There is a great need in the profession for such a method by means of which an existing, obsolete drilling island or production platform can be dismantled in a cost- efficient and environmentally sound manner. The objective of the invention is to provide such a method. To this end the method according to the invention is characterized in that use is made of a floating container, boat or vessel, by which adjustable jack-up legs are shipped out to the drilling island or production platform to be dismantled, in that the jack-up legs are fastened to the construction and positioned such that they substantially support the construction, in that the supporting structure is uncou- pled from the construction and at least an upper part of the supporting structure is removed, in that the jack-up legs are subsequently adjusted in such a manner that the construction is lowered and comes to rest on the floating container, boat or vessel or on a floating body, and in that subsequently the lower portion of the supporting structure resting on the seabed is removed.

This provides a very elegant method of disman- tling an obsolete drilling island or production platform or the like that is practical and simple to execute.

Thanks to the simplicity of the method it can also be car- ried out at very low costs, while safeguarding the envi- ronment in all respects. With the method according to the invention, the marine environment stays indeed absolutely free from remnants of the obsolete drilling island or pro- duction platform.

For an effective execution of the method accord- ing to the invention it is desirable for the adjustable jack-up legs chosen to be of the type comprised of leg elements and guide members to be welded to the construc- tion, the guide members being equipped for moving along the leg elements.

In addition to the floating container, boat or vessel by means of which the adjustable jack-up legs are shipped, it is also possible to use other vessels or the like in the method according to the invention, for exam- ple, for the removal of remnants from the drilling island or production platform, or which may serve as carrier for a crane or the like with which the parts of the drilling island or production platform are removed.

However, according to the invention it is pre- ferred for the floating container, boat or vessel to be embodied with a crane by which means the upper part of the supporting structure is removed, and that this upper part is placed upon the floating container, boat or vessel by means of the crane.

To further improve the efficiency of the method according to the invention, the floating container, boat or vessel is positioned between the jack-up legs and under the construction before the jack-up legs are adjusted for the lowering of the construction, subsequent to which the construction is lowered until it rests upon the floating container, boat or vessel. Thus the floating container, boat or vessel not only fulfils the task of shipping the jack-up legs, but serves also to transport the parts of the drilling island or production platform to be disman- tled. The method according to the invention can thus be carried out economically with the very same floating con- tainer, boat or vessel.

It is also desirable if once the construction rests on the floating container, boat or vessel, the jack- up legs are lifted from the seabed.

The method according to the invention may advan- tageously further be realized such that the lower portion

of the supporting structure is lifted from the seabed and placed on the floating container, boat or vessel by means of the crane.

Hereinbelow the invention will be further eluci- dated by means of a non-limiting exemplary embodiment with reference to the drawings. In the figures 1 to 12 the drawings show the method according to the invention in various stages of execution.

Similar parts in the figures are indicated by identical reference numbers.

Referring first to figure 1, an example is shown of a drilling platform 1 that is to be dismantled. Said drilling platform 1 is provided with a construction 3 ris- ing above the surface of the sea 2, and a supporting structure 4 with legs 5 supporting the construction 3 and resting on the seabed 6. The method for the demounting of the drilling platform uses, for example, a floating con- tainer 7, by means of which adjustable jack-up legs 8 are shipped to the drilling island 1 that is to be dismantled.

Figures 2 to 4 show that after the jack-up legs 8 have been delivered by the floating container, they are fastened at several sides of the drilling island 1 to the construction 3.

Figure 5 then shows the situation where the jack-up legs 8 have been positioned such that they sub- stantially support the construction 3, in order to take the weight off the supporting structure 4 thereunder. Sub- sequently, at this stage the supporting structure 4 may be uncoupled from the construction 3 allowing at least an up- per portion 9 to be removed from the supporting structure 4, as shown in figure 6. The removal of the upper portion 9 occurs with the aid of a crane 10 mounted on the float- ing container 7. Said crane 10 is then operated such as to place the upper portion 9 from the supporting structure 4 onto the floating container 7, as shown in figure 7. With respect to the jack-up legs 8, it is remarked that these are preferably of the type comprising leg elements 8' (see figure 3) and guide members, for example, shafts 8''that

are equipped for movement along the leg elements 8', that can be welded to the construction 3. This is a simple and reliable manner for the vertical adjustment of the con- struction 3.

Further, with reference to figure 8, it is shown that prior to the adjustment of the jack-up legs 8 for lowering the construction 3, the floating container 7 is positioned between the jack-up legs 8 and under the con- struction 3, subsequent to which the construction 3 is lowered until it rests upon the floating container 7, as shown in figure 9.

Figure 10 subsequently shows that the jack-up legs 8,8'are lifted from the seabed 6, such that the construction 3 with the adjustable jack-up legs 8 coupled thereto, rests entirely on the floating container 7.

Figure 11, subsequently shows that the crane 10 of the floating container 7, is used for lifting the lower portion 11 of the supporting structure 4 from the seabed 6 on which it is still resting, after which it is in the fi- nal stage placed upon the floating container 7, as shown in figure 12. This completes the removal of the drilling island 1. Any further dismantling may be carried out later on shore.