Abstract

The replacement of metallic pipes used in the energy industry by corrosion-free, lighter and flexible, non-metallic pipes has increased dramatically over the past decades. Single layer thermoplastic pipes have been used extensively in low pressure water and gas applications whereas three-layer thermoplastic composite pipes (TCP) increasingly are being adopted in higher pressure applications, such as oil and gas flowlines and water injection lines. The majority of TCP are joined by metallic connectors, unlike single layer thermoplastic pipes which are usually joined by electrofusion welding. Extending the capability of electrofusion coupling to high pressure TCP offers the advantage of having a full non-metallic system (i.e., corrosion-free), improved sealing capacity and potentially a lower cost solution for some selected applications. However, joining TCP using electrofusion welding poses a number of challenges. Among the important considerations, special attention should be given to the behaviour of the reinforced composite laminate layer during the welding process, as excessive heat may disturb the material microstructure and hence the mechanical properties of the pipe. Finite element modelling is required to understand the effect of welding parameters on the thermal response of TCP during electrofusion welding. In this study, a 2D multiscale model representing the pipe/fitting assembly is developed using COMSOL Multiphysics software. The purpose of this model is to simulate the transient heat transfer during electrofusion welding of Glass-PE TCP with the aim of optimizing the welding parameters and coupler design to achieve the required long-term joint performance. Temperature dependent material properties, thermal surface resistivity, material phase change and thermal expansion are accounted for in the model. A special attention is given to the heat source model in order to better simulate the heat input into the pipe material during the welding process. A sensitivity analysis is carried out to determine the influence of various parameters on the heat transfer including the power input, heating wire size and depth, contact between the fitting and the pipe, and thickness of the external pipe layer. An experimental validation is conducted with a real pipe/fitting, and the temperature profile is recorded during the entire welding cycle utilizing an infrared camera and pre-installed thermocouples at different depths and locations in the pipe/fitting assembly. The results of the experiment are used to calibrate the key model parameters at different welding times.