Numerical modelling of fully coupled vortex-induced vibration of elastically mounted 3d offshore spar platform

Abstract

This thesis presents a comprehensive investigation into the numerical analysis of canonical flow problems and the subsequent development of a vortex-induced vibration (VIV) model for offshore spar platforms. The study utilizes in-house MATLAB coding and the OpenFOAM computational fluid dynamics (CFD) software, incorporating turbulence modelling techniques such as Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) to address the complexities associated with turbulent flows. In the initial part of the thesis, a systematic analysis of canonical flow problems is conducted to establish a solid foundation for understanding fluid dynamics phenomena. Several canonical flow scenarios, including lid-driven cavity, and flow past a static cylinder, are considered. The governing fluid dynamics equations are solved using a meticulously developed in-house MATLAB code, which employs appropriate numerical schemes and boundary conditions. The accuracy and reliability of the code are validated by comparing the obtained results with existing analytical solutions and experimental data. This is further extended to an in-house VIV model with an elastically mounted cylinder experiencing the fluid flow. The second segment of the thesis focuses on addressing the vortex-induced vibration phenomenon prevalent in offshore spar platforms. VIV poses a significant concern for these structures, as the excessive vibration can lead to fatigue damage and structural failure. Mooring lines and other attachments can also get damaged in case of tethered floating structures. In order to simulate and predict VIV behaviour accurately, a specialized 3D fully-coupled VIV model is developed utilizing the C++ based open-source flow solver OpenFOAM. The model incorporates turbulence modelling techniques, including RANS and LES, to capture the complex turbulent flow features and their impact on VIV. This allows for a more realistic representation of the flow characteristics around the offshore spar platform. The RANS is used in the case of two-way coupled model, whereas, LES is employed for one-way coupled vibration model. The developed VIV model with turbulence modelling is meticulously validated against available experimental data. The validation process involves a comprehensive comparison of predicted VIV response amplitudes, RMS (root mean square) lift coefficient for different reduced velocities, with the corresponding measured values, ensuring the accuracy and reliability of the model. The successful validation of the VIV model with turbulence modelling confirms its capability to accurately predict the VIV behaviour of offshore spar platforms under turbulent flow conditions. Furthermore, the VIV study extended to a multi degrees of freedom system, and effect of the spring stiffness on VIV response amplitude, frequency, etc. are studied.The thesis concludes by providing a detailed discussion on the findings, limitations, and potential areas for future research. The inclusion of turbulence modelling using RANS and LES in the VIV model enhances the accuracy of predicting the VIV characteristics in turbulent flow environments. Furthermore, the numerical results obtained from solving canonical flow problems using the in- house MATLAB code and incorporating turbulence modelling techniques demonstrate the effectiveness of the approach in accurately simulating and analysing a wide range of fluid dynamics phenomena.