Fracture Mechanics Based Fatigue Assessment of an HPHT Valve Body

Abstract

The development of HPHT oilfield equipment has typically resulted in the construction of heavy-walled designs, where the increase in rated working pressure is accommodated by an increase in sectional thickness. This manner of design, however, is limited by practical difficulties which arise in the areas of manufacturing, handling/lifting, and uniformity of through-thickness material properties. Designs of more efficient size and weight may be developed by relaxing assumed design factors and hydrotest pressures, but this requires more rigorous analysis, validation, and QA measures. In particular, designers must address the fatigue susceptibility of HPHT equipment which, even in purely static conditions, may fail under cycles of shut-in pressure alone. These failures typically originate from stress risers such as cross-bores, seat pockets, or transitions in bore diameter, which exhibit complex stress states under the action of internal pressure. A fracture mechanics (FM) based analysis of such features has presented a longstanding challenge to designers and analysts as general solutions for their KI and σref are not presently available. It is therefore the objective of this paper to provide a useful methodology for conducting FM-based analysis of arbitrary geometry using the KI and σref solutions provided in API 579-1/ASME FFS-1. The method is presented in the form of a case study which describes the FM-based fatigue analysis of a seat pocket radius within a valve body. Here, the mode I behavior of a hypothetical surface-breaking, semi-elliptical flaw located at the seat pocket radius is evaluated by means of 3D finite element analysis. This method generally comprises two parts. The first involves the development of a 3D finite element model similar to what would be used in a conventional durability analysis. From this model, stresses are extracted along an anticipated fracture plane and used in conjunction with a weight function method to derive KI and σref from solutions provided in API 579-1/ASME FFS-1. These solutions are then used to compute the number of cycles to unstable fracture. The second part involves the direct incorporation of cracks into the finite element model. The approach benefits from a submodeling technique which reduces computational expense and allows the method to be used on complex structures. The numerical model is used in conjunction with conventional linear-elastic fracture mechanics assumptions to derive KI solutions for the geometry of interest. These KI results are used to confirm the conservatism of the code-based solutions and, thereby, the conservatism of the previous FM analysis. The method described in this paper allows designers to rapidly develop and execute FM-based fatigue analyses of arbitrary geometric features in timeframes similar to those associated with traditional S-N analysis.