Offshore pipeline projects have been expanded to deeper water region and the linepipes are required to have higher resistance against collapse by external pressure. The collapse resistance is mainly dominated by pipe geometry and compressive yield strength. For deep water application, diameter to thickness ratio (D/t) and pipe roundness are key factors. On the other hand, the mechanical properties in each circumferential position is dramatically changed by cyclic deformation through a pipe forming process. Therefore, in order to improve compressive yield strength of pipes, it is important to consider the Bauschinger effect caused by pipe expansion. The mechanism of this effect is understood that internal stress is generated by accumulation of dislocation and it reduces reverse flow stress. In this study, the microscopic deformation behavior was analyzed from FEM calculation, it was found that multi-phases microstructure enhanced the microscopic heterogeneous deformation adjacent to the boundary between soft and hard phases. Therefore, homogenized microstructure inhibits the Bauschinger effect. In addition, the materials of offshore pipeline should have other properties such as low temperature toughness and sour resistance. It is well known that fine grained microstructure improves the lower temperature toughness. For achieving high compressive yield strength and good lower temperature toughness, the effect of chemistry and rolling condition were investigated to obtain fine and homogeneous microstructure. Based on laboratory results, mill trial tests were carried out for Grade X65 linepipes with heavy gauge by TMCP. Full scale collapse test was also conducted after pipe coating heating. In this paper, material design concept and its mechanical properties of developed pipes were introduced.

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