A METHOD AND VESSEL FOR REMOVING OFFSHORE STRUCTURES

摘要

A method for removing or installing an offshore jacket structure (15) in a body of water utilises a vessel (1) having a generally planar main buoyancy section (2) having in plan view substantially the outline of a delta with an extension (4, 5) at the apex and auxiliary buoyancy sections (3, 8) extending transversely of the main buoyancy section at the ends of the base of the delta. The bottom parts (8) of the auxiliary buoyancy sections (3) are pro-vided with a rounded outer surface (11) and contain heavy fixed ballast (12). By suitable ballasting the vessel (1) may be rotated so that the main section (2) forms an angle less than 90 degrees with the water surface (17) as it is brought close to the jacket structure with the auxiliary sections (3) straddling the jacket structure. In this position the vessel is rotated towards the jacket structure while its rounded bottom portion (11) rolls on the seabed (16), thus permitting good control of the movements of the vessel in a critical phase of the removal process. Once the jacket structure (15) has been connected securely to the vessel (1), the vessel is rotated back to the initial position while still in contact with the seabed by de-ballasting the auxiliary sections (3) before it is brought to the surface. The vessel is made from stiffened flat steel plates and can therefore be efficiently manufactured in commonly equipped shipyards.