Global ultimate strength assessment (GUSA) for lifetime extension of ageing offshore structures

Abstract

Malaysia is the second largest oil and gas producer in Southeast Asia. The majority of jacket platforms in Malaysia have exceeded their design life with various types of underwater structure irregularities. Therefore, it is essential to address the reliability of the jacket platforms in Malaysia due to ageing and increasing environmental loading. Global Ultimate Strength Assessment (GUSA) methodology was established to support detailed reassessment applied in managing safety, integrity analysis and reliability by evaluating the ageing and existing platform loading. It is a tool for the high-end analysis of structures for risk based assessment and has been accepted by most of the major marine operators in the offshore industry. The main purposes of this analysis are to manage the structure’s risk level over its remaining service life and to initiate cost efficient inspection or mitigation actions, if required. Probabilistic models which are derived from structural reliability methods with the result from pushover analysis, are used to determine the annual probability of failure of the structure over its remaining service life. The outcome of these analyses can efficiently assist in understanding the structure failure mechanism and correctly define relevant type of mitigations required. In this paper, the reassessment of an ageing platform over 30 years old, still in production is presented to demonstrate GUSA capability to perform life extension evaluation. Due to the demand to prolong the production for a further 25 years, it has been evaluated in design level analysis in early stage. With the major modifications such as extension deck for multipurpose pump and outboard conductors have given rise to overstressed and fatigue issues.