Prediction of Hydrodynamic Loading on Floating Offshore Platforms

This dissertation reports on the prediction of the unsteady hydrodynamic forces that act on bluff bodies due to the action of a steady current. Bridge piers and tower structures are common bluff bodies in civil engineering, and a floating Tension-Leg Platform (TLP) is another example of bluff bodies frequently being used in offshore engineering. In this study, the predictions are obtained from the numerical simulations which were performed using a state-of-the-art, computational fluid dynamics (CFD) code which solves the three-dimensional, time-dependent form of the equations governing conservation of mass and momentum. FLOW-3D is a CFD commercial code developed by Flow Science Inc., which we modified and compiled this code to include the proposed modifications in the k-epsilon model. The FLOW-3D code also possesses a treatment to track the movement of the free surface using the Volume of Fluid algorithm as opposed to adopting the usual solid-lid approximation.;The effects of turbulence were accounted for using two very different approaches which are Large-Eddy Simulations (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS). The former approach finds suitable approximations to account for the influence of the small-scale eddies by incorporating a sub-grid scale model. Generally the small eddies are universal, random, homogeneous and isotropic, but the large energy-carrying eddies were directly computed since unlike small eddies they do not have universal characteristics. The latter approach utilized a two-equation turbulence closure that has been extended to capture the occurrence and consequences of vortex shedding from bluff bodies.;The turbulence closures in k-epsilon model fail to capture the correct level of fluctuations in the pressure field associated with the vortex shedding. This deficiency is due to not considering the effects of the interactions between the large-scale organized periodicity of the mean flow and the random, small-scale high-frequency motions that characterize turbulence. To overcome this failure, the effects of the mean-flow periodicity on the energy-transfer process was accounted by modifying the dissipation production term in the turbulence energy dissipation equation due to the actions of the vortex shedding. The turbulence closure with the proposed modifications is to be called the modified k-epsilon model. We explored in this dissertation the capabilities of each modeling approach when applied to a large-scale structure of the type frequently encountered in practice.;The modified k-epsilon model was employed in earlier studies to model the hydrodynamics of a two dimensional square cylinder surrounded by a flow. In this dissertation, the proposed modifications in k-epsilon model were utilized to model the wake region downstream of the 2-D and 3-D square cylinders due to the action of a steady current for Reynolds numbers ranging from 10 4 to 105. The comparisons between the global parameters such as the Strouhal number, mean and r.m.s. values of lift and drag coefficients predicted by the modified k-epsilon model with the measured values from the experiments and results from alternative closures such as Large-Eddy Simulations verified the capability of the modified model to accurately predict the unsteady hydrodynamic forces and hence the flow characteristics for the mentioned higher Reynolds numbers.;Three dimensionality of vortices in the wake of a square cylinder was also studied in this dissertation by performing simulations using the modified k-epsilon model and Large-Eddy Simulation for Reynolds number of 10 5. Similarly, both of the models predicted different magnitudes and frequencies for the time series of the global parameters such as the drag and lift coefficients at different heights of the cylinder which confirmed the existence of three dimensional non-uniform vortices in the wake of the cylinder.;In addition, hydrodynamic loads on the various elements of a TLP, and of the effects of the free surface movement on this loading were quantified due to the effects of different uniform current incidences. The test case chosen for this purposes of this assessment was the case of a conventional TLP in a steady current at Reynolds numbers of 7.5x106 and 7.5x107 which are representative of those encountered in deep-sea operations. Experiments and field observations have indicated that the resulting flows exhibit a number of complicated features due to the interactions between the shed vortices and the various structural components of the TLP. Many but not all of these features were captured by the present computations.;Loading on the columns of the TLP were found to be highly affected by the presence of a moving free surface. This was concluded from the comparisons made between the time series of drag and lift coefficients for the TLP members predicted by the modified k-epsilon model and LES by utilizing the volume of fluid technique to track the evolution of the free surface. In addition, the extent of the shielding from the front members of the TLP, located upstream, on the aft columns and pontoons was discussed by analyzing the predicted streamwise and transverse hydrodynamic forces of the aft members.;The predicted hydrodynamic loading on the TLP members were shown to be very dependent on the current's angle of incidence. This was illustrated by demonstrating the flow field's perturbations in the wake of the TLP members by changing the TLP flow angle of incidence by 45°. Finally, a large discrepancy was shown to exist between the global parameters by comparing the engineering design code estimates for the columns and pontoons of the TLP with the predictions obtained from the present and previous studies, where in estimating the drag coefficients for the pontoons, the engineering design code was concluded to be conservative, while it underestimates the same parameter for the columns which was shown to be attributed to the substantial interference effects as discussed in this dissertation.