Risk-Based Inspection of a Crude Oil Import/Export Line: The Corrosion Engineer’s Role
This paper uses the case study of a line used to import and export stabilized crude oil to illustrate the steps, the benefits and the role of a corrosion engineer for a typical semi-quantitative Risk-Based Inspection (RBI) process. The line was originally designed for dry service, but currently receives a significant amount of produced water which accumulates in deadlegs with resultant deterioration. Historically, this line also experienced a number of high profile failures attributed to microbial activity, preferential weld decay and corrosion under insulation. Consequently the HSE issued an improvement notice which included a requirement for an RBI study to mitigate these threats and to provide operational integrity assurance. The scope of the RBI review considered in this paper covers only the corrosion circuit, which is from the tank isolation valve to the storage tanks. On the basis of the identified threats and the associated risks, the following inspection, monitoring, and repair (IMR) actions were recommended by the RBI team: (1) an intermediate, non-invasive, rolling inspection at a 12-monthly frequency. This includes a 100% ultrasonic testing (UT) or saturated low frequency eddy current (SLOFEC) scan of all susceptible deadlegs and a USFD (using time of flight diffraction, ToFD) of all welds on the deadlegs; (2) a thorough, non-invasive inspection at a 36-monthly frequency. This includes a full line walk and general visual inspection; a 100% UT scan or SLOFEC on all deadlegs; inspection of small bore branches for fatigue cracks with magnetic particle inspection (MPI) or eddy current (EC) Inspection and visual examination of all accessible supports; (3) a survey to identify corrosion monitoring locations for probes, coupons and sampling points; and (4) Identification and deletion of all unnecessary deadlegs during the next turn around (TAR) maintenance. These actions have since been implemented and provide the largest single contribution to the significant reduction in the number of failures/anomalies experienced on this line. The primary role of the corrosion engineer in this process is to ensure that all the credible threats for this circuit (generic and from inspection history) are identified, risk assessed, and appropriate inspection/monitoring techniques and frequencies identified, and assigned in order to mitigate the high risk threats.
KeywordsFailure mechanism Inspections Corrosion
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