Fatigue Capacity of Steel Cylindrical Bodies and Conduits Subjected to Cyclic Pressure

Abstract

This paper presents a code review (API 17G, ASME VIII Div 2 and ASME VIII Div 3) for addressing the fatigue capacity of steel cylindrical bodies and conduits subjected to cyclic pressure only. The fatigue capacity for pipes with a yield strength of 75ksi and 90ksi and with rated working pressures (RWP) ranging from 5ksi to 30ksi have been considered using both the S-N approach and the fracture mechanic approach. The S-N based fatigue lives from API 17G are found to be much longer than the corresponding S-N based fatigue lives from ASME VIII Div 2 and Div 3 and by the fracture mechanic (FM) approach as required in ASME VIII Div 3 for vessels where a leak-before-break condition can not be demonstrated. The S-N predicted fatigue lives are found to decrease with increasing RWP while the FM based fatigue lives are found to be rather independent of the RWP. The S-N based fatigue lives from ASME VIII Div 2 and Div 3 for free corrosion conditions are found to be shorter than the corresponding FM based fatigue lives for RWPs ≥ 25ksi and ≥ 20ksi, respectively. Based on this work, it is recommended to establish the fatigue capacity of steel cylindrical bodies and conduits subjected to cyclic pressure using either ASME VIII Div 2 or Div 3. The FM approach is considered to give a lower bound fatigue life as the number of cycles to initiate a crack is disregarded. Guidance on when it is considered applicable to use the API 17G criterion is given in the conclusion section of this paper.