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

Abstract

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 reel-lay installation leads to stress concentration due to the 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 (Yuan and Kyriakides, 2015, “Liner Wrinkling and Collapse of Girth-Welded Bi-Material Pipe Under Bending,” Appl. Ocean Res., 50, pp. 209–216. 10.1016/j.apor.2015.01.018). 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 the 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.