Akihiko Tanizawa

Grade X65 and X70 Heavy Wall (up to 37mm in Thick) UOE Linepipe for Sour Service

Based on the increasing demand for natural gas to be extracted under severe environmental conditions, requirements for thicker sour service pipe steels with high strength (X65 grades or more) are increasing. In order to achieve both a resistance to hydrogen induced cracking (HIC) and better mechanical properties, it is important to obtain a homogeneous microstructure. For this purpose, we manufactured API X65 and X70 heavy wall (up to 37mm in thick) UOE linepipe for sour service utilizing an advanced thermo mechanical control process (TMCP) employing the theoretical maximum cooling rate with water (the ‘ultimate cooling rate’) and homogeneous temperature distribution in accelerated cooling. For applications in deeper water, higher pipe thickness to diameter ratio (t/D) is required. However, during pipe forming, these high ratio pipes suffer higher plastic strain in the vicinity of the surface. This plastic strain causes increasing surface hardness and HIC resistance may deteriorate. It is important to evaluate the effect on strain systematically based on the difference of microstructure morphologies. Therefore, the effect of bending strain on HIC properties was investigated through a simulated laboratory bending test with a strain of up to 8.9%. It was found that homogeneous bainitic microstructure can prevent HIC even under a higher bending strain. On the other hand, crack area ratio (CAR) was almost doubled when 5% strain was applied near the surface area in the material in which inclusion morphologies were not optimized.Copyright

Heavy Gauge UOE Pipe With Improved Compressive Strength for Offshore Pipeline

Offshore gas pipeline development has been expanding toward deeper water region that requires pipes to have strong resistance against collapse by external pressure. Collapse pressure is mainly dominated by pipe roundness and compressive strength. In order to improve compressive strength, it is quite important to understand the Bauschinger effect caused by cyclic deformation during pipe forming. Compressive strength is reduced by the Bauschinger effect since compression in the circumferential direction is applied after the pipe expansion. Therefore, prevention of Bauschinger effect is an important issue for improving compressive strength of pipes.In this paper, the effect of microstructure on the Bauschinger effect was investigated. It was proved that microstructure that consists of a hard second phase shows a large strength reduction in reverse loading, since a mixed microstructure with soft phase and hard phase enhances the Bauschinger effect. In order to obtain homogeneous bainitic microstructure, advanced plate production technology, where heat treatment on-line process (HOP) is applied after accelerated cooling, was developed. The steel produced by HOP process exhibits a fine bainitic microstructure with very low amount of hard second phase such as MA constituent. It was demonstrated that the trial produced pipe with HOP process has a higher compressive strength than conventional pipes.In addition to the fundamental study on compressive strength, further investigations were conducted to optimize other material properties for offshore linepipe, such as DWTT property, resistance to hydrogen induced cracking and HAZ toughness to comply with DNV requirements. Production tests of Grade X65 linepipe with the 38mm WT and 876mm OD was carried out. Material and mechanical properties of these heavy gauge linepipes were introduced.Copyright