NL2018364B1

NL2018364B1 - Offshore support vessel

Info

Publication number: NL2018364B1
Application number: NL2018364A
Authority: NL
Inventors: Everaerts Wouter; Anne Montulet Alice
Original assignee: G Tec Offshore
Priority date: 2017-02-13
Filing date: 2017-02-13
Publication date: 2018-09-04

Abstract

An offshore support vessel for surveying, including UXO (unexploded ordnance) clearance works is described, comprising a hull, a main deck and provided with hoisting means for equipment lifting. The equipment at least comprises an airlift device adapted to be brought close to an underwater bottom by the hoisting means to suck up bottom material around a surveyed object, whereby the hoisting means comprise active heave compensation means for holding an underside of the equipment at a substantially constant distance from the underwater bottom. A method for offshore surveying, including UXO clearance works, is also described.

Description

FIELD OF THE INVENTION

The invention relates to an offshore support vessel, in particular a vessel that is also equipped to carry out geological survey and coring/sampling activities. The invention in particular relates to an offshore support vessel adapted to carry out surveying and preparing a seabed for installations of offshore wind farms. Surveying includes UXO (unexploded ordnance) clearance works.

BACKGROUND OF THE INVENTION

Offshore drilling and support vessels are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor. These vessels search for unexploited oil and gas fields but are less apt to carry out other surveying works, including but not limited to offshore geotechnical site investigation works, offshore drilling works, ROV (remotely operated vehicle) inspection works, UXO (unexploded ordnance) clearance works, offshore support, anchor handling and towage and grouting supply for instance.

hi particular when carrying out UXO clearance works, it is generally desirable to provide an offshore support vessel that facilitates operation with a high degree of safety. Indeed, unexploded ordnance represents a large risk and extreme control over operations is required to avoid unintended explosion of the ordnance.

In order to efficiently support the operations performed by the vessel, equipment for carrying out the operations needs to be loaded onto the vessel and/or unloaded from the vessel. Moreover, equipment frequently has to be moved across the vessel, e.g. between various storage areas and support floor(s) for instance. Due to the size and weight of typical equipment, this equipment is often loaded and/or repositioned on the vessel by means of cranes that may be part of the support vessel itself. While drilling vessels may include a certain amount of storage area on deck, this may not be sufficient for all equipment, in particular in case of an offshore support vessel, which indeed has to carry a lot of cargo, such as containers for carrying out analysis of the surveyed and sampled sea bed.

It is an aim of the invention to provide an offshore support vessel that allows carrying out geological survey and coring/sampling activities in an efficient and safe way, in particular UXO clearance works. A particular aim is to provide adequate equipment (active heave compensated airlift, grab) to safely perform UXO clearance works, performed from a DP (drill-)ship with moonpool or other DP vessel with (knuckle boom) cranes

BRIEF SUMMARY OF THE INVENTION

The invention thereto provides an offshore support vessel according to claim 1. The offshore support vessel in accordance with the invention comprises a hull, a main deck and hoisting means for equipment lifting, the equipment at least comprising an airlift device adapted to be brought close to an underwater bottom by the hoisting means to suck up bottom material around a surveyed object, whereby the hoisting means comprise heave compensation means for holding an underside of the equipment at a substantially constant distance from the underwater bottom.

The support vessel is advantageously used in offshore surveying and UXO clearance works and allows an efficient and safe removal of bottom material around a surveyed object without actually contacting the object, also in challenging environmental offshore conditions in high tidal waters (for instance up to 13 m tide).

In an embodiment of the support vessel, the vessel further comprises at least one further deck at a higher level than the main deck and provided with a drilling tower.

Another embodiment of the invention provides a vessel that further comprises a moon pool defining an upwardly extending opening between a main deck and the hull, and an airlift device adapted to be brought close to the underwater bottom through the moon pool.

In order to increase safety, the offshore support vessel in an embodiment of the invention is characterized in that at least the airlift device of the equipment is provided with a locating system adapted to identify the position of the airlift device relative to the vessel’s position. The vessel’s position itself is preferably kept at a fixed location above a surveyed object to be cleared by the action of a Dynamic Positioning (DP) system provided on the vessel for maintaining the vessel’s position.

The combination of the DP system and the locating system for the airlift device allows a very accurate positioning of at least the airlift device with respect to a surveyed object to be cleared.

The object to be cleared may be identified by any means in the art. In most cases, the object to be surveyed and optionally cleared will be ferromagnetic, and in a practical embodiment of the vessel at least the airlift device of the equipment is provided with an identifying system for ferromagnetic surveyed objects, such as magnetometers.

A further improved embodiment provides an offshore support vessel wherein at least the airlift device of the equipment is provided with a visual identifying system for surveyed objects, such as cameras.

The offshore support vessel is conveniently used for surveying, identifying, clearing and optionally performing geotechnical bottom tests and taking bottom samples and analysing these on the vessel itself. The main deck or another work deck of the vessel is thereto provided with a plurality of containers that have each been furnished with analysing equipment and appliances. In this embodiment, the support vessel may be used as an offshore floating laboratory.

Apart from clearing an object such as an UXO by the airlift device, the support vessel of the invention may also carry out other functions such as surveying and identifying objects positioned on or in the underwater bottom. A particularly suitable embodiment of the invention relates to an offshore support vessel wherein the equipment further comprises a remotely operated vehicle (ROV) fitted with an identifying system for ferromagnetic surveyed objects, such as magnetometers and adapted to be brought close to an underwater bottom by a launching and recovery system (LARS) to search for objects and to localize objects, prior to the removal of the surrounding or overlaying soils by the airlift device. The ROV is preferably equipped with a locating system and/or identifying system as also used on the airlift device, and further with mechanical manipulators or hydraulic cutting tools to remove small objects. Optionally the ROV is equipped with hydraulic dredging tools to be able to clear away bottom material around a surveyed object.

In a typical process the ROV would be launched (at the side of the vessel) prior to the launch of the ROA (through moonpool). The ROV would first search for the magnetic object (using the magnetometers mounted on the ROV). If the object would not be identified because it is overlaid by or embedded in the soils, the soil would be removed by the ROA. Then the object itself can be removed by the grab (launched from the rig mast or crane), or can be removed by the ROV using its manipulators (optionally cables can be cut into sections by hydraulic cutters mounted on the ROV) and then be removed.

In an embodiment, the ROV is launched by a launching and recovery system (LARS) preferably including a tether management system (TMS). The LARS and TMS systems are preferably built on a dedicated raised deck or foundation floor.

Once an object has been identified and bottom material around the object has been cleared away by the heave compensated airlift, the object may, if desired, be removed from the seabed. To this end an embodiment of the invention provides an offshore support vessel wherein the equipment further comprises a mechanical grabber adapted to be brought close to an underwater bottom through the moon pool by the hoisting means to take up surveyed objects. The mechanical grabber is preferably equipped with a locating system and/or identifying system as also used on the airlift device.

The mechanical grabber is lifted by the hoisting means and is, for this reason, preferably also heave compensated by the heave compensating means present on the vessel.

The versatile offshore support vessel can carry out a multitude of tasks in an efficient way, among others by providing at least one further deck at a higher level than the main deck, which further deck is provided with a drilling tower and hoisting means for equipment lifting. This construction saves space on the main deck and also allows for swiftly interchanging equipment on the main deck for lowering into the water by the hoisting means. The further deck or mezzanine deck also allows storage of consumables, such as synthetic based fluid (SBF) and equipment such as cone penetrometer testing equipment (CPT) below the drill floor. A dedicated launching mechanism mounted above the large moonpool offers a quick, smooth and safe switch between drilling mode, seabed CPT mode and UXO clearance mode.

An embodiment of the support vessel comprises a hoisting means such as a knuckle boom crane, whereby the airlift device or ROA is lowered at the side of the vessel by the knuckle boom crane, but still using the active heave compensation system.

A particularly suitable embodiment of the invention provides for this purpose an offshore support vessel wherein the main deck comprises at least one mobile working floor adapted to be loaded with a piece of the equipment and to slide across the main deck from a storage position on the main deck towards a position over the moon pool. The working floor(s) may for instance be moved across the main deck on skid beams provided for this purpose.

An embodiment wherein the offshore support vessel further comprises at least one equipment lift configured for transporting equipment between the main deck and the further deck can be used to position a piece of equipment onto one of the mobile working floors where after the mobile working floor is arranged over the moon pool. The piece of equipment is then attached to the hoisting means and lowered into the water.

The mobile working floors may also be used to carry away cleared bottom material and/or objects by providing a floor with a container.

The DP system for maintaining the vessel’s position in an embodiment of the invention is known per se to one skilled in the art, and a preferred active DP system comprises a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyrocompasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. The computer program contains a mathematical model of the vessel that includes information pertaining to the wind and current drag of the vessel and the location of the thrusters. This knowledge, combined with the sensor information, allows the computer to calculate the required steering angle and thruster output for each thruster. The DP system is preferably used in an absolute sense in that the position is locked to a fixed point over the sea bed.

The locating system for determining the position of the equipment such as the air lift device in an embodiment comprises an acoustic wave emitter positioned on the vessel and acoustic wave receivers positioned on at least the airlift device of the equipment. Such a system provides an accurate yet practical determination of the equipment’s position under water. A suitable acoustic system is known in the art as a USBL. It calculates the position of under-water equipment relative to the position of the vessel. As the vessel’s position is preferably determined by a DGPS system, known per se, the absolute position of the under-water equipment can be derived.

The invention in another aspect provides a method for offshore surveying, including UXO (unexploded ordnance) clearance works. The method comprises providing a support vessel in accordance with any one of the preceding claims, lifting equipment with the hoisting means and bringing the equipment at least comprising an airlift device close to an underwater bottom through the moon pool and suck up bottom material around a surveyed object, whereby an underside of the equipment is held at a substantially constant distance from the underwater bottom by the heave compensation means.

Generally, the offshore support vessel may be oblong having two ends in the form of a bow and a stern. The vessel may comprise a hull and a superstructure extending upwards above the hull. In some embodiments, the vessel comprises a midship portion between the ends. The support vessel comprises a further (drill) floor, and may comprise one or more drilling support structures such as a derrick, a mast and/or the like and a hoisting system configured to raise or lower equipment through a well centre and/or through another work centre in the drill floor. Examples of hoisting systems include draw-works hoisting systems and hydraulic hoisting systems. The support vessel may further comprise one or more top-drives and/or other equipment for imparting torque on a drill string or on another piece of equipment.

The moon pool defines an opening in the hull to the sea through which equipment may be lowered from the further floor towards the seabed so as to allow surveying, clearing and/or removal operations in the vicinity of the seabed. The hull may comprise upwardly extending walls that surround and define a periphery of the moon pool. The moon pool may form a hole in the main deck and in all optional decks below the main deck and, optionally, in one or more further decks above the main deck. The moon pool may be positioned at a central portion of the vessel, e.g. such that the centreline of the vessel extends through the moon pool and such that the longitudinal centre of the vessel extends to the moon pool.

The further deck provided above the main deck may be formed as a part of a superstructure extending above the hull of the vessel. The further deck may be a drill floor deck from which equipment may be lowered in the sea. It may provide an open deck floor allowing efficient movement of equipment to and from the well centre(s). The term well centre refers to a hole in the floor of the further deck through which the equipment, such as the airlift device can be lowered by the hoisting means towards the seabed. The further deck floor may also include a deck area where human operators and movable equipment such as forklifts can move around. A launch of equipment through a moonpool or well centre is generally smoother than a launch over board (aside of the vessel) as within the moonpool waves are dampened and the moonpool protects against currents and provides a relatively quiet mass of water.

The airlift device according to the invention may comprise a tubular piece of equipment that is advanced through the well centre and moon pool towaids the sea floor during one or more stages of the surveying, in particular the clearing operation. The airlift device may comprise one of more straight tubular elements. More tubular elements can be joined to form a string of tubular elements. The airlift device may comprise a source of pressurized air that is introduced at a bottom side of the tubular element to sustain an upwardly directed flow of water and air. This flow provides the suction force to clear away bottom material from the sea bed. The airlift device may be equipped with a side structure that extends about perpendicular to the longitudinal axis of the tubular element and onto which the locating system and/or identifying system is mounted. The airlift device and in particular its tubular element(s) may have varying lengths and diameters, whereby the length may be 4 m and more for instance. The tubular element of the airlift device is typically suspended from a cable attached to the hoisting system of the vessel.

In order to diminish interference with the object identifying system (e.g. magnetometers), the airlift device is preferably at least partly made from non-magnetic materials.

The airlift device may be located on one of the outboard sides of the moon pool, i.e. on the starboard or the port side of the moon pool, or on the further deck, or on another part of the main deck. The airlift device is preferably carried towards the moon pool by one of the movable working floors.

In some embodiments, the support vessel comprises one or more hoisting devices such as cranes, for instance boom cranes that are configured to transfer equipment from a supply vessel to a deck location on the support vessel.

The support vessel according to the invention comprises hoisting means comprising heave compensation means for holding an underside of the equipment at a substantially constant distance from the underwater bottom. Preferably, active heave compensation is used. Active heave compensation is used on the hoisting or lifting equipment in the context of the invention to reduce the influence of waves upon the offshore surveying and clearing operations. Active heave compensation differs from passive heave compensation by having a control system that actively acts to compensate for any movement at a specific point, and using power to achieve this.

The aim of the active heave compensation of the vessel is to keep a piece of equipment, in particular an underside thereof, in particular the airlift device, held by the hoisting means on the support vessel substantially motionless with regard to the seabed, in particular to an object on or in the seabed. A preferred embodiment uses a Motion Reference Unit or MRU to detect the current vessel’s displacements and rotations in all directions. A control system, for instance PLC or computer based, then calculates how the active parts of the heave compensation system are to react to the detected movement. The active heave compensation system preferably comprises a real time system that is adapted to calculate and compensate for any displacement in a matter of seconds.

The active parts of the heave compensation system may be any system known to one skilled in the art. Suitable embodiments comprise electric and hydraulic winch systems, the latter being preferred. In both systems, any movement caused by waves is compensated by automatically driving the winch in the opposite direction at the same speed. A hoisting means attached to the winch will thus keep its position relative to the seabed. In hydraulic winches, movement is controlled by controlling the oil flow from pump(s) to the winch so that the target position is maintained. In some embodiments, it may be needed to calculate the vertical displacement and/or the velocity of an underside part of a piece of equipment, such as the airlift device, apart from the hoisting means position, in order to actively heave compensate the piece of equipment.

The active heave compensation system in a preferred embodiment cooperates with the DP system of the vessel to compensate for any vertical as well as horizontal movement of the underside of the airlift device, or any other piece of equipment that is lowered towards the sea floor by the hoisting means. The heave compensation preferably is able to keep the underside of the equipment relatively steady with respect to an object in or on the seabed, whereby deviations of a few centimeters only are expected in waves up to 3 m and more.

The embodiments of the invention described in this patent application can be combined in any possible combination of these embodiments, and each embodiment can individually form the subject-matter of a divisional patent application.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be elucidated with reference to the following figures, without however being limited thereto. In the figures:

FIG. 1 illustrates a side view of an embodiment of the offshore support vessel according to the invention;

FIG. 2 illustrates a cross-sectional view of a part of an offshore support vessel according to an embodiment of the invention; and

FIG. 3 illustrates a detailed cross-sectional view of a heave compensation system in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to figure 1, an embodiment of the offshore support vessel 1 comprises a hull 2 that is partly submerged in the water under a water line 20, a main deck 3 and a drill deck 4 at a higher level than the main deck 3. The drill deck 4 is supported by a number of poles 40 that rest on the main deck 3. The drill deck 4 is provided with a drilling tower 5 and hoisting means 6 for lifting the main drive 63. The hoisting means 6 comprise pulling cables 60 that are wound up on a winch 61 located on the drill deck 4. The pulling cables 60 run over a number of sheaves 62 provided at an upper end of the drilling tower 5 and vertically downwards towards the drill and main decks (4, 3). As shown in figure 2, the pulling cables 60 at an outer end carry a main drive 63 provided with another sheave 64. Winding up the winch 61 (or pulling on the cables 60) will bring the main drive 63 to an upward position. Unwinding the winch 61 (or lowering the cables 60) will bring the main drive 63 to a lower position. The sheave 64 is instrumental in the heave compensation system as will be explained below.

The equipment to be lifted by the hoisting means 6 at least comprises an airlift device 7 of tubular shape. The airlift device 7 is adapted to be brought close to an underwater bottom 8 by the hoisting means 6 to suck up bottom material 80 around a surveyed object 81. Figure 3 shows several positions of the airlift device 7 in a water mass subject to current 21, but it will be understood that, although more than one airlift device 7 may be used, a typical usage of the invented support vessel 1 uses one airlift device 7 at a time.

The support vessel 1 is advantageously used in offshore surveying and UXO clearance works and inter alia allows an efficient and safe removal of bottom material 81 around the surveyed object 81 without actually contacting the object 80. To this end, the hoisting means 6 comprise heave compensation means 9 for holding an underside 70 of the airlift device 7 at a substantially constant distance 82 from the underwater bottom 8. The distance 82 may be varied but typically comprises tenths of cm. The heave compensation means 9 in the embodiment shown comprises an active heave compensation system equipped with a control system that actively acts to compensate for any movement at a specific point, and using power to achieve this. A Motion Reference Unit or MRU 92 may be used to detect the current displacements and rotations in all directions of the vessel 1 as a result of wave motion. A PLC controller 93 may then calculate how an active part of the heave compensation system 9 is to react to the detected movement. The active part of the heave compensation system 9 in the embodiment shown comprises a hydraulic winch system 90. A heave compensation cable 91 is wound around the winch 90 and carried over the sheave 64 and vertically downward towards the main deck 3. The cable 91 is attached to an upper side of the airlift device 7 by a method explained further below. The movement (rotation) of the winch 90 may be controlled by controlling the oil flow from a pump 94 to the winch 90. The pump 94 is controlled by the PLC controller 93 which receives its input from the MRU 92, and, optionally from other input parameters such as the vertical position, displacement and/or velocity of an underside part of the airlift device 7, apart from the sheave’s 64 position, which is substantially controlled by the cables 60 and hoisting winch 61.

As shown in figures 2 and 3, the vessel 1 may be provided with a moon pool 22 to protect against currents 21 and provide a relatively quiet mass of water. The sidewalls 23 of the moonpool 22 define an upwardly extending opening between the main deck 3 and the hull 2. In this embodiment, the airlift device 7 is to be brought close to the underwater bottom 8 through the moon pool 22, as shown in figures 2 and 3. The invention is not limited to such an embodiment however, and the airlift device 7 - or any other piece of equipment - may be brought over a side board by any type of hoisting and/or launching means, such as by a knuckle crane 30, or by a launching and recovery system (LARS), known per se and not shown in the figures.

The active heave compensation system 90 allows keeping the airlift device 7 at a substantially constant height 82, relative to the sea bottom 8. In order to increase the accuracy of the airlift device’s position, the airlift device 7 may further be equipped with a locating system 74 adapted to identify the position of the airlift device relative to the position of the vessel 1. The vessel’s position itself is then preferably kept at a fixed location above a surveyed object 81 to be cleared by the action of a Dynamic Positioning (DP) system provided on the vessel 1 for maintaining the vessel’s position through the operation of its thrusters 24. Such a DP system is known to one skilled in the art and is not described further herein.

In an embodiment of the vessel comprising hoisting means such as a knuckle boom crane for lowering the airlift device 7 over a side board of the vessel, the station keeping of the airlift device 7 during horizontal displacements of the vessel preferably is realized by remote activation of the hoisting means’ joints or knuckles by the airlift operator, rather than by moving the vessel by its DP system.

The airlift device 7 generally is an oblong structure comprising a hollow tubular shaft, optionally build up from one or more straight tubular elements that are mutually connected The tubular shaft 70 of the airlift device 7 is suspended from the heave compensated cable 91 running over the sheave 64 of the hoisting system 6 of the vessel 1 and connected to the heave compensation winch 90. The airlift device 7 further comprises a source of pressurized air, whereby the air is introduced at a bottom side of the tubular shaft to create and sustain an upwardly directed flow 71 of water and air bubbles in an inner cavity of the hollow tubular shaft 70. In clayey soils, in addition to the compressed air, high pressure water might be injected through high pressure nozzles mounted on the suction head of the air lift system into the soils, loosening the soils before they are transported through the inner pipe 70. This flow provides the suction force to clear away bottom material 80 from the sea bed 8. The airlift device 7 may be equipped with a lateral support structure 72 in the form of arms that extend about perpendicular to a longitudinal axis 73 of the tubular shaft 70 and onto which the locating system 74 and/or an identifying system 75 is mounted. The airlift device 7 and in particular its tubular shaft 70 may have varying lengths and diameters, whereby the length may be 4 m and more. To be able to operate, the airlift device 7 is equipped with umbilicals for providing power to the locating system 74, the identifying system 75, optionally camera’s, and the pressurized air and optionally pressurized water supply, and allowing data transfer between both systems (74, 75) and a computer provided on the vessel 1 for storing data and for control.

The airlift device 7 when not in use may be located on one of the outboard sides of the moon pool 22, i.e. on the starboard or the port side of the moon pool 22, or on the drill deck 4, or on another part of the main deck 3.

The locating system 71 for determining the position of the airlift device 7 may comprise an acoustic wave emitter positioned on the vessel 1 and acoustic wave receivers positioned on the support structure 72 of the airlift device 7. Such a system provides an accurate yet practical determination of the airlift device’s position under the water line 20.

In order to identify any object 81 to be cleared, the support structure 72 of the airlift device 7 may be provided with an identifying system for ferromagnetic surveyed objects, such as magnetometers 75. A further improved embodiment provides an airlift device 7 with a visual identifying system (not shown) for survey cd objects, such as cameras and in case of low visibility blue view acoustic cameras.

The offshore support vessel 1 is conveniently used for surveying, identifying, clearing and optionally performing geotechnical sub bottom tests and taking bottom samples and analysing these on the vessel 1 itself. The main deck 3 or another work deck of the vessel is thereto provided with a plurality of containers 25 that have each been furnished with analysing equipment and appliances. Containers 25 may for instance comprise a hydraulic power unit (container 25a), electrical converter equipment (container 25b), an airbank (container 25c), a meeting room (container 25d), a drill workshop (container 25e), and so on.

The offshore support vessel 1 may also comprise other equipment on board, besides the airlift device 7. Suitable equipment to be used in surveying and clearing of objects 81 for instance comprise a remotely operated vehicle (ROV) adapted to be brought close to the underwater bottom 8 by a ROV launch and recovery system to identify surveyed objects 81. The ROV may be equipped with a locating system 74 and/or identifying system 75 as also used on the airlift device 7, and further with mechanical manipulators or hydraulic cutting tools to remove small objects or cables. Optionally the ROV may be equipped with hydraulic dredging tools to be able to clear away bottom material 80 around a surveyed object 8.

Another useful piece of equipment comprises a mechanical grabber 10 to clear the object 81 from the seabed 8, once bottom material 81 was removed by the airlift device

7. The mechanical grabber 10 is adapted to be brought close to the underwater bottom 8 by providing a hoisting eye 11 that cooperates with the hoisting means 90 to take up the mechanical grabber 10 and position it over a surveyed object 81 to be cleared. The mechanical grabber 10 may also be equipped with a locating system 74 and/or identifying system 75 as also used on the airlift device 7.

The versatile support vessel 1 of the invention can carry out a multitude of tasks in an efficient way, among others by providing the drill deck 4 at a higher level than the main deck 3. This construction saves space on the main deck 3 and also allows interchanging equipment (7, 10) on the main deck 3 for lowering into the water by the hoisting means 6 and 90. The main deck 3 or drill deck 4 also allows storage of consumables, such as synthetic based fluid (SBF) and equipment such as cone penetrometer testing equipment 12 (CPT) below the drill floor 4.

The pieces of equipment (7, 10, 12) are brought under the water line 20 by attaching them to the heave compensation cable 91. The equipment (7, 10 ,12) is brought under the tackle of the heave compensation cable 91 by one or more mobile working floors (26a, 26b) provided on the main deck 3. The mobile working floors (26a, 26b) are adapted to be loaded with a piece of the equipment (7, 10, 12) and to slide across the main deck 3 from a storage position on the main deck 3 towards a desired position in which they may be attached to the heave compensation cable 91, for instance over the moon pool 22. The working floors (26a, 26b) may be moved across the main deck 3 by providing wheels 27 on the working floors (26a, 26b) and let these roll or slide on skid beams 28 provided on the work deck 3 for this purpose. The skid beams 28 or rails are provided in such directions that they allow movement of the mobile working floors (26a, 26b) to and from the moonpool 22 in the indicated directions (29a, 29b) or of a side board of the vessel 1.

The support vessel 1 may further comprise other equipment lifts, such as knuckle crane 30 or other boom cranes configured for transporting equipment between the main deck 3 and the drill deck 4 for instance, or to the water, or across the main deck 3. Such lifts can also be used to position a piece of equipment onto one of the mobile working floors (26a, 26b) where after the mobile working floor (26a, 26b) is moved to its desired position, for instance for attaching the heave compensated cable 91 to a piece of equipment and lowering it into the water. The mobile working floors (26a, 26b) may also be used to carry away cleared bottom material 81 and/or objects 8 by providing a floor (26a, 26b) with a container.

The heave compensated system of the invention allows surveying and clearing UXO and other objects provided on or in the sea bed 8. The method comprises providing a support vessel 1 as described above, lifting equipment such as the airlift device 7 with 5 the hoisting means 6 and 90 and bringing it close to the underwater bottom or sea bed 8.

The airlift device 7 sucks up bottom material 81 around a surveyed object 8 while an underside of the airlift device 7 is kept at a substantially constant distance 82 from the sea bed 8 by the heave compensation system 9 (90, 64, 91,92, 93, 94).

Claims

CONCLUSIONS

An offshore research support vessel, including clearing operations of UXO (non-exploded shooting devices), comprising a hull, a main deck and hoisting means for lifting equipment, the equipment comprising at least one airlift device adapted to be brought close to an underwater bottom by the hoisting means for sucking up bottom material around an examined object, the hoisting means comprising active swell compensation means for keeping a bottom side of the equipment at a substantially constant distance from the underwater bottom.

An offshore support vessel according to claim 1, wherein at least the airlift device of the equipment is provided with a location system, arranged to determine the position of the airlift device relative to the position of the vessel.

An offshore support vessel according to claim 1 or 2, wherein the vessel comprises a dynamic positioning system (DP) for maintaining the position of the vessel.

An offshore support vessel according to any one of the preceding claims, wherein the equipment further comprises a mechanical gripper adapted to be brought with the hoisting means through the moonpool or overboard of the vessel close to an underwater bottom to receive examined objects.

An offshore support vessel according to any of the preceding claims, further comprising at least one further deck at a higher level than the main deck and provided with a derrick.

An offshore support vessel according to any one of the preceding claims, further comprising a moonpool defining an upwardly directed opening between a main deck and the hull, and an airlift device adapted to be brought through the moonpool close to the underwater bottom.

7.

An offshore support vessel according to any one of the preceding claims, wherein at least the airlift device of the equipment is provided with an identification system for ferromagnetically examined objects, such as magnetometers.

An offshore support vessel according to any of the preceding claims, wherein at least the airlift device of the equipment is provided with a visual identification system for examined objects, such as optical and acoustic cameras.

An offshore support vessel according to any one of the preceding claims, wherein the equipment further comprises a remotely controlled vehicle adapted to be passed through the moonpool with an ROV launch and recovery system close to an underwater bottom to identify examined objects.

An offshore support vessel according to any one of the preceding claims, wherein the main deck comprises at least one mobile work floor, adapted to be loaded with a piece of equipment and to slide over the main deck from a storage position on the main deck to a position above the moonpool.

An offshore support vessel according to any of the preceding claims, further comprising at least one equipment lift adapted to transport the equipment between the main deck and the further deck.

An offshore support vessel according to any one of the preceding claims, wherein the localization system comprises an acoustic wave receiver placed on the vessel and acoustic wave receivers placed at least on the airlift device of the equipment.

An offshore support vessel according to any one of the preceding claims, wherein the mechanical gripper is suspended in an A-frame.

An offshore support vessel according to any of the preceding claims, wherein the remotely controlled vehicle and / or the grab are provided with the location system and / or the identification system.

A method for offshore surveying, including performing clearing operations of UXO (non-exploded shooting devices), the method comprising providing a support vessel according to any of the preceding claims, lifting the equipment with the hoisting means and passing through the moonpool close to bringing the equipment to an underwater bottom,

At least comprising an airlift device, and sucking up bottom material around an examined object, wherein a bottom side of the equipment is kept at a substantially constant distance from the underwater bottom by the swell compensating means.

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