The Ultimate Limit State of Offshore Platforms Using Reserve and Residual Strength Principles
Authors: Lalani M. , Shuttleworth E.P. 
Published: October 2008

Offshore steel structures are designed on a component rather than system basis. Accordingly, offshore installations have varying system reserve strengths even if they have been designed to the same code. The use of system reserve strength has increasingly been recognized in recent times as an important decision-making tool in the design of new structures and, more importantly, on Inspection, maintenance and Repair (IMR) operations for existing installations. This paper describes the development of system reserve and residual strength technology by reference to a recently completed Phase 1 of a major joint industry project supported by eight oil companies and the UK Department of Energy.

Sources of reserve and residual strength are described, together with previous work undertaken in this area. Phase 1 of the joint industry project is described in detail. The project comprised ultimate load tests on four large scale tubular frames which, at 15m high by 6m wide, are considered to be the largest tubular frames ever tested in a controlled manner. The paper describes the development and calibration of a powerful non-linear frame analysis program. Example frames are analyzed and reported, demonstrating the use of the analysis program. Exploitation of the developed reserve strength technology for new structures is described, and the use of this technology within the premise of Assessment Procedures for existing installations is described. Finally, Phase II of the joint industry project is explained, which concentrates on addressing further the unusual and unexpected frame behavioral characteristics in the presence of tubular joint failures.

For offshore steel structures, the reserve and residual strength depends on a number of different issues such as redundancy, exceedence of minimum requirements, material reserve, code safety factors, over-design, non-structural requirements and foundation reserve. For new structures, the developed technology reported herein can be used to enable full benefits of redundancy to be taken without compromising safety. For existing installations, which are often reanalyzes in response to certification requirements or to assess structural integrity following superstructure modifications or substructure damage, the techniques described represent an important decision-making tool for cost effective I!4Roperations, leading to the minimization of costly remedial measures.

1. INTRODUCTION

Offshore steel structures are designed on the basis of traditional structural engineering practice. A combination of loads is applied to the structure and the internal forces in each brace member are established. Each member and joint is checked against allowable strengths given in design codes. The structure is considered to meet the selected code standards if all the individual components satisfy the code requirements. All codes, whether they are based on permissible stress design or limit state design, address the design of individual members and joints; the allowable strengths are essentially derived from the large database on isolated joints and tubular beam-column members. Implicit within this design procedure is the premise that failure of one member or joint to satisfy code requirements constitutes non-compliance with the relevant code.