Merging ASME and API Design Methods for Subsea Equipment Up To 25,000 PSI Working Pressure
Authors: Kocurek Chris George , Pathak Parth , Melancon Cole , Sohn Sangjoon 
Published: April 2012

A current challenge in the oil industry is the design of subsea equipment forpressures more than 15000 psi. Current standard, American Petroleum Institute, Specification, 17D (API 17D) for designing subsea equipment is limited to 15000psi working pressure. One of the key recommendations of API TR PER15K (draft)is the utilization of the American Society of Mechanical Engineers (ASME)Boiler and Pressure Vessel Codes (BPVC) for designing pressure vessels forpressures above 15000 psi. This paper proposes a design methodology combiningthe relevant API and ASME design codes for the design of subsea equipment forpressures more than 15000 psi.

Specific guidance is provided in this paper to safely utilize the ASME designmethods with API materials. These approaches allow for the increase of ASMEdesign test pressures to match API while satisfying ASME and API designallowable limits. Methods and guidance are provided for the use of stressclassification, stress linearization, protection against general plasticcollapse, local collapse, buckling and cyclic loading. Recommendations are madefor Load Resistance Design Factors to accommodate the difference in hydrostatictest pressure between ASME and API. Additionally, approaches using bothtraditional stress-based fatigue analyses methods and fracture mechanics theoryare compared. The design of closure bolting conforming to API requirements isintegrated with the ASME methods, along with the recommendation ofnon-destructive examination (NDE) requirements to align with the recommendedstress and fatigue design factors.

An example design evaluation of a pressure containing API 6A, 4 in. 20 ksi type6BX flange is presented for a design pressure of 20000 psi with bolt preload asrecommended in API 17D. The results show the existing API methods are adequateup to 25000 psi and the design verification methods meet the recommendations ofAPI TR PER15K.

Introduction

Subsea Wellhead and Christmas Tree equipment rated up to 15000 psi can bedesigned using methods recommended in API 17D. However, this standard does notprovide guidance for equipment rated at pressures more than 15000 psi. Giventhe current trends in the industry, fields are being explored which requireequipment rated above 15000 psi.

The design methods given in API 17D refer the design methods of API 6A whichare valid up to working pressures of 20000 psi. The ASME BPVC Section VIII, Div. 2 and Div. 3 provide design methods for high pressure vessels. Thesedesign methods can be used for designing the equipment; however the design mustmeet the API design allowable limits, material and test requirements. ASME BPVChas its own design allowables, material and test requirements which are to beused along with the design methods outlined in the ASME codes. But since theyare different from the requirements put forth by the API design codes, a designmethod combing the ASME design techniques with API material and testrequirements which satisfy both the codes has been suggested in this paper.