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 KISCC and / or KIH needs to be
considered.
A practical approach based on recent TWI experience is presented.
Keywords: Fatigue, Corrosion, Lateral buckling, Pipeline, Girth weld, Sour, Seawater, Inhibitor,
Fracture toughness