Towards 6 Degrees of Freedom Seakeeping Simulations Using a Fully Nonlinear Potential Flow Method

Licentiate thesis, 2020

In recent years, the International Maritime Organization introduced a new set of rules in order to try to reduce emissions of ships by improving their efficiency. To assess the energy efficiency of a new ship, the regulations require to estimate the Energy Efficiency Design Index (EEDI), which represent the amount of Carbon Dioxide produced per mile in relation to the amount of cargo carried, and verify that it is smaller than a prescribed value. For a proper evaluation of the EEDI, it is necessary to estimate the added resistance in waves with high accuracy. There are different ways to evaluate added resistance: empirical methods, adding a safety factor to the calm water resistance called sea margin, numerical simulations and model test experiments. Nowadays, the most used way during the design stage to do that is employing numerical simulations. Numerical simulations are not only used for the estimation of added resistance, but also to predict ship motions. If the motions are known at an early design stage, it is possible to modify the design of a ship to minimize them in order to improve the performance of the ship and to increase the safety and comfort of those who are on board.
The main objective of the PhD project is to evaluate added resistance and ship motions in oblique waves. In the work presented in this thesis, an existing fully nonlinear unsteady potential flow method is used to perform seakeeping numerical simulations in head and beam sea. Since viscosity is disregarded in potential flow methods but it is still very important for some cases, such as roll motion, viscous damping coefficients were added into the equation of motions. For the last two papers, an unstructured adaptive grid refinement, a nonlinear decomposition of the velocity potential, a formulation for the acceleration potential and a Barnes-Hut algorithm were introduced in the code. The method has been used to simulate roll motion in beam sea, parametric rolling, added resistance and ship motions in head waves as well as ship-ship interaction in calm water. Numerical results were compared with experiments and with other methods.
Overall, the method presented here proved to be able to handle the tested scenarios, showing a good agreement between the simulations and the experiments. The work summarized in this thesis contributed to a better understanding of the numerical method used and helped to outline the next steps to be taken in order to achieve numerical seakeeping simulations in 6 degrees of freedom in oblique waves.