The prediction of burn-through during in-service welding of gas pipelines

Abstract

Numerical methods have a useful role in the assessment of welding conditions for the safe in-service welding of high-pressure gas pipelines. These have been used for the prediction of thermal cycles leading to an estimate of heat-affected-zone (HAZ) hardness and possible cracking. In earlier work burn-through limits have only been considered indirectly, i.e. based on the maximum temperature of the pipe wall. In this paper, two significant research aspects of the numerical simulation of in-service welding have been addressed as follows: 1. A new mathematical description of a heat-source has been formed to represent the common in-service welding process, i.e. vertical-up and vertical-down manual metal arc (MMA) welding with hydrogen controlled electrodes. Empirical relationships between welding process inputs, weld bead size and weld bead shape define the weldment geometry and control the heat source co-ordinates. Finite element models using this heat-source have given good correlation with experimental and field welds. With this approach adequate agreement between predicted weld penetration, weld cooling times and HAZ hardness, has been made. 2. The prediction of burn-through has been achieved using a full thermo-elastic–plastic model, but this leads to lengthy calculations. Here, a new approximate method of predicting burn-through has been developed and shown to give industrially useful results. This is based on translating the temperature field into an effective cavity in the pipe-wall thickness, and using this information to calculate a safe working pressure during in-service welding.