The invention relates to vessels for lifting heavy loads offshore, and particu- larly to vessels for decommissioning offshore platforms with a platform deck placed on jacket legs. A method for using the vessel is also disclosed.

As the offshore oilfields around the world are depleted and when further ex- ploitation is no longer profitable, platforms will have to be closed or shut down.

It is a requirement from various authorities that all platforms that has been taken out of service and can be removed should be taken on shore for dis- posal.

Some of the topsides consist of several modules installed separately. These could be removed individually in a reverse installation process. This would require several lifts using expensive SSCVs including preparations and engi- neering.

Studies have shown that 30-50% of the cost for platform removal is due to offshore operations support. Prior to removal, safety checks, internal seafas- tening, physical separation of connections and preparations of lifting attach- ments have to be carried out. These are time consuming tasks also introduc- ing need for accommodation and messing services, crane and power supply, hence removing the topsides in single lifts has a potential for cost saving.

By lifting a topside in one single lift, preparations are only required for one op- eration. A lot of the work onboard the topside, e. g. separating the modules, could then be carried out much cheaper onshore.

Vessels with various designs have been proposed for this purpose.

Norwegian patent NO 306385 shows a method and a transportation device for use in connection with installation or removal of offshore platform decks on or

from jackets. The platform decks are removed from the jacket in one lift, and the transportation device comprises pontoons with controllable buoyancy.

Norwegian patent NO 831997 shows a floating device for the lifting and trans- portation of a load, and a method for the same. A platform deck can be re- moved from a jacket in one lift, and the transportation device comprises pon- toons with controllable buoyancy.

Building new vessels is however quite expensive, and necessitates many ma- rine systems in addition to fabrication of the steel.

At the same time, there are a large number of drilling, construction and ac- commodation vessels laid up waiting for work. They are already equipped with systems for safety, ballasting, mooring, towing, and propulsion in addition to accommodation, messing and large deck areas. These systems have also been tested, certified and used.

By attaching a lifting structure to an existing vessel, the cost of a lifting or de- commissioning vessel can be substantially reduced. It is expected that a weight of 5000-10000 tonnes of new steel can be saved. The existing vessel is relative cheap due to the current situation in the North Sea. The integration between the existing and new structure should and can be kept to a minimum to control the cost.

Making a lifting structure suitable for attachment to various vessel types will introduce an additional feature and keep the dayrates to a minimum.

By integrating these extra services in the lifting vessel the requirement for ad- ditional vessels during the offshore preparations will be limited.

Accordingly, the present invention relates to an lifting vessel with controllable buoyancy for lifting offshore platforms placed on a jackets with jacket legs, the vessel having two, in a longitudinal direction, first substantially parallel pon- toons and a between the two first substantially parallel pontoons and an, at

one end of said first pontoons, an extending transversal second pontoon for connecting said first pontoons to form a first U, two longitudinal, substantially parallel first deck beams and an, at one end of said first deck beams, extend- ing transversal second deck beam for connecting said first deck beams to form a second U and columns extending between the first U and the second U to form a unitary construction where the first U and the second U are substantially parallel ; said vessel is adapted to be rigidly attached to a second semi-submersible vessel.

The invention relates to using an existing vessel, (drilling, accommodation etc.) together with the steel unit or vessel of the invention. Both the existing vessel and the steel lifting unit of the invention are preferably semi submersible.

To this existing vessel, the steel unit or lifting vessel of the invention, with suf- ficient strength and buoyancy to lift platform decks or topsides, is secured. The steel unit comprises at least two longitudinal pontoons, at least one transversal pontoon, two or more, preferably four, columns, at least two longitudinal deck beams, and at least one transversal deck beam. The steel unit is secured to the exiting vessel with its at least two longitudinal pontoons as extensions of the pontoons of the existing vessel. The pontoons of the unit are intercon- nected to form a substantially U shaped pontoon, and the three deck beams are interconnected to form a substantially U shaped deck beam. The two U shaped constructions are interconnected with the columns to form the unit.

The steel unit is preferably constructed to carry all the weight of the platform by is self, but the possibility of letting the existing vessel carry some of the load should also be contemplated. The number of intersections between the exist- ing vessel and the new steel unit should be kept to a minimum for simplicity.

The ballasting systems, anchoring systems, cranes, quarters, power supplies, lighting systems, deck area are features of the existing system that can be util- ised of the joint vessel. This will provide for considerable cost and resource savings, as compared to the production of an entirely new vessel.

Due to the facilities of the existing vessel, the joint vessel is suitable for the initial preparations necessary before the lift is made. A footpath or bridge can be placed between the vessel and the platform to be moved.

Semi-submersible vessels are used due to their inherent stability.

It is crucial for the operation of the vessel to maintain a stable or fixed position in relation to the platform to be lifted.

In this docking and load transfer condition three different methods available for maintaining the position.

The first and preferred method, is connecting the vessel to jacket legs through flexible connections.

The flexible connection is achieved with a fender system of the invention. The fender system is preferably hydraulically controlled and is placed between the vessel and the jacket legs of the platform. The stiffness or resiliency of the fenders is controlled and optimised to dampen the movements without inflict- ing damage to the jacket. With this system, the vessel may move freely in a substantially vertical direction. The fender system can be adjusted to follow the inclination of the jacket legs before, during and after the platform has been lifted from the jacket.

If the vessel is connected to the jacket through a flexible connection, the static loads due to the mean wave drift, wind and current have to be transferred through the connections.

The stiffness of the connections has to be sufficiently soft to avoid the dynamic responses as imposed of rigid connections between the bodies. If the connec- tions are too soft, the second order dynamic responses of the vessel increase so much, that it becomes impossible to align the load transfer points of the lifting vessel and the topside. The static offset has also to be compensated.

The connection system also has to be made sufficiently flexible to cope with the varying draft of the lifting vessel from docking, through load transfer and to lifting off.

The use of flexible connectors is a feasible solution covering all jacket types for docking, lift off and through all phases of the lift off operation.

The second way of maintaining the position between the vessel and the plat- form is with the vessel fixed to the jacket legs or topside frames.

All jackets are stiff structures and the dynamic responses are controlled by the stiffness, in contrary to a moored vessel that has a low stiffness and the dy- namic responses are controlled by the mass.

In addition jacket structures are slender structures, and attract therefore low hydrodynamic forces. The natural periods of these structures in operation ranges from 0.5-3.0 seconds depending on the topsides weight, water depth, piles and soil conditions and number and distance between the legs.

By connecting a vessel with a displacement of 30.000 to 100.000 tonnes rig- idly to the jacket in moderate seastates (significant wave heights up to 3m), the additional hydrodynamic loads will be in the same order as the total envi- ronmental loads on the jacket including the topsides.

The increased mass (displaced mass + added mass of the lifting vessel) will also change the natural periods of the combined structures, which means a considerable increase in the dynamic response of the jacket.

This method is therefore possibly less suitable than the first method described.

It is also questionable whether the jacket would be capable of withstanding the applied loads, without collapsing.

The third way is not to connect to the jacket and vessel at all, and control the position of the vessel either by dynamic positioning, mooring lines or tugs.

The position of the vessel is, until load transfer starts, only controlled by a catenary mooring (or attached to tugs) or by dynamic positioning. The vessel motions cannot be to large in order to position the respective load transfer points at the jacket/topsides with the vessel and to maintain the position enve- lope during the main part of the load transfer phase.

A vessel moored with catenary moorings have stiffness in the region of 30- 40t/m, which means that the second order motion responses are much greater than the first order motions and the resulting responses are too excessive to carry out the operation. A line failure would also have a high probability of re- sulting in a catastrophic consequence due to the likelihood of collision be- tween the vessel and the jacket.

A vessel maintaining position by dynamic positioning would also have prob- lems to operate within the small footprint as required for positioning and load transfer. The risk of drive off and drift off of DP systems are very high (1.5- 2.5x10-5 pr. DP hour) in addition during load transfer between the jacket and vessel the input signals to the computer analysing the external loads will be corrupted. This results in unstable responses, which again results in a high probability of failure.

The use of dynamic positioning during topside removal is therefore considered to be a less suitable solution than the first positioning system.

The vessel also comprises a load bearing system that incorporates lifting beams, controllable jacking systems, low friction pads and a horizontal load transmittal system. The lifting beams are designed to be thread under the plat- form to be lifted, and to extend between the deck beams. The jacks of the jacking system are attached to the lifting beams and are placed under the solid parts of the platform. For all topsides, the strong load bearing points are adja- cent to the jacket legs. It is for 4-legged jackets sufficient therefore to lift the topsides supported from the outside.

For larger jackets (12 legged jackets) most of the topsides do not have suffi- cient strong load bearing frame to transfer the entire load through the outer support points. The load bearing system has therefore either to be extracted to transfer the loads through all jacket support points/or alternatively to rein- force the topside to transfer the loads to the outer points. Studies have re- vealed that massive reinforcement of topsides is required.

It is therefore recommendable with support points adjacent to all the jacket legs. The jacking system controls the force applied to the various parts of the platform.

The lifting beams may be adjusted back and forth on the deck beams by means of a wire system, hydraulic motors or hydraulic cylinders.

The anti friction pads are placed between the jacks and the platform to prevent horizontal loads on the jacks. The platform is held in the horizontal direction by the horizontal load transmittal system.

To safely be able to remove the platform or topside from the jacket, jacket leg coupling means, well known in the art, are provided. The coupling means comprises an upper and a lower part releasably connected. The upper and lower part of the coupling means can be released from each other by using any suitable method such as hydraulically controlled cutting of bolts, with fran- gible elements/shear pins or slips. The coupling means are secured to the jacket legs with mechanical fasteners, welding or other suitable securing means. It is an essential feature to be able to cut the jacket leg between the upper and lower part of the coupling means. Disconnection of the coupling means can be initiated automatically at a predetermined load, or manually in response to a signal from an operator. The object is to hold the platform to the jacket until everything is ready for lift off.

An embodiment of the invention will now be described with reference to the accompanying drawings where :

figure 1 shows a perspective view of a lifting decommissioning vessel accord- ing to the present invention ; figure 2 shows a perspective view of the steel unit in more detail ; figure 3 shows a perspective view of the steel unit in more detail showing a platform deck, jacket legs, parts of the hydraulic jacking system and the con- trolled fenders; and figure 4 shows a perspective view of a platform jacket with coupling means attached to the jacket legs.

Fig. 1 shows the lifting vessel (1) for removal of platforms in one lift including a semi-submersible vessel (2) of a drilling, construction or accommodation type (for instance an Aker H3 type). The steel unit of the invention (3) is attached to the existing semi-submersible vessel to form the complete lifting or decommis- sioning vessel (1). The vessel (1) can provide for offshore preparations, for lifting, accommodation for working personnel, power and crane supply, ROV supporting operations and ballasting.

Fig. 2 shows the steel unit of the invention with two longitudinal pontoons (4), one transversal pontoon (5), four columns (6), two longitudinal deck beams (7), one transversal deck beam (8) and six lifting beams (9). This unit may have a 40.100-tonn displacement and a topside lifting capacity of 12.000 ton- nes.

On fig. 3, the vessel of the invention is shown with its fender system (10) se- cured to the four columns (6). The deck of the platform (11) to be lifted is shown only as a steel frame for better clarity. Jacks of a jacking system (12) extend between the lifting beams (9) and strong points of the platform (11).

Low friction pads are placed between the jacks and the platform (11). Refer- ence numeral (13) denotes load transferral systems that hold the platform (11) in a horizontal direction.

The jacket with jacket legs is shown on fig. 4 with coupling means (14) con- nected to the jacket legs.

When a platform is to be lifted, the lifting vessel is manoeuvred below the plat- form to be lifted by means of winches and/or tugs and is, in a horizontal direc- tion, held to the jacket legs of the platform with the fender system (10). The vessel (1) is free to move vertically. The lifting beams (9), supported on the deck beams (7) are thread under the platform (11) and directed towards strong points on the platform (11). Ballast in the lifting vessel (1) is then pumped out, and the weight of the platform (11) is supported of the lifting vessel (1). The jacking system (12) controls the load on the contacting points between the vessel (1) and the platform (11). The horizontal load transferral systems (13) are applied to hold the platform (11). The coupling means (14) subsequently disconnects the platform from the jacket when a predetermined load is applied to the platform (11) or the coupling means (14), and the platform is lifted off the jacket. The platform is now free to be hauled to a place where it can be transferred to a barge, and further be brought on shore with any suitable sys- tem known in the art.