High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. This paper presents the results of a fatigue based fracture mechanics assessment case study conducted on a fully clad subsea HPHT component. The component was assumed to be constructed from F22 low alloy steel internally clad with alloy 625 and exposed to 20ksi (137.8MPa) and 400°F (204°C) internal pressure and temperature.
A number of different assessment methods were evaluated as part of this study, including standard failure assessment diagram (FAD) based assessment methods, such as those found in API 579-1/ASME FFS-1 and BS 7910, as well as finite element (i.e. crack mesh) methods.
A detailed description of the finite element analysis (FEA) of the uncracked and cracked component is provided. An internal surface flaw assumed to be exposed to sour production fluids was evaluated. The results of the fatigue and fracture assessments are summarized along with the key differences between the assessment methods adopted. The sensitivity of the assessment results to other variables such as welding residual stresses is also discussed.