Comparative Evaluation Of A Concrete And Steel Hull For A Deepwater Gulf Of Mexico Tension Leg Platform

Abstract

ABSTRACT This paper outlines the concept development and comparison of a steel hull and a concrete hull TLP designed for the same Gulf of Mexico (GOM) conditions in 3,000 ft. water depth. The global performance of the two TLP's is evaluated to the same level of detail using widely accepted analytical procedures. The global performance analysis and recent model tests confirm that the results are comparable and with no significant advantages nor disadvantages for the steel vs. the concrete alternative. Innovative construction and assembly methods for the concrete hull, specifically engineered for the inshore shallow water GOM conditions are highlighted. A cost efficient construction technique based on local construction has been developed for the concrete alternative. cost evaluation confirms that a concrete TLP is a competitive alternative to a steel hull TLP. INTRODUCTION The success of concrete gravity based structures in the North Sea is indisputable. Lately, there has been considerable industry interest in the use of concrete construction also for floating platform hulls. Several publications on concrete versus steel for floaters, and in particular, Tension Leg Platforms (TLP), have been presented. in general, the comparisons have been promising for the concrete alternatives. The previous studies comparing steel and concrete TLP hulls have, however focused on large TLP's. This will be the first published study of the use of concrete for a smaller "second generation" TLP, designed specifically for GOM construction. The results, of the extensive work on both steel and concrete TLP's will provide valuable information to a designer/operator to make an objective assessment of the relative merits of the two materials. The decisions by Conoco Norway and Norsk Hydro to have concrete hulls for the Heidrun and Troll Oil platforms, both in the North Sea, were a breakthrough for floating concrete structures. Both the Heidrun TLP and the Troll Oil Semisubmersible are very large floaters. Heidrun carries some 90,000 st above top of columns and has a displacement of 290,000 st while Troll Oil carries 32,000 st and has a displacement of 270,000 st. Both platforms are scheduled for installation in 1995. Considerable experience has been accumulated over the years in the analysis and design of steel hull TLP's, Hutton, Jolliet, Snorre and Auger are all installed and the fifth steel TLP is in the detail engineering phase while Heidrun is the only concrete hull TLP so far. The present study is based on state of the art technology. The comparative evaluations are based on a steel TLP already developed for the GOM and a concrete TLP designed with an identical design basis. Although the development has addressed most aspects related to a TLP concept, the main focus in this presentation is on comparing global performance of a steel and a concrete TLP and construction/installation scenarios of the concrete TLP.