15K plus, The Limits of Established Pressure Vessel Design Methods

Abstract

Abstract The majority of oilfield pressure containing Wellhead and Tree equipment hasbeen designed with guidance from API 6A and 17D. However, their design methodsare not the most appropriate for High Pressure High Temperature (HPHT)applications. This paper discusses the limitations of established PressureVessel design methods and presents more suitable methods for HPHT. Unfortunately, these methods are not readily intuitive or straightforward. Sothis paper also addresses the challenges currently being faced by designers ofHPHT oilfield equipment. Recent design evaluations of HPHT equipment haverevealed significantly different (and sometimes unexpected) results from thevarious design and analysis methods that are available. Several industrystandard design and analysis methods are discussed and compared, using simpleHPHT equipment geometries to illustrate the authors' findings. Although work isstill in process, this paper shares some significant findings that should berecognized and understood by HPHT equipment designers in order to ensure thattheir design methods are appropriate for the application. In conclusion, on thebasis of these findings the authors determined that many of the established andcommonly used oilfield methods of Pressure Vessel design are not the mostappropriate for HPHT applications. They are not accurate for thick-walledcomponents and their use is open to interpretation. Thus, it is recommendedthat designers carefully review and understand their design methods to ensurethat they are accurately predicting the stresses, deflections and fatigue lifeof the equipment under development. Introduction As exploration for oil and gas has moved into deeper, hotter and higherpressure zones, the design and stress analysis for oilfield equipment becomesever more critical and challenging. The higher pressures, and therefore thickerwall sections, mean the thin-wall assumptions of the past are no longer valid. Historically, designers could overdesign the equipment to provide a robustproduct with liberal Design Margins. However, for the newer HPHT oilfieldequipment, there is literally no room for overdesigned equipment and with thefailure mode shifting towards fatigue and fast fracture, very accurate andrealistic stress analysis and design methods are essential to achieve safe, reliable and cost effective equipment designs. Another aspect critical to HPHT design are the relative movement betweencomponents and stress relaxation phenomenon for seals and their sealinglocations. But this subject is beyond the scope of this paper, focusing insteadon pressure vessel design requirements.