A computer program, WINPOST-MULT, is developed for the dynamic analysis of a multiple-body floating system coupled with mooring lines and risers in the presence of waves, winds and currents. The coupled dynamics program for a single platform is extended for analyzing multiple-body systems by including all the platforms, mooring lines and risers in a combined matrix equation in the time domain. Compared to the iteration method between multiple bodies, the combined matrix method can include the full hydrodynamic interactions among bodies. The floating platform is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass coefficients, and radiation damping coefficients are calculated from the hydrodynamics program WAMIT for multiple bodies. Then, the time series of wave forces are generated in the time domain based on the two-term Volterra model. The wind forces are separately generated from the input wind spectrum and wind force formula. The current is included in Morison's drag force formula. In case of FPSO, the wind and current forces are generated using the respective coefficients given in the OCIMF data sheet. A finite element method is derived for the long elastic element of an arbitrary shape and material. This newly developed computer program is first applied to the system of a turret-moored FPSO and a shuttle tanker in tandem mooring. The dynamics of the turret-moored FPSO in waves, winds and currents are verified against independent computation and OTRC experiment. Then, the simulations for the FPSO-shuttle system with a hawser connection are carried out and the results are compared with the simplified methods without considering or partially including hydrodynamic interactions.

Tie-down systems are used to fasten drilling rigs to the deck of offshore structures during harsh environmental conditions such as hurricanes. During Hurricane Ivan (2004) and Katrina (2005), a number of offshore structures were moved and several tie-down systems were damaged. In the present study, the reaction force and connection capacity of tie-down systems for a TLP and SPAR are investigated. The environmental conditions are taken from the API Bulletin 2INT-MET which has been updated after several major storms during 2004-2005. The hydrodynamic coefficients of the TLP and SPAR are obtained using a 3D diffraction/radiation panel method. The motions of the TLP and SPAR are then simulated in the time domain by using the hull-mooring-riser coupled dynamic analysis tool CHARM3D. Based on the simulated motion and acceleration time series, the inertial and gravity loads on derrick and skid base footing are calculated. In addition to the inertial-gravity loads, wind forces exerted on the derrick are also calculated. All the external forces and resultant hull motions are simulated for 100-year, 200-year and 1000-year storms to observe the derrick structural integrity with increasing environmental intensity. Various environmental headings are also considered to find the maximum reaction forces. In the present method, the phase differences between gravity-inertia forces and wind forces are taken into consideration to obtain more realistic loads on derrick and skid base footings. This research shows that the maximum and minimum load values are appreciably higher for the SPAR. In addition, the direction of external forces is also important to determine maximum reaction forces on footings. The capacities of the clamps in slip, bolt tension, and bolt shear can be also analyzed using the resultant data to provide guidance on appropriate design values.

A computer program, WINPOST-MULT, is developed for the dynamic analysis of a multiple-body floating system coupled with mooring lines and risers in the presence of waves, winds and currents. The coupled dynamics program for a single platform is extended for analyzing multiple-body systems by including all the platforms, mooring lines and risers in a combined matrix equation in the time domain. Compared to the iteration method between multiple bodies, the combined matrix method can include the full hydrodynamic interactions among bodies. The floating platform is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass coefficients, and radiation damping coefficients are calculated from the hydrodynamics program WAMIT for multiple bodies. Then, the time series of wave forces are generated in the time domain based on the two-term Volterra model. The wind forces are separately generated from the input wind spectrum and wind force formula. The current is included in Morison's drag force formula. In case of FPSO, the wind and current forces are generated using the respective coefficients given in the OCIMF data sheet. A finite element method is derived for the long elastic element of an arbitrary shape and material. This newly developed computer program is first applied to the system of a turret-moored FPSO and a shuttle tanker in tandem mooring. The dynamics of the turret-moored FPSO in waves, winds and currents are verified against independent computation and OTRC experiment. Then, the simulations for the FPSO-shuttle system with a hawser connection are carried out and the results are compared with the simplified methods without considering or partially including hydrodynamic interactions.

The increased interest in the offshore wind resource in both industry and academic and the extension of the wind field where offshore wind turbine can be deployed has stimulated quite a number of offshore wind turbines concepts. This thesis presents a design of mono-column platform supported for 5 MW baseline wind turbine developed by the National Renewable Energy Laboratory (NREL), with a single tether anchored to the seabed. The design, based on the pioneer concept SWAY, results from parametric optimized design processes which account for important design considerations in the static and dynamic view, such as the stability, natural frequency, performance requirements as well as the economic feasibility. Fully coupled aero-hydro-servo-elastic model is established in the time-domain simulation tool FAST (Fatigue, Aerodynamics, Structures, and Turbulence) with the hydrodynamic coefficients from HydroGen, an indoor program providing same outputs as the commercial software WAMIT. The optimized model is verified by imitating the frequency-domain approach in FAST and thus comparing the results with the frequency-domain calculations. A number of simulations with various wind and wave conditions are run to explore the effect of wind speed and wave significant height in various water depths. By modifying the optimized model to a downwind turbine with the nacelle rigidly mounted on the tower and the single tether connected to the platform by a subsea swivel, the modified models are more closed to the original SWAY-concept wind turbine. These models are compared based on the platform motion, tether tension, displacement, nacelle velocity and acceleration, resonant behavior as well as the damping of the coupled systems. The results of these comparisons prove the advantage of the modified model in performance. The modified model has also clarified itself a good candidate for deep water deployment.