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There are increasing numbers of ageing offshore structures in the South China Sea water which come along with various types of underwater structure anomalies, such as joint crack, member flooding, shallow gas, subsidence and etc. This phenomenon brings an immediate attention to PETRONAS to rectify the anomalies through various Strengthening, Modification and Repair (SMR) options. However, execution of SMR option is often a very costly operation. Optimum mitigation measure shall be established through proper detailed structural integrity reassessment activities with in-depth understanding of structural failure mechanism.Global Ultimate Strength Assessment (GUSA) methodology was established to support re-assesment activities comprises of three (3) integrated analyses i.e., Non-linear Plastic Collapse Analysis, Member Importance Analysis and Structural Reliability Analysis. These analyses are to quantify the ultimate strength of the structure through Reserve Strength Ratio (RSR) and Probability of Failure (P f ). The outcome of these analyses can efficiently assist in understanding the structure failure mechanism and correctly define relevant type of mitigation required.The paper elaborates GUSA methodology adopted and highlights few case studies to give an insight on how this assessment is undertaken in demonstrating fitness for purpose of the structure and defining the optimum mitigation measure.

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Currently there are more than 200 existing offshore structures in Malaysian waters. These structures were designed for life of 25 years however majority of these structures had exceeded this design life. Throughout the operating life these structures had been subjected to modification/extension of structural serviceability to accommodate additional weight and damage associated with South China Sea tropical storms & seasonal monsoons and deteriorations of the structure materials due to aging. This had pointed that some of these structures may not meet today requirements for integrity & reliability. In order to ensure the safety of these structures are continuously intact there is a need for reassessment on the ability of these older structures to perform original function and accommodate new requirement as per corporate expectation. A sophisticated and rational requalification procedure has been established to ensure the safety is maintained for life extension. The present paper will focus on the established reassessment procedures incorporating the Structural Integrity Management (SIM) processes. The issues with regards to acceptance criteria will be elaborated and methods in performing the assessment activities are addressed in stepwise manner. The paper will also highlight several issues inherited in the adopted processes and provide some new developments to support the process in addressing the constraint faced. The adoption of the established approach has enabled the requalification of these existing structures satisfies structural integrity compliance and has resulted in: No costly strengthening, No costly underwater inspection, No de-rating of facility, No prevention of further/future upgrading, Extension of Platform life for continuous production/operation.

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