In connection with the removal of discarded oil and gas installations offshore many different challenges-economical as well as technical-are encountered.

The removal of such installations represents a large expenditure for the oil companies and it is therefore of great importance that operations of this kind can be accomplished as fast and effectively as possible. Generally working at sea is more difficult and more expensive and the aim is therefore to have as much as possible of the disassembling done ashore.

The most likely way of removing several installations is by a specially designed lifting vessel which has the ability to both lift and float decks as well as jackets (platform substructures) to the mainland for further dismounting and destruction.

By for instance removal of a platform fixed to the seabed with substructure of steel frame (so called"jacket") it is assumed that this could be done with only two lifting/floating operations. First the deck is cut away from the substructure (with continuous support from the lifting vessel) and is then floated ashore where it can be dismounted faster and safer than what would be the case at sea.

Thereafter the same is done for the whole of or parts of the substructure.

The said method for removing a platform deck gives big economic gains.

However, in some cases it is necessary that the deck is reinforced to be able to bear the lifting and the floating to the shore. The lifting vessel is flexible as to size and weight of the deck, but in some cases the vessel will not offer a support as good as the natural substructure did. Because the gain by lifting/floating the deck

in one operation is so high, it is a desire in these cases to reinforce the deck to rearrange the forces out to the support points of the lifting vessel instead of disassemble/unload the deck at sea and float this to land in several operations.

The far greatest number of deck structures consist of a framework of steel.

One way of reinforcement of such a structure is of course mounting (welding) new elements on the existing. This invention is an alternative to this solution.

An object of this invention is to give cost savings and time savings compared to previous solutions as for instance installing new steel elements.

Another object of the method is that it should be flexible with regard to geometry and loads on the structure which shall be reinforced.

The objects of this invention are achieved by a method for reinforcement of an offshore installation as indicated in the introduction of the description, and which is characterised by that the forces from the lacking support points are transferred to the other support points by means of pre-tensioning cables which are cast into concrete and are tensioned after the curing of the concrete.

Preferred embodiments of the method are further more thoroughly explained in claims 2 and 3.

The invention will now be further explained with reference to the enclosed figure which shows an embodiment of the present invention.

Referring to the figure, an offshore installation (structure) 1, which can be a platform deck construction, is shown with three supports (support points) 2,3,4.

The natural support point 2 for the deck construction cannot be used when the deck is being lifted and transported. Consequently, the forces from the lacking support point 2 have to be transferred to the other support points 3,4. This is done by placing a formwork 7 in the form of plastic or steel pipe with thin walls extending from the fastening point of support 3 to the deck structure 1 and to the lower area of the support 2 and further to the fastening point of the support 4 to the deck structure 1. One or more pre-tensioning cables 5 are centrally introduced into the formwork cavity and fastened in the fastening area of the support 3,4 to the deck structure. The fastening of the pre-tensioning cable 5 in these areas is adjustable so that the degree of pretensioning the cable can be adjusted.

Concrete 6 is poured (filled) into the formwork 7 and thus encompasses pre- tensioning cable 5. After the curing of the concrete 6 the cable 5 is tightened in each end, so that the concrete 6 is pressurised. Tension in the cable 5 is achieved

in this way and compression in the surrounding concrete 6 is achieved. During loading the tensioning force in the tension cable 5 will be further increased, while the compressive force in the concrete 6 is reduced. After loading the degree, of pre-tensioning still can be adjusted, although an adjustment of the pre-tension after the cables have been definitely tightened (100%) normally is not done. One point here is that variation of the force gives very small variation of tension in the pre-tensioning cables. Almost all variation takes place in the concrete (as long as the concrete is under pressure). This is, however, known in all pre-tensioned structures.

Preferably the formwork pipes can be steel pipes with thin walls which are resistive to expansion of the diameter when the concrete is pressurised. This gives a 3-dimensional tension in the concrete and consequently a higher design tension, which again leads to a smaller cross section.

The pre-tensioned method will give savings compared to mounting/welding in new elements by the fact that the pre-tensioning force will give support for a lift, which causes less deflections in the platform deck. The need for reinforcement of the actual deck will be reduced. This has been demonstrated in an analysis which is briefly mentioned below.

The principle of pre-tensioning of concrete structures is old and well tested.

This is often used in connection with bridges and other structures where deflection generally would be a problem. The difference of this invention from traditional pretensioning is that it is not an integrated part of the original structure, but is installed as a reinforcing element afterwards to make up for a structure problem.

Analysis of the invention have been carried out in connection with reinforcement of a deck on an offshore installation, as mentioned above. The enclosed figure is a simple principal drawing showing how the invention functions in the computed case. The figure illustrates the storage of the deck structure 1 on a lifting vessel on two support points 3,4. On the middle of the deck on the other hand a natural support point 2 has been removed and thus the deck has to be reinforced.

The figure explains the static manner of operation of the pre-tensioning cable 5. A pre-tensioning cable or a pre-tensioning bar is a very simple structure consisting of a cable of steel having high tensile (stretch) capacity and which is cast into concrete. As mentioned above the invention has a large degree of

flexibility with regard to load, since the level of tension can be adjusted to the load on the deck. Theoretically the most ideal tensioning force is the force which together with the load gives a tension level equal to zero in the concrete.

Referring to the figure the process according to the invention will be carried out step by step as follows : 1. Mounting of formwork pipes 7 and cables 5 2. Concrete 8 is poured into the formwork pipe 7 at the break point 3. The concrete 8 is poured for 1 day/night 4. The cables 5 are pre-tensioned to some degree 5. The rest of the formwork pipes 7 are cast out 6. The concrete 6 is cured for 3 days and nights 7. The cables 5 are tensioned up to 100% Total time for mounting is estimated to be 5-6 days.