The onset of pipewall failure during “in-service” welding of gas pipelines

This study demonstrates a numerical model to predict the onset of pipe-wall failure for circumferential fillet and longitudinal weld. Numerical methods have been used in the assessment of welding conditions for the safe ‘in-service’ welding of high-pressure gas pipelines. This has given useful information for the prediction of thermal cycles leading to an estimate of the heat-affected zone (HAZ) hardness and possible cracking. This paper also discusses a new mathematical description of a heat-source representing, a commonly used, manual metal arc (MMA) welding with hydrogen-controlled electrodes. Empirical relationships between welding process inputs, weld bead sizes and shapes define the weldment geometry and control the heat source co-ordinates. The formation of a new three dimensional power density distribution function for a low-hydrogen electrode is also presented. Finite element models using this heat-source have given acceptable correlation with experimental and field welds. The prediction of burn-through has been achieved using thermo-elastoplastic model. This study has investigated earlier work and translates the temperature field into an effective radial deflection in the pipe-wall thickness. This information can be used to calculate a safe working pressure during ‘in-service’ welding. The simulation of the early stages of pipe-wall failure of circumferential fillet welding using a thermo-elastic plastic analysis has been successfully demonstrated.