Decommissioning of offshore oil and gas installations

Abstract

The offshore oil and gas industry had its beginnings in the Gulf of Mexico in 1947. The first offshore development used a multipiled steel jacket to support the topside production facilities, a design which has since been used extensively. Now there are more than 7000 drilling and production platforms located on the Continental Shelves of 53 countries [1]. Some of these structures have been installed in areas of deep water and treacherous climates, and consequently structure designs have adapted to withstand the environmental conditions of these areas. Some typical designs are shown in Figs 5.1-5.5. In the North Sea, which is an area that experiences some extreme environmental conditions, more than 200 structures have been installed, about 25% of which are in water depths greater than 75 m and can be exposed to maximum storm wave heights of 30 m. This combination of deep waters and extreme storm forces dictates large structures, some with component weights that exceed 50 000 tonnes [6]. One of the world’s largest gravity base structures (GBS) is scheduled to be installed off the coast of Canada in 1996. It is designed to withstand impacts by icebergs and will weigh approximately 1.5 million tonnes including ballast [7]. Now, as oil and gas fields begin to deplete their reserves, the concern has turned to the removal and disposal of these structures at the end of their producing lives. Estimates indicate that the cost of some removals may exceed the cost of the original installation. The structures located on the Norwegian Continental Shelf contain only 1% of the world’s offshore structures, but will account for nearly 20% of the worldwide removal costs [4]. Innovative removal and disposal techniques must be developed to limit costs and minimize the impact on the environment.