Reliability assessment for subsea HIPPS valves with partial stroke testing
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High integrity pressure protection systems (HIPPS) have been applied in subsea applications to prevent accidents due to over-pressurization. A subsea HIPPS can be regarded as a complete functional loop consisting of sensors, logic solvers and actuating elements. It is important to test subsea HIPPS components with specified time intervals, so as to ensure their safety instrumented functions (SIFs). International standards IEC61508 and IEC61511, as well as many previous research works have proposed the reliability analysis methods for both SIS in general and HIPPS in particular, however, few of them focus on the different operational issues of HIPPSs in subsea. Partial stroke testing (PST) has been adapted for HIPPSs, as a means to test without stopping the subsea production process. This article proposes an extended model for quantitative reliability assessment, in consideration of the effect of not being able to carry out immediate repair of failures detected associated with PST. Approximation formulas for the average probability of failure on demand (PFDavg) in low-demand mode for HIPPS valves are developed by introducing the nonconstant failure rate following Weibull distributions and partial stroke testing coverage. A case study is carried out in order to demonstrate the proposed approximate formulas, and the numerical results of approximations are compared with those from Petri nets simulation.