Non-Destructive Examination Limits for Fracture Mechanics Design of High Pressure High Temperature Subsea Equipment

Abstract The design and operation of High Pressure / High Temperature (HP/HT) equipment in the oil and gas industry using API 17TR8 drives requirements for fracture mechanics fatigue verifications. The inputs used as part of the fracture mechanics assessment require to be defined in order to ensure the integrity of the assumptions and results. Such assessments assume the HP/HT product may include an initial flaw that is equal to the size of the minimum indication detectable by the defined Non-destructive Examination (NDE) technique used as part of the part quality requirements. This size of this initial flaw can have a large impact on the overall fatigue life of the product and can influence the operating conditions of the subsea operator. Therefore, the smaller the initial crack size that is detectable the longer the subsea operating window and the harder the conditions that can be tolerated. Historically, in the subsea oil and gas industry, API 6A limits on detectable flaws have been assumed but these limits do not define the Probability of Detection (POD) or confidence level of the NDE techniques used. This paper outlines a test methodology for deriving the POD and confidence level of NDE techniques based on the use EDM notches in sample plates. Results of the testing show the ability of the NDE techniques used in the test to detect initial EDM notches that align with the API 6A limits of 1/16″ long flaws with a POD of 90% and with a confidence level of 80%. The impact of using this size of initial flaw on the fracture mechanics assessment of a HP/HT product is outlined with significant improvements in fatigue life achieved.