in firing line above the mobile working deck.

The hoisting device comprises a heave compensation system. It is submitted that heave compensation systems for hoisting devices are generally known in the prior art and typically comprise active and passive heave compensation provided by hydraulic cylinders supporting sheaves guiding the hoist cable. Because these type of heave compensation systems for hoisting devices are generally known, they will not be discussed in detail herein.

The mobile deck compensator is configured to lift the mobile working deck out of the lowered position, and to move the mobile working deck between the lowered position and an elevated position. Furthermore, the mobile deck compensator is configured to provide a heave compensated motion of the mobile working deck when the mobile working deck is in the elevated position. When providing heave compensation, the mobile working deck is lowered and raised relative to the elevated position to compensate for vessel motions due to the heave of the waves. The mobile deck compensator can lift the mobile working deck relative to the elevated position into a heave compensation maximum height position, and lower the mobile working deck relative to the elevated position into a heave compensation minimum height position.

For example, in an embodiment, in the elevated positon of the mobile working deck, the distance between the mobile working deck surface of the mobile working deck and the deck surface of the deck is 5 meters, in the heave compensation maximum height positon of the mobile working deck, the distance between the mobile working deck surface of the mobile working deck and the deck surface of the deck is 7,5 meters, and in the heave

compensation minimum height positon of the mobile working deck, the distance between the mobile working deck surface of the mobile working deck and the deck surface of the deck is 2,5 meters.

In an embodiment, the distance between a bottom side of the mobile working deck, when supported in an elevated position, preferably the heave compensation maximum height position, and the deck surface of the vessel is at least 3 meter, preferably at least 4 meter, to enable large, i.e. having a height of more than 2 m, objects to be moved into the firing line and below the mobile working deck.

In an embodiment, the mobile deck compensator comprises two or more, preferably four, hydraulic mobile deck compensator cylinders. Preferably, the cylinders have a work line parallel to the firing line.

In an embodiment of a vessel according to the invention, the mobile deck compensator comprises a crane or multi-purpose tower comprising a heave compensated hoisting device adapted for supporting the mobile working deck. In such an embodiment, a second hoisting device has to be provided for lowering and lifting the objects.

In an embodiment, the mobile deck compensator, more in particular the two or more hydraulic mobile deck compensator cylinders of the mobile deck compensator, is

hydraulically connected via a hydraulic conduit to the heave compensation system of the hoisting device, such that the mobile working deck and the travelling block move

synchronously when heave compensated.

In an embodiment, the mobile deck compensator comprises in series:

- a lift cylinder, configured to lift the mobile working deck out of the lowered position and to move the mobile working deck between the lowered position and an elevated position, and

- a heave compensation cylinder, configured to provide a heave compensated motion of the mobile working deck when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position.

In embodiments, the lift cylinder is connected to the floating hull and to the heave compensation cylinder, and the heave compensation cylinder is connected to the lift cylinder and to the mobile working deck. It is also conceivable that the lift cylinder is connected to the mobile working deck and to the heave compensation cylinder, and the heave compensation cylinder is connected to the lift cylinder and to the floating hull.

The connected lift cylinder and heave compensation cylinder allow a large operational range of the mobile deck compensator, combined with a compact design that can be stored in the floating hull, in particular in a deck box of a semisubmersible. This arrangement allows for a relatively reduced length of the heave compensation cylinder, or cylinders, as this cylinder(s) only has to have a stroke length attuned to the expected heave motion compensation. The lifting of the mobile working deck to the elevated position for example avoids any risk of the working deck reaching its stationary resting position during heave motion operation, and for example allows for passing of lines, pipes, etc. from underneath the working deck, e.g. from the diverter and/or a rotary control device (RCD) to locations outside of the moonpool, e.g. onto the upper deck.

For example a single lift cylinder supports two heave compensation cylinders, e.g. the single lift cylinder in between the two heave compensation cylinders. For example a working lift cylinder is secured with a rod directed downwards, and each heave compensation cylinder is secured with its cylinder body to the cylinder body of the lift cylinder, e.g. via a frame, and has its rod directed upward to the working deck.

In an embodiment, the mobile deck compensator is configured to support the mobile working deck in the elevated position, preferably in the heave compensation maximum height position, while an object is allowed to move into the firing line below mobile working deck. Preferably, the mobile deck compensator is configured to provide sufficient clearance between the deck and the bottom side of the mobile working deck for introducing an object from the deck into the firing line and below the mobile working deck, when the latter is in the elevated position. Preferably, the mobile working deck is configured to support the mobile working deck in the maximum heave compensation height position to provide additional clearance, and allow for introducing an object into the firing line and below the mobile working deck. Once in this position, the object can be coupled with the travelling block of the hoisting device and/or the mobile working deck and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires, to enable the object to move along the firing line below the mobile working deck.

In embodiments, the object is moved from the deck surface of the vessel in a substantially horizontal direction into the firing line. It is also conceivable that the object is stored above or below the deck surface, and that the object is moved in a non-horizontal movement path.

Preferably, the vessel is provided with an object loading device for introducing an object below the mobile working deck and supporting the object above the moonpool and in the firing line, such that the object can be coupled with the travelling block of the hoisting device and/or the mobile working deck and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires, to enable the object to move along the firing line below the mobile working deck.

In an embodiment, the object loading device is a dedicated crane or vehicle, configured to pick up and lift an object located on the deck, move the object towards the moonpool, and support the object above the moonpool in the firing line. In an embodiment, the object loading device is a heavy duty robot arm, configured to pick up an object located on the deck and to, in an extended configuration, support the object above the moonpool in the firing line. The object loading device e.g. comprises a cart track comprising tracks for moving a cart and/or the object, while suspended from or supported by the cart track, into the firing line and below the mobile working deck, wherein preferably the cart track is located on the deck surface, or below the deck surface on a moonpool deck. Preferably the cart and/ or object is skid or slid in a substantially horizontal direction into the firing line. Preferably, the cart track comprises tracks on opposite sides of the firing line, for moving a cart and/or the object, while suspended from or supported by the cart track into the firing line and below the mobile working deck.

Possibly, the object are supported on a cart, or are supported directly on the tracks, i.e. an object with an integrated cart. In a further embodiment, the mobile working deck is provided with a cart track as well, the tracks of which align with tracks of the cart track on the deck surface of the vessel when the mobile working deck is in the lowered position, to enable a cart or object to be moved along said tracks from the deck surface of the vessel directly onto the mobile working deck, and vice versa, in a substantially horizontal direction.

In an embodiment, the object loading device comprises a sliding deck section, preferably supported on a track comprising tracks on opposite sides of the firing line, wherein the sliding deck section is located adjacent a moonpool opening in the deck surface, and which sliding deck section can be slid over the moonpool opening in the deck surface for moving the object, while supported by the sliding deck section, into the firing line and below the mobile working deck, preferably when the mobile working deck is in the elevated position, preferably the heave compensation maximum height position, to allow for the object to be coupled with the travelling block and/or the mobile deck and/or a coupling device, e.g. a tubular, supported by the mobile deck and/or guide wires.

In an embodiment wherein the mobile deck compensator comprises two or more hydraulic mobile deck compensator cylinders, located on opposite sides of the moonpool, the sliding deck section is configured to be positioned above the moonpool between the hydraulic mobile deck compensation cylinders.

In embodiments, the cart track is located on the deck surface of the vessel, or alternatively the cart track is located below the deck surface of the vessel on a lower deck level, e.g. a so-called moonpool deck.

The tracks of the cart track, e.g. rails or skid rails, preferably extend on opposite sides of the firing line, preferably along opposite sides of the moonpool. In an embodiment, the object loading device comprises a removable track or sliding deck section that can be placed over the moonpool to support an object in the firing line above the moonpool and below the mobile working deck. Thus, the firing line passes between the track. A cart supporting an object, or an object supported directly on the tracks, is with one end supported on one of the tracks, e.g. on one of the rails, and with another end on the opposite track, e.g. the other rail. The track can thus support an object in the firing line.

In an embodiment, a second floor is provided below the mobile working deck, to support an object above the moonpool in the firing line. In such an embodiment for example a skid track, comprising tracks on the deck of the vessel and tracks on the second floor, which tracks are aligned when the mobile working deck is supported in the elevated position, preferably is supported in the maximum heave compensation height position, to enable an object to be skidded from the deck onto the second floor.

Preferably, the mobile working deck is configured to support a tubular clamping device in the firing line, to thus enable the mobile working deck to support a tubular, or a string of tubulars, in the firing line. Furthermore, in such an embodiment, the mobile deck

compensator can be used to lower and lift the supported tubular, and objects connected thereto, for example other tubulars or a BOP, along the firing line.

The mobile working deck is preferably configured to removable support a tubular clamping device, such that it can be moved away from the firing line to enable the travelling block and hoist cable to pass through the mobile working deck and lift and lower objects through the working deck and/or below the working deck. In an embodiment, the tubular clamping device can be lifted into and out of a working position in the mobile working deck, and can be removed from the mobile working deck when not in use. In another embodiment, the tubular clamping device can be slid between an active position in which it is aligned with the firing line and a parking position in which it is out of line with the firing line. In such an embodiment the tubular support clamp may for example be supported in a sliding deck section, and by sliding the deck section the tubular support clamp can be moved into and out of the firing line.

In an embodiment, the mobile working deck is configured to support tools, for example a slip device or gimbal device, in the firing line in a similar way as set out above in reference to the tubular clamping device. In an embodiment of the claimed invention, the vessel furthermore comprises at least one guide wire winch and a guide wire connected to said guide wire winch, which guide wire is to be connected to the subsea well, e.g. to a subsea stack mounted on that well or to a well template, such that it extends parallel to and spaced from the firing line, and wherein the guide wire winch comprises a heave compensation system adapted to keep the guide wire, when fixed to a subsea well, under substantially constant tension for guiding an object being lowered towards, or lifted from, the sea floor along the firing line.

In an embodiment of the claimed invention, the vessel comprises at least one guide wire winch, mounted to the mobile working deck, and a guide wire connected to said guide wire winch, which guide wire is to be connected to the subsea well, e.g. to a subsea stack mounted on that well or to a well template, such that it extends parallel to and spaced from the firing line, and wherein the guide wire winch comprises a heave compensation system adapted to keep the guide wire, when fixed to a subsea well, under substantially constant tension when the mobile working deck is held in a fixed position relative to the vessel, to provide a guide, parallel to and offset from the firing line, for guiding the object being lowered towards, or lifted from, the sea floor along the firing line.

In such an embodiment, when the mobile deck compensator provides the mobile working deck with heave compensation, this heave compensation also provides the guide wire winch with heave compensation, and can thus be sued to keep the guide wire, when fixed to a subsea well, under substantially constant tension. This is beneficial since it is not required to continuously roll the guide wire on and of the guide wire winch, or over a heave

compensated sheave, to provide heave compensation, which will lengthen the lifetime of the guide wire.

In embodiments wherein at least one guide wire winch is mounted to the mobile working deck, it is conceivable that a guide wire winch is provided onto the working deck surface. In preferred embodiments, the at least one guide wire winch is located below the mobile working deck surface of the mobile working deck. The guide wire winch not being mounted onto the deck surface of the mobile working deck enables to provide the mobile working deck with a clean deck surface, which is in particular beneficial when the mobile working deck is in the lowered position and the deck surface of the mobile working deck is level with the deck surface of the deck of the vessel.

In an embodiment, the guide wire winch is moveably mounted, to enable the position of the guide wire winch to be adapted relative to guide wire anchor points at the well, such that the guide wire extends parallel to the firing line. In addition or as an alternative, guide wire guides, for example moveable sheaves, can be provided on the mobile working deck to allow for the trajectory of the guide wire to be adjusted, in particular to make the guide wire extend parallel to the firing line.

The working deck surface of a vessel according to the invention preferably comprises a hoist cable opening aligned with the firing line, for passing through hoist cable of the hoisting device. In embodiments, the hoist cable opening of the mobile working deck is large enough for passing through the travelling block.

In an alternative embodiment, the mobile working deck comprises a frame defining a central passage for passing through the object supported by the hoisting device, and one or more movable, e.g. pivotable or removable, deck segments.

In embodiments, the movable deck segments can be provided in a hoist position in which a hoist cable opening is provided, large enough for passing through the hoist cable, or hoist cables in case of a multi fall hoisting configuration. In a travelling block position of the moveable deck segments, the movable deck segments have moved to allow a travelling block to pass though the central opening in the mobile working deck, which travelling block position of the movable deck segments creates an opening which is large enough for passing through the travelling block.

In embodiments, the movable deck segments are moveable to a cover position in which the movable deck segments cover said central passage to provide a deck surface over that opening.

In embodiments, the movable deck segments are movable to an open position in which said central opening is open. Preferably, said central opening is large enough to pass through a large sized object to be lowered towards the sea floor. For example in such an embodiment, part of a subsea stack to be installed on top of a well head can be assembled in the firing line and on the mobile working deck, be lifted from the mobile working deck using the hoisting device, after which the moveable deck segments are moved to the open position and the object can be lowered through the mobile working deck and through the moonpool towards the sea floor. Preferably, the open position of the movable deck segments allows for passing through objects having a foot print of four by four meters. It is submitted that the size of such an object pass through opening is significantly larger than the size of a travelling block pass through opening. The moveable deck segments can be temporarily removed, or can for example be pivoted or slid out of the way.

Thus, the mobile working deck can be utilised prior to the object being lowered, i.e. to assemble the subsea stack, and while and after the object is being lowered. Thus, an highly efficient use of deck space is possible.

In an embodiment of a vessel according to the invention, a second floor is suspended below the mobile deck. The second floor allows for objects to be supported below the mobile working deck, and to provide access to the space below the mobile working deck.

Preferably, the second floor is level with the deck surface of the vessel when the mobile deck is in an elevated position, preferably the heave compensation maximum height position. Thus, an object can be moved from the deck surface of the vessel, onto the second floor in a substantially horizontal direction, when the mobile working deck is in an elevated position, preferably the heave compensation maximum height position. Once the object is located on the second floor, it can be coupled with the hoisting device, i.e. the travelling block, or with an object supported by the hoisting device or the mobile working deck, for example a tubular, and/or with guide wires extending between the vessel, preferably between the mobile working deck of the vessel, and the subsea well bore.

In an embodiment of the vessel according to the invention, the second floor comprises a floor frame defining a central passage, and one or more movable floor segments, which are movable , e.g. pivotable or removable, between a cover position wherein the floor segments cover said central passage to provide a deck surface over that passage, and an open position in which said central opening is open, e.g. for passing through the object supported by the hoisting device.

In addition or as an alternative, the second floor can be disconnected and removed in its entirety, once the object is supported by the hoisting device and is ready to be lowered towards the subsea well bore.

In an embodiment of a vessel according to the claimed invention, the vessel comprises a heave compensation control system, which heave compensation control system controls the heave compensation system of the hoisting device, the mobile deck compensator and if present, a heave compensation system of a guide wire winch. Possibly, synchronous heave compensation is provided. A heave compensation control system of the same applicant has been described in WO2016/062812.

Preferably, at least one of a drill string slip device, a riser spider device, and/or a diverter is supported by the mobile working deck, wherein said drill string slip device is configured to support a suspended drill string within a riser, wherein the riser spider device is configured to support a suspended riser, e.g. during assembly and disassembly of a riser, and wherein the diverter is configured to divert a hydrocarbon and/or drilling mud stream from a subsea wellbore to the vessel.

In an embodiment of a vessel according to the claimed invention, a riser tensioning system is provided for supporting a riser at an upper end thereof, located in the moonpool; the riser extending along the firing line between the subsea wellbore and the vessel. Possibly, riser tensioning cylinders are provided in the hull of the vessel, in the deckbox structure of a semisubmersible, which are connected to a telescopic joint which is provided at the upper end of the riser.

A riser tensioning system provides a near constant upward force on the riser independent of the movement of the floating vessel. When the riser is connected to a wellhead on the sea bed the riser tensioning system manages the differential movements between the riser and the vessel. If there were no riser tensioning system and the vessel moves downward because of heave, the riser would buckle; and if the vessel rises then high forces would be transmitted to the riser and it would stretch and be damaged.

The weight of the riser is suspended form the riser tensioning system. Possibly, the riser can be provided with buoyancy means, e.g. floating cans, to support at least part of the weight of the riser.

In embodiments, a rotating control device and / or a diverter is suspended from the mobile working deck, to which a top end of the riser, in particular of the telescopic joint is attached, preferably via a flex joint.

In embodiments wherein the position of the mobile working deck is fixed with respect to the vessel, in particular in the lowered position thereof, the differential movements between riser and vessel can be absorbed by the telescopic joint. In embodiments wherein the mobile deck compensator provides a heave compensated motion of the mobile working deck, and the telescopic joint is connected to the mobile working deck, then generally there is no need for the telescopic joint to absorb differential movements. In such an embodiment, the telescopic joint can be locked, fixing the position of the mobile working deck relative to the riser, and the mobile deck compensator is used to support the mobile working deck and optionally part of the weight of the riser. Thus, the mobile deck compensator can be used to absorb the differential movements between riser and vessel and thus act as a riser tensioning system.

It is also conceivable that the telescopic joint is not locked and that the telescopic joint still absorbs some relative movements.

It is conceivable that during assembly, the mobile working deck can be used to support the riser.

In an embodiment, the vessel is a semi-submersible drilling vessel and the floating hull is embodied as a deckbox structure, the semi-submersible drilling vessel comprising one or more pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple support columns extending upward from the one or more pontoons and supporting thereon the deckbox structure. The deckbox structure is the construction supporting the deck surface of the semisubmersible.

In embodiments, the semi-submersible vessel comprises a moonpool deck located below the deck surface in the deck box structure. Advantageously, a cart track is located below the deck surface of the vessel, preferably located on the moonpool deck, wherein the cart track comprises tracks, e.g. rails or skid rails, for moving a cart and/or the object, while suspended from or supported by the cart track into the firing line and below the mobile working deck to allow for the object to be coupled with the travelling block and/or the mobile deck and/or a coupling device, e.g. a tubular, supported by the mobile deck and/or guide wires. Preferably, the tracks extend on opposite sides of the firing line.

In such an embodiment, the moonpool deck and the cart track for moving an object into the firing line are located below the deck surface, and thus below the mobile working deck when in the lowered position. Possibly, the mobile working deck does not need to be lifted to enable the object to be moved into or out of the firing line below the mobile working deck. In an embodiment, the vessel furthermore comprises a cart track provided on the deck surface of the vessel, the cart track comprising tracks for moving a cart and/or the object, while supported by the cart track, in a substantially horizontal direction from the deck surface into the firing line and below the mobile working deck in the elevated position, preferably the heave compensation maximum height position, to allow for the object to be coupled with the travelling block and/or the mobile deck and/or a coupling device, e.g. a tubular, supported by the mobile deck and/or guide wires.

In such an embodiment, the mobile deck compensator is configured to provide sufficient clearance between the deck surface and the mobile working deck, more in particular a bottom side, i.e. the side facing the sea, of the mobile working deck, for the object to be moved into the firing line to pass under the mobile working deck.

The invention furthermore provides methods that are performed using the vessel according to the claimed invention.

An exemplary method for installing an object on to the seafloor using a vessel according to the invention comprises the steps:

positioning an object in the below the mobile working deck and in the firing line;

- connecting the object to the travelling block of the hoisting device and/or the mobile working deck and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires; and

lowering the object towards the seafloor using the hoisting device.

Possibly, the method further comprises the step of lifting the mobile working deck out of the lowered position into the elevated position, preferably the heave compensation maximum height position.

A further method according to the claimed invention comprises the steps:

- connecting at least one guide wire from a guide wire winch located on the mobile working deck to a subsea well, e.g. a well template;

using a heave compensation system of the guide wire winch to keep the guide wire under a substantially constant tension, while the mobile deck is supported in an elevated position, preferably the heave compensation maximum height position, and;

- slidably connecting the object with the at least one guide wire. Preferably, the method further comprises the step of using the mobile deck compensator to provide a heave compensated motion of the mobile working deck and, instead of the heave compensation system of the guide wire winch, to keep the guide wire under a substantially constant tension.

In an embodiment, a method for installing an object on to the seafloor using a vessel according to the claimed invention, the method comprises the steps:

positioning an object in the firing line, preferably while the mobile working deck is supported in an elevated position, preferably is while the mobile working deck is supported in the heave compensation maximum height position;

- connecting the object to the travelling block of the hoisting device;

use the mobile deck compensator to lift the mobile working deck into the elevated position, and to provide heave compensated motion of the mobile working deck; using the heave compensation system of the hoisting device to provide a compensated motion of the travelling block and therefore of the object supported by the travelling block;

preferably connecting the object with guide wires; and

lowering the object towards the seafloor using the hoisting device.

The invention furthermore provides a method for using a wellbore-related activity tool, the wellbore-related activity tool comprising a tool winch a tool wire and a tool, in a subsea well using a vessel comprising at least one guide wire winch according to claim 6, the method comprising the steps:

- connecting the at least one guide wire to the subsea well and use the heave

compensation system of the guide wire winch to keep the guide wire under a substantially constant tension;

positioning the wellbore-related activity tool on the deck surface of the mobile working deck;

positioning the tool below the mobile working deck, the tool wire passing through an opening in the working deck;

connecting the tool with the at least one guide wire;

using the mobile deck compensator to lift the mobile working deck into the elevated position and to provide the mobile working deck with heave compensated motion; using the mobile deck compensator, instead of the heave compensation system of the guide wire winch, to keep the guide wire under a substantially constant tension; and

use the tool winch to lower the tool towards the subsea well bore. The invention is further elucidated in relation to the drawings, in which:

Fig. 1 shows a schematic side view in cross section of an exemplary embodiment of a vessel according to the invention in a first working position;

Fig. 2 shows a schematic side view in cross section of an exemplary embodiment of a vessel according to the invention in a second working position;

Fig. 3 shows a schematic side view in cross section of the vessel of Figure 1 in a third working position;

Fig. 4 shows a schematic side view in cross section of the vessel of Figure 1 in a fourth working position;

Fig. 5 shows a schematic side view in cross section of the vessel of Figure 1 in an alternative working position;

Fig. 6 shows a further alternative configuration of the vessel according to the invention; and

Fig. 7 shows a further alternative configuration of the vessel according to the invention;

Fig. 8 shows a further alternative configuration of the vessel according to the invention;

Fig. 9 shows a detail of the configuration of fig. 8,

Fig. 10 shows the vessel of fig. 8 in another configuration;

Fig. 11 shows the vessel of fig. 8 in another configuration;

Figs. 12a-12c show the vessel of fig. 10 in distinct positions.

First, the invention will be further elucidated on the basis of the exemplary embodiment of an apparatus according to the invention as shown in Figs. 1-5. In the figures, the same reference signs have been used for elements that correspond in the various figures. The reference sign shave been linked to the various aspects in the list of reference signs as well. Once such an element has been described with respect to one figure, the description of that element is incorporated by reference into the descriptions of corresponding elements of other figures unless the text or context indicates otherwise.

Fig. 1 shows a schematic side view in cross section of an exemplary embodiment of a vessel 1 according to the invention in a first working position of lowering an object towards the sea floor. Figures 2, 3 and 4 show the same vessel in a subsequent second working position and third working position.

The vessel 1 according to claim 1 is configured for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object 8 on a subsea well bore 2. According to the claimed invention, the vessel 1 comprises a floating hull 3, a hoisting device 7, a mobile working deck 15 and a mobile deck compensator 18.

The floating hull 3 has a deck surface 6, which is here the main deck. The floating hull 3 furthermore has a moonpool 4, providing access to the sea for letting down objects into the sea and lifting objects out of the sea. Above the moonpool, the deck 6 has a moonpool opening 27. A firing line 9 extends through the moonpool.

The vessel 1 furthermore comprises a mobile working deck 15. The mobile working deck 15 has a working deck surface 16. Figure 1 shows the mobile working deck in a lowered position. In the shown lowered position, the mobile working deck 15 covers the moonpool 4. In the lowered positon, the mobile working deck surface 16 is level with the deck surface 6 of the vessel 1 , i.e. the mobile working deck surface 16 and the deck surface 6 form one large deck surface that can be integrally used. In the preferred embodiment shown, the working deck surface 16 of the mobile working deck 15 is even flush with the deck surface 6. Thus, a person or an object can pass from the deck onto the mobile working deck in a horizontal direction. No step or lowering has to be negotiated.

A mobile deck compensator 18 is connected with the floating hull 3 of the vessel 1 and the mobile working deck 15.

In the exemplary embodiment shown, the mobile deck compensator 18 comprises four hydraulic mobile deck compensator cylinders 31. These hydraulic mobile deck compensator cylinders 31 are each located at a corner of the mobile working deck, and are with one end connected with the floating hull 3 of the vessel 1 and with the other end to the mobile working deck 15. One pair of hydraulic mobile working deck cylinders 31 is located on one side of the moonpool 4. These cylinders are visible in the figures. One pair of hydraulic mobile working deck cylinders is located on an opposite side of the moonpool 4. These cylinders are not visible in the figure because they are located behind the two hydraulic mobile working deck cylinders 31 that are visible.

The mobile deck compensator 18 is configured to lift the mobile working deck 15 out of the lowered position, shown in figure 1 , and into an elevated positon, shown in figures 2-4. Thus, the mobile deck compensator 18 according to the claimed invention can move the mobile working deck 15 between the lowered position and an elevated position.

Furthermore, according to the claimed invention, the mobile deck compensator 18 is configured to provide a heave compensated motion of the mobile working deck when lifted in the elevated position, moving the mobile working deck relative to the elevated position between a heave compensation maximum height position and the heave compensation minimum height position. Thus, the mobile deck compensator can keep the mobile working deck, when lifted in the elevated position, substantially stationary relative to the sea floor.

Possibly, the distance between a bottom side 44 of the mobile working deck 15, when supported in the heave compensation maximum height position, and the deck surface 6 of the vessel 1 is at least 3 meter, preferably at least 4 meter, to enable large, i.e. having a height of more than 2 meter, objects to be moved into the firing line 9 and below the mobile working deck 15.

A hoisting device 7 is provided which is configured for lowering and lifting an object 8 along the firing line 9 and via the moonpool towards and from a sea floor 10 respectively. In the embodiment shown, the hoisting device 7 is a multipurpose tower. The multipurpose tower comprises a mast that is mounted on the floating hull 3, and that supports a hoisting winch 11 and a hoist cable 12 connected to the hoisting winch 11. A travelling block 13, for connecting with the object to be lowered to, or lifted from, the sea floor, is supported by the hoist cable 12.

The hoisting device is configured to support the hoist cable 12 along a firing line 9, i.e. a virtual line indicating the vertical trajectory along which an object can be lowered through the moonpool. In the figures, the firing line coincides with the section of hoist cable extending between the multipurpose tower, more in particular a crown block of the multipurpose tower, and the travelling block.

The hoisting device 7 furthermore comprises a heave motion compensation system 14, adapted to provide heave compensation for the hoist cable to provide a heave compensated motion of the travelling block. It is submitted that heave compensation systems for hoisting devices are generally known in the prior art and typically comprise active and passive heave compensation provided by hydraulic cylinders supporting sheaves guiding the hoist cable. Because these type of heave compensation systems for hoisting devices are generally known, it will not be discussed in detail herein.

Here, the mobile working deck 15 is provided with a hoist cable opening 17, aligned with the firing line 9 for passing through the travelling block 13 and the hoist cable 12 of the hoisting device 7. In the embodiment shown, an object loading device is provided for introducing an object below the mobile working deck 15 and in the firing line 9, here embodied as a sliding deck section 24. The sliding deck section 25 is supported on a track 24 comprising tracks 26 on opposite sides of the firing line 9, here between the two pairs of hydraulic mobile working deck cylinders 31. The sliding deck section is located adjacent a moonpool opening 27 (shown in fig. 2) in the deck surface 6, which position is shown in figures 1 and 2.

The sliding deck section 25 can be slid over the moonpool opening 27 in the deck surface 6 for moving an object 8, while supported by the sliding deck section, into the firing line 9 and below the mobile working deck 15, when the mobile working deck is in the elevated position, which is shown in figures 2-5. It is submitted that the mobile deck compensator is configured to support the mobile working deck in the heave compensation maximum height position, to provide additional clearance between the deck surface and the bottom side of the mobile working deck, for introducing large sized objects into the firing line and below the mobile working deck.

Once positioned below the mobile working deck, the object is coupled with the travelling block, to enable the hoisting device to move the object along the firing line below the mobile working deck, more in particular, to lower the object 8 towards the sea floor, more in particular towards the subsea well bore 2.

In the embodiment shown, the vessel 1 furthermore comprises two guide wire winches 28, mounted to the mobile working deck 15, and a guide wire 29 connected to said guide wire winch.

The guide wires 29 are connected to the subsea well bore 2, in the embodiment shown to a well template 46, such that the guide wires extends parallel to and spaced from the firing line 9. The guide wire winches 28 each comprise a heave compensation system 30 adapted to keep the guide wire 29, when fixed to a subsea well, under substantially constant tension for guiding an object being lowered towards, or lifted from, the sea floor along the firing line.

When the mobile deck compensator provides the mobile working deck with heave compensation, this heave compensation also provides the guide wire winch with heave compensation, and can thus be used to keep the guide wire, when fixed to a subsea well, under substantially constant tension when the mobile working deck is held in a fixed position relative to the vessel. This is beneficial since it is not required to continuously roll the guide wire on and of the guide wire winch, or over a heave compensated sheeve, to provide heave compensation, which will lengthen the lifetime of the guide wire.

In the preferred embodiment shown, the guide wire winches 28 are located below the deck surface 16 of the mobile working deck 15, which provide the mobile working deck with a clean deck surface, which is in particular beneficial when the mobile working deck is in the lowered position and the deck surface of the mobile working deck is level with the deck surface of the deck of the vessel, see figure 1.

It is submitted that in the situation shown in figure 1 , i.e. with the guide wires 29 connected to the subsea well and the mobile working deck in the lower, thus fixed, position, the heave compensation system of the guide wire winches is used to keep the guide wires under a substantially constant tension. Also, while the mobile working deck is in the elevated position and an object is being moved into the firing line, the mobile working deck will be in a fixed positon, and the heave compensation system of the guide wire winches is used to keep the guide wires under a substantially constant tension. Once the mobile working deck is heave compensated, the guide wire winches no longer need to provide heave compensation.

The vessel 1 allows for installing the object 8 on to the seafloor, more in particular on the subsea well bore.

Once the travelling block 13 has been positioned below the mobile working deck 15, the hoist cable passing through the hoist cable opening 17 in the mobile working deck, the travelling block 13 can be connected to an object located below the mobile working deck, see figure 2.

The object loading device, in the embodiment shown the sliding deck section, can be used for positioning an object in the below the mobile working deck, when raised in the elevated position, and in the firing line 9, see figures 2 and 3. In the embodiment of the vessel shown, the object can be moved in a substantially horizontal direction from the deck surface 6 into the firing line 9 and below the mobile working deck 15

After the object 8 has been connected with the travelling block 13, and with the guide wires 29, the object is lifted from the sliding deck section 24, which is subsequently retracted, such that the object can be lowered towards the subsea well bore 2 using the hoisting device 7. While it is lowered the object is guide by the guide wires, and is thus positioned correctly relative to the subsea well bore, see figure 4. Figure 5 shows a schematic side view in cross section of the vessel of figure 1 in an alternative working position.

Here, the vessel 1 is used for performing well bore related activities using a wellbore-related activity tool 47. In the shown embodiment, the wellbore-related activity tool comprises a tool winch 48 a tool wire 49 and a tool 50, in a subsea well bore 2.

A well bore related activity tool can for example be a coiled tubing tool or a wire line tool, which are generally known tools. With such a tool, the tool winch can be used for lowering and lifting the tool wire and tool winch. In addition, it is possible that a separate tool transportation device 51 is used, for example opposite tracks configured for engaging the tool wire between them and moving the tool wire to lift and lower the tool.

The guide wires 29 have been connected to the subsea well bore 2, more in particular to the well template 46 of the subsea well bore, and can thus be used for guiding the tool 50.

It is submitted that since the heave compensated mobile working deck can be used for supporting the wellbore-related activity tool, there is no need for providing the wellbore- related activity tool with a dedicated heave compensation device.

Once the wellbore-related activity tool 47 has positioned the tool 50 below the mobile working deck 15, the tool wire passing through an opening, e.g. the hoist cable opening, in the working deck it can be connected to the guide wires 29.

Subsequently, the mobile deck compensator can be used to lift the mobile working deck into the elevated position, and to provided the mobile working deck with heave compensated motion. When the mobile working deck is heave compensated, both the guide wires and the well bore activity tool are in a substantially fixed position relative to the well bore. Thus, no additional heave compensation, for example by the heave compensation device of the guide wire winches, is required.

In the embodiment shown in figure 5, the tool winch 48 is used in combination with a tool transportation device 51 , comprising opposed tracks, to lower the tool towards the subsea well bore. The guide wires guide the tool, and make sure it lands in a correct position on the well bore, in the embodiment shown on the object 7 mounted on the well bore. It is submitted that even when guide wires are used, a remotely operated vehicle (ROV), may be deployed to accurately position and/or install objects and or tools on the subsea well bore.

Figure 6 shows yet a further alternative configuration according to the invention, wherein the vessel is a semi-submersible drilling vessel and wherein the floating hull 3 is embodied as a deckbox structure, the semi-submersible drilling vessel comprising one or more pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple support columns extending upward from the one or more pontoons and supporting thereon the deckbox structure. The deckbox structure has a deck surface 6, and another lower deck surface labelled a moonpool deck 19.

An object loading device is provided for introducing the object 8 in the firing line and below the mobile working deck 15, such that the object 8 can be coupled with the travelling block 13 of the hoisting device 7 and/or the mobile working deck 15 and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires, to enable the object to move along the firing line 9 below the mobile working deck 15. Here, the object loading device comprises a cart track 20 comprising tracks 21 for moving a cart 22 with the object 8, and/or the object, while suspended from or supported by the cart track, into the firing line and below the mobile working deck 15. Here, the cart track 20 is located below the deck surface 6 on the moonpool deck 19.

Figure 7 show yet further alternative configuration of the vessel according to the invention. A second floor 36 is suspended below the mobile working deck 15, which is here level with the deck surface 6 of the vessel 1 when the mobile working deck 15 is in an elevated position, preferably the heave compensation maximum height position. The object 8 is allowed to move over the deck surface 6 onto the second floor 36, where it can be coupled with the travelling block 13 of the hoisting device 7 and/or the mobile working deck 15 and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires, to enable the object to move along the firing line 9 below the mobile working deck 15.

In figs. 8-12c yet a further alternative vessel according to the invention is shown in cross- sectional view. In fig. 8 a detail of a semi-submersible vessel 1 is shown , comprising a deckbox structure 3. Not shown is that the semi-submersible drilling vessel comprises one or more pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple support columns extending upward from the one or more pontoons and supporting thereon the deckbox structure. The deckbox structure 3 has a moonpool 4 through which a firing line 9 extends. The deckbox structure 3 has a deck surface 6. A hoisting device is not shown.

A mobile working deck 15 is provided, having a working deck surface 16, which in a lowered position of the mobile working deck 15, as shown in fig. 8, is level, here flush, with the deck surface 6.

A mobile deck compensator 18 is provided, here according to a preferred embodiment, which is shown in further detail in fig. 9. The mobile deck compensator comprises a lift cylinder 101 , configured to lift the mobile working deck 15 out of the lowered position and to move the mobile working deck between the lowered position and an elevated position. The mobile deck compensator further comprises a heave compensation cylinder 102, configured to provide a heave compensated motion of the mobile working deck 15 when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position as is visible in fig. 12c, and a heave compensation minimum height position, as is visible in fig. 12b. The mobile working deck 15 is supported by two sets of a lift cylinder and two heave compensation cylinders.

In the shown configuration, the lift cylinder 101 is connected to the floating hull 3 and to two heave compensation cylinders 102, and the heave compensation cylinders 102 are connected to the lift cylinder 101 and to the mobile working deck 15.

It is also conceivable that the lift cylinder 101 is connected to the mobile working deck 15 and to the heave compensation cylinder 102, and the heave compensation cylinder 102 is connected to the lift cylinder 101 and to the floating hull 3.

The connected lift cylinder and heave compensation cylinders allow a large operational range of the mobile deck compensator, combined with a compact design that can be stored in the floating hull, in particular in a deckbox of a semisubmersible. In fig. 9 it this operation range is visible: on the right-hand side of fig. 9 the piston 10T of the lift cylinder 101 is shown in an extended position, as well as piston 102’ of the heave compensation cylinders 102. On the left-hand side of fig. 9 both pistons are retracted and hence not visible.

In fig. 8 it is visible that in the shown embodiment of the vessel 1 a riser tensioning system is provided for supporting a riser (shown in figs. 10, 12a-12c) at an upper end thereof, located in the moonpool 4; the riser extending along the firing line between the subsea wellbore and the vessel. Riser tensioning cylinders 110 are provided in the deckbox structure 3 of a semisubmersible. In fig 10 it is visible that these are connected to a telescopic joint 115 which is provided at the upper end of the riser. Here, the outer part of the telescopic joint is labelled 115a and the inner part 115c. Therebetween a ring 115b is provided which is supported by the riser tensioning cylinders 110. As such, the weight of the riser is suspended form the riser tensioning system. Possibly, the riser can be provided with buoyancy means, e.g. floating cans, to support at least part of the weight of the riser.

In fig. 8 is visible that a rotating control device 112 and a diverter 113 is suspended from the mobile working deck 15, to which a top end of the riser, in particular of the telescopic joint 115c is attached, preferably via a flex joint 116, visible in fig. 10. Also visible is a work floor 114, which is here suspended from the mobile working deck 15.

In embodiments wherein the position of the mobile working deck is fixed with respect to the vessel, in particular in the lowered position thereof, as shown in fig. 10, the differential movements between riser and vessel can be absorbed by the telescopic joint.

In figs. 12a-12c is shown that the mobile deck compensator 18 with lift cylinder 101 and heave compensation cylinders 102 provides a heave compensated motion of the mobile working deck 15, and the telescopic joint is connected to the mobile working deck. In this situation there is generally no need for the telescopic joint to absorb differential movements. In such an embodiment, the telescopic joint can be locked, fixing the position of the mobile working deck relative to the riser. Thus, the mobile deck compensator can be used to absorb the differential movements between riser and vessel and thus act as a riser tensioning system.

In fig. 12a, the mobile working deck has moved out of the lowered position by the lift cylinder 101 , of which now the piston 10T is visible, to an elevated position. Heave compensated motion of the mobile working deck 15 is provided by the heave compensation cylinders 102. In fig. 12c, the mobile deck 15 has moved to a heave compensation maximum height position, in which the pistons 102’ of the heave compensation cylinders are fully extended, similar to the situation in the right-hand part of fig. 9. In fig. 12b, the mobile deck is in a heave compensation minimum height position, with the pistons 102’ of the heave

compensation cylinders 102 fully retracted.

In fig. 11 , it is shown that a BOP 140 has skidded from a parking position (in which the BOP 140 protrudes above deck surface 6), via a cart 122 on track 121 into the firing line 9 and below the mobile working deck 15, which is here in the heave compensation maximum height position with fully extended pistons 102’ of the heave compensation cylinders 102 and fully extended pistons 101’ of the lift cylinders 101. In an embodiment multiple vertically mounted mobile deck compensator cylinders are arranged between the deckbox structure and the mobile working deck, e.g. two sets of multiple compensator cylinders, e.g. two pairs, supporting the mobile working deck.

Preferably the mobile deck compensator cylinders are configured to provide a heave compensated motion of the working deck relative to the deckbox structure. In an

embodiment a first set of working deck compensator cylinders is arranged outward of a first BOP handling cart rails, relative to the moonpool, and a second set of working deck compensator cylinders is arranged outward of a second BOP handling cart rails, relative to the moonpool, so as to allow for passing a subsea BOP on a BOP handling cart in between the first and second set of working deck compensator cylinders. For example the working deck compensator cylinders are extendable to raise the working deck from its lowered stationary resting position into an elevated position so as to allow for passage of the subsea BOP from the BOP storage room into the firing line underneath the elevated working deck. As preferred mobile deck compensator cylinder not only allow for raising and lowering of the working deck but also for heave compensation motion of the working deck, e.g. with a riser connected via a locked telescopic joint to the working deck.

Claims

1. Vessel (1) for performing subsea wellbore related activities, wherein the vessel comprises:

- a floating hull (3) having a main deck (5) with a deck surface (6) and a moonpool (4) with a firing line (9) extending there through,

- a hoisting device (7), e.g. a crane or multipurpose tower, which hoisting device (7) is mounted on the floating hull (3) and is configured for lowering and lifting an object (8) along the firing line (9) and via the moonpool towards and from a sea floor (10) respectively, said hoisting device comprising:

o a hoisting winch (11) and hoist cable (12) connected to said hoisting winch, wherein the hoisting device is configured to support the hoist cable (12) along the firing line (9);

o a travelling block (13) supported by the hoist cable (12) for connecting with the object (8); and

o a heave compensation system (14) adapted to provide heave compensation for the hoist cable (12) to provide a heave compensated motion of the travelling block (13);

- a vertically mobile working deck (15), the mobile working deck having a working deck surface (16), which mobile working deck in a lowered position thereof covers the moonpool (4), and which working deck surface (16) of the mobile working deck (15) is level, preferably flush, with the deck surface (6) of the main deck (5) of the hull (3);

- a mobile deck compensator (18), which mobile deck compensator is connected

between the floating hull (3) of the vessel (1) and the mobile working deck (15), wherein the mobile deck compensator (18) is configured to lift the mobile working deck (15) out of the lowered position, to move the mobile working deck between the lowered position and an elevated position, and to provide a heave compensated motion of the mobile working deck (15) when lifted in an elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position.

2. Vessel according to claim 1 , wherein the mobile deck compensator comprises two or more, preferably four, hydraulic mobile deck compensator cylinders, wherein preferably the cylinders have a work line parallel to the firing line.

3. Vessel according to claim 2, wherein the hydraulic mobile deck compensator cylinders of the mobile deck compensator, are hydraulically connected via a hydraulic conduit to the heave compensation system of the hoisting device, such that the mobile working deck and the travelling block move synchronously when heave compensated.

4. Vessel according to one or more of the preceding claims, wherein the mobile deck compensator (18) comprises in series:

a lift cylinder (101), configured to lift the mobile working deck (15) out of the lowered position and to move the mobile working deck between the lowered position and an elevated position, and

a heave compensation cylinder (102), configured to provide a heave compensated motion of the mobile working deck (15) when lifted in the elevated position, moving the mobile deck between a heave compensation maximum height position and a heave compensation minimum height position.

5. Vessel according to claim 4, wherein the lift cylinder (101) is connected to the floating hull (3) and to the heave compensation cylinder (102), and wherein the heave compensation cylinder (102) is connected to the lift cylinder (101) and to the mobile working deck (15).

6. Vessel according to claim 4, wherein the lift cylinder (101) is connected to the mobile working deck (15) and to the heave compensation cylinder (102), and wherein the heave compensation cylinder (102) is connected to the lift cylinder (101) and to the floating hull (3).

7. Vessel according to one or more of the preceding claims, wherein a single lift cylinder supports two heave compensation cylinders, e.g. the single lift cylinder in between the two heave compensation cylinders.

8. Vessel according to one or more of the preceding claims, wherein the mobile deck compensator comprises a crane or multi-purpose tower comprising a heave compensated hoisting device adapted for supporting the mobile working deck.

9. Vessel according to one or more of the preceding claims, wherein the mobile working deck is configured to support a tubular clamping device in the firing line, and wherein the mobile deck compensator is operable to lower and lift the supported tubular, and objects connected thereto, for example other tubulars or a BOP, along the firing line.

10. Vessel according one or more of the preceding claims, wherein the vessel is a semi- submersible drilling vessel and wherein the floating hull (3) is embodied as a deckbox structure, the semi-submersible drilling vessel comprising one or more pontoons, e.g. two parallel pontoons or a ring pontoon, and multiple support columns extending upward from the one or more pontoons and supporting thereon the deckbox structure.

11. Vessel according to one or more of the preceding claims, wherein the distance between a bottom side (44) of the mobile working deck (15), when supported in the heave

compensation maximum height position, and the deck surface (6) of the vessel (1) is at least 3 meter, preferably at least 4 meter, to enable large, i.e. having a height of more than 2 meter, objects (8) to be moved into the firing line (9) and below the mobile working deck (15).

12. Method for installing an object onto the seafloor or wellhead using a vessel according to one or more of the preceding claims, the method comprising the steps:

lifting the mobile working deck out of the lowered position into the heave

compensation maximum height position,

positioning an object below the mobile working deck and in the firing line;

- connecting the object to the travelling block (13) of the hoisting device (7) and/or the mobile working deck (15) and/or a coupling device, e.g. a tubular, supported by the mobile deck, and/or guide wires; and

lowering the object towards the seafloor.

13. The method according to claim 12, the method further comprising the steps:

using a heave compensation system of the guide wire winch to keep the guide wire under a substantially constant tension, while the mobile deck is supported in the heave compensation maximum height position, and;

- slidably connecting the object with the at least one guide wire.

14. The method according to claim 12, the method further comprising the steps:

using the mobile deck compensator to provide a heave compensated motion of the mobile working deck and to keep the guide wire under a substantially constant tension while the mobile deck is supported in the heave compensation maximum height position, and

- slidably connecting the object with the at least one guide wire.