Local Failure Analysis of HPHT Subsea Tree Components due to Triaxial Stress

Structures can fail prematurely and potentially suddenly when the stress state is in triaxial tension. Triaxial stresses commonly occur in notches, crack tips and thread roots. Yielding criteria such as Tresca and von Mises are based on the difference in the principal stresses and therefore reduce as the three principal stresses become more equal in magnitude. Therefore, structures need to be analyzed for failure due to triaxial stress state when conducting a plastic collapse study. One of the requirements of the ASME Section VIII Divisions 2 and 3, Design by Analysis approach is to check for the local failure of the component due to triaxial stress. Based on the ASME approach, local failure is analyzed by calculating the allowable total equivalent strain and summing the plastic strain history through a damage accumulation function. In this paper, two common HPHT subsea tree components, a gate valve body and a load bearing shoulder are analyzed for ASME local failure criteria.