Numerical analysis of wave-induced responses of floating bridge pontoons with bilge boxes
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- Avhandlinger 
In the offshore industry it has become more common to install damping ”devices” on large displacement structures like FPSO and SPAR platforms. The damping devices are often circular discs on the bottom of the structure and it is installed to reduce the motion of the structure, mainly the heave motion. In this thesis a bottom disc is installed on a bridge pontoon supposed to be located in a fjord in Norway. The objective is to study the motion behavior of the pontoon, study how it is dependent on the Cd-coefficient and develop a simple numerical model for estimating the KCnumber by vertical oscillation velocity of the pontoon. A surface model is created in the hydrodynamic analysis tool Ansys Aqwa and run for several different disc ratios in a Sea State given by the Norwegian Public Road Administration (NPRA). The main purpose of the flange is to increase the added mass of the structure and shift the Natural Period away from the wave spectrum. Several simulations with different disc ratios is performed and the natural period is shifted away from the wave spectrum when the disc ratio is increasing. However, a downside by adding the bottom disc is that a cancellation effect of the damping load is occurring at certain periods. By adopting KC-numbers and Cd-coefficients from earlier studies on similar structures, a procedure is developed for estimating the KC-number for the pontoon. By using linear potential theory and Morison drag linearization, motion analysis is performed in Aqwa by adding Morison drag elements on to the disc to obtain the effect of drag loads on the pontoon. A second and harsher sea state is tested for the same pontoon for comparison and the same procedure is carried out for this sea state. It is found that the pontoon is quite independent of the Cd- coefficient for the initial moderate sea state, where the resulting vertical velocities are very small and are giving very small values of KC-number. For the second sea state the peak period is shifted and interacting more with the resulting motion RAOs of the pontoon, resulting in larger response and hence larger vertical velocities. The resulting KC-numbers for the second sea state is within range of the initially assumed KC-numbers.
Master thesis - Technical University of Denmark. DTU Mechanical Engineering. Section of Fluid Mechanics, Coastal and Maritime Engineering.