Beyond Design Criteria – Evaluating Mooring Design Integrity and Establishing Threshold Limits for Offshore Floating Structures Post-Tropical Storm PABUK

Abstract This technical paper assesses the mooring integrity of a floating structure unit in offshore Peninsular Malaysia post-Tropical Storm PABUK. The study focuses on a Floating Storage and Offloading (FSO) unit, experiencing significant forces during the storm, which recorded the highest wave height and wind speed in the region. Using industry software, the paper reviews design criteria for station-keeping analysis under intact and damage conditions during the 100-year return period. Initial analysis reveals criteria were below recent data recorded for PABUK. Sensitivity analysis studies incrementally increased environmental load cases in all directions of attacking waves and winds. Mooring line strength capacity was evaluated against Abnormal (ALS) criteria, considering survival from significant wave heights of 5m to 9m, equivalent to over a 10,000-year return period. Results indicate mooring lines could perform above design values, surviving the storm without failure. The study establishes a threshold capacity for surviving future storm events and introduces a corresponding integrity alert level for the floating structure unit. The paper highlights the novelty of determining the threshold limit for a floating structure unit, enabling data-driven decision-making for operators. Successful mitigation during the storm is attributed to a measurement system providing real-time storm characteristics for emergency response. The paper explores the historical context of tropical storms in the South China Sea, focusing on Tropical Storm PABUK's impact on PETRONAS operational waters. It discusses the development stages of tropical storms, their geographical distribution, and analyzes real-time metocean observations during PABUK's passage. The study emphasizes the importance of weather warnings, forecast accuracy, and risk assessments for enhancing preparedness in the face of tropical storms. The methodology and analysis section details mooring analysis using OrcaFlex software, considering Response Amplitude Operators (RAO) and conducting a time-domain approach. The study evaluates mooring chain tensions, rolling behavior, and excursion offsets, providing a holistic understanding of the challenges posed by tropical storms. In conclusion, the paper contributes valuable insights to offshore engineering, risk management, and weather forecasting. Recommendations include enhanced forecasting, reevaluating offshore structure resilience, continued monitoring, adaptive design strategies, and collaborative research initiatives. The findings aim to inform industries operating in tropical storm-prone regions, fostering resilience, safety, and sustainability.