Design Methods for Subsea Equipment for Extreme and Survival Load Conditions

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25352, “Design Method and Load- and Resistance-Factor Design for High-Pressure/High-Temperature Subsea Equipment for Extreme and Survival Load Conditions,” by Parth D. Pathak and Samuel L. Taylor, OneSubsea, prepared for the 2014 Offshore Technology Conference, Houston, 5–8 May. The paper has not been peer reviewed. This paper posits a design method for subsea equipment governed by American Petroleum Institute (API) Specification (spec.) 17 (and particularly 17D). Fatigue-life estimation is also addressed for these conditions. An analysis of a subsea-tree component is reviewed, evaluating its combined load capacity for normal, extreme, and survival conditions. Additionally, the fatigue life-cycle estimation for this component is included. Introduction The API spec. 17D and spec. 6A provisions have been the governing terms for subsea trees and wellheads, providing the design guidelines for normal operating conditions up to 15,000 psi. The recently published API Technical Report (TR) 1PER15K-1 and TR 17TR8, the latter still under development, address the design methodology for normal operating high-pressure/ high-temperature (HP/HT) conditions. But these specifications and technical reports do not provide design guidelines for extreme and survival operating conditions. This paper tries to address design methods for subsea equipment that falls under API spec. 17, with focus on subsea trees and wellheads for such conditions. API Recommended Practice (RP) 17G, which is intended for design of completion and workover risers, addresses these extreme and survival load operating conditions. It provides the equipment’s normal, extreme, and survival operating capacities on the basis of safety factors applied to the ultimate capacity. It also provides allowables based on yield strength with condition factors for extreme and survival conditions. This paper provides a methodology whereby the design factors for the extreme and survival load conditions from API RP 17G are adapted so that they can be applied to API spec. 17D equipment. Because testing the API spec. 17D equipment to its ultimate capacity is not easily feasible, an elastic/plastic analysis method based on American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section VIII, Division 2 or Division 3 (ASME Division 2 or 3) is suggested to determine the ultimate capacity of the equipment.