Engineering critical assessments (ECA) have increasingly become a routine part of pipeline design to determine tolerable flaw sizes for weld defects. These assessments are now being applied to pipeline systems in deeper water with increased loadings arising from responses to thermal and pressure cycling. Often these are flowline systems in which fatigue damage is exacerbated by the presence of aggressive internal conditions.
Deep water flowlines operating at high temperatures and pressures need to be designed to accommodate issues such as significant end expansion, walking and lateral buckling. Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion.
Currently for a number of offshore sour service pipelines, Engineering Critical Assessment (ECA) for flaw acceptance criteria are performed using knockdown factors up to 50X on fatigue crack growth rates. The toughness values used for the analysis are performed assuming a CTOD of 0.1 mm. In order to develop material chemistry and heat treatment procedures for offshore pipelines, it is important to be able to understand the fracture toughness behavior as well in sour environments.
In order to evaluate the safety of offshore flowlines and risers applications fatigue and fracture toughness properties of X80 parent pipe material in sour environments were determined. The data developed was used to develop an ECA framework for flowlines and riser applications.