Structural Reliability Analysis and Quantitative Risk Assessment for Optimizing Cost of Offshore Structural Integrity Management and Life Extension Programme without Air Diving Operation

Offshore platforms in Gulf of Thailand have been aging and reaching their end of service life. To safely operate in aging platforms beyond their original intended service life, all possible operation risks need to be assessed through design review, condition assessment, inspection strategy and technique for the platform life extension stage. Qualitative or Semi-quantitative risk assessment was generally utilized by adopting scoring rule based on the condition assessment through inspection data/anomalies. A general subsea inspection campaign requested a costly additional air diving campaign for ACFM for joints with low fatigue life without a consideration of past inspection results. In practice, the routine inspection should be arranged in such a way that the operation risk meets the as-low-as practically possible (ALARP) criteria and yet minimize the cost, time and resources required for inspection. A quantitative risk assessment (QRA) and structural reliability analysis (SRA) are applied within PTTEP to evaluate and predict platform's risk development through time dependent probability of system failure. Inspection strategy including interval (when), locations (where), and suitable equipment and technique (how) are quantitatively incorporated into a mathematical model. The system probability of failure incorporating the system degradation is evaluated and inspection is triggered when the risk exceeds the allowable threshold. A complementary statistical decision model to evaluate the suitable inspection interval considered cost of failure, loss of production, and inspection is also employed. Results of quantitative method shows that the inspection interval can be prolonged, particularly, for well-designed and well-maintained platforms with reducing demand of additional air diving campaign. For most platform with relatively low to moderate consequences of failure, the inspection interval can be extended up until 10 years with routine general visual inspection (GVI), Flooded Member Detection (FMD) operated by remotely operated vehicle (ROV), provided that no significant anomalies are reported during the entire platform life cycle. In Summary, the approaches minimize the demand for additional air diving operation leading a reduction in additional cost, operational risk, and carbon footprint at least 250MT. The offshore structural integrity management and life extension programme for platform after service life by QRA is considered as new leveraged paradigm and delivers the cutting-edge integrity management programme leading to the most optimized operational cost with philosophy of safety and sustainability.