Design Method Combing API and ASME Codes for Subsea Equipment for HPHT Conditions Up to 25,000 psi Working Pressure and Temperature to 400?

Abstract

Abstract A current challenge in the oil industry is the design of subsea equipment for pressures more than 15000 psi and temperatures more than 250°F. This combination of pressure and temperature has been fairly accepted as the start of the HPHT region. Current standard, American Petroleum Institute, Specification, 17D (API 17D) for designing subsea equipment is limited to 15000 psi working pressure and provides little guidance on temperature conditions above 250°F. This paper demonstrates a design methodology which combines the API and American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) for designing an example subsea pressure containing component for HPHT conditions above 15000 psi and with temperatures more than 250°F. This paper shows the evaluation of combined load capacity chart for an API 17D (API 6A, 4?? 20K) flange flow-loop for design pressure of 20,000 psi and temperature of 350°F with external tension and bending loads. Both the linear elastic and elastic-plastic methods for protection against plastic collapse are used to determine the structural capacity of the flange body. These methods combine the API material and design allowables and ASME design methods. Stress classification and linearization are used for the evaluation of design capacities with linear methods. Modified Load Resistance Design Factors are used to both evaluate design capacities and to account for the difference in ASME and API hydrostatic test pressures with elastic-plastic methods. The structural capacity is combined with thermal analysis to determine the effects of high temperature to the flange capacity. To assess the cyclic loading capacity of the flange, stress-based fatigue analysis and fracture mechanics analysis are also compared. The results obtained are comparable to existing API TR 6AF1 charts. This work has been performed to demonstrate both the acceptance of existing methods for HPHT conditions, but to also introduce the advanced ASME design methods for designing API 17D subsea equipment. The methods presented are acceptable for designing equipment up to working pressures of 25,000 psi and temperatures up to 400°F.