Liner Wrinkling and Buckling of Girth-welded Lined Pipe Under Monotonic and Cyclic Bending: Effect of Triple Point Imperfections

Abstract It is well recognized that girth welds of carbon steel pipe lined with a thin layer of a corrosion resistant alloy constitute a weakness. Plastic bending to levels such as those imposed by reeling installation leads to stress concentration due to mismatch of properties between the carrier steel, the liner alloy and the weld. Furthermore, the constraint of the weld causes a local periodic separation of the liner from the carrier, which triggers wrinkling and subsequently large-amplitude buckles 1. The present analysis shows that replacing the contact stress of manufacture by a low level of constant internal pressure does not alter the induced disturbance or its consequences under bending; and that the growth of liner separation accelerates when the liner achieves a moment maximum (critical curvature). In addition, the presence of small geometric imperfections in the neighborhood of the weld was shown to reduce the curvature at which the stability of the liner becomes critical. Cyclic bending causes progressive accumulation of liner separation adjacent to the girth weld. The evolution of events as the number of cycles, N, increases is similar to that of monotonic bending with N replacing curvature. The rate of growth of liner separation depends on the amplitude of the imperfection, the internal pressure and the curvature of the bending cycle. It was observed that, when liner separation reaches the level at which the instability becomes critical under monotonic bending its rate of growth per cycle accelerates. Thus, monitoring liner separation during cycling can guide design.