Investigation of the Stress-Strain Behaviour of Large-Diameter X100 Linepipe in View of Strain-Based Design Requirements

The use of high strength steels is considered as the best economical option to transport large gas volumes under high pressure from remote areas to the market. Exploration of new energy resources located in areas of complex ground and ambient climate imposes strict requirements on pipeline material and design. One of the major research issues in such areas is differential ground movement, which may be associated with large longitudinal straining in addition to plastic circumferential elongation. Hence, common design principles need thorough re-consideration, notably with respect to strain hardening properties of both base metal and girth welds. The present paper addresses several characteristics of axial and circumferential stress-strain behaviour as it is encountered in high-grade UOE line pipe. Two delivery states are taken into account, namely the “as expanded” as well as the “as coated” state. In a first experimental step, the effect of thermal cycle of the anti-corrosion coating process on stress-strain behaviour is simulated subjecting pipe material to temperatures in the range of 180° up to 250° C. In a second experimental step, stress-strain behaviour in both axial and transverse direction is mapped along the pipe production process in order to assess when and to what extent plastic strain capacity is lost during cold forming. The experimental work is complemented by instrumented ring expansion tests and instrumented burst tests. In a third future step, stress-strain information measured in both directions will be analyzed using a theoretical model based on Hill’s plasticity in order to clarify in which way circumferential stress-strain behaviour may impose constraints on strain capacity of axial direction. Within the scope of this paper, first and foremost, underlying principles are outlined and discussed and indications with respect to modelling implications given. Based upon these three sequential investigatory steps, it will be possible to draw conclusions with respect to stress-strain behaviour of parent material and the pipe forming process and to show that unfavourable effects triggered by coating do not show within the structure while they might do in material tests.