Optimization of Deepwater Steel Structures

Abstract

ABSTRACT This paper describes an optimization method for the design of fixed or compliant structures to be installed in large water depths and whose designs are dominated by the fatigue loading resulting from low period sea states. The method increases the range of application of jacket type structures, until the extreme environmental conditions generate prohibitive overturning moments at the mudline. The optimization of the geometry of the structure to minimize the hydrodynamic forces is complemented by a complete dynamic and fatigue analysis methods. The dynamic analysis can be either in the time domain or in the frequency domain and considers all wind, wave and current forces. Linear relationships between internal forces of the members of the structure have been established and permits the simplification of the overall fatigue analysis. The method was applied on a jacket for 350 m (1150 ft) of water and the main results are presented. INTRODUCTION Oil production in deep water is a major objective of today's offshore industry. To minimize the cost of the equipment of a deep water field, it is necessary to design and build optimized offshore structures. When the water depth increases, the governing criteria for the design of the structures to be installed differ from what they are in shallower waters. The overall flexibility of any fixed or compliant structure increases with the water depth. Thus, the natural periods of vibration become close to the Mean period of waves. Then dynamic and fatigue effects become the governing criteria for the design of such structures. The purpose of this paper is to describe a method for the optimization and the fatigue analysis of any deep water structure. In a first step, starting from a preliminary design fitting with all production criteria, the structure is optimized with respect to the hydrodynamic forces, in such a way that the wave period inducing the minimum external forces will correspond to the natural period of the structure. In a second step, the structure defined in the first step is fully analysed using a 3-dimensional model to verify its dynamic behaviour and to establish its fatigue life. The fatigue analysis can be optimized by the use of the "correlations" defined in the dynamic analysis and permit a refined calculation of the hot spot stress areas. METHODOLOGY GENERAL The basic tool for structure optimization is a set of computer programs performing a dynamic analysis and computing the response of the structure to waves wind and current. The main computer program works in the frequency domain or in the time domain and calculates the response of a lumped mass model of the platform. The hydrodynamic forces can be computed on all members of a jacket type platform.