Low cycle fatigue strength of X65 pipeline girth welds

During pipe laying at large water depth using S-lay over a stinger, the pipe may be subjected to reversed plastic strains which could lead to low cycle fatigue failure. X65 pipeline girth welds with wall thickness 22mm were tested in cyclic strain control. Undermatched and overmatched welds were tested. Two types of cyclic strain programs were applied. In the first test series the welds were subjected to a tensile-tensile cyclic strain, simulating pipe going over the stinger, to validate that a pipe going over a stinger would not violate criteria for low cycle fatigue design. In the second test series, the specimens were initially pre-strained to 2% and subsequently fatigue-cycled with a maximum strain of 2% to obtain design criteria for a pipe laying stationary over the stinger for a period of time, subjected to cyclic loads due to waves. The results are presented in terms of Δε-N curves, with proposed design curves for the two types of welds.