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

Abstract

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.

Misapplication of Pressure Vessel Codes in Forensic Applications

“…FEA can also be used to assess in-service equipment for useful remaining life after modifications or repairs 8 . Implicit nonlinear FEA has been used to determine whether pressure vessels and piping designed and built to other codes can be considered equivalent to ASME BPVC and under what conditions 16 .This technique has been used in pressure equipment failures 17 . Use of the FEA is not only well established in the practice of engineering, but it has also been accepted by the courts 18,19 .…”