Stress concentration analysis of butt welds with variable wall thickness of spanning pipelines caused by additional loads

In the field of oil and natural gas gathering and transportation, the pipeline is likely to be span under the geological load. With the influence of gravity, internal pressure and other additional loads the stress concentration at the variable wall thickness weld on the spanning pipeline would further threaten the safe operation of pipelines. In this paper, finite element method is used to establish the numerical model of variable wall thickness butt weld of free spanning pipelines. The SCF(Stress Concentration Factor) is used to reflect the stress concentration degree of the weld with variable wall thickness under the affect of additional stress, and the stress distribution of variable wall thickness welds is studied. The influence of spanning pipe length, welding positions, internal pressure, thickness ratios and conical surface lengths on the SCF of the spanning pipe are obtained. The results illustrate that under the action of additional stress the existence of variable wall thickness weld has a great influence on the welding root at the bottom of the pipeline, which is not conducive to the stress of the spanning pipeline. The change of pipe spanning length has great influence on the SCF at the weld root at the bottom of the pipe. As the length of the spanning increases, the SCF growth rate at the root of the weld is greater. The position of variable wall thickness weld has a certain impact on the SCF of spanning pipeline. As the variable wall thickness weld is close to the mid-span position of pipeline, the stress concentration part of the weld gradually transfers from the top of the pipe to the bottom of the pipe. The change of pipeline internal pressure has little influence on SCF. The wall thickness ratio and the conical surface length have a great influence on the pipeline, which is the main influencing factor of the stress concentration of the spanning pipeline. Reducing the wall thickness ratio or increasing the conical surface length can effectively reduce the stress concentration of the variable wall thickness weld.