Mitsuhiro Okatsu

Development of High Strength Linepipe With Excellent Deformability

Extensive studies to develop high deformability linepipe have been conducted. In case of linepipes laid at seismic region, higher resistance to buckling against large strain induced by earthquake related ground movements are required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, base on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine M-A constituents particles was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure were evaluated by axial compression test, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Tensile properties after thermal coating of developed high deformability pipe was also investigate. It was shown that increase in yield strength by thermal strain aging was minimized and round-house type stress-strain curve was maintained for the linepipe manufactured by new conceptional TMCP process.Copyright

Effects of Geometric Imperfection on Bending Capacity of X80 Linepipe

Validation of finite element modeling to predict bending capacity of linepipes and effects of geometric imperfection on the bending capacity are presented. A bending test of an X80 linepipe was conducted to discuss the validation and investigate the effects. The geometric imperfection of the linepipe about the outside diameter, the wall thickness and the longitudinal blister of the linepipe was measured in the round. Consequently, the results obtained by FEA taking into account the geometric imperfection present good agreement with the experimental data. And the moment capacity is virtually independent of the geometric imperfection however the strain capacity of the linepipe is quite susceptible to the geometric imperfection.Copyright

Mass Production and Installation of X100 Linepipe for Strain-Based Design Application

Continuous efforts have been made for the realization of strain-based design pipeline using high grade linepipe materials. Two demonstrative constructions of the pipelines using X100 linepipe proved sufficient materials properties for strain-based design and high quality field welding with good productivity. In order to verify further applicability of high strain X100 linepipe for long distance transmission, large scale installation of X100 pipeline was accomplished. Mass production of X100 linepipe of about 2,000 metric tons with the size of 42″ OD and 14.3mm wall thick was successfully conducted by applying recent developed TMCP process including accelerated cooling and online heat treatment process and UOE pipe forming. Field girth welding was safely completed by the dual tandem pulsed GMAW, and sufficient girth weld properties were demonstrated. This paper will describe material development and mass production results of X100 linepipe for strain-based design which specifying longitudinal tensile properties such as Y/T ratio and uniform elongation. In order to securely specify the shape of stress-strain curve without Luders elongation, material parameter “stress ratio” was introduced for the material specification for compressive strain capacity. Stringent base metal requirements were imposed for base metal material properties in this project. One of the most challenging aspects in developing high strain linepipe is to balance uniform elongation and Charpy absorbed energy. Dual phase microstructure is essential to improve strain capacity, but this may lead to lower Charpy absorbed energy. Therefore, precise control of microstructure by controlling plate manufacturing parameter was required. In addition, on-line heating process subsequently after accelerated cooling enabled increase of Charpy energy without deteriorating uniform elongation. Girth weld properties were closely evaluated using the X100 pipe in as UOE condition and after external coating. All the material properties of base metal and girth weldment of the X100 linepipes used for this project fulfill the stringent requirement for strain-based design consideration to prevent buckling and weld fracture.Copyright

Design Concept and Production of High Deformability Linepipe

Extensive studies to develop high deformability linepipe have been conducted. In the case of linepipes laid in seismic region or permafrost field, higher resistance to buckling against large strain induced by ground movement is required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, based on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine MA constituents was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure was evaluated by axial compression and bending tests, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Plate manufacturing technologies for producing recent high strength linepipe steel and the concept for microstructure control for improving deformability were also introduced in this paper.Copyright

Material Development and Strain Capacity of Grade X100 High Strain Linepipe Produced by Heat Treatment Online Process

Linepipes installed in permafrost ground or seismic region, where larger strains can be expected by ground movement, are required to have sufficient strain capacity in order to prevent local buckling or girth weld fracture. On the other hand, strain capacity of linepipes usually degreases with increasing strength, and this is one of the reasons for preventing wider use of high-grade linepipe for high strain application. Furthermore, external coating is necessary for corrosion resistance of pipe, but coating heat can cause strain-aged hardening, which results in increased yield strength and Y/T. Therefore, there is a strong demand for developing high strength linepipe for a high strain application with resistance to strain-aged hardening. Extensive studies to develop Grade X100 high strain linepipe have been conducted. One of the key technologies for improving strain capacity is dual-phase microstructural control. Steel plate with the microstructure including bainite and dispersed martensite-austenite constituent (MA) can be obtained by applying accelerated cooling followed by heat treatment online process (HOP). HOP is the induction heating process that enables rapid heating of the steel plates. Variety of microstructural control, such as fine carbide precipitation and MA formation, can be utilized by this newly developed heating process. One of the significant features of the HOP process is to improve resistance to strain-aged hardening. Increase in yield strength by coating can be minimized even for the Grade X100 linepipe. Trial production of X100 high strain linepipe with the size of 36″ OD and 15mm WT was conducted by applying the HOP process. Microstructural characteristics and mechanical properties of developed X100 linepipe are introduced in this paper. In order to evaluate compressive strain capacity of the developed pipe, full-scale pipe bending test was carried out by using the trial X100 high strain linepipe after external coating. Full scale bending test of developed X100 linepipe demonstrated sufficient compressive strain capacity even after external coating.Copyright

High Strength Linepipe With Excellent HAZ Toughness

The API 5L-X65 steel plates for low temperature service were produced using the thermo-mechanical control process (TMCP) with the optimum micro-alloying addition. Featuring of the additions are as low amount of titanium, calcium, niobium, and vanadium as possible, for high heat affected zone (HAZ) toughness and strength. Controlling titanium and nitrogen and the Ti/N ratio, a large number of TiN dispersed finely are formed in steel and the austenite grain size near a weld fusion line is refined remarkably owing to strong pinning effect of TiN. Calcium addition promotes ferrite nucleation, so that increase in fine polygonal ferrites makes microstructure of HAZ much finer. Niobium and vanadium content are reduced, because carbide precipitates are formed when the coarse grain HAZ is reheated around 700 degree C and the precipitation hardening deteriorates HAZ toughness. The trial manufacturing of the 19.5mm, 26.9mm and 31.4mm thick X65 grade UOE pipes was finalized with the satisfactory results. The toughness of longitudinal submerged-arc welds was more than 50 J in Charpy V-notch impact test at −30°C.Copyright

Production of Grade X80 High Strain Linepipes for Seismic Region Application

In order to achieve safety and reliability of long-distance gas transmission pipeline installed in seismic region while obtaining economical benefit by reducing material and construction cost, it is essential to apply the high-strength linepipes with sufficient strain capacity against buckling and weld fracture by seismic ground movement. At the same time, it is quite important to develop appropriate material requirement for strain capacity depending on the pipe dimension and strain demand of the region where the pipeline is installed. Grade X80 heavy gauge linepipes with excellent deformability were mass produced by applying advanced plate manufacturing technologies. These linepipes exhibit low Y/T and high uniform elongation in the longitudinal direction even after pipe coating. Strain capacity of the pipe against bending deformation with internal pressure was verified by conducting full scale pipe bending testing. In this paper, production results of high strain X80 linepipes for the application in long-distance pipelines in seismic region and full scale pipe bending and hydraulic burst test results were introduced.Copyright

Study on Unstable Brittle Crack Arrest Toughness of Extremely-Low Carbon Bainitic Steel Plates

The extremely-low carbon bainitic steel (ELCB steel) is a high strength steel with about 0.02 mass% or less carbon. In this research, unstable brittle crack arrest toughness of ELCB steel plates was investigated by temperature-gradient ESSO tests, compared with that of conventional TMCP steel plates. Both of ELCB and TMCP steel plates without pre-straining had sufficient crack-arrest toughness at 0°C. After 10% prestraining, the TMCP steel plate had not sufficient crack-arrest toughness at 0 °C . The ELCB steel plates, however, maintained high crack arrest toughness at 0°C. even after 10% pre-straining. ELCB steel were also different from TMCP steels in the correlation between transition temperature of crack arrest toughness and fracture appearance transition temperature (vTrs) obtained by Charpy impact test. When the vTrs of an ELCB steel and that of a TMCP steel were the same value, crack arrest toughness of an ELCB steel was higher than that of a TMCP steel. In the cross section of the ESSO test piece of the ELCB steels, many sub-cracks and micro-crack branching were observed. However, in the cross section of the ESSO test piece of the conventional TMCP steels, there were few subcracks and branching. Initiation of sub-cracks and branching around the main crack tip reduces the stress intensity factor of the main crack. It was considered that the above features of the ELCB steel were caused by initiation of sub-cracks and branching at the tip of the main brittle crack.Copyright

Mechanical Properties of X80 Grade UOE Pipe for Long-Term Exposure at Elevated Temperature

Recently, X80 grade UOE pipes have been planned to apply to steam injecting oil sand recovery systems to increase the volume of steam to be injected and lowering installation cost. The pipes for systems are subjected to high temperature for a long time, such as 350-C, for 20 years. Before real applications of the pipes, it is important to ensure the reliability of the pipes during and after long-term operations. In this study, in order to establish simulation conditions for 350-C × 20 years of operation, the change in microstructure and resulting mechanical properties of X80 grade pipes after a long-term exposure at elevated temperatures were investigated. Then, mechanical properties of the pipes subjected to the established simulated condition were examined. Change in the microstructure was quite small after exposure of 400-C and lower temperatures. Tensile strengths of the base metal and seam weld after up to 400-C of heat treatment can be arranged with the Larson-Miller parameter composed with temperature and holding time of the heat treatments. Therefore, heat treatments at 400-C for shorter than 20 years can be simulation conditions for the operation condition of the systems. As a result of mechanical tests simulating long-term exposure, satisfied performance of X80 grade pipes can be obtained.Copyright