The installation of offshore platforms is of relevance in connection with the development of offshore oil and gas fields.

The removal of offshore platforms, consisting of topsides and jacket, is a field which will become increasingly important in the years to come, as a number of offshore platforms will be taken out of service.

Offshore platforms can be removed by being split into smaller parts, for example by means of cutting torches or blasting, whereupon each of these smaller parts can be hoisted on board a barge or a ship and transported away.

Regardless of how the division is performed, however, the costs associated with the splitting and the subsequent transport in smaller parts are high.

It is therefore desirable to split up a platform which has to be removed into the fewest possible parts, and a natural division is to divide it into topsides and jacket, which are removed separately. In the case of small platforms with a low weight this is relatively uncomplicated, while in the case of large, heavy platforms it entails lifting with huge and expensive crane vessels, if it is possible at all within the scope of what is achievable in practical terms.

Topsides has been transferred to platform jackets by building the topsides on barges in the form of tankers. These barges have been moved over a ballastable, submerged platform jacket, whereupon by deballasting the jacket the topsides has been transferred to the jacket. The use may be envisaged per se of a reversal of this procedure for the removal of the platform topsides, provided the jacket can be submerged. Consequently, in the case of platform jackets which are located on the seabed a reversal of this procedure cannot be employed.

NO 160 424 describes a device with adjustable buoyancy for lifting and transport during operations at sea. The device consists of floats which together form an oblong, ballastable structure with a U-shaped cross section and an opening in the bottom. After ballasting of the device, a barge with a platform topsides can be brought into the recess, whereupon when the device

is deballasted, gripping devices in the device's long sides are caused to lift : the platform topsides from the barge. After moving the device to a position where a jacket is located in the recess, the device is ballasted, with the result that the topsides is lowered on to the jacket.

The sea is usually in motion, and during a transfer of a platform topsides as described in NO 160 424 both the device and the barge with the platform topsides will be influenced by this motion and themselves be set in motion.

On account of their different shapes and different positions in the sea the movements of the device and the barge in practice will not coincide, with the result that the transfer of the topsides from the barge to the device may be imprecise, which can make the transfer problematical. Moreover, the motion of the device and the barge can cause lateral impacts during the transfer.

These impacts can be extremely heavy, particularly in the case of large topsides, and in unfortunate situations the impacts can be so great that the device, the barge or the topsides incur damage.

The object of the invention is to provide a method and a transporter for removal of a platform topsides from an associated jacket where the platform topsides is first transferred to a transporter and then transferred to a barge, where the transfer from the transporter to the barge should be performed in a precise manner and without lateral impacts.

The object is further to provide a transporter which can also be used for removal of a platform jacket, and transport of other heavy objects at sea.

A further object is that the same transporter should also be able to be employed for installation of a jacket for an offshore platform, and installation of a topsides on the jacket.

The first object is achieved according to the invention with a method and a transporter of the type mentioned in the introduction, characterized by the features which are stated in the claims.

The second object is achieved with a transporter of the type mentioned in the introduction, which in addition to being characterized by the features which are stated in the claims, is also characterized by the features which are stated in the claims in the applicant's copending Norwegian patent applications Nos.

97 3561 and 97 3563.

The third object is achieved by employing a transporter according to the invention in a method which corresponds to the method according to the invention, but where the actions are performed conversely and the steps are performed in reverse order.

The method according to the invention comprises the following steps: a) a ballastable transporter with horizontal contact sections and an intermediate recess is lowered by means of ballasting and moved in under the platform topsides, with the result that the jacket is located in the recess and the contact sections are located under the topsides, b) the transporter is deballasted so that it is raised and the contact sections come into abutment against the topsides's underside, lifting the topsides from the jacket, c) the transporter with the topsides are moved away from the jacket, d) a barge is moved in under the topsides, e) the transporter is ballasted with the result that the topsides is lowered on to the barge and the transporter's contact sections are released from the topsides's underside, f) the barge with the topsides sail away.

The transporter according to the invention consists of an oblong, ballastable structure with a U-shaped cross section and a prismatic enveloping surface, consisting of two long sides, an intermediate underside and an opposite open upper side. At one end the underside has a recess which can hold the jacket when the transporter is located with the underside horizontally down in the water, and the long sides on the side of the recess have contact sections which, when the transporter is deballasted, can be brought into abutment against the platform topsides and lift it.

In a preferred embodiment the transporter's long sides in areas under the contact sections have guide rails for attaching to slide blocks on a barge, which guide rails extend in the direction perpendicular to the underside, for vertical control of the barge's movement when it is located under the contact sections and the transporter is located with its underside horizontally down in the sea.

The transporter according to the invention makes it possible to remove large, heavy platform topsides and platform topsides which are located high above the surface of the sea. In the method according to the invention the transporter according to the invention is employed to remove a platform topsides from an associated jacket in a rational and economical manner, whereupon the platform topsides is transported by means of a barge to a suitable receiver, such as a ship-breaker.

The actual transfer of the transporter will be conducted by tugs. The transporter can be positioned with winches which can vary the length of the wires between the transporter and the platform jacket or another fixed point.

Further features and advantages will be presented in the detailed description.

The transporter according to the invention can also be used in the installation of an offshore platform topsides on to an associated jacket. In the installation of a platform topsides a method is employed which corresponds to the method according to the invention, but where the steps are performed in reverse order and the actions which are performed within each step are implemented conversely. This is an obvious variant of the method according to the invention, and will not be explained in detail. It should be understood, however, that the inventive concept also covers this alternative.

The invention will now be explained in more detail in association with a description of individual specific embodiments, and with reference to the drawing, in which: fig. 1 is a perspective view of a transporter according to the invention, figs. 2a-e illustrate a sequence in which a transporter according to the invention removes a platform topsides from an associated jacket, and transfers the topsides to a barge, fig. 3 illustrates shock-absorbing elements for cushioning impacts between the transporter and the platform topsides, and fig. 4 illustrates guide rails for vertical control of the barge's motion.

Fig. 1 illustrates a transporter 1 according to the invention, comprising an underside 3 and a long side 2 arranged on each side of the underside, the

three sides together thus forming an oblong structure with a U-shaped cross section.

Lower longitudinal pontoons 9 are provided in the corner areas between the underside 3 and the long sides 2, and upper longitudinal pontoons 8 are provided in the long sides'upper sections. Ballasting chambers are provided in each of the ends of the pontoons 8,9, which ballasting chambers preferably extend in towards the pontoons'central areas. Transverse elements 7, which are also in the form of pontoons and contain ballasting chambers, bind together the longitudinal pontoons 8,9.

Together with an open upper side opposite the underside 3 and two open short sides, the oblong structure forms a body with a prismatic enveloping surface. The ballasting chambers are located in the points of intersection between the long sides 2, the underside 3, the open upper side and the open short sides, with the result that one ballasting chamber is provided in each of the corners of the prismatic enveloping surface 8. During ballasting/deballasting of the transporter it is thereby possible to rotate it into the desired position in the sea.

The long sides 2 project at one end of the transporter past the underside 3, thus forming a recess 11 in the underside. In the extension of the recess 11 the lower longitudinal pontoons 9 project past the long sides 2 in sections 12, which also contain ballasting chambers.

On each side of the recess 11, forming part of the long sides 2, there are arranged contact sections in the form of longitudinal lifting beams 63 which are located at a distance from the transporter's underside 3 and parallel thereto. The lifting beams 63 are adapted to corresponding not shown contact sections on the underside of a platform topsides 16, and during deballasting of the transporter can be brought into abutment against these corresponding contact sections and lift the platform topsides. The lifting beams 63 are connected to their respective long side's upper area by tension elements 78 in the form of tension rods, and to their respective long side's lower area by pressure elements 79 in the form of columns. The weight of the platform topsides 16 is thereby distributed between the tension and pressure elements, and distributed on the pontoons in the transporter's long sides 2 and underside 3.

The illustrated lifting beams 63 are only one of several possible designs, since the contact sections may also be in the form of, for example, brackets or beams which project from the long sides 2, or the lifting may be performed directly by the columns'79 surfaces.

The transporter also comprises two longitudinal pontoons 24 in the underside 3. The longitudinal pontoons 24 are provided with grooves or rails 58 to support a sliding saddle 52 for attaching a platform jacket to the transporter's underside 3. In fig. 1 the transporter further comprises fixed cutting equipment, illustrated schematically in the form of four boxes 62 for cutting the jacket after it is attached to the sliding saddle. The function of the sliding saddle 52 and cutting equipment 62 in the utilization of the transporter is to remove the platform jacket, which function is not an object of this patent application.

In addition, and not illustrated, the transporter comprises piping, valves, pumps with motors and control equipment for performing the ballasting/deballasting. The ballasting/deballasting are remotely controlled by means of not shown communication equipment. Alternatively, the ballasting/deballasting are carried out by an umbilical which contains both pressurised water for ballasting and pressurised air for deballasting, together with pressurised fluid for operation and control of the valves.

When transporting the transporter, which is implemented with tugs, it is advantageous to deballast the transporter, thus ensuring that it offers the least possible motional resistance. In most cases this will mean that it is advantageous to place the transporter in a position where it is located with the underside 3 down, as high as possible in the sea.

In a preferred embodiment, not shown, the underside 3 between the lower longitudinal pontoons 9 is arranged displaced towards the transporter's central area. The result of this is that the underside is at least partly located above the sea when the transporter is lying deballasted in the sea with the lower longitudinal pontoons down, thus causing the underside during transport to offer little or no resistance against the motion in the sea.

Figs. 2a-e illustrate a sequence where a transporter according to the invention removes a platform topsides from an associated jacket, and transfers the topsides to a barge.

Fig. 2a illustrates the transporter 1 according to the invention lying horizontally in the sea 18, beside a platform topsides 16 which is standing on a jacket 17 which in turn is standing on the seabed 19. The transporter is ballasted and lying with the underside 3 with the recess 11 and the lower longitudinal pontoons 9 under water. The long sides 2 are moved on to the outside of the platform topsides 16, with the lifting beams 63 under the platform topsides, with the result that the jacket 17 is located in the underside's recess 11.

The topsides 16 is transferred to the transporter by deballasting the transporter so that it is raised and the contact sections 63 come into abutment against the topsides's underside. When the transporter is further deballasted the weight of the topsides is transferred from the jacket to the lifting beams, with the result that the transporter lifts the topsides off the jacket.

In fig. 2b the transporter 1 with the topsides 16 are removed from the jacket 17. The transporter is ballasted in such a manner that it is lying partly submerged. The projecting sections 12 of the pontoons, see fig. 1, are, however, deballasted in their position under the surface, providing buoyancy which supplies an aligning moment, counteracting the weight of the topsides and helping to keep the transporter horizontal.

Fig. 2c illustrates the transporter completely removed from the jacket. A barge 75 is lying in the sea in front of the transporter. The barge is illustrated here as a pipe structure, but may per se be of any type.

In fig. 2d the barge has been moved in under the topsides, and the transporter is ballasted in such a manner that the topsides has been lowered on to the barge. The transporter's contact sections are thereby released from the topsides's underside.

In fig. 2e the barge with the topsides have been removed from the transporter for further transport of the topsides.

Fig. 3 illustrates a preferred embodiment of the transporter in connection with the design of the contact sections and the transfer of the forces between the topsides and the transporter. Support beams 76 extend here under the platform topsides, and secured thereto, and are arranged in such a manner that, when they abut against the lifting beams, they remain with their ends on

the lifting beams. Between the support beams 76 and the contact sections in the form of lifting beams 63 there are provided shock-absorbing elements 80 for cushioning of impacts between the topsides and the transporter. It is also possible to use support beams without employing shock-absorbing elements.

In this case the support beams are brought into abutment directly against the lifting beams.

The shock-absorbing elements 80 may be of any type, and may also be employed when the platform topsides is lifted without support beams. In this case the shock-absorbing elements 80 are placed directly between the lifting beams 63 and the topsides.

Fig. 4 illustrates a preferred embodiment of the transporter, viewed from one of the short sides, in connection with the barge 75.

Here, in areas under the contact sections 63, the long sides 2 are provided with guide rails 77 which extend in the direction perpendicular to the underside 3, i. e. vertically when the transporter is located with the underside 3 horizontally down in the sea 18. In a preferred embodiment of the method according to the invention, slide blocks 81 on the barge 75 are moved into the vertical guides 77, with the result that the barge can only move vertically relative to the transporter. Thus when transferring the topsides 16 from the transporter to the barge a highly advantageous control is achieved of the relative motion between the barge and the topsides. After the transfer of the topsides to the barge, the barge is released from the vertical guides, thus enabling it to freely sail away.

Further advantageous embodiments and applications of the transporter, associated with the removal of a jacket for a platform topsides and transport of other heavy objects at sea, are disclosed and described in the applicant's simultaneously submitted Norwegian patent applications Nos. 97 3561 and 97 3563.