As offshore pipeline projects have expanded to deeper water regions with depths of more than 2 000 m, higher resistance against collapse by external pressure is now required in linepipe. Collapse resistance is mainly controlled by the pipe geometry and compressive yield strength. In UOE pipe, the compressive yield strength along the circumferential direction changes dramatically due to tensile pre-strain that occurs in pipe forming processes such as the expansion process. In order to improve the compressive yield strength of pipes, it is important to consider the Bauschinger effect caused by pipe expansion. As the mechanism of this effect, it is understood that internal stress is generated by the accumulation of dislocations, and this reduces reverse flow stress. Compressive yield strength is also changed by the thermal cycle associated with application of fusion-bond epoxy in pipe anti-corrosion coating by induction heating. In the typical thermal heat cycle of this coating process, the maximum heating temperature is from 200 °C to 250 °C. In this case, compressive yield strength increases as an effect of the thermal cycle, resulting in increased collapse resistance. Thus, for deep water application of UEO linepipe, it is important to clarify the conflicting effects of the Bauschinger effect and the thermal heat cycle on compressive yield strength. During installation of deep water pipelines by a method such as J-lay, curvature is imposed on the pipe axis, but the circumferential bending that leads to ovalization is determined by the interaction of the curvature of bending deformation. This bending deformation decreases collapse resistance. The interaction of external pressure and bending is also important when evaluating collapse. Against this background, this study discusses the collapse criteria for coated linepipe and their bending interaction against collapse based on a full-scale collapse test under external pressure with and without bending loading. The effect of the thermal heat cycle on linepipe collapse criteria is also discussed based on the results of tensile pre-strain tests with simulation of the thermal cycle and a collapse calculation by FEA.

As offshore pipeline projects have expanded to deeper water regions with depths of more than 2,000 m, linepipes are required to have higher resistance against collapse by external pressure. Collapse resistance is mainly controlled by the pipe geometry and compressive yield strength. In UOE pipe, the compressive yield strength along the circumferential direction changes dramatically due to tensile pre-strain that occurs in pipe forming processes such as the expansion process. In order to improve the compressive yield strength of pipes, it is important to consider the Bauschinger effect caused by pipe expansion. As the mechanism of this effect, it is understood that internal stress is generated by the accumulation of dislocations, and this reduces reverse flow stress. Compressive yield strength is also changed by the thermal cycle associated with application of fusion-bond epoxy in pipe anti-corrosion coating by the induction heating process. In the typical thermal heat cycle of this coating process, the maximum heat temperature is from 200 °C to 250 °C. In this case, compressive yield strength increases as an effect of the thermal cycle, resulting in increased collapse resistance. Thus, for deep water application of UEO linepipe, it is important to clarify the conflicting effects of the Bauschinger effect and the thermal heat cycle on compressive yield strength. Based on this background, in this study, the combined effect of the Bauschinger effect and the thermal heat cycle on compressive stress is investigated by conducting tensile pre-strain tests and simulation of the thermal cycle associated with coating. Compressive yield strength was obtained for several pre-strain and thermal cycle conditions, and the collapse pressure was calculated by an FE analysis based on the obtained compressive yield strength. This study discusses the effect of tensile pre-strain on the collapse pressure of linepipes with these simulated thermal heat cycles.

The single specimen offset potential drop method has been applied to obtain the internal hydrogen-induced threshold stress intensity factor K IH in Cr-Mo steels by using small-sized 12.7-mm-thick ASTM CT-specimens (1/2T-CT).The subcommittee on Hydrogen Embrittlement in the Material Division of the Japan Pressure Vessel Research Council (JPVRC) had been investigating internal hydrogen embrittlement by using 25.4mm thick compact tension specimens (1T-CT(1/2): B = 25.4mm, W = 25.4mm, 2 H = 30.5mm), to enable an adequate supply of hydrogen from the uncracked region of the specimen near a crack tip to the crack tip zone during the experiment under normal atmospheric conditions. In the present study, tests were performed on 1/2T-CT specimens under a mixed load control and grip-displacement control aimed at achieving a reduction in the test execution time. It was found that the threshold stress intensity factors for the 1/2T-CT specimens exhibited virtually the same behavior as those obtained in the previous studies using the 1T-CT (1/2) specimens.

Demand for double jointing technology is increasing to improve pipeline construction productivity. Submerged arc welding (SAW) utilized for double jointing is likely to cause a much wider heat affected zone (HAZ) than those of typical field welding by gas metal arc welding (GMAW), and it should be taken into account for strain-based design of high strength line-pipes. However, guidelines for SAW welds properties to ensure strain capacity of high strength line-pipes such as X80 have not been established yet. In this study, a submerged arc weld joint was produced using tensile strength (TS) over-matching welding consumable. API standard type transverse weld tension test was conducted to measure local elongation at weld metal, HAZ, and base material. Elongation at weld metal increases prior to base material, but soon after that elongation at the HAZ softening region and base material adjacent to the HAZ catch up with the elongation in the weld metal, and finally, deformation concentrates at the HAZ softening region before final fracture. Deformation behavior of the joint was analyzed to verify applicability to double jointing girth welds for strain-based design. From finite element (FE) analysis of notched wide plate test which characterizes tensile strain capacity of a pipeline, it is suggested that ductile crack would not initiate before base material start necking in this particular TS over-matching weld joint in which the defect size is 1mm of notch depth and 25mm of notch length. Thus, the weld joint would be applicable for double jointing girth welds based on strain-based design.

The growing demand of the transport of gas and oil under severe conditions, such as in deepwater and onshore in cold climates, requires heavy wall thickness line pipes with both high strength and excellent toughness at low temperature in order to reduce the cost of gas transportation and constructions. In particular, when developing and efficiently manufacturing high strength line pipes with heavy wall thickness, achieving excellent heat affected zone (HAZ) toughness of seam weld is one of the key subjects. In general, steel plate for heavy wall thickness line pipes relatively contains large amount of alloy elements to secure required mechanical properties, whereas addition of alloy elements cause deterioration of HAZ toughness of seam weld. This paper deals with the method of improving HAZ toughness in UOE pipe seam weld. Relationship between microstructure and toughness of simulated HAZ was investigated, and volume fraction of martensite-austenite constituent was reduced by optimization of chemical composition of steel plates. Moreover, low heat input double submerged arc welding (DSAW) process with using welding wires with smaller diameter was newly developed to enhance HAZ toughness. With this new DSAW process, decrease of HAZ width and reduction of austenite grain size were also achieved. Based on the above knowledge, the manufacturing condition was optimized and heavy wall thickness X70 UOE pipe with excellent toughness was successfully developed.

Conventionally, 18-8 stainless steel has been used for the facility of cryogenic service in LNG plant because of the excellent toughness. Although 18-8 stainless steel piping requires pipe looping to relax thermal expansion in the cryogenic piping of LNG plant, 36% Ni steel (generally called as Invar) piping does not require pipe looping due to its low coefficient of linear expansion of about one-tenth of that of 18-8 stainless steel. Elimination of pipe looping makes significant reduction of the piping cost by reducing the number of elbows, welding points and adoption of small diameter of piping. Furthermore, it is possible to reduce the tunnel diameter, when the piping is installed in a tunnel. However, conventionally, it has been known that there is a difficulty in adopting the material from the viewpoint of weldability, which is reheat cracking at the weld metal in multi-layer welding. So, we have developed an excellent welding technology for 36% Ni steel with a new welding consumable. In order to adopt 36% Ni steel to the cryogenic piping in LNG plant, comprehensive experiments to find out the properties such as mechanical properties, corrosion resistance, weldability, nondestructive inspection and thermal stress analysis were conducted. By using the results the appropriate welding procedures were established and 36% Ni steel was used for the Ohgishima LNG receiving terminal of Tokyo Gas Company.