EP0726987A1

EP0726987A1 - A method and vessel for removal of platforms at sea

Info

Publication number: EP0726987A1
Application number: EP94904016A
Authority: EP
Inventors: Brian Stead; Ove Tobias Gudmestad
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 1993-11-22
Filing date: 1993-11-22
Publication date: 1996-08-21
Also published as: AU5823794A; WO1995014824A1

Abstract

This invention relates to a vessel for removing offshore platform constructions characterized in that it comprises a main part (1) having an essentially U-shaped horizontal shape, a foundation (4) being located on the deck of the main part (1), another per se independent smaller part (2) adapted to be drawn into the outer part of the U-shaped main part (1) and fastened therein to form a unitary sea-going vessel. The invention also relates to a method for removing platform constructions at sea using a vessel according to the invention characterized in that the vessel is transported and anchored in the vicinity of the platform to be removed, the vessel parts (1 and 2) are temporarily separated and the U-shaped part of the main part (1) is positioned around the platform underbody, the second part (2) is then put back in place in the outer part of the U-shaped part of the main part, and fastened there, the cross beams (5) are moved under the platform construction close to the main beams of the underbody for support against the platform deck and fastened thereto and to the foundation parts (4a, 4b), the platform underbody is fastened to the vessel by means of previously installed wire (6), the vessel is deballasted, the platform legs are cut at a level underneath the vessel, the vessel with the platform is then towed to a suitable dismantling yard.

Description

A method and vessel for removal of platforms at sea.

This invention relates to a vessel for removing platform constructions at sea, as well as a method for achieving this. Removing platforms at sea has been increasingly actuali- zed as more and more oilfields are emptied and stop produc¬ ing. For the sake of the enviroment it will be necessary to remove the deck constructions of the platforms. In addition considerations to the shipping traffic indicate that in most cases the platform underbodies must be removed. Also inter- national rules are being prepared demanding that platform underbodies must be removed at least down to a depth of 55 meters below sea level.

Existing technology for removing platforms at sea are based upon the use of lifting vessels. Most lifting vessels are limited to smaller lifts. However there exist a few large crane vessels capable of handling heavy lifts at sea, but these large cranes are expensive : \se . There is there¬ fore a need for improved technology fc removing platforms at sea. This is especially important in the North Sea where operations like platform removal must be stopped for long periods because of the weather conditions like wind and large waves.

The Dutch company Allseas Engineering BV has proposed a method where two hulls are connected so as to enable moving in towards the platform and lifting the deck construction off. Allseas Engineering BV in the Norwegian patent applica¬ tion nr. 89.3941 has also proposed using a heave compensating construction developed by them to ensure that the operations are possible in small waves. In addition the above mentioned company has the Nor¬ wegian patent nr 169.731 relating to a method and arrange: nt for manouvering a superstructure element in relation to a permanent construction in water. This may have things in common with this invention, but the patent publication is primarily concerned with the installation or building of a platform or similar constructions. This known method and arrangement are however very complicated in that at least one floating body shall be in a liquid reservoir. This is unnec- essarily expensive when regarding platform removal, but may be necessary when installing a superstructure element. In addition, large forces will be involved in this element when the said vessel elements move in relation to each other. Problems with such forces will also be present when the installation described in the patent publication involves a simple U-shaped vessel.

GB 2.231.843 concerns a unit for removing platforms comprised by a number of foating bodies adapted to be put together at sea, preferrably under water. This is an extr- e ly complicated method. The unit is also not suitable for transportation of the platform to the shore. When the plat¬ form is released it must be transferred to another vehicle for transportation. Indeed it is described that the demount- ing of platform- and tower legs are cut and the floating bodies are simultaneously deballasted. As will be shown below this invention provides a stable vessel for this opera¬ tion.

Other examples of more or less known techniques of interest are found in the British patent publications nr.

2.083.112, 2.084.228, 2.156.286, 2.165.188 og 2.168.293, as well as Norwegian Patent nr. 160.424.

The object og this invention is to provide a solution which in simple and inexpensive manner can be used for the removing of platforms at sea. This is done using a simple vessel construction that can be used for executing the comp¬ lete operation, both the removing of a platform with legs, e.g. down to a depth of 55 meters below sealevel, and the following transportation of the hole unit in a safe way to the shore, so that dismanteling and dividing may be done in a protected enviroment.

This is obtained by a vessel characterized in that it comprises a main part, having an essentially U-shaped hori¬ zontal shape, a foundation being located on the deck of the main part, another per se independant part shaped as a wedge both in the horizontal and vertical direction, adapted to be drawn into the outer portion of the U-shaped main part and fastened, to form a unitary seagoing vessel. On each side of the inner U-shaped part of the main part there is preferrably mounted a combined foundation/ballast tank. The foundation supports movable crossbeams, capable of being pushed under the platform construction for optimized support of the platform deck. The foundation with ballast tanks also gives the vessel ekstra buoyancy.

The present invention differs significantly from the previously proposed solutions and represents in addition a lot of advantages when compared to them. The present invention will demand far less in investment kosts than the known solutions. Moreover the possibility that tensions develop in the platform deck on removal (cata¬ maran effect) will be redused since the platform is supported midships. During the platform transportation the mentioned catama¬ ran effect will be almost eliminated because of the shape of the vessel.

The present invention also concerns a method for remov¬ ing platform constructions at sea using a vessel as described in the claims characterized in that the vessel is transported and anchored in the vicinity of the platform to be removed, the vessel parts are temporarily separated and the U-shaped part of the main part is positioned around the platform underbody the second part is then put back in place in the outer portion of the U-shaped part of the main part, and fastened there, the cross beams are positioned under the platform construction close to the main girders of the under¬ body for support against the platform deck and fastened to it and the foundation parts, the platform underbody is fastened to the vessel by means of previously installed wire, the vessel is deballasted, the platform legs are cut at a level under the vessel, the vessel with the platform is then towed to a suitable dismantling yard.

The invention will in the following be descibed with reference to the enclosed drawings where:

Fig. la) shows a schematic drawing of the vessel with a platform deck structure and underbody fastened to it. Fig. lb) shows the same arrangement as seen from above. Fig. 2a) shows a horizontal sectional view of the main part moved in position around the platform and anchored. Fig. 2b) shows a horizontal sectional view of the part of the vessel meant to be pulled into the outer U- shaped part of the main part of the towing vessels.

Fig. 2c) shows a sectional elevation through the hull of the vessels in the moment they are connected. Fig. 3 shows a part of a platform, where the level of the cutting is shown. Fig. 4 shows a preferred embodiment for dismanteling.

In the following the vessel will be described in detail. Moreover the method according to the invention will be pres¬ ented with reference to the mentioned vessel.

The vessel shown in the figures la,1b comprises two hull parts, a main hull 1 and a smaller hull 2. The main hull has an essentially U-shaped horizontal section, and will with its dominant size give the largest contribution to buoyancy. The main hull is preferrably made in steel, but other materials such as concrete, may be used. The main hull may comprise different types of equipment and machinery for operating them (winches, windlass, chains and ballast tanks) .

The smaller part of the hull 2 is in itself independant adapted to be pulled into the outer portion of the U-shaped part of the main hull 15 such that the hull parts 1,2 thereby form a seagoing vessel with great strength and buoyancy. The pulling of the hull part 2 may be performed using towing vessels 18. The two hull parts may be fastened to each other in different ways, e.g. by activating hydraulic units 3. On the deck of the main part there is placed a founda¬ tion 4. In a preferred embodiment the foundation 4 is com¬ prised by two foundation parts positioned on each side of the inner area of the U-shaped part of the main hull. The foun¬ dation parts 4a,4b comprises ballast tanks and are in a preferred embodiment made out of steel. The height and length of the foundation are adapted to the normal size and distance of the platforms to be removed. The foundation also supports movable crossbeams 5, that can be pushed under the platform deck construction for opti¬ mal support of the platform deck.

When removing platforms the crossbeams will be built or rebuilt to be adapted to the specific platform, to admit optimum support of the platform deck and to admit a simple temporary fastening. The crossbeams may be stored on one of the foundations when not in use.

By a method for removing platforms according to this invention the vessel is first transported out to, and an¬ chored in the vicinity of a platform 10 to be removed, in anticipation of an acceptable wheather window. By a given wheather window the operation is started by temporarily separating the hull part 2 from the main hull 1. This is done by loosening wires/chains held e.g. by hydraulic units. After this part 2 is ballasted to lower the deck in relation to the main deck, and then pulled out.

The main hull part l is then manoeuvered in position around the platform underbody and drawn in place with the windlasses 9. The hull part 2 is then pulled back into place in the outer portion of the U-shaped part of the main hull, with a lower freeboard than hull part 1, as illustrated by figure 2c. Then part 2 is deballasted until it gains a positiv buoyancy in relation to the main part of the hull. The hull parts are secured to each other by tightening the wire/chain with the hydraulicaly operated units 3.

At the same time as hull part 2 is manoeuvered in place the crossbeams 5 are pushed between the legs of the underbody so as all the crossbeams rest on the foundation parts 4a,b. The crossbeams are then securely fastened to both the founda¬ tion parts 4a,b and the platform deck by using techniques that are known per se. To hinder damages to the platform deck by vertical movement of the vessel the beams 5 may be provided with compensators/strong springs 11. The wires 6 are fastened to the platform underbody in advance, a distance above the point where the platform is cut. The wires are taken on board the ship and tightened with winch 7. The vessel is then deballasted to take a previously calculated weight, representing a part of the weight of the platform deck and underbody down to a depth of e.g. 55 met¬ ers, which is the international requirement for free sailing depth. Hereby the beams 5 are resting up under the platform deck and the underbody is in addition held by the wires 6. After this a fast deballastation of the remaining wateramount is performed in such a way that the total outlet amount corresponds to the net weight of the platform deck and under- body down to a depth of 55 meters. The fast deballastation can be obtained by quickly opening the valves/sluices in the foundations/ballast tanks 4.

At the same time or within a short periode after the fast deballastation the platform legs 20 are cut at a level below the vessel. Thereby one almost eliminates unwanted vertical movements in the vessel. In a preferred embodiment the legs are cut by explosion, with charges positioned in such a way that a part 21 of some meters is blown away, and out from its original position in the underbody. The advantage with this fast deballastation described here is related to the fact that the platform legs are put under pressure during this operation. This happens partially to avoid collision between platform legs and parts of legs remaining after the cutting. The last part of the deballast- ation is done fast to avoid the legs being subjected to stress for a long periode of time before they are cut.

The vessel will now float with platform deck and under¬ body down to a depth of for exampel 55 meters, and the plat¬ form deck and underbody are ready to be towed to a suitable dismantling yard in protected waters.

The vessel with the platform is then taken to a land establishement with special facilities for dismanteling platforms, and all dismanteling is thereby move ashore.

When dismantling a platform that has been removed accor- ding to the above mentioned method, the main part is first anchored in front to underwater supports 30 installed befor- hand. Then the hull part 2 is removed and the main part 1 is pulled forward with anchors and lines fastened to land until the underbody 10 legs are positioned above the foundation supports 30. Then part 1 is ballasted such that the legs 20 are lowered down into the supports and fastened by filling cavities in the supports 30 with e.g. concrete. Part 1 is then ballasted until the whole platform weight is removed and the main hull may be pulled away for preparation before the next commision.

Claims

C L A I S

1. Vessel for removing offshore platform constructions c h a r a c t e r i z e d in comprising:

A main part (1) , having an essentially U-shaped horizon¬ tal shape, a foundation (4) being located on the deck of the main part (1), another per se independent smaller part (2) , shaped as a wedge both in the horizontal and vertical direction, adapted to be pulled into the outer part of the U-shaped main part and fastened therein, to form a unitary seagoing vessel.

2. Vessel according to claim 1, c h a r a c t e r i z e d in that the foundation (4) is formed by two parts (4a,b) located on either side of the U- shaped part of the deck of the main part (1) .

3. Vessel according to claim 1-2, c h a r a c t e r i z e d in that the foundation (4) comp¬ rises ballast tanks.

4. Vessel according to claim 1-3, c h a r a c t e r i z e d in that the foundation (4) forms a support for movable cross beams (5) , for engagement against the platform deck.

5. Vessel according to claim 4, c h a r a c t e r i z e d in that the movable cross beams (5) are provided with compensators/strong springs (11) .

6. Method for removing offshore platform constructions using a vessel as stated in claims 1-5, c h a r a c t e r i z e d in that a) the vessel is transported and anchored in the vicinity of a platform to be removed. b) the vessel parts (1) and (2) are temporarily separated and the U-shaped part of the main part (1) is positioned around the platform underbody c) the second part (2) is then put back in place in the outer portion of the U-shaped part of the main part, and fastened there, d) the cross beams (5) are pushed under the platform const¬ ruction close to the main beams of the underbody for support against the platform deck and fastened to it and the foundation parts (4a,4b), e) the platform underbody is fastened to the vessel by means of previously installed wire (6) , f) the vessel is deballasted, g) the platform legs (20) are cut at a level underneath the vessel, h) the vessel with the platform is then towed to a suitable dismantling yard.

7. Method according to claim 6, c h a r a c t e r i z e d in that preferrably a last part of the deballasting is performed fast by quickly opening valv¬ es/sluices in the foundation (4) .

8. Method according to claim 7, c h a r a c t e r i z e d in that the platform legs (20) are cut at the same time as or immediately after the fast debal¬ lasting step.