NL2005736C2 - Method to demolish a metal structure positioned on or connected to the seabed. - Google Patents

Description

METHOD TO DEMOLISH A METAL STRUCTURE POSITIONED ON OR CONNECTED TO THE SEABED

The invention is directed to a method to demolish a metal 5 structure positioned on or connected to the seabed and extending partly above the sea level, wherein the part of the structure extending above the sea level comprises at least a crane support. The invention is also directed to the metal structure itself.

Metal structures positioned on or connected to the seabed and 10 extending partly above the sea level are for example oil or gas production or drilling platforms and the like. At the end of their commercial life such structures have to be removed. Such removal is also referred to as decommissioning. Various methods have been developed for this purpose. For example publications like EP-A-15 2070813, FR-A-2921894, WO-A-2009090321 and WO-A-2005123499 describe methods wherein the entire or at least the top part of the structure is disconnected as one piece and transported to shore. At a location on shore the structure is subsequently demolished. A disadvantage of these methods is that dedicated vessels have to be 20 developed and build in order to lift and transport each structure.

There is a desire to demolish such structures in a more simple manner.

This is achieved by means of the following method. Method to demolish a metal structure positioned on or connected to the seabed 25 and extending partly above the sea level, wherein the part of the structure extending above the sea level comprises at least a crane support by performing the following steps: (i) placing a moveable demolition unit suited to cut through metal objects on the crane support, 30 (ii) using the demolition unit to cut away parts of the structure.

The invention is also directed to a metal structure positioned on or connected to the seabed and extending partly above the sea level, wherein the part of the structure extending above the sea level 2 comprises a crane support and a moveable demolition unit placed on said support which demolition unit is suited to cut through metal..

Applicants have found that by using the existing crane support on the metal structure demolition can be performed at the location of the 5 metal structure in a more easy and simple manner. Moving the structure or large parts of the structure to a location on shore is thus avoided. It has been found that the crane support on off-shore metal structures have such a design that they can carry a heavy and and suited demolition unit as will be described in detail below.

10 The metal structure positioned on or connected to the seabed and extending partly above the sea level may be an oil or gas production or drilling platform or any other off-shore installation comprising a crane support, which is, for example, at its end of its economical lifetime and needs to be decommissioned. The platform may be of the so-called 15 tension leg, SPAR, compliant tower, or fixed type platforms. The metal structures comprises at least a crane support. If a crane is positioned on the crane support the crane is removed from said crane support prior to performing step (i). The metal structure may even comprise a second crane support and respective crane. Such cranes are 20 sometimes referred to as hoisting cranes. Examples of a crane and crane support is a derrick crane as described in EP-A-077638. Other possible crane types are fixed boom cranes or lattice boom cranes.

The crane support is an integral part of the metal structure. Examples of platforms are described in WO-A-9520074, EP-A-0039590 and GB-25 A-2203782.

If a crane is still positioned on the crane support of step (i) it is preferably removed using a crane as present on a barge alongside near the metal structure. The barge is preferably fixed alongside by means of jack up legs to i.e. the sea floor, i.e. a jack-up barge. The 30 barge may also be a vessel as described in WO-A-2010114359, wherein the motion of the waves are compensated for.

In step (i) a moveable demolition unit suited to cut (shear) through metal objects is placed on the crane support. The demolition unit suitably comprises a base which fits on the crane support, a moveable 3 arm and a head equipped to cut metal. By moveable demolition unit is meant that the arm is moveable such that the head can reach the various parts of the structure to be demolished. The base of the demolition unit is made such that it fits on the crane support. Such a 5 design will depend on the design of the existing crane support and its manufacture is within the skills of the skilled person. The moveable arm is suitably a boom which can move using hydraulics and which is of a dimension such that the head can reach the various parts of the structure which is to be demolished. The boom may comprise of 10 between 1 to 4 interconnected elements which are moveable relative to each other. The head suitably comprises of a lower and upper jaw, suitably powered by hydraulics. Typically a hydraulic actuating member is connected to at least one of the jaws, and actuation of the hydraulic actuator causes the first and second jaws to open and close, and to 15 engage in interleaving and shearing relationship. Each of the jaws may for example have a shear knife attached along an inner edge and guide plates similarly attached for permitting the jaws to interleave in close, shearing relationship.

In principle every type of demolition unit may be used in the 20 present invention which is capable of shearing the structure. Examples of possible shear heads and arm combinations are described in US-A-20050115076, EP-A-1565629, EP-A-1546473, US-A-6298560, EP-A-1239965, EP-A-1218600, US-A-5992023, EP-A-1133603, US-A-5926958, US-A-5894666, US-A-5860214, EP-A-751260 and EP-A-25 754820.

In step (ii) the demolition unit is used to cut away parts of the metal structure. In step (ii) also other non-metal parts of the structure are suitably cut using the demolition unit. Two or more different heads of said unit may be used to accommodate for different parts of the 30 structure. It can for example be advantageous to use a heavy duty shearing head for the larger parts of the structure and use a smaller shearing head for the parts having a smaller diameter of the structure. Shearing heads for different materials may also be used. For example 4 special shearing heads adapted to cut reinforced concrete may be used in alternating use with a shearing head adapted to cut metal.

The loose parts which are cut or sheared from the structure in step (ii), also referred to as scrap, are preferably collected and stored in 5 a container. If the structure to be demolished is equipped with at least two crane supports it is preferred to use such a crane, positioned on the respective second crane support, to move the cut away parts obtained in step (ii). This crane is suitably equipped with a grab. This grab is used to collect scrap and put it in containers which are placed 10 for example on the deck of the structure. The containers are subsequently put on a transport barge floating near the structure for transport to shore. Moving the containers may be performed by the crane on the second crane support, by exchanging the grab for a hook, or suitably by a crane positioned on barge alongside the metal 15 structure. This barge may be a barge as described above.

Large parts of the structure, like for example the heli-deck, if present, may be cut loose and removed as a whole.

Step (ii) is continued until the crane support on which the demolition unit is placed looses its integrity. At that moment the 20 demolition unit is removed from the crane support and demolition of the remaining parts of the structure, both above and under the sea level, is preferably continued in a step (iii) using a demolition unit placed on a barge, preferably fixed to the sea floor by means of jack-up legs or positioned on a vessel as described in WO-A-2010114359 . The 25 demolition unit as used in step (ii) may be used in step (iii). Because the remaining structure in step (iii) is typically of a larger dimension a more heavy demolition unit is suitably used in step (iii).

The invention will be illustrated making use of the following figures.

30 Figure 1 shows an oil production platform 1 as the metal structure provided with a support structure 2, a deck 3, a derrick 4 and a first and second crane 5 and 6 positioned on respective crane supports 7 and 8. The platform 1 extends partly above sea level 9 and is positioned on sea bed 10.

5

Figure 2 shows the same platform 1 and a barge 11 fixed to sea bed 10 by means of a jack-up legs 12. Barge 11 is provided with a crane 13 which is used to remove crane 5 from its support 7. Subsequently crane 13 is used to place a demolition unit 14, as shown 5 in Figure 3, on crane support 7.

Figure 3 shows platform 1 with a demolition unit 14 positioned on crane support 7. Demolition unit 14 has a base 20 provided with space for the operator of said unit, a boom 17 and a shear head 18. Figure 3 also shows crane 6 provided with a grab 19 which can collect the scrap 10 in container 15. Container 15 when filled with scrap are placed on transport barge 16.

Claims (9)

A method for demolishing a metal structure positioned on, or connected to, the sea floor and extending partially above the sea level, the part of the structure extending above the sea level comprising at least one crane support by perform the following steps: i. placing on the crane support a movable demolition unit suitable for cutting through metal objects, ii. using the demolition unit to cut away parts of the structure. The method of claim 1, wherein the structure comprises a crane on the crane support, and wherein, prior to step i., The crane is removed from the crane support. 3. Method as claimed in any of the claims 1-2, wherein a tap 20 is used to adjust the in step ii. moving cut away parts. 4. Method as claimed in claim 3, wherein the crane is a crane that is positioned on a crane support, wherein the crane support forms part of the metal structure. A method according to any one of claims 1-4, wherein in a subsequent step iii. the metal structure is demolished by using a demolition unit which is placed on a pontoon or on a trawl. 6. Method as claimed in any of the claims 1-5, wherein the demolition unit is provided with a base, with a movable arm, and with a head which is suitable or equipped for cutting metal. 7. Metal structure positioned on or connected to the seabed and partially extending above sea level, the part of the structure extending above sea level comprising at least one crane support as well as a movable demolition unit placed on the support, and wherein the demolition unit is suitable for cutting through metal. A metal structure according to claim 7, wherein the demolition unit is provided with a base, with a movable arm, and with a head suitable or equipped to cut metal. 9. Metal structure as claimed in claim 8, wherein the head is provided with a lower and an upper claw which are hydraulically driven, wherein a hydraulic actuator is connected to at least one of the claws, and wherein the activation of the hydraulic actuator ensures that the first and second jaws open and close relative to each other, and thus perform a cooperating and cutting movement with each other.