SAFEBUCK JIP: Critical Aspects of the Fatigue Limit State for Pipelines Designed to Laterally Buckle

Abstract

Abstract In pipelines designed to laterally buckle, start-up and shut-down cycles result in significant axial cyclic stress ranges experienced at the buckle crown. The fatigue performance of girth welds in this high-stress low-cycle regime is therefore often a critical aspect of overall pipeline design. Although thermal cycling is usually intended to be entirely elastic, stress ranges may approach or even marginally exceed the uniaxial yield stress, particularly early in life. Under these conditions, material response may differ from that conventionally seen under low-stress high-cycle fatigue loading, such as that resulting from wave or VIV loading. In particular, the possibility of cyclic softening needs to be considered, and specific boundaries set to ensure such behaviour is avoided. The specific nature of the fatigue loading associated with lateral buckling, also presents a significant challenge when considering the likely corrosion fatigue performance of girth welds exposed to either seawater or sour produced fluids. Corrosion fatigue performance is known to depend on the frequency of cyclic loading, with lower frequency loading incurring greater fatigue damage in each cycle. Unfortunately the cyclic loading frequency associated with lateral buckling is very low (at least several hours per cycle) and this is beyond the range of conventional laboratory testing. Special techniques and methods of analysis are therefore needed to determine an appropriate fatigue life reduction factor for use in design. This paper presents the results of experimental work carried out to investigate these two critical aspects of material behaviour, and summarises the corresponding SAFEBUCK design guidance. Introduction Fatigue is often a critical aspect in the overall design of pipelines designed to laterally buckle. The stress ranges associated with start-up and shutdown cycles are often a significant fraction of the material's yield stress. Although the number of such cycles is relatively low, compared to other potential sources of fatigue, the anticipated levels of fatigue damage are high. This is partly due to the very high levels of stress (fatigue life is inversely proportional to the stress range raised to the power of 3 or 5) and partly due environmental considerations, where fatigue lives in seawater or sour environment are potentially an order of magnitude (or more) lower than expected in air. Lateral buckling analyses often show that on first load, the stress experienced at the buckle crown approaches or even marginally exceeds the uniaxial yield stress. Peak loads and stress ranges typically decay during subsequent loading cycles. Although a small amount of plasticity may be tolerable on first load, it is important to ensure that cyclic plasticity is avoided. Conventional (stress based) design approaches would not be applicable in this instance. High-cycle (low-stress) fatigue design curves are usually associated with a number of cycles greater than 104 [2]. Although these curves may be extrapolated backwards into the lower cycle region, it is clear that some care is needed, and it is important to set appropriate limits (on the operational stress range) to ensure that determined estimates of life remain conservative.