Design of Pipelines Subject to Lateral Buckling to Resist Corrosion Fatigue

High strain, low cycle, fatigue at girth welds needs consideration for subsea flowlines carrying hot fluids which operate in ‘lateral buckling’ mode, which is becoming  more common. With corrosive fluids, the concern at the weld root is corrosion fatigue, and weld caps generally operate in sea water, with cathodic polarisation.
Fatigue testing in production fluids (sweet and sour) and seawater for steel catenary riser design, has used frequencies which are not relevant to lateral buckling. Exploration of the effects of frequency, (low frequency commonly makes corrosion fatigue more severe) using fracture mechanics specimens, must allow for potential differences between deep crack behaviour, and the conditions pertaining for the majority of life. This may be particularly important under scaling conditions, and/or in the presence of inhibitor.
The production environment varies both with position and time. The most highly corrosive conditions may not be most aggressive for corrosion fatigue. However, the environment for project specific fatigue testing needs to be that at the time when straining is occurring, rather than the most conservative. For end of life, the critical combination of K and K_ISCC and / or K_IH needs to be considered.
A practical approach based on recent TWI experience is presented.