Reliability modelling of subsea gate valves

Offshore Installations comprise complex, expensive and potentially hazardous facilities which need to be maintained and operated in a proper and safe manner. Valves are one of the most widely used components on offshore installations and their reliability is important to both to safety and to the minimisation of down time. During discussions with offshore operators and valve manufacturers on the significance of valve failure to safety, it has been found that valve failures are frequent occurrences and not restricted to a few isolated incidents. The problem is wide spread in the petroleum industry. This industry experience can be regarded as unacceptable _ The success of a reliability and risk analysis exercise is ultimately dependent on the availability of historical reliability data. This study conmed the findings of a number of recent surveys which have revealed a dramatic lack of knowledge and understanding of valve mechanical failure mechanisms amongst petroleum related industry. Furthermore it was found that historical reliability data was generally scarce and inadequate for thorough studies. As a result of this situation, it was felt necessary to develop a approach to mechanical valve reliability modelling which was predictive i nature and which would take into account design and operational factors. This research project concentrates on the modelling and prediction of reliability of valves and includes a case study of subsea hydraulically operated gate valves. The modelling process includes procedures for the identification of vulnerable areas in valve design and its relation to the operating and environmental conditions. A number of statistical models have been developed based on stress strength interference concepts. These models enable the probability of failure to be described in terms of the physical characteristics of mechanical valve deterioration for specific failure modes such a internal leakage, fail to close and fail to open. The models require information both on design and on deterioration performance the latter of which is poorly understood. Provided judgement can be utilised to describe the ageing parameters governing failures, the models enable engineers to predict the time dependent reliability of this type of gate valve together with uncertainty at the design stage. I addition to the fundamental modelling work, the implications of reliability management systems have been investigated. The role of condition monitoring and the use of monitoring data for both for anticipation of failure and for updating the predicted reliability has been discussed. The role of failure reporting systems to provide improved understanding of failure mechanisms if valves was considered to be a important element of a reliability management system and vital to the future improvement in valve reliability. No evidence of any systematic failure reporting system was found in the offshore industry.