Development of Design Criteria and NDE Method for HPHT Equipment

Subsea production systems have been using API-specified methods and detection criteria for nondestructive examination (NDE) for equipment up to 15,000-psi rated working pressure. With higher pressure requirements and use of methods for evaluation of the fatigue life of HPHT equipment, existing criteria cannot be sufficient to achieve the desired fatigue life. This paper demonstrates more stringent design criteria and a detection method that was developed to achieve HPHT fatigue life, yet is cost and schedule effective. When fracture mechanics was used for fatigue life estimation, an initial flaw size is stated as a starting point in determining design life. Using API-specified NDE criteria of 1/16-inch detection of surface flaw, project design life was not achieved for certain components without a major redesign and retest. Smaller flaw lengths were preferred in critical areas instead of standard flaw sizes. This created a need for a high-sensitivity penetrant inspection technique to effectively detect this new minimum flaw size in critical areas. Various methods of NDE were considered, and a conclusion was taken in selecting the best inspection method. Fracture mechanics and finite element analysis required a minimum detectable flaw size length of 1/32 inch to meet the project design life without changing equipment technology. By using surface NDE with fluorescent liquid dye penetrant of sensitivity levels 2 and 3, all seeded flaws of 1/32-inch were consistently detected in the validation test coupon, which enabled the use of this stricter criteria for the analysis. Detailed procedures were established, and validation testing results were documented with photographs of detected flaws. Seeded flaw coupons were manufactured for validation of procedures and to train the various facilities that will use these techniques for production equipment. Additionally, procedures and guidelines were provided to inspectors and inspection facilities to ensure proper implementation of the methods. Measurement system analysis for repeatability and reproducibility was conducted at the facilities. This enabled the fatigue design of the HPHT equipment to advance beyond the boundaries of traditional methods and acceptance criteria set by current industry standards. New and tighter acceptance criteria were developed to improve HPHT fatigue life. High-sensitivity penetrant inspection technique, capable of detecting flaw sizes as small as 1/32-inch, was established and implemented. This inspection technique is not common to the oil and gas industry because of the ability of standard methods to readily detect the API-required criteria. The method has improved detection capabilities and has the potential to move toward adopting advanced design methods to address HPHT requirements.