Quantitative risk assessment of submarine pipeline instability

Abstract

Ensuring the on-bottom stability of the submarine pipeline is very important for safety concern during the operational stage of submarine pipeline. Due to the action of the external factors, e.g. strong current, wave and soil liquefaction, the submarine pipeline may have lateral displacement, vertical floatation or sinking. Although the submerged weight of submarine pipeline is designed to meet the requirement of on-bottom stability, the loss risk of pipeline on-bottom stability still exists due to the change of ocean environment or seabed. This paper presents a reliability-based assessment methodology for submarine pipeline instability. Firstly, a mechanical model of pipeline on-bottom state is built considering the hydrodynamic loads and pipe-soil interaction, which is the static forces equilibrium equations essentially, and a detailed analysis of loading condition of pipeline on seabed is also conducted. Then, based on the reliability theory, the limit state equations of pipeline instability are developed through converting the forces equilibrium equations. Because the parameters in limit state functions possess random uncertainty, such as wave and current loads, etc., the specific probability distributions are employed to present the random uncertainty of the parameters in limit state functions. The Monte Carlo method is then employed to solve the limit state equations for assessing the pipeline instability probability. The risk level of pipeline instability is judged using the risk ranking in DNV-RP-F107. The case study indicates that the proposed risk assessment methodology for pipeline instability possesses a good application performance. In essence, the assessment results could provide a powerful support for risk management or decision-making of submarine pipeline instability.