Reliability and availability assessment of seabed storage tanks using fault tree analysis

• The concept of subsea production system with seabed storage tank (SST) was proposed.
• The reliability assessment for the SST was estimated using a fault tree analysis.
• The repair time for SST was estimated in accordance with MIL-HDBK 472 Procedure V.
• Consequently, the availability of the SST was approximately 91.8%.

This study proposes the concept of subsea production systems with a seabed storage tank to provide an alternative to conventional floating facilities and performs the reliability, maintainability and availability study for the seabed storage tank. The reliability assessment of the seabed storage tank performs a four-step procedure. A four-step procedure is to define the system boundary, collect the reliability data, construct a fault tree and estimate the reliability. The failure and repair data are obtained from the component of the lowest level of the seabed storage tank because the seabed storage tank is a new system unknown reliability data. Reliability of the seabed storage tank is estimated with a consideration of critical events. The total failure frequency of the SST is estimated approximately 2.27· 10−4/hour. The maintainability analysis is estimated in accordance with MIL-HDBK 472 Procedure V. The elements of repair time is obtained from the subsea JIP 2000 and RAM study of field data. Active repair time from OREDA 2009 is regarded as the repair/replace of repair time elements. The total repair time is mainly effected by the preparation time of repair time elements. The system availability for the seabed storage tank under normal operation is calculated as approximately 91.8%.