Modelling of Induction Heating for Offshore Pipeline Field Joints: A Systematic Approach for Development of a Simulation Model Leading Towards Understanding of Induction Heating Parameters for Optimised Heat Profile Design

Induction heating is used to heat offshore pipeline field joints to a required temperature for application of anti-corrosion coatings. The objective of induction heating is to obtain a uniform surface temperature of typically 230 ± 10 °C in a time-span of minutes. The desired temperature profile of the field joint, also known as heat profile is usually obtained in an systematic experimental approach by fine tuning of the coil geometry and power settings. The experimental approach contains many individual experiments which is time and resource consuming, especially whenever a complex geometry (a collar) is present in the field joint. For the experimental approach, Heerema Marine Contractors mainly relies on know-how about induction heating provided by subcontractors. In future, development of deep water oil fields will result in high operational flow line temperatures. This development increases the performance requirements for anti-corrosion coatings, leading towards increased requirements for more equally heated field joint surfaces. The challenges of uniform surface heating of collars, the dependence on subcontractors and the future developments are the motivation for this research project. Heerema Marine Contractors has the ambition to improve control of the induction heating process, to be able to deliver good quality flow lines required for future developments. The objective of this research project is the development of an induction heating prediction model based on understanding of the underlying physics. Modelling of induction heating is a complex field of engineering which only gained interest since the 1990's due to major improvements in computing power of PCs. Little knowledge is available in literature about development of an accurate induction heating prediction model. Therefore it was decided to use a systematic approach in which a prediction model was developed from a basic cylinder model by various steps into a full-scale model including a complex geometry. Firstly, material properties were obtained. Literature and analysis of Maxwell's equations showed that accurate evaluation of material properties and its temperature dependence is a key towards accurate predictions of the induction heating process. Secondly, a basic model was developed based on a simplified geometry of the field joint. Due to limited time and resources it was chosen to perform frequency-transient analyses. Therefore non-linearities in magnetic fields were linearised. This model was verified by use of a semi-analytical equations and found to be suitable for further development. After verification of the basic model, the model was used to model small-scale geometries including collar. It was concluded that hysteresis losses can have a significant contribution in high frequency induction heating and must be taken into account in modelling to obtain accurate temperature predictions. A simplified method was introduced to account for hysteresis losses in linearised magnetic fields. After validation with high frequency experiments, the model predicted for 90% of all data within ±10 °C of experimental data. The largest offset with respect to experimental data was considered to be the minimum achieved accuracy. The accuracy was Tpredicted = Tmeasured ± 13 °C. The small-scale model was used as basis for a full-scale model in which a typical offshore pipeline field joint was modelled. Due to high magnetic field intensities it was concluded that saturation in the magnetic flux density has a significant effect on the induction heating process. The model was validated by use of experiments. Two validation steps were included: first without collar and second with collar. The model predicted for 88% of all data within ± 10 °C of experimental data based on a field joint without collar. Based on the largest offset with experimental data the accuracy of the model was Tpredicted = Tmeasured ± 15 °C. For modelling of a field joint with collar, accurate results could only be obtained in validation for a limited range of the magnetic field intensity due to non-linear limitations in the solver technique. The required accuracy of ±10 °C was too ambitious for the model developed in the thesis project. However, the limitations can be overcome when a full time-dependent solver would be used, which results in long simulation times. During small-scale experiments it was observed that the induction heating process is very sensitive and therefore repeatability of the induction heating process is limited. During full-scale experiments for the validation of the model, large temperature deviations on the field joint were observed on positions that should have had an equal temperature. The precision of experimental measurements was limited, since deviations up to 11 \degree C were present. Therefore it was concluded that further development and validation of an accurate prediction model is limited by the experimental precision. The research project was the start of a greater project within Heerema Marine Contractors, in which the model developed during the thesis project can be further developed for optimisation of coil design. For further development it is recommended to improve the experimental methods to obtain validation data. A second recommendation is accurate determination of material properties of the alloys used in the field joint. Furthermore a time dependent solver should be selected to improve the non-linear magnetic relationship simulations for high magnetic field intensity around the collar.Aerospace EngineeringAerospace Structures and Material