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Master's thesis in Offshore Technology Subsea pipelines, when exposed to free spans, can experience vortex-induced vibrations. This phenomenon was described as a resonance condition occurring when the vortex shedding frequency and the natural frequency of a structure approach common oscillation frequency. Fatigue life of the pipeline can be adversely affected by a high amplitude oscillations attributed to the vortex-induced vibrations. A numerical study has been performed on the effects of wall proximity on the vortex shedding of an elastically mounted circular cylinder. In addition, the study was extended to investigate the influence of a second cylinder with a smaller diameter rigidly coupled with the large cylinder. Such configurations can be regarded as a model of a subsea pipeline or, in case of coupled cylinders, a subsea piggyback pipeline in a free span situation. A series of two dimensional, numerical studies using open source CFD code OPENFOAM has been performed. Simulations were performed in two flow regimes, a laminar vortex street regime at Reynolds number Re = 200 and an upper transition regime at Re = 3.6×10e6. A range of reduced velocities covering a frequency lock-in phenomenon was investigated. Hydrodynamic forces and response amplitudes were mapped with respect to the reduced velocity. Furthermore, a study of the phase differences between the hydrodynamic forces and cylinder displacements was conducted. The motions of the cylinder were recorded and presented on the trajectory plots. The frequency components of hydrodynamic forces and displacements were analyzed with the FFT algorithm in the frequency domain. In order to gain insight into the effects of the shear layers interaction in the area around the oscillating cylinder, flow visualizations of the numerical simulations were analyzed.

Floating offshore wind turbines (FOWTs) are installed to effectively capture energy from offshore wind in deeper water areas where building bottom-fixed offshore wind turbines is not practical. The present study focuses on the application and fatigue life of suspended inter-array power cables in floating offshore wind farms. A spar type and a semi-submersible type FOWTs are modeled in the dynamic analysis software OrcaFlex and validated against reference studies. The power cable connecting the FOWTs is installed fully suspended with attached buoys. The impacts of marine growth on the power cable and the non-linear bending behavior of the cable are considered in the analyses. Representative environmental conditions for the North Sea are applied to assess the dynamic responses of the model. Different cable configurations connecting two spar FOWTs are evaluated based on Fitness Factors using static and dynamic analyses. The number of buoys is identified as the main parameter determining the maximum effective tension and the minimum bending radii of the cables. The vertical excursion of the suspended cable depends on the buoy spacing. The effect of cable length and marine growth on cable tensions and curvatures is small. Two semi-submersible FOWTs are connected using the most suitable cable configuration obtained for the two spar FOWTs. The impact of the floater type on the tension and curvature of the power cable is determined to be very small. A two-turbine configuration with various environmental load directions is identified to be representative of the suspended power cables design in whole wind farms, according to analyses of power cable behavior in various offshore wind farm layouts. To investigate the fatigue life of the power cable, the relationships between the axial tensions and curvatures of the power cable and its stresses are obtained by cross-section analysis in UFLEX. The optimum cable in the spar setup has a fatigue life of 85 years, with curvature-induced stresses being identified as the primary cause of the fatigue damage. The power cable's fatigue life reduces when marine growth effects are considered. A decrease in the number of buoys attached to the cable and the use of semi-submersible FOWTs increase the fatigue life. The armor wires are the critical components, and the highest fatigue damage always occurs adjacent to the buoys. The largest fatigue damage results from environmental loads approaching the power cable from a crossflow direction.

Numerical simulations of turbulent flow around stationary and vibrating cylinders at Re = 3900 are performed using the Large Eddy Simulation (LES) approach. Detailed convergence and validation studies confirm feasibility of the present model to accurately predict the Vortex-Induced Vibrations (VIV) and turbulent wake characteristics. The thesis is divided into seven chapters. First a brief introduction to the topic is given. Chapter 2 gives a brief review of the theory for flow around circular cylinders. In chapter 3 the theory of Computational Fluid Dynamics (CFD) is given. In chapter 4 a series of simulations for a stationary cylinder configuration with different mesh and time step parameters are performed to find an optimal balance between the computational cost and accuracy. Furthermore, simulation results are compared with an extensive dataset from multiple experimental studies showing a very good agreement of the present simulation results with the experimental data. In chapter 5 simulations of a self-excited cylinder vibrating in the cross-flow direction are performed at three different values of reduced velocity. The flow fields are analyzed and different vortex shedding modes are categorized and then compared with the vortex shedding map derived from forced oscillation tests. Proper Orthogonal Decomposition (POD) is performed on sampled data in the wake of the cylinder to analyze the turbulent flow. The present numerical model is found to give accurate predictions with respect to essential parameters affecting the fluid-structure interaction of circular cylinder in the subcritical regime. In chapter 6 a conclusion of the results is presented and chapter 7 presents recommendation for future work.

Master's thesis in Offshore Technology Two degree-of-freedom (2-DoF) flow-induced vibrations (FIV) of a single cylinder and two rigidly coupled cylinders are numerically investigated at the Reynolds number of 3.6 × 10^6. Two-dimensional (2D) Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are performed combined with the k-ω SST turbulence model. A low mass-damping system is considered with the mass ratio set to 2 and the damping ratio to zero. For the coupled cylinders configurations, the diameter ratio is set to 0.25. A parametric study is performed to analyse the influence of the reduced velocity (Ur), the position angle (α) of the small cylinder relative to the large cylinder and the gap ratio (G⁄D) between the cylinders on the FIV response of the system. For the single cylinder, the simulations are performed for 2.5≤Ur≤12, and for the coupled cylinders, for 2≤Ur≤12. To analyse the effect of α, three values are considered, [α=0°, 90°, 180°] with a constant value of G⁄D=0.1. The effect of G⁄D is studied for the α=90° configuration in which the G⁄D=0.25 and G⁄D=0.5 configurations are analysed in addition to the G⁄D=0.1 cases. A good grid convergence is obtained and the numerical model is validated against published results for a stationary single cylinder. It is found that the lock-in regime extends beyond Ur=12 for the α=0°, 180° and α=90° at G⁄D=0.1, 0.25 configurations compared with the single cylinder configuration. Galloping response is observed when the small cylinder is placed at α=90°, with G⁄D=0.1, and at α=180°. In addition, for the α=90° configuration, the results obtained for C_D, C_L and A_(y,max)⁄D converge to those of the single cylinder with the increase of G⁄D.

Master thesis in Offshore technology : subsea technology Free-spanning subsea pipelines may experience vortex-induced vibrations (VIV) when exposed to fluid flow. These resonant motions can cause fatigue-related problems and in some cases failure of the pipe. A common configuration is to have two pipelines laid next to each other. This further complicates the dynamic response, as the downstream pipeline is affected by the flow-interference induced by the upstream pipe. Two cylinders in tandem arrangement in the vicinity of a horizontal plane wall have been investigated numerically using 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a k-ω SST turbulence model. The cylinders are modelled as elastically supported with two-degrees-of-freedom (2DoF) which allow for both transverse and inline vibrations. Based on the cylinder diameter, D, the Reynolds number investigated in the present study is Re=3.6×10^6, corresponding to the transcritical flow regime. The effects of three main parameters are investigated; (i) gap ratio, G^*, is the gap distance from the horizontal boundary to the cylinder surface, (ii) cylinder spacing, L^*, is the horizontal surface-to-surface distance of the tandem cylinders (iii) reduced velocity, U_r, is the ratio of flow velocity to vibration frequency and diameter. Numerical simulations are performed for 〖U_r={4,5},G〗^*={1.5,2},L^*={4,5,6} using the open source CFD code OpenFOAM. The mass ratio is set to m^*=10, damping ratio to ζ=0, and boundary layer thickness δ/D=0.48. The model is validated against similar published studies. The single static cylinder model is in a good agreement with similar numerical studies. The model is modified to include a second downstream cylinder and allow for 2DoF vibrations. A second mesh sensitivity study is performed to determine the appropriate mesh density for the tandem case. Hydrodynamic forces, response amplitudes, motion trajectories, flow fields and frequency power spectra for displacements and hydrodynamic coefficients are analyzed and presented with respect to the parameter matrix defined in the study.