Economic and Performance Evaluation of Concrete vs. Steel TLP's for the Gulf of Mexico
Authors: Rajabi F. , Tuturea D.P. , Mangiavacchi A. 
Published: October 2008

In recent years, in response to persistent low oil prices, the demand for affordab1e technology has led to a considerable effort intended to reduce the overall oil field development costs. One of the aspects of this search for innovation resulted in studies on the use of concrete for Tension Leg Platforms (TLP) hulls. These studies have evidenced a promising potential for an overall cost reduction for the installed platform. This paper presents a cost comparison of a series of TLP configurations, with steel and concrete hulls for potential application in the Gulf of Mexico. The paper includes a description of the sizing strategy, detail s of the configurations developed, global cost estimates and sensitivity of cost estimates to water depth and payload variations.

1.0 INTRODUCTION

The unpredictable and dramatic reduction of oil prices in the mid eighties has forced operators and designers to explore innovative ways of developing oil fields at affordable cost. The need for cost reductions while moving into deeper waters or harsher environments presents an unusual set of challenges, which can only be met by developing alternative, unconventional strategies. The Tension Leg Platform (TLP) technology as a means of developing oil fields was first proven with the Hutton Field installation, in the UK sector of the North Sea. However, the search for cheaper alternatives has been leading designers towards variations to the basic early concepts. One notable result is the first Tension Leg Wellhead Platform (TLWP), just recently installed in the Gulf of Mexico, in a water depth of 1,760 (FT). Another promising direction forth is type of structura1 concept is the use of concrete rather than steel as structural material for the hull. A great deal of work in this conceptual development has been carried out, and the recent announcement by Conoco Norway of the forthcoming use of a concrete TLP for the Heidrun Field development, in the Norwegian sector of the North Sea, has considerably heightened the interest in this evolutionary application,[l], [2], [3]. Concrete has already been used Quite extensively for offshore applications, particularly in the North Sea. Some of the heaviest and largest offshore platforms have concrete Substructures. The long term performance of the material has proven Quite satisfactory, with very 1ittle need for expensive underwater maintenance. A number of other advantages have been claimed for this material, which is, in fact, experiencing a newwave of popularity in the UK sector of the North Sea even for relatively shallow water applications. A number of potentia1 benefits have been suggested from the use of concrete for TLP hull s. Among others:–The shorter fabrication schedule, compared to a steel hull, with possible economic benefits;–The possibility of increasing the platform operating draft, without substantial structural penalties. This in turn may lead to higher displacement and payload carrying capacity than a comparable steel TLP, and eventual lower tendon dynamic loads;–A high resistance to damage from impact loads;–The inherent resistance to corrosion, with consequent decreased requirements for inspection and maintenance;