Numerical Study on the In-Service Welding Stress of X80 Steel Natural Gas Pipeline

In this study, the welding stress of in-service welding on the X80 steel pipeline was investigated using the 3D finite element method. The parameters of heat source and axial and hoop welding stress were verified in the experiment. As shown in the results, in the heat-affected-zone (HAZ) location of the pipeline and sleeve, the outer wall was predominantly under compressive stress, while the inner wall was mainly subjected to tensile stress. The hoop stress (<i>σ_h</i>) is greater than the axial stress (<i>σ_a</i>). The maximum hoop stress is primarily concentrated at the connection point between the fillet weld and the sleeve, and its value exceeds the yield strength of X80 steel. Excluding the start-end region, the axial stress distributed in the circumferential direction remains at an almost constant value. The hoop stress values exhibit an approximately symmetric distribution, with relatively higher values at 0° and 180° and relatively lower values at 90° and 270°. Compared with axial stress, the influence of natural gas pressure and flow rate on the hoop stress of the pipeline is more pronounced. When the natural gas pressure increases from 0.5 MPa to 2.5 MPa and the flow rate increases from 1 m/s to 20 m/s, the hoop stress of the pipeline increases by 3.18% and 15.42%, respectively. Moreover, the influence of the preheating temperature on the axial stress of the sleeve is highly prominent. When the preheating temperature is elevated from 20 °C to 300 °C, the axial welding stress of the sleeve increases by 115.3%. These results indicate that maintaining the natural gas pressure at 1 MPa, keeping the flow rate below 12 m/s, and controlling the preheating temperature at approximately 50 °C can enhance the performance of the fillet weld during the in-service welding of X80 steel pipelines.