Development of an approach to incorporate proportional hazard modelling into a risk-based inspection methodology for an optimal inspection policy

Abstract

The health and safety of pressure vessels are a concern in the power, chemical, petrochemical, and other industries handling gases or liquids at high temperatures. The content of a pressure vessel usually is at a substantially different pressure than the ambient pressure, and if not handled carefully it can lead to fatal accidents such as an explosion. Therefore, industry decision makers rely on a risk-based approach to perform inspection and maintenance on the pressure vessel. According to the Risk-Based Inspection and Maintenance Procedures project (RIMAP) for the European industry, risk has two main components: the probability of failure (PoF) and the consequence of failure (CoF). As one of these risk drivers, a more accurate estimation of the PoF will contribute to a more accurate risk assessment. Current methods to estimate the probability of failure are either time-based or founded on expert judgement. This work proposes enhancements to the quantitative risk assessment for the probability of failure ( PoF) and the consequence of failure (CoF) through the utilization of a newly proposed methodology. The proposed methodology consists of incorporating the proportional hazard model (PHM), which is a statistical procedure to estimate the risk of failure for a component subject to condition monitoring, into the risk-based inspection (RBI) methodology so that the PoF estimation can be enhanced to optimize inspection policies. To achieve the overall goal of this work, case studies applying the PHM to determine the PoF for real-time condition data components, are discussed. Also, considering the consequences of failure due to accidents which can occur in pressure vessels using steam and water as reference material, boiling expanding vapour explosions (BLEVEs) are especially important due to their severity and diverse effects such as overpressure, thermal radiation and missile ejection. By way of example this work considers only the overpressure due to BLEVE to model the CoF. The first benefit of this work is that by incorporating PHM with the RBI approach, the PHM uses real-time condition data, to allow dynamic decision-making on inspection and maintenance planning. An additional advantage of the PHM is that where traditional techniques might not give an accurate estimation of the remaining useful life to plan an inspection, the PHM method can consider the condition as well as the age of the component. Another benefit of this work is that risk-based inspection is presently one of the best methodologies to provide an inspection schedule and ensure the mechanical integrity of pressure vessels. RBI usually provides an inspection schedule based on calendar or usage time intervals. This work however optimizes the inspection schedule on pressure vessels, by incorporating proportional hazard modelling (PHM) into RBI methodology as stated above. The work presented here comprises the application of the newly proposed methodology in the context of pressure vessels, considering the important challenge of possible explosion accidents due to boiling liquid expanding vapour explosion (BLEVE) as the consequence of failure calculations. The proposed risk management methodology incorporates a quantitative assessment of the Probability of Failure (PoF), based on Proportional Hazard Modelling (PHM), and the Consequence of Failure (CoF), of an explosion event. The unmitigated risk is thereby quantified by means of a risk matrix, which enables evaluating and deciding on suitable risk mitigation strategies.