High strength steel line pipes have different mechanical properties (plasticity, ductility and toughness) in each direction. This anisotropy is developed by thermo mechanical control process (TMCP) in a heavy plate mill and cold forming process in a pipe mill. In this study, experimental work was carried out using both as received and pre-strained (up to 6% along T direction) steel plates for X100 grade pipes, in order to analyze anisotropic toughness. Compact tests were conducted in the longitudinal (L) and transverse (T) directions. Furthermore, a new damage model based on the GTN model was proposed, in order to represent anisotropic damage behavior in high strength line pipe steels.
As consumption of energy is increasing worldwide, the demand for development of oil and gas resources in remote locations becomes strong. These development areas are often far from major consumers because the potential locations are harsh environments. Environmental loads by offshore ice, discontinuous permafrost and seismic activity impose a strain demand on the pipeline structures transporting the oil and gas from these remote resources to the population centers. While stress-based design of pipeline is normally preferred, the nature of these environmental loads makes strain-based design (SBD) a necessity in these types of harsh environments. Accurate prediction of the environmentally imposed strain by pipeline designers and accommodation of this strain by installation of advanced steels for pipeline is essential to operate a safe and reliable pipeline. The basic materials requirements for the SBD line pipe steels are generally control of longitudinal yield strength, low yield to tensile strength (Y/T) ratio, high strain-hardening exponent, high uniform elongation, and good toughness (Glover, 2004). Additionally, aging effects on tensile properties during the coating process must be minimized and fully characterized (Shinohara, 2005; Timms, 2005).
