WO2010109243A2

WO2010109243A2 - Apparatus and method for handling a submersible item

Info

Publication number: WO2010109243A2
Application number: PCT/GB2010/050507
Authority: WO
Inventors: Julian Romuald Tomas
Original assignee: Subsea 7 Limited
Priority date: 2009-03-26
Filing date: 2010-03-25
Publication date: 2010-09-30
Also published as: GB0905198D0; WO2010109243A3

Abstract

A submersible barge (1) is disclosed and a method of its use for handling a submersible item and transferring it to and from the barge underwater. The barge has a deck (10) for supporting the item, at least two columns (20) spaced apart on the deck (10) and a buoyancy control system. The buoyancy control system controls the submerging of the deck of the barge to permit transfer of the item to or from the deck (10) while the item is submerged. The barge is useful for facilitating delivery of large and heavy submersible items to an underwater location, or for recovering them. Transferring the item onto the deck of the submersible barge wherein the deck and the item are both submerged allows a more stable transfer. The deck is typically supported in the water by buoyancy in the deck and in the columns (20).

Description

APPARATUS AND METHOD FOR HANDLING A SUBMERSIBLE ITEM

This invention relates to apparatus and a method for handing a submersible item. Embodiments of the invention are particularly although not exclusively applicable to deployment and recovery of large pieces of oil and gas well equipment such wellheads onto a sea bed.

In the development of oil and gas fields, it is often necessary to land large and heavy pieces equipment on the bed below a body of water, such as well heads, manifold, anchors etc which are deployed on the sea bed.

Often these pieces of equipment are very large and heavy and their size and mass can cause problems in handling them from a deck of a vessel and deploying them into the water, or recovering them from it.

The invention provides a submersible barge for handling a submersible item, the barge having a deck for supporting the item, at least two columns arranged at spaced apart locations on the deck and a buoyancy control system incorporating buoyancy devices, wherein the buoyancy control system is adapted to submerge the deck of the barge and support while submerged, to permit transfer of the item to or from the deck while the item is submerged.

The barge is typically used for delivering a submersible item to an underwater location, but can be used also for recovering an item from such a location.

The present invention also provides a method of handling a submersible item and delivering it from a submersible barge to an underwater location, the method comprising placing the item on a deck of the barge, transporting the barge to a point above the underwater location and transferring the item from the deck of the barge to the underwater location, wherein the item is submerged when being transferred from the deck of the barge.

This process can be used in reverse in the case of recovery of the item to the surface of the water. Thus the invention also provides a method of recovering a submersible item from an underwater location, the method comprising transferring the item onto the deck of a submersible barge wherein the barge and the item are both submerged when the item is transferred to the deck of the barge.

Typically during the recovery operation, the barge can be raised to the surface (e.g. by de-ballasting) after the item has been transferred to the deck, to recover the item to a surface or onshore location.

Optionally the item is a subsea installation, such as a wellhead, a manifold, a piece of control equipment, or a piece of pipework or other large underwater structure such as a buoyancy tank for a hybrid riser, which during initial deployment phases may be flooded but after connection to a riser string can be made buoyant and can remain in the water column.

The deck is typically on an upper surface of a pontoon.

The deck is typically supported in the water by buoyancy. Typically the buoyancy is arranged in the deck in fixed buoyancy tanks, which define a fixed volume and have a fixed amount of buoyancy, and variable buoyancy tanks, which define a volume in which the buoyancy can be varied, for example, by flooding the variable tanks to admit water into them and thereby decrease the buoyancy, or by increasing the air (or other gas) pressure in the tanks to expel water from the variable tanks in order to increase their buoyancy. Typically the fixed buoyancy tanks have sufficient buoyancy to support the deck with the payload of the item when the item is submerged. Typically the variable tanks are flooded to submerge the deck and transfer the item under water. Typically the fixed buoyancy tanks are located close to the centre of gravity of the deck, and optionally the variable tanks are located around the fixed buoyancy tanks. Tanks can be filled or emptied by pumping water using an onboard pumping system, either powered by an onboard engine or by an umbilical supplied outside source such as an attending vessel or tug.

The geometry of the tank structures is typically optimised to withstand hydrostatic differential pressure in an efficient manner and be resistant to fatigue type stresses in particular. The tanks may be located in the pontoon. The pontoon may be constructed using a longitudinal and transverse series of watertight bulkheads using multidirectionally stiffened flat plates or pressure resistant cross sections such as cylinders and shape stiffened plates i.e. hemispherical shells to form end caps or bulkheads.

The fixed tanks can have the same or a similar configuration or shape, and the variable tanks can also have the same or a similar configuration or shape, which might be different from the shape or configuration of the fixed tanks. In one embodiment, the fixed tanks have a regular polygonal arrangement, with at least 5 sides, e.g. 6, 7, 8 or more sides. In one embodiment, the fixed tanks have an even number of sides and can typically be hexagonal in plan view. The fixed tanks can be grouped together, in a general honeycomb arrangement, optionally close to the centreline of the barge. The arrangement of tanks can optionally be symmetrical between port and starboard, and between fore and aft. The top and base of the tanks is also typically reinforced by hexagonal (or other multisided) panels.

The pontoon can be generally planar, but in some embodiments, the pontoon can be formed from one or more interconnected tubular members, which can incorporate or comprise the tanks. The pontoon can optionally have a flat upper surface forming the deck. The tubular members forming the pontoon can be arranged in a parallel configuration, typically aligned with the long axis of the barge. The tubular members can be interconnected by cross brace members that can also form or contain buoyancy tanks, which can provide fixed or variable buoyancy.

Typically the tanks are formed from subdivisions of the pontoon below the deck. The subdivisions typically form closed compartments within the pontoon. The compartments are typically reinforced so as to have walls that are resistant to pressures than prevail during submersion. Typically the walls can resist 3-4 atmospheres of pressure in some embodiments, although in other embodiments the walls can resist up to 5-6 atmospheres of pressure, so that the subdivided pontoon and the deck can be submerged with the item to be delivered, and can sink to a depth sufficient to submerge the item completely without applying a hydrostatic pressure to the compartment walls that is sufficient to crush or distort the walls substantially. Typically the deck and the pontoon are able to withstand hydrostatic pressures at depths of up to and excess of 20-35m, or even deeper, e.g. 50-6Om in some embodiments, so that the barge can deliver large items while accommodating the draft of a lifting vessel located on the sea surface above the barge deck during delivery.

The deck can be generally rectangular in plan view. The columns can be arranged at opposite ends of the barge, typically at opposite ends of the rectangular deck. Optionally the barge can have one column at each corner of the deck, and in one embodiment the barge can have four columns.

The deck can have column supporting foundations adapted to support the columns and optionally arranged to permit detachment of the columns from the deck. The detachment of the columns permits the columns to be modular and to allow different configurations of columns to be attached to any particular deck.

Optionally the height of the columns above the deck is selected to be higher than the height of the item while supported on the deck.

Typically the item to be delivered or recovered is a submersible, negatively buoyant item which sinks in water and requires to be supported from a buoyant source (e.g. a lifting vessel or the barge) before being lowered to the sea bed.

The columns are typically supported in the water by buoyancy. Typically the buoyancy is arranged in the columns in fixed buoyancy tanks and variable buoyancy tanks in the same manner as the deck. Typically the fixed buoyancy tanks have sufficient buoyancy to support the barge with the payload of the item when the item is submerged. Typically the fixed buoyancy tanks are located in the upper sections of the columns, spaced apart from the deck.

Typically the tanks are formed from subdivisions of the columns. The subdivisions typically form closed compartments of the columns. The compartments are typically reinforced with walls resistant to up to and exceeding 3-6 atmospheres in some embodiments. The deeper the handover of load from a vessel to the barge the less effect the topside weather will have on the operation, so 50-6Om submersion (5-6 atmospheres of pressure) could be contemplated in some embodiments. The columns can be submerged with the item to be delivered, and can sink to a depth sufficient to submerge the item completely without applying a hydrostatic pressure to the compartment walls that is sufficient to crush or distort the walls substantially. Typically the columns are able to withstand hydrostatic pressures at working depths of up to and exceeding 20-6Om.

At least one column typically incorporates a control centre at the top of the column. This control centre can constitute a fixed buoyancy tank, and can include controls for the buoyancy system and the other components of the barge.

At least one of the columns may include a lifting device such as a winch device, and can optionally incorporate disconnection mechanisms to separate the column (or at least a portion of the column) from the deck into two separate disconnected portions. The winch or other lifting device can be connected between the disconnectable portions, and thus can be activated to raise and lower the deck relative to the column or column portion, which can optionally remain at least partially afloat during operations. In some embodiments, the column can be connected to the pontoon by means of a pile or leg that is received within the column in the same manner as a piston is received within a chamber, so that when the column leg is disconnected from the pontoon and extended to lower the pontoon away from the column, the pile or leg improves the stability of the pontoon. The leg could optionally supply additional compartmented displacement buoyancy volumes. The leg can be keyed to the column so that axial movement of the two is permitted but lateral or other types of movement are restricted. The leg can be moved by lifting devices such as winches, or by hydraulic or pneumatic pressure. Instead of being received inside the column, the leg can be mounted on the outer surface of the column in some embodiments.

The deck can optionally have a releasable section such as a grille that is adapted to be lowered from the deck while the deck remains at a fixed draft in the water, so that the releasable section can be lowered to the sea bed or other desired underwater location. The releasable section is typically adapted to support a number of items to be delivered to the underwater location. Placing the items together on the releasable section and delivering the releasable section as a whole in concert with the items can reduce the time required for craning the items to the sea bed. The releasable section can be held on the deck by means of clamps that can optionally be released remotely from the control centre or remotely from an attending vessel, allowing the releasable section to be lowered through the deck without lifting. Typically the barge can therefore be used to deploy pallets of items onto the sea bed in the vicinity of a work site prior to arrival of a lifting vessel, so that the lifting vessel does not need to transfer each item from the surface to the sea bed, but merely needs to move the items laterally to the required position for use, thereby reducing requirements for vessel time and vertical crane or winch block travel.

In certain embodiments, the item is transferred from the deck of the barge, optionally to the sea bed, while submerged under the draft of a lifting vessel. Submerging the item to an extent that it clears the draft of the lifting vessel means that the lifting vessel can use its moon pool for the lifting, and the stability of the lifting operation can be improved, allowing the use of a smaller lifting vessel. A typical installation sequence would be

1. Load the item onto the barge at quayside.

2. Optional wet Factory Acceptance Tests / Site Integration Tests.

3. Tow barge to the field. 4. Ballast barge down to submerge item.

5. Align Installation vessel relative to the barge.

6. Moor up installation vessel to barge or use relative dynamic positioning to maintain relative positions.

7. Connect installation vessel lifting equipment to the submerged load on the submerged barge deck.

8. Transfer weight of submerged load to installation vessel by lifting or ballasting.

9. Release load fastenings from barge deck.

10. Separate the barge and the installation vessel with load. 11. Installation vessel deploys item to worksite.

Typically the barge travels to the area above the worksite on the surface of the water, and is typically towed, although the barge could be powered in some configurations. In some instances, the barge could be submerged with the item or other payload in order to weather storms or high sea states before reaching the area above the worksite.

Optionally the items can be provided on or in a skid or pallet which can be locked onto the deck using optional interlocking formations on the skid/pallet and the deck. Typically the interlocking formations are adapted to be disconnected, typically by command from the control centre of the barge, or remotely from an attending vessel.

In one embodiment the barge can have a crane, typically a gantry crane (although cantilever crane designs could be used as well or instead) that optionally extends between the columns at opposite ends. The crane can typically lift items from the deck and deliver them to the underwater location. Typically the crane can move relative to the deck on a gantry mounted on the braces between the columns. Optionally, the crane can move on rails that extend over the side of the deck so that it can move items from the deck and lower them from the barge without an external crane, e.g. on another vessel.

In one embodiment the barge can have spud anchors or other supports to stabilise it during; wet tests, loading or lifting operations.

In most embodiments the barge can be unpowered, but in some embodiments the barge can incorporate one or more propulsion units to maintain station relative to the lifting vessel, or to propel the barge to the installation location.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a perspective view of a first barge; Fig. 2 shows a side elevation of the Fig. 1 barge;

Fig. 3 shows an end elevation of the barge;

Fig. 4 shows a plan view of the barge;

Fig. 5 shows a perspective view of the barge on a quayside;

Fig. 6 shows a perspective view of the Fig. 5 barge being loaded with a subsea manifold;

Fig. 7 shows a side view of the Fig. 5 barge submerged to a deep draft in which the manifold is submerged for a quayside FAT (factory acceptance test);

Fig. 8 shows a perspective view of the barge being towed to a deployment site; Fig. 9 shows the barge being manoeuvred into position to align with a lifting vessel;

Fig 10 shows a side elevation of the barge and manifold being submerged beneath the draft of the lifting vessel; Fig. 11 and 12 show sequential perspective and side views of the lifting vessel being manoeuvred over the top of the barge and the manifold;

Fig. 13 shows an end view of the lifting vessel connecting to the manifold while the manifold is supported on the deck of the barge beneath the draft of the lifting vessel; Alternative configuration showing barge being held in position by two close coupled attendant tugs.

Fig 14 shows a side view of the barge in the configuration shown in

Fig 13; Fig 15 is a plan view of the barge in the configuration shown in Fig

13,

Fig 16 shows a perspective view of the barge in the configuration shown in Fig 13, with the manifold connected to the vessel by means of lifting strops connected to side davits of the vessel; Fig 17 shows a perspective submerged view with the barge ballasted to sink below the supported manifold;

Fig. 18 and 19 show perspective and side views of the barge moving away from the suspended manifold;

Fig. 20 shows an alternative configuration of the Fig. 1 barge with a lifting vessel suspending the manifold through the vessel moon pool;

Fig. 21 shows a perspective view of the Fig 20 configuration;

Fig. 22 shows a combined view of the lifting vessel lowering the manifold to the sea bed while the barge is towed to its next destination; Fig. 23 shows a plan view of a further lifting vessel holding station alongside a further barge, which is submerged and carrying a range of items to be delivered to the sea bed;

Fig 24, 25 and 26 shows end, perspective and side views of the Fig 23 configuration;

Fig 27 and 28 show perspective and plan views of a further configuration of the barge;

Fig 29 and 30 show perspective and plan views of another configuration of the barge; Figs 31 -33 show perspective, side and end views of a further configuration of barge submerged beneath a lifting vessel and in a pre-deployment configuration;

Fig 34 and Fig 35 shown side and perspective views of the Fig 31 barge; Fig 36 shows a plan view of the Fig 31 barge, showing a partial section through the deck;

Fig 37, 38 and 39 show perspective, end and side views of the Fig 31 barge in a deployment configuration;

Fig 40 shows a perspective view of a further design of barge; Fig 41 shows a side view of the Fig 40 barge;

Fig 42 shows an end view of the Fig 40 barge;

Fig 43 shows a plan view of the Fig 40 barge;

Fig 44 shows a perspective view of a further design of barge;

Fig 45 shows a side view of the Fig 44 barge; Fig 46 shows an end view of the Fig 44 barge; and

Fig 47 shows a plan view of the Fig 44 barge.

Referring now to the drawings, Fig 1 shows a barge 1 having a deck 10 of a pontoon 11 in the general form of a rectangle, and having four corners, each having a foundation 19 for a column 20. The example barge shown in Fig. 1 has a length of 100m, a breadth of 30 m and a depth of 6m. The in air draft (height above the water surface) of the barge 1 is approximately 33m in the configuration shown, but this can be varied in accordance with the mass and shape of the payload to be carried. One column 20 extends upward from each of the foundations 19. The columns 20 are optionally made up of modular blocks 20a stacked on top of one another to the required height. Different heights can be suitable for different payloads or sizes of items. Typically the height of the columns is the same on each barge unit, and is kept as low as possible to minimize the sail area and reduce the centre of gravity of the barge. The columns 20 at each end are typically connected by a brace 21 , which enhances the stability of the columns. Optionally a bridge or control centre 22 can be provided on one or each of the braces 21. The braces can optionally have fixed buoyancy tanks capable of supporting the weight of the barge 1 when fully loaded and mooring points 23 to facilitate mooring of the barge 1 to other vessels.

The modular blocks 20s forming the columns 20 can comprise buoyancy tanks. Optionally some of the blocks 20a can be fixed buoyancy tanks, typically those near to the top braces 21 , and some of the blocks 20 can comprise variable buoyancy tanks and can be operated with the buoyancy control system to adapt the draft of the barge 1 in accordance with the buoyancy in the variable tanks of the barge 1. More variable buoyancy blocks 20a can be added if necessary to provide more variability to the buoyancy control system.

The pontoon 11 is formed of a hull that may be subdivided into internal compartments in a general honeycomb structure as shown in Fig 36 and 37. The internal compartments define buoyancy tanks that enable the barge 1 to adopt different drafts in the water, depending on the amount of buoyancy in the tanks, and the load on the barge 1. Typically the pontoon 11 has two sets of tanks: one set of tanks with fixed buoyancy, which are sealed and have a fixed volume and buoyancy, and one set of tanks with variable buoyancy, which have a valve to admit pressurised gas such as air, and optionally have a port to admit water. The tanks are flooded with water by opening the ports to enable ingress of seawater or the like to submerge the barge, and are purged by pressurised gas to drive water out of the tanks in order to raise the barge or to change its draft in the water. Variable buoyancy tanks may be filled or emptied by use of pumps. Typically the variable buoyancy tanks are located around the fixed buoyancy tanks, which are typically located at or near to the centerpoint of the barge 1.

The walls of the internal compartments can be reinforced so that they withstand the hydrostatic pressure of the surrounding water when the barge is submerged. This is typically achieved by forming the tanks with multisided panels typically in the form of a regular polygon, such as a hexagon. The geometry of the tank structures within the hydrodynamic hull envelope is optimised to withstand hydrostatic differential pressure and be resistant to fatigue type stresses in particular. It may be constructed using longitudinal and transverse series of watertight bulkheads using multidirectionally stiffened flat plates or more pressure resistant cross sections such as cylinders and shape stiffened plates.

Another construction method could involve use of cylindrical or polygonal sections orientated vertically or horizontally along the longitudinal or transverse axis of the pontoon. These could be configured and subdivided by watertight and pressure resistant bulkheads into individual floodable volumes of optimal size to provide adequate control of the ballasting requirements. Each floodable volume could include at least one flooded water level indicator sensor (or an equivalent sensor) to provide remote feedback as to the flooded status of each tank.

The pontoon 11 can typically be reinforced with cross-braces and stiffening members to withstand loads applied to the pontoon 11 by the sea.

The deck of the pontoon could be structurally reinforced to allow for payload weight distribution during loading and frequent seafastening operations.

The barge is typically equipped with a total ballast pumping capacity of 1500 m3/hour @ 5 bar pressure. The ballast system is optionally operated locally from a pump room (not shown) in the pontoon 11 , but can also be operated remotely from a control centre located on top of the super structure.

The ballast system consists of an array of ballast tanks, all typically located in the pontoon. A number of the tanks are designed to maintain fixed volume at the fully submerged draft. These are located centrally and symmetrically. Each tank is typically connected with a separate ballast pipe running to the ballast system in the pump room. All tanks are typically connected to a remote sounding system.

The barge 1 is typically loaded with the item to be delivered to the sea bed when floating at a quayside. By way of example, Fig. 5 shows a manifold 4 waiting to be loaded onto the barge 1. The manifold 4 typically has a mass in air of 300 tonnes and has typical dimensions of 30m x 22m x 16m. Some loads can be craned onto the barge 1 at the quayside, but very heavy loads like the manifold 4 are typically delivered from the quayside to the barge by means of trolleys, hydraulic transporters and/or spacer frames (not shown). Once the manifold 4 has been wheeled onto the barge (e.g. using hydraulic transporters and spacer frames or the like) and secured in position as shown in Fig. 6, the buoyancy control system on the barge 1 can be activated to flood the variable tanks and allow the barge 1 to submerge with the manifold as shown in Fig. 7. This allows a factory acceptance test (FAT) and other wet tests to be completed at the quayside without the requirement to transfer the manifold separately into the water and without requiring a graving dock or the like. Once all wet tests have been completed, the variable tanks in the barge 1 are typically purged with compressed gas, or pumped dry, to increase their buoyancy and raise the deck 10 to the surface once more. The fixed tanks of course do not change their buoyancy. The barge 1 can then be towed by one or more tugs 2 to the vicinity above the underwater location for installation of the manifold, where it can meet with an installation and lifting vessel 3. Thus embodiments of the invention can thereby avoid the need to load the item to be delivered onto a lifting vessel, thus freeing up lifting vessel deck area and lifting vessel time on location.

Once the barge 1 has reached the deployment site and the lifting vessel 3 is available, the tug(s) 2 align the barge 1 with the prevailing weather to facilitate engagement with the lifting vessel 3, and the barge 1 is once more submerged with the manifold 4 by controlled pumping, so that the depth of the submerged manifold on the deck of the barge is typically deeper than the draft of the lifting vessel 3. The lifting vessel 3 is then moored to the mooring points 23 on the braces 21 , using fore and aft mooring points to allow adjustment of the relative positions of the barge 1 and the vessel 3. During the transfer, the moorings facilitate relative positioning while the tug(s) 2 typically maintains headway and directional control. Alternatively, the moorings can facilitate relative positioning while twin tugs hold the barge 1 in position. The lifting vessel 3 is then moved into position for lifting operations, with the centre of gravity of the vessel 3 located over the manifold 4, to stabilise the vessel 3 during lifting. This is typically achieved by manipulating the fore and aft mooring lines to change the relative positions of the barge 1 and the vessel 3, but thrusters or screws can be used, either from the vessel 3 or the tug(s) 2 or both. Where tugs are used to move the barge, the towlines are typically designed and ballasted to keep the tow bridle submerged and clear of the thrusters on the tugs and the vessel 3.

Once the lifting vessel 3 is moored to the barge and is in position for lifting, pre-hgged lifting strops are optionally passed over from the barge topsides using the vessel crane and are connected to deck winches via deployment davits on the sides of the vessel 3, as shown in Fig 16.

Once a secure connection has been made (and optionally tested) between the lifting vessel 3 and the manifold 4, the fastenings connecting the manifold 4 to the barge 1 are released (optionally remotely e.g. hydraulically) and the barge 1 then typically floods its variable buoyancy tanks to increase its draft to suspend the manifold from the strops and side davits on the lifting vessel 3. Alternatively the manifold 4 can be lifted from the vessel 3 by means one or more deck winches or cranes, to rise clear of the deck 10 of the barge 1 as shown in Fig 17. Typically the vessel can operate a plurality of deck winches (and/or cranes) that can be used in cooperation with one another to raise and/or lower the manifold 4 in the water. Because the manifold 4 is already in the water when the load is borne by the lifting vessel, it behaves in a more predictable manner than if the manifold 4 was to be lifted from the deck in air and then deployed into the water. The required crane capacity (lift and reach) are also typically reduced.

Once the load of the manifold 4 is suspended from the lifting vessel 3 and unsupported by the barge 1 , and there is a sufficient clearance between the two, the barge 1 and the lifting vessel 3 can move apart from one another as shown in Fig 17 and Fig 18, to move the manifold clear of the barge 1 so that it can be lowered to the seabed from the vessel 3.

Alternatively, if the vessel 3 is lifting and lowering the manifold 4 from its moonpool, pre-hgged lifting strops are passed over from the barge 1 topsides using the lifting vessel's crane and passed into the moon-pool using pre-hgged messenger lines as shown in Figs 20 and 21. The remainder of the deployment sequence is the same as described above with respect to figs 13-19, except that the winches are routed through the moonpool, allowing use of alternative winches in addition to or instead of the deck winches where vessel design allows.

Figs 23-26 show an alternative method of deployment of tall items from the barge 1 where the lifting vessel is arranged alongside the barge 1 , instead of above it. This is useful for deployment of tall items or with lifting vessels with very deep drafts, and can be accomplished without necessarily mooring the barge 1 and the vessel 3 together; typically a pair of tugs 2 keep station with the barge 1 and the vessel 3 typically holds position with its dynamic positioning system. As shown in Figs 23-26, the items such as suction piles 5 are lifted from the deck 10 of the barge 1 by means of the vessel crane 3c deploying over the side of the vessel 3, and typically lifting the items from the deck 10 of the barge 1 , although the barge 1 could be ballasted down to increase its depth and clear the suction pile 5 from the deck 10. The crane 3c then lowers the item 5 between the vessel 3 and the barge 1.

In one modified embodiment shown in Fig 27 and 28, the modified barge 1 a optionally has a set of rails 31 mounted on the top of the braces 21 a, which extends parallel to the braces and perpendicular to the long edges of the deck 10a. The rails 31 extend over the ends of the braces 21 in a cantilever manner as shown best in Fig 27. The rails 31 support a gantry 32 that can travel perpendicular to the rails 31 and in Fig 27 is shown in a position in which it is outboard of the deck 10. The gantry supports one or more crane blocks 33 that can pick up items 6 from the deck 10a and swing them overboard to lower them to the sea bed. This can be achieved without the assistance of a lifting vessel and while the item is submerged as disclosed in previous embodiments.

A further modification is shown in Figs 29 and 30, in which the modified barge 1 b has a modified pontoon 11 b, with a releasable grille 13 that is supported by winch lines 14 from the pontoon 11 b. The pontoon 11 b can optionally have winches (not shown) and can have more than one releasable section, or the whole of the deck can optionally comprise a releasable section. Optionally the releasable section can be smaller than the grille 13 shown in the figures. Optionally the grille can be a skidable pallet that can be loaded with the item in the fabrication site, for example in a quayside factory, delivered to the quayside by lorry and latched down to the deck when in place on the barge. Optionally the gantry crane on the barge can be used to deploy payload items through openings in the pontoon deck, without necessarily being arranged on cantilever rails.

The releasable section formed by the grille 13 can be lowered to the sea bed as shown in Fig 29, while the deck and the items thereon are submerged as disclosed in previous embodiments, and without the presence of a lifting vessel, as the winch controls can be situated in the control centre 22b. Thus the barge 1 b can be used to deploy to the sea bed a number of items secured onto the grille and leave these pallets in the vicinity of a subsea worksite, so that at a later time, a lifting vessel can be deployed to lift the items from the grille and convey them to the worksite for installation, assembly or use. This optional feature of the releasable section can therefore reduce the dependency of the barge on a separate lifting vessel. By combining the embodiments in figs 27-28 and 29-30 a modified embodiment can be conceived with both a crane and a releaseable section, reducing the barge dependency on a lifting vessel still further.

A further embodiment of a barge 101 is shown in Figs 31 -39. The barge 101 has similar features to the barge 1 , and common features are given the same reference number, but increased by 100. These common features will not be described in detail here.

The barge 101 differs from the barge 1 in the design of the columns 220, which are modular like the columns 20, but which have an axial bore extending up each column to receive a leg 125 that is connected to each foundation 119 of the pontoon 111. The legs 125 slide within the bores of the columns between a contracted configuration shown in Fig 34 and 35 to an extended configuration shown in Fig 37-39. In the contracted configuration shown in figs 34 and 35, the legs 125 are retained within the columns and in the extended configuration they extend out of the columns to space the pontoon 111 from the braces 121. The extension of the legs 125 in the bores of the columns 120 is optionally powered by winches or hydraulic cylinders (not shown) or pneumatically or hydraulically. The legs are typically keyed to the columns to restrain relative movement of the legs 125 and columns 120 apart from axial sliding of the two.

Additional leg sections can be added to the "minimal length" configuration, thus extending the deployment draft of the barge pontoon and the load. In such embodiments, the hull compartments are typically rated at a suitable depth to withstand the additional hydrostatic pressure resulting from the deeper draft.

When the barge 101 is moved into place as described for previous embodiments, and the payload 4 is connected to the lifting vessel 3 (but still supported by the deck 110) variable buoyancy tanks are flooded to account for the reduced barge payload, and the legs 125 are typically extended to lower the deck 110 away from the floating columns 120 so that the deck no longer supports the payload 4, which is then suspended from the lifting vessel 3. The barge 101 can then be moved away from beneath the payload (and optionally the lifting vessel) as previously described. The provision of the extendable legs 125 and especially having them keyed to the columns improves the stability of the barge during handover operations.

Figs 40-43 show a further embodiment of a barge 201 , which has extendable legs 225 that are received within columns 220 like the previous embodiment. Components of the barge 201 that are similar to the barge 101 are given the same reference number with 100 added, and will hereinafter not be discussed in detail. The barge 201 differs from the barge 101 in the design of the pontoon. In the barge 201 the pontoon is formed from three interconnected tubular components 211 which are arranged parallel to one another and parallel to the long axis X-X of the barge 201. The tubular members 211 are interconnected by cross braces 216 formed from tubular sections, which space the tubular members from one another laterally in the barge 201. The tubular members 211 and the cross braces 216 together form the deck 210 of the barge 201. The tubular members 211 and the cross braces 216 can form or contain buoyancy tanks that operate in a similar manner to those described for earlier embodiments.

Additional tanks (not shown) could be arranged along the waterline and could further enhance stability of the barge while under tow in transit.

The gaps between the tubular members 211 permit the passage of water through the deck between the tubular members 211 , so during ballasting of the barge 201 , the barge is less affected by hydrodynamic effects of water displacement (heave excitation) and the stability of the barge is increased during ballasting and de-ballasting. Also, the towing resistance of the barge 201 is reduced by the parallel arrangement of the tubular members 211.

Figs 44-47 show a further embodiment of a barge 301 , which is loaded with the manifold 4, ready for surface towing. The barge 301 has extendable legs 325 that are received within columns 320 like the previous embodiment. Components of the barge 301 that are similar to the barge 201 are given the same reference number with 100 added, and will hereinafter not be discussed in detail. The barge 301 differs from the barge 201 in the design of the pontoon. In the barge 301 the pontoon is formed from three interconnected tubular components 311 which are arranged parallel to one another and parallel to the long axis X-X of the barge 301. The tubular members 311 are interconnected by cross braces 316, which space the tubular members from one another laterally in the barge 301. The tubular members 311 and the cross braces are topped with a flat plat surface forming the deck 310, with apertures between the tubular members 311 and the cross braces 316. The tubular members 311 and the cross braces 316 can form or contain buoyancy tanks that operate in a similar manner to those described for earlier embodiments.

The gaps between the tubular members 311 permit the passage of water through the deck between the tubular members 311 , so during ballasting, the barge 301 is less affected by hydrodynamic effects of water displacement (heave excitation) and the stability of the barge is increased during ballasting and de-ballasting. Also, the towing resistance of the barge 301 is reduced by the parallel arrangement of the tubular members 311.

In the last two embodiments, additional buoyancy can be accommodated between the tubular members if desired. The design permits deeper submergence for taller items to be loaded onto the deck. Also, flat deck grillage can be applied to different sections of the deck to accommodate loads of particular shapes.

Embodiments of the invention have the advantage that the item to be delivered to the sea bed does not need to be lifted from the deck of the vessel 3 and lowered into the water. Therefore, factors relating to the changes in load when passing through the air water interface can be managed more easily. Also, the deployment of the submerged item from the submerged deck of the barge avoids dynamic forces resulting from the free surface effects of the water, wave induced drag and displacement associated inertia loadings. Thus, embodiments of the invention allow the handling of heavy loads and their delivery to and recovery from the sea bed without requiring heavy lift cranes.

By adding water permeable sides or fences to the barge pontoon, various scrap or salvaged items can be brought to the surface in a gentle manner with reduced risk of disintegration typically associated with through water lifts of structurally uncertain status.

Palletised open top removable containers can be used as submersible skips. A recovery vessel could lift the full pallet or skip from the seabed to the depth of the submersible barge deck, transfer the load to the barge deck, disengage from the barge and allow the barge to ballast up bringing the payload to the surface in a safe and controlled manner.

Modifications and improvements can be incorporated without departing from the scope of the invention.

Claims

1 A submersible barge for handling a submersible item, the barge having a deck for supporting the item, at least two columns arranged at spaced apart locations on the deck and a buoyancy control system incorporating buoyancy devices, wherein the buoyancy control system is adapted to submerge the deck of the barge and support while submerged, to permit transfer of the item to or from the deck while the item is submerged.

2 A barge as claimed in claim 1 , wherein the buoyancy devices comprise fixed buoyancy tanks, which define a fixed volume and have a fixed amount of buoyancy, and variable buoyancy tanks, which define a volume in which the buoyancy can be varied.

3 A barge as claimed in claim 2, wherein the fixed buoyancy tanks are provided in the deck and have sufficient buoyancy to support the deck with the payload of the item when the item is submerged.

4 A barge as claimed in claim 2 or claim 3, wherein the variable buoyancy tanks are adapted to be flooded to submerge the deck and transfer the item under water.

5 A barge as claimed in any one of claims 2-4 wherein the deck comprises fixed and variable buoyancy tanks and wherein the fixed buoyancy tanks are disposed closer to the centre of gravity of the deck than the variable buoyancy tanks. 6 A barge as claimed in any one of claims 2-5 wherein the buoyancy tanks are arranged in polygons with adjacent buoyancy tanks being provided with common walls having pressure resistant cross sections.

7 A barge as claimed in any one of claims 2-6, wherein the tanks are arranged symmetrically in the deck.

8 A barge as claimed in any preceding claim, wherein the barge has a long axis, and wherein the deck is formed from at least two tubular members interconnected with one another, and arranged parallel to one another, in alignment with the long axis of the barge.

9 A barge as claimed in any preceding claim, wherein the deck is rectangular and wherein the columns are arranged at corners of the rectangular deck.

10 A barge as claimed in any preceding claim, wherein the columns are releasably connected to the deck allowing connection and detachment of the columns from the deck.

11 A barge as claimed in any preceding claim, wherein the columns incorporate buoyancy tanks.

12 A barge as claimed in claim 11 , wherein the columns incorporate fixed buoyancy tanks and variable buoyancy tanks.

13 A barge as claimed in claim 11 or 12, wherein the fixed buoyancy tanks have sufficient buoyancy to support the barge with the payload of the item when the item is submerged. 14 A barge as claimed in claim 13, wherein the fixed buoyancy tanks are spaced apart from the deck in upper sections of the columns.

15 A barge as claimed in any preceding claim, wherein the buoyancy control system includes at least one floodable buoyancy device having at least one sensor to provide remote feedback as to the flooded status of the floodable buoyancy device.

16 A barge as claimed in any one of claims 11 -15, wherein at least one column incorporates a control centre at the top of the column, the control centre comprising a fixed buoyancy tank, and having controls for controlling at least the buoyancy system.

17 A barge as claimed in any preceding claim, having a lifting device to transfer the submersible item between the deck and the underwater location, and wherein the columns support the lifting device above the submersible item.

18 A barge as claimed in claim 17, wherein the lifting device comprises a gantry crane extending between the columns at opposite ends of the deck.

19 A barge as claimed in claim 18, wherein the gantry crane has a cable guide device and a lifting cable and wherein the cable guide device guides the lifting cable between the crane and the submersible item on the deck, and wherein the cable guide device is configured to move between a first position that above the deck and a second position that is not above the deck. 20 A barge as claimed in any preceding claim, wherein at least one column incorporates a disconnection mechanism to separate at least a portion of the column from the deck into two separate disconnected portions.

21 A barge as claimed in claim 20, wherein a lifting device is connected between the disconnectable portions, and can be activated to raise and lower the deck relative to the column or column portion, which can remain at least partially afloat while the deck is submerged and lowered.

22 A barge as claimed in claim 20 or 21 , wherein the column is connected to the deck by means of a leg and wherein one end of the leg is connected to the deck and the other end of the leg slides relative to the disconnected portion of the column.

23 A barge as claimed in any preceding claim, wherein the deck has a releasable section adapted to support a number of items to be delivered to the underwater location, and wherein the releasable section is connected to the deck by a winch device, and wherein the winch device is adapted to lower the releasable section from the deck while the deck is submerged.

24 A barge as claimed in any preceding claim, wherein the deck is formed with drainage channels.

25 A barge as claimed in any preceding claim, wherein the deck is formed by the upper surface of at least two parallel tubular members that are spaced apart laterally by interconnecting cross brace members, and wherein the tubular members are arranged parallel to a long axis of the barge. 26 A method of handling a submersible item and moving it between a submersible barge and an underwater location, the method comprising carrying the item on a deck of the barge, and transferring the item between the deck of the barge and the underwater location, wherein the item is submerged when being transferred between the deck of the barge and the underwater location.

27 A method as claimed in claim 26, wherein the submersible item is recovered from the underwater location to the deck of a barge, wherein the barge and the item are both submerged when the item is transferred to the deck of the barge.

28 A method as claimed in claim 26 or 27, wherein the item is transferred between the deck of the barge and the underwater location while submerged under the draft of a lifting vessel.

29 A method as claimed in claim 28, wherein the lifting vessel has a moonpool, and wherein the lifting vessel provides at least one lifting device to transfer the submersible item between the deck and the underwater location, and wherein a lifting cable between the submersible item and the lifting device of the lifting vessel is routed through the lifting vessel's moonpool.