Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been applied to offshore pipelines by using the reel-lay method and onshore in extremely low temperature environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the applicability of these HFW linepipes to seismic regions, a series of full-scale tests such as the bending test with internal pressure and the uniaxial compression test were conducted according to the seismic design code of the Japan Gas Association (JGA). Based on these full-scale tests, the safety performance of high quality HFW linepipe when applied to seismic regions is discussed in comparison with the mechanical properties obtained in small-scale tests, such as the tensile and compression properties of the base material and weld seam, focusing especially on the compressive and tensile strain capacity of HFW linepipes from the viewpoints of full-scale performance and geometrical imperfections. The results of the bending test under internal pressure and the uniaxial compression test without internal pressure complied with the JGA seismic design code for permanent ground deformation induced by lateral spreading and surface faults. In addition, a full-pipe tension test was also conducted in order to investigate the tensile strain capacity of HFW linepipes for axial deformation.

The reel-lay method is a fast and cost efficient installation process for offshore rigid steel pipelines. Pipelines installed by the reel-lay method are plastically deformed due to reeling, unreeling, aligning and straightening during pipeline installation. Therefore, local buckling is one of the major concerns from a view point of integrity in linepipes, especially around girth welds where strength mismatching arises due to adjacent pipes with different yield strength. One the other hand, the change in mechanical properties of linepipes during reel-lay, including coating process (e.g. 250°C) and long time exposure (e.g. 250°C aging) after installation is also important in order to guarantee safety of linepipes. Furthermore fracture toughness at girth weld Coarse-grain HAZ (CGHAZ) after reeling and aging should be clear to prevent brittle fracture of offshore linepipes in service. In this study, full scale reeling simulation tests of girth welded X60 HFW (High Frequency electric resistance Welded) linepipes with OD; 323.9mm and WT;15.9mm after full body heat treatment (coating simulation) were conducted at 5cycles and 2cycles reeling and straightening situations when yield strength mismatches are present between adjacent pipes around girth welds. Localized strain concentration was observed near girth welds by strength mismatching of adjacent pipes. DNV ovality increased with increasing reeling and straightening cycles, however the ovality did not exceed 10%, which was a criterion value for local buckling, after 5cycles reeling simulation test. The change ratio of wall thickness after full scale reeling simulation tests were about ±2% (within DNV-OS-F101 tolerance) regardless of circumferential and longitudinal direction of pipes. Longitudinal tensile properties could be characterized by axial last introduced plastic strain. That is, in the positive number of last introduced plastic strain, YS and Y/T increased, while uEl decreased by work hardening effect. On the other hand YS and Y/T decreased, while uEl increased at the negative number of last introduced plastic strain by Bauschinger effect. Circumferential tensile properties could be also characterized by axial last introduced plastic strain. Yield strength and Y/T slightly increased while uniform elongation slightly decreased by aging at longitudinal and circumferential direction. Tensile properties did not change irrespective of the number of cycles of reeling simulation tests. After 5cycles reeling simulation test and aging, girth welded CGHAZ CTOD values were over 0.4mm at −20°C and Charpy absorbed energy were over 200J at −30°V, therefore, it was considered that the brittle fracture of the girth welded linepipe unlikely occur at reeling and aging situation in this study.

Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been used in pipelines in reel-lay applications and low temperature service environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the applicability of these HFW linepipes to the seismic environment, a series of full-scale tests such as bending test with internal pressure and uniaxial compression test were conducted according to the seismic design code in Japan gas association (JGA). Based on the above-mentioned full-scale tests, the safety performance of high quality HFW linepipe to apply to the seismic region is discussed in comparison with the mechanical properties in the small-scale tests such as the tensile and compression property of the base material and weld seam, especially focused on the strain capacity of HFW linepipe from the view points of full-scale performance and geometrical imperfection. Test results of the bending test with internal pressure and the uniaxial compression were complied with the JGA seismic design code for the permanent ground deformation induced by lateral spreading and surface faults.

This paper presents experimental and analytical results focusing on the strain limit of X80 linepipe. Ductile crack growth behavior from a girth weld notch is simulated by FE analysis based on a proposed damage model and is compared with the experimental results. The simulation model for ductile crack growth accompanied by penetration through the wall thickness consists of two criteria. One is a criterion for ductile crack initiation from the notch-tip, which is described by the plastic strain at the notch tip, because the onset of ductile cracking can be expressed by constant plastic strain independent of the shape and size of the components and the loading mode. The other is a damage-based criterion for simulating ductile crack extension associated with damage evolution influenced by plastic strain in accordance with the stress triaxiality ahead of the extending crack tip. The proposed simulation model is applicable to prediction of ductile crack growth behaviors from a circumferentially-notched girth welded pipe with high internal pressure, which is subjected to tensile loading or bending (post-buckling) deformation.

Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been used in pipelines in reel-lay applications and low temperature service environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the safety performance of these HFW linepipes, a series of full-scale tests including a hydrostatic burst test at low temperature, fatigue test and tension test under high internal pressure was conducted, together with small-scale tests such as impact energy and standard fracture toughness tests, which are generally used in mill production and pre-qualification tests. The Charpy transition curve of the developed HFW pipe occurred at a temperature much lower than −45°C. Based on the low-temperature hydrostatic burst test with a notched weld seam at −45°C, the weld of the HFW linepipe presented the same level of leak-before-break (LBB) behavior, as observed in UOE pipes. A full-pipe fatigue test of HFW pipes with repeated internal pressurizing was conducted. The fatigue strength of the developed HFW linepipe shows better performance than butt weld joints because of the smoothness at its weld portion, which is achieved by mechanical grinding of the weld reinforcement. Full-scale pipe tensile tests of girth welded joints were performed with an artificial surface notch at the heat affected zone in the girth weld. The influence of internal pressure was clearly observed in these tests. Based on the above-mentioned full-scale tests, the safety performance of high quality HFW linepipe is discussed in comparison with the mechanical properties in the small-scale tests such as the Charpy and standard fracture toughness tests, especially when the notch was placed in the welded seam.

Reel-lay method is widely recognized cost effective installation process for offshore pipelines. In the process of reel-lay, cyclic plastic strain is expected at pipelines due to reeling, unreeling, aligning and straightening. In order to maintain integrity of pipelines subjected to large bending deformation it is necessary to understand the bending deformability and change in mechanical properties of pipelines. Advanced HFW (High Frequency electric resistance Welded) linepipe “MightySeam ® ” has superior low temperature toughness at seam weld and base material and has some advantages in application to offshore pipelines from a viewpoint of change in toughness due to pre-straining, coatings and long time exposure. In this paper full scale reeling simulation tests of X65 HFW linepipe were conducted in order to study bending deformability and the change in mechanical properties. The tests were performed by bending pipe alternately to reeling former and straitening former with different radii. Girth welding were applied to test pipes in order to investigate effect of strain concentration around girth weld by strength mismatch or weld reinforcement. Small scale reeling simulation tests with/without aging were also conducted and the results were compared to those of full scale reeling simulation tests. In full scale reeling simulation test, some strain concentration arose around girth weld by strength mismatch and local contact of weld reinforcement to former. Some ovalizations were advanced during tests, however local buckling could not be recognized throughout pipes even if strain concentration occur around girth welds. Tensile properties were changed proportional to last introduced plastic strain as a result of Bauschinger effect and work hardening at strain history. Low temperature toughness of base material and seam weld did not change significantly by pre-straining or aging.

One general method for improving the fatigue strength of welded joints is introduction of compressive residual stress by peening. However, there is concern that the fatigue strength of the welded joint may decrease if excessive preloading is applied after peening. It has been found that fatigue strength decreased after applying compressive preloading to a welded joint due to cancellation of the compressive stress at the weld toe. In the present research, the influence of excessive preloading on the fatigue strength of welded joints with compressive residual stress at the weld toe was clarified by experiments using hammer peening with an improved pin. When hammer peening was applied to welded joints, increasing the radius of the weld toe reduced the decrease of compressive residual stress due to excessive preloading. As a result, the decrease of the fatigue strength of the welded joint was also reduced.

This paper presents the experimental and analytical results focused on the compressive and tensile strain capacity of X80 linepipe. A full-scale bending test of girth welded 48″ OD X80 linepipes was conducted to investigate the compressive strain limit regarding to the local buckling and tensile strain limit regarding to the girth weld fracture. As for the compressive buckling behavior, one large developing wrinkle and some small wrinkles on the pipe surface were captured relatively well from observation and strain distribution measurement after pipe reaches its endurable maximum bending moment. The tensile strain limit is discussed from the viewpoint of competition of two fracture phenomena: ductile crack initiation / propagation from an artificial notch at the HAZ of the girth weld, and strain concentration and necking / rupture in the base material. The ductile crack growth behavior from the girth weld notch is simulated by FE-analysis based on the proposed damage model, and compared with the experimental results. In this report, it is also demonstrated that the simulation model can be applicable to predicting ductile crack growth behaviors from a circumferentially notched girth welded pipe with internal high pressure subjected to post-buckling loading.

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.