GB2280150A - Stabilising marine lifting equipment - Google Patents

Abstract

A system for performing marine lifting operations comprises a lift vessel (16) equipped with lifting gear (22, 24) and adapted to be coupled by means of a jib (26) to an adjacent vessel (12) which acts as a counterweight to stabilise the lift vessel while a load (10) is lifted from a barge (14). The lift vessel may be a pontoon, a tug or a dynamically positioned vessel. The jib may be adapted to be compressible and extensible, and its end may be mounted on a movable carriage on the lift vessel. <IMAGE>

Description

Improvements in or relatina to Marine Lifting The present invention relates to improved methods and apparatus for marine lifting operations, and is particularly, but not exclusively concerned with the lifting of relatively heavy loads during the deployment of subsea structures from transport barges.

Marine lifting operations carried out by Diving Support Vessels (DSV's) typically involve the deployment ("load-out") of subsea structures from dumb transport barges. Heavy lift operations, involving relatively heavy loads, account for a very small proportion of DSV work, typically less than 10%. Accordingly, the cranage carried by a DSV will typically be limited to a capacity of about 100 tons, and specialised heavy lift vessels (often semi-submersible) are employed for heavy lift operations. However, the specialised nature of such vessels, their limited use and the short duration of normal heavy lift operations make them very expensive to operate.

It is an object of the present invention to provide marine lifting apparatus and methods which allow relatively heavy loads to be handled in a more costeffective manner than has been possible hitherto. In its preferred embodiment, the invention provides for upgrading the lift capability of a DSV for deploying subsea structures from loadout barges onto the seabed.

In accordance with a first aspect of the invention there is provided a marine lifting vessel comprising an elongate, buoyant hull having lift rigging mounted thereon and including coupling means adapted to releasably couple the vessel alongside an adjacent vessel, whereby, in use, transverse stability is imparted to said hull by virtue of said coupling to said adjacent vessel.

Preferably, said lift rigging includes a generally vertical load-bearing structure, such as.a mast or frame.

Preferably also, said lift rigging further includes a pivotable lifting structure (such as a jib or frame) extending, in use upwardly and outwardly from a first longitudinal side of said hull.

Preferably also, said coupling means comprises a pivotable structure (such as a jib or frame) extending, in use, outwardly from a second longitudinal side of said hull for pivotable connection to said adjacent vessel.

In one embodiment, the vessel is a pontoon. Preferably, the pontoon is at least semi-submersible.

Preferably also, the pontoon is adapted to be tethered to a barge for transit to and from an operational site.

Preferably also, the pontoon is equipped with at least one winch.

Preferably also, said winch is powered and controlled, in use, from said adjacent vessel.

In another embodiment, the vessel is in the form of a tug, and may be used to tow the load on a barge to the deployment site. In this case the lifting winch is preferably controlled from the adjacent vessel, and may also be powered from the adjacent vessel if desired.

In still another embodiment, the vessel is in the form of a dynamically positioned vessel (DPV). Preferably the DPV provides both power and control for the lifting gear. In this case the adjacent vessel is preferably a tug, but may alternatively be a dumb barge.

In accordance with a second aspect of the invention, a method of performing marine lifting operations comprises coupling a lifting vessel in accordance with the first aspect of the invention to an adjacent vessel which imparts transverse stability to said lifting vessel during lifting of a load.

Preferably, where said vessel is a pontoon it is tethered to a barge for transit to and from the operational site.

Alternatively, said vessel is a tug which may tow the barge to the work site.

Preferably also, the lifting operation is powered and/or controlled from said adjacent vessel.

Alternatively, said vessel is a DPV, and said adjacent vessel is a tug or dumb barge.

In this case, the lifting operation is preferably powered and controlled from the DPV.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figs la to ld are schematic end views illustrating a first embodiment using a heavy lift pontoon in accordance with the invention together with a Diving Support Vessel (DSV) and a loadout barge in the deployment of a subsea structure; Fig. 2 is a schematic plan view of the apparatus of Fig. 1, corresponding to Fig.

lb; Figs. 3a to 3d are end views similar to those of Fig. 1 but illustrating a second embodiment of the invention utilising a tug as lift vessel; Fig. 4 is a schematic plan view of the apparatus of Fig. 3; Figs. 5a to 5d are end views similar to those of Figs. 1 and 3 but illustrating a third embodiment of the invention utilising a dynamically positioned vessel (DPV) as lift vessel and a tug as the adjacent counterweight vessel; Figs. 6a to 6e are end views similar to that of Fig. 3b, illustrating relative movements of a DSV and adjacent lift vessel under different conditions of roll and heave; Figs. 7a to 7d are end views similar to those of Fig. 3, illustrating preferred features in greater detail; Fig. 8 is a plan view of the apparatus of Fig. 7; and Fig. 9 is a plan view similar to Fig. 8, illustrating relative fore and aft motions of the DSV with respect to the lift vessel.

Referring now to Figs. 1 and 2, a system for the deployment of subsea structures, such as a wellhead protection structure 10, onto the seabed, utilises a standard DSV 12, a standard loadout barge 14, and a heavy lift pontoon 16 in accordance with the invention.

The barge 14 is towed to the lift site by a tug (not shown), carrying the structure 10 to be deployed. The pontoon 16 may be tethered to the side of the barge by any suitable means 20 and towed therewith.

The pontoon is fitted with a vertical load-bearing mast or frame 22 extending upwardly from a topmost deck surface, a pivotable lifting jib or frame 24 extending upwardly and outwardly from the deck surface on the barge side thereof, and a pivotable coupling jib or frame 26 extending upwardly and outwardly from the deck surface on the opposite (DSV) side thereof. The coupling jib 26 is raised in transit, as seen in Fig.

la, and the lifting jib may optionally be tethered by a tie back 28 connected to the far side of the barge 14.

All of the lift rigging could be stowed for transit, but is preferably erected prior to sailaway. Various rigging configurations may be employed; e.g. single or twin mast, shear legs, single or twin DSV deck couplings etc, subject to optimisation.

At the lift site, the pontoon 16 is detached from the barge 14, and the coupling jib 26 is lowered and coupled to the deck edge of the DSV 12 by any suitable pivotable connection means (suitably a universal-joint arrangement, not shown), as seen in Figs. lb and 2.

Fore and aft mooring lines 30 also tether the pontoon 16 to the DSV 12. The structure 10 is raised from the barge deck, the barge 14 is moved away, and the structure 10 is lowered to the seabed, as seen in Fig.

lc. When the lift is complete, the pontoon 16 is uncoupled from the DSV, the coupling jib 26 is raised, and the pontoon is re-attached to the barge for return to port.

The main lifting winch(es) are preferably mounted on the pontoon 16, together with auxiliary winches for controlling the positions of the jibs 24, 26, with lift power and control supplied from the DSV. The pontoon is not vessel-specific, and can be used with a variety of DSV's, which will require only minor adaptation including the fitting of suitable connector means for receiving the coupling jib 26. Similar minimal adaptation is required for coupling to existing barges.

The pontoon 16 is essentially unmanned and may be completely submersible, with any onboard winches or power systems enclosed in fully watertight compartments or else maintained at locations above the waterline.

The pontoon 16 is particularly suited to the low freeboard characteristics of monohull DSV's, whereas heavy-lift operations are more normally associated with semi-submersibles. However, the pontoon may also be used with semi-submersible vessels. Typical lift capacities for the pontoon may be 300 - 500 tonnes, at 25m hook height above sea-level. Different capacities are also possible.

The pontoon 16 is preferably of long, slender shape, being relatively narrow so as to minimise drag when towed alongside the barge 14 and sufficiently long for longitudinal stability under high hook load. The pontoon might be 40m long by 10m wide for use with a standard 300 x 100 foot barge. The transverse (roll) stability of the pontoon 16 is derived from its being secured at deck level to a larger, more stable vessel (either the barge 14 or the DSV 16). Stability control of the pontoon 16 is transferred from the barge 14 to the DSV 12 immediately before making the lift. Other pontoon hull configurations may be employed; e.g. long, in waterline, or shorter semi-submersible, subject to optimisation.

Because the pontoon 16 is much smaller than conventional heavy-lift DSV's, the lift will be softer, minimising the motion loads induced during the lift.

This may be of particular benefit in picking the load off the barge.

In the present embodiment, the invention serves to upgrade the lift capability of a given DSV. The pontoon provides a low-cost, load bearing, buoyant lift support with inherent longitudinal stability and with transverse stability provided by virtue of its connection to a larger adjacent vessel. As discussed, the configuration of the hull of the pontoon itself and of the lift rigging and coupling structure may be varied, and the invention may find application in marine lifting operations other than subsea structure deployment from barges, and in combination with vessels other than DSV's.

Turning to Figs. 3 and 4, there is illustrated a second embodiment of the invention in which the pontoon 16 is replaced by a modified tug 116. The tug 116 is fitted with load handling equipment 22, 24, 26 and pivotal connection means as in the first embodiment. The actual deployment of the load is achieved in exactly the same manner as in the first embodiment. However, for transit the barge 14 with the load 10 aboard is towed to the deployment site by the tug 116.

Thus, fewer vessels are required in total. The modifications to a standard tug are relatively minor, and the tug can operate in its normal role when not required as a lift vessel.

Referring now to Figs. 5a to Sd, in a third embodiment the lifting vessel comprises a modified dynamically positioned vessel (DPV) 216. The DPV 316 is fitted with load handling equipment 22, 24, 26 and pivotal connection means similar to those fitted to the tug 116 in the second embodiment. In this case a tug 112 is used as the adjacent counterweight vessel, replacing the DSV 12 of the first and second embodiments. The actual deployment of the load is achieved in exactly the same manner as in the first and second embodiments.

The barge 14 with the load 10 aboard may again be towed to the deployment site by the tug 112. The lifting operation will be both powered and controlled from the DPV 216.

A tug will normally have greater clear deck space available than a DPV, so that in the third embodiment the coupling jib 26 may be connected to a point in the centre of the deck of the tug 112, rather than to the deck edge as would normally be necessary with a DSV.

Alternatively, the tug 112 in this arrangement could be replaced by a dumb barge (not shown). Again, the coupling jib could be connected to the centre of the barge deck, rather than its edge.

In all of the foregoing embodiments it is desirable that the coupling jib 26 be compressible and extensible, within predetermined limits, to accommodate relative roll and heave motions between the lift vessel 16, 116, 216 and the counterweight vessel 12, 112. This is illustrated schematically in Fig. 6 for the case where the lift vessel 116 is a tug and the counterweight vessel 12 is a DSV. Fig. 6a shows the relative positions of the vessels in still water. Fig.

6b shows a 2 degree roll of the lift vessel 116, resulting in (in this example) a 0.8 metre compression of the coupling jib 26. Fig. 6b shows an equal and opposite roll resulting in a 0.8 metre extension of the jib 26. Figs. 6d and 6e show, respectively, positive and negative heave motions of 1.2 metres, again resulting in 0.8 metre compression and extension of the jib 26 respectively. Hydraulic rams (not shown) may be incorporated into the jib 26 to accommodate a predetermined degree of compression/extension.

Figs. 7 to 9 show a more detailed embodiment of the invention using a tug 316 as the lift vessel and a DSV 12 as the counterweight vessel, and schematically illustrates the connection procedure. In this case the end of the coupling jib 326 at the lift vessel is connected to a carriage (not shown) movable on a track (not shown) extending across the beam of the lift vessel 316, so that the position of the jib end may vary along the length of the track. A hydraulic ram, as discussed above, is preferably located between the jib 326 and the carriage.

Mounting the jib 326 on hydraulic rams and tracks simplifies deployment offshore, limits the dynamic loads imposed on the jib itself and also improves motion performance under load.

The connection of the jib 326 to the lift vessel 316 is preferably by a single point universal joint or swivel coupling, as at the coupling of the jib to the DSV.

This limits the loading caused by differential fore and aft movements between the two vessels. Such movements and the corresponding movement of the jib 326 are illustrated in Fig. 9.

The connection procedure is as follows. Initially, as seen in Fig. 7a, the jib end would be located at the centre of the deck of the tug 316, and the jib would be lowered by the tug to the position shown in Fig. 7b. At this point the weight of the jib would be taken by a crane on the DSV, as seen in Fig. 7c, pulled in and coupled to the DSV. Finally, the jib 326 would be extended by movement of the carriage on its track to the edge of the deck of the tug 316 and locked in this position prior to execution of the lift.

At this stage strops (cables) 328 are preferably also connected between the top of the lifting frame 322 and the edge of the DSV adjacent the end of the jib 326.

The strops 328 may be connected to the universal joint (not shown) to which the DSV end of the jib 326 is connected, but are preferably connected directly to the DSV by means of padeyes or the like (not shown). There are preferably at least two strops 328, but possibly more. Connection directly to the DSV enables the system to be stable without recourse to a complex universal joint, and eliminates one possible cause of single point failure by introducing a degree of structural redundancy.

Figs. 8 and 9 also illustrate a preferred arrangement of mooring lines 330 between the lift and counterweight vessels.

All of the features discussed above in relation to Figs. 7 to 9 are equally applicable to previously described embodiments of the invention utilising different types and combinations of lift and counterweight vessels.

In general, for all of the embodiments of the invention, it is preferable for as much as possible of the equipment dedicated to heavy lifting operations to be located on the lift vessel, so as to avoid duplication of special purpose equipment on multiple potential counterweight vessels.

Improvements and modifications may be incorporated without departing from the scope of the invention.

Claims (25)

Claims

1. A marine lifting vessel comprising an elongate, buoyant hull having lift rigging mounted thereon and including coupling means adapted to releasably couple the vessel alongside an adjacent vessel, whereby, in use, transverse stability is imparted to said hull by virtue of said coupling to said adjacent vessel.

2. A vessel as claimed in Claim 1 wherein said lift rigging includes a generally vertical load-bearing structure, such as a mast or frame.

3. A vessel as claimed in Claim 1 or Claim 2 wherein said lift rigging further includes a pivotable lifting structure (such as a jib or frame) extending, in use upwardly and outwardly from a first longitudinal side of.said hull.

4. A vessel as claimed in Claim 1, Claim 2 or Claim 3 wherein said coupling means comprises a pivotable structure (such as a jib or frame) extending, in use, outwardly from a second longitudinal side of said hull for pivotable connection to said adjacent vessel.

5. A vessel as Claimed in any preceding Claim wherein the vessel is a pontoon.

6. A vessel as claimed in Claim 5 wherein the pontoon is at least semi-submersible.

7. A vessel as claimed in Claim 5 or Claim 6 wherein the pontoon is adapted to be tethered to a barge for transit to and from an operational site.

8. A vessel as Claimed in Claim 5, Claim 6 or Claim 7 wherein the pontoon is equipped with at least one winch.

9. A vessel as claimed in Claim 8 wherein said winch is powered and controlled, in use, from said adjacent vessel.

10. A vessel as claimed in any one of Claims 1 to 4 wherein the vessel is in the form of a tug, and may be used to tow the load on a barge to the deployment site.

11. A vessel as claimed in Claim 10 wherein said tug includes lifting winch means adapted to be controlled and/or powered from the adjacent vessel.

12. A vessel as claimed in any one of Claims 1 to 4 wherein the vessel is in the form of a dynamically positioned vessel (DPV).

13. A vessel as claimed in Claim 12 wherein the DPV provides both power and control for the lifting gear.

14. A vessel as claimed in any preceding Claim wherein said coupling means comprises an elongate member which is compressible and extensible by a predetermined amount in response to loads arising from relative roll and or heave movements of the vessel and adjacent vessel in use.

15. A vessel as claimed in any preceding Claim wherein said coupling means comprises an elongate member connected to carriage means adapted for movement across the beam of said vessel.

16. A vessel as claimed in any preceding Claim wherein said coupling means comprises an elongate member coupled to said vessel at a single point.

17. A vessel as claimed in any preceding Claim wherein said coupling means comprises an elongate member adapted to be coupled to said adjacent vessel at a single point.

18. A method of performing marine lifting operations comprising coupling a lifting vessel as claimed in any one of Claims 1 to 17 to an adjacent vessel which imparts transverse stability to said lifting vessel during lifting of a load.

19. A method as claimed in Claim 18 wherein said vessel is a pontoon and is tethered to a barge for transit to and from the operational site.

20. A method as claimed in Claim 18 wherein said vessel is a tug which may tow the barge to the work site.

21. A method as claimed in any one of Claims 18 to 20 wherein the lifting operation is powered and/or controlled from said adjacent vessel.

22. A method as claimed in Claim 18 wherein said vessel is a dynamically positioned vessel and said adjacent vessel is a tug or a barge.

23. A method as claimed in Claim 22 wherein the lifting operation is powered and controlled from the DPV.

24. A marine lifting vessel substantially as hereinbefore described with reference to Figs. 1 and 2, or to Figs. 3 and 4 or to Fig. 5 or to Figs. 6 to 9 of the accompanying drawings.

25. A method of performing marine lifting operations substantially as hereinbefore described with reference to Figs. 1 and 2, or to Figs. 3 and 4 or to Fig. 5 or to Figs. 6 to 9 of the accompanying drawings.