Fundamentals of risk based inspection — a practical approach

New API structural integrity management recommended practice and post-hurricane inspection bulletin, MMS proposed changes to CFR´s on decommissioning, and increased interest in developing risk based inspection plans for offshore assets require better understanding of reliability principles. This paper will discuss the fundamentals of RBI from a practical viewpoint and gives details of recently published applicable DNV offshore standards. Offshore structures must be inspected to maintain an acceptable safety level throughout their lifetime. Inspections have traditionally been based upon experience, and judgment of likelihood and consequence of failure. Risk based inspection planning (RBI) for structures, as developed by DNV, represents a systematic, qualitative and quantitative approach which combines theoretical models, test results and in-service experiences. The method is specially developed for application to all types of offshore structures including jackets, jack-ups, TLPs, FPSOs, spars, semi-submersibles, GBSS, and subsea templates. The basis of RBI is to prioritize individual items and systems by considering the associated risks. Attention is given to high risk items, while low risk items receive a more appropriately lesser level of inspection. RBI focuses on cost optimization in all associated activities, to ensure a cost optimal inspection program. The RBI analysis is performed in two steps, comprising a risk screening and a subsequent inspection scheduling. Dedicated software models are utilized to establish the Risk Matrix and to calculate the ¿time to next inspection¿. The final deliverable of an RBI analysis is an inspection plan, in which inspection efforts are prioritized from an overall risk perspective.