Comparing ASME and PD5500 Fatigue Analysis Methods: A Case Study

Abstract This study compares the fatigue life assessments of two existing pressure vessels designed to the BS 5500, 1994 & AMD 1995 CAT.1 standard that is now superseded and ASME Sec VIII Div. 2, respectively. The vessels have been in service for two-phase oil and gas separation. The objective is to compare the fatigue life predictive methods and explain the differences in approach and treatment of stresses between the two design Codes. Increased oil and gas production from newly added wells has led to higher liquid slug loads on the inlet nozzle. To evaluate the impact of these modified loads, fatigue design limits were calculated for both separators using their original design codes and an alternative code. Cyclic loads on the nozzle have substantially increased due to liquid slugs, exceeding the original design limits. This study investigates the nozzle’s suitability for extended service under these modified loading conditions, thereby qualifying the inlet separator. Using Finite Element Analysis (FEA) and both design codes, we investigated the nozzle’s fatigue life under these modified loading conditions. Fatigue life estimate for inlet nozzles were determined using the modern design standards ASME Sec VIII Div. 2 and PD 5500, employing both equivalent stress and maximum principal stress criteria. Our findings reveal significant differences in fatigue life predictions between ASME and PD 5500, primarily attributed to their distinct stress criteria and approaches. This study provides valuable insights for assessing the remaining life of existing pressure vessels under modified or elevated loading conditions and informs future design practices.