Influence of Natural Gas Pipeline Explosion on Material Performance of X80 Steel Pipe Laid in One Ditch

Due to geographical restrictions, it is often necessary to lay two pipelines in the same ditch during pipeline construction. However, as developments in natural gas pipelines increasingly incorporate higher grades of steel, larger diameters, and higher pressures, the consequences of an accident caused by leakage, explosion, or ignition become progressively more severe. The explosion occurred in one pipe would affect the other pipe laid in the same ditch, resulting in subsequent explosion. In this study, a full-scale explosion test was conducted on two X80 steel pipes with diameters of 1422 mm and 1219 mm, laid in one ditch and set up in parallel arrangement, with a spacing of 36 m between one another, and a burying depth of 1.5 m. Some X80 steel samples were selected to assess materials performance in different areas of a given pipe in the same ditch by deploying selective explosions. The mechanical performance tests on a number of points including tensile strength, impact, and hardness were executed by the Mechanical Testing and Sensing (MTS) universal testing machine. Drop-Weight Tear Test (DWTT) of the selected samples were executed by the DWTT pendulum test machine. Metallography and microstructure analysis of the steel samples was performed by the MEF4M metallographic microscope and image analysis system. Results indicate that limited effects of jet fire on the macroscopic tensile properties and impact toughness of the pipelines, but a more obvious softening effect on the steel surface material. Drop-Weight Tear Test and microscopic observation revealed that jet fire can significantly reduce crack propagation resistance on the surface of the pipeline, pointing to deformation behavior and other concerns that could impact pipeline safety and efficacy. The results of this experimentation can help to improve pipeline construction, resulting in better safety.

Effect of Notch Type in Drop-Weight Tear Test Specimen on Crack Propagation and Arrest Behavior

The drop-weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle fracture propagation. However, in the recent years there is an ambiguity in its evaluation if inverse fracture appears on the specimen fracture surfaces. Although cause of the inverse fracture is not fully understood, compressive pre-straining near the impact hammer and existing tiny split have been discussed as a possible cause. In this paper, machined notch in brittle weld DWTT for X65 was performed and compared with various notch types of DWTTs such as conventional DWTT specimen with a pressed notch (PN), a chevron notch (CN) and a static pre-cracked (SPC). The fracture appearances were compared with different strength X65 - X80 grades linepipes and with different initial notch types. The frequency of the inverse fracture appeared in these DWTTs were different in each material and each specimen types, but there were no cases where the inverse fracture did not occurs. The purpose of DWTT is to evaluate the brittle crack arrestability of the material in a pressurized linepipe. A large scale brittle crack arrest test, so called West Jeferson test is generally used to reproduce crack propagation and arrest behavior in an actual pipeline material. A middle scale test so called Crack Arrest Temperature (CAT) test was also proposed to check the shear area fraction measured in DWTT with API rating with that the local shear lip thickness fraction in those tests. CAT test can well reproduce crack propagation and arrest behavior under the condition of brittle crack initiation from the initial notch.

Inverse Fracture in DWTT and Brittle Crack Behavior in Large-Scale Brittle Crack Arrest Test

The drop weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle fracture propagation. Although there is an ambiguity in the evaluation of DWTT results if inverse fracture appears on the fracture surfaces, the cause of inverse fracture is not yet fully understood. In the present work, DWTTs were performed with X65, X70, and X80 steel linepipes. In addition to the conventional DWTT specimen with a pressed notch (PN), PN specimens with a back slot (BS) and specimens with a chevron notch (CN) or static precrack (SPC) were also examined, and the fracture appearances in different strengths and different initial notch types were compared. Although the frequency of inverse fracture in these DWTTs was different with each material and each specimen type, there was no material or specimen type that was entirely free from inverse fracture. The purpose of the DWTT is to evaluate the brittle crack arrestability of the material in a pressurized linepipe. Therefore, the DWTT results should be examined with a running brittle crack arrest (BCA) test. A large-scale BCA test with temperature gradient was also performed with the X65 mother plate, and the shear area fraction measured in the DWTT fracture surface was compared with the local shear lip thickness fraction in the BCA test. Based on the results, the count of inverse fracture in the DWTT was discussed in comparison with the long BCA behavior in the BCA test.