Fully Nonlinear Unsteady Three-Dimensional Boundary Element Method for Ship Motions in Waves

Kjellberg, Martin.

PhD Thesis. Chalmers University of Technology, Göteborg, n.d. 2013.


The prediction of the behavior of ships in a seaway has been an important topic in ship design since the beginning of modern naval architecture. In recent years, computer simulations using numerical methods based on mathematical models of the ship hydrodynamics problem has become a valuable tool for researchers and engineers working in this field. A large variety of methods with different levels of sophistication and accuracy already exist today, all being suitable for specific problems. However, with the current trend of larger, more advanced and optimized designs and focus turning more and more towards safety and energy efficiency, nonlinear phenomena are becoming more important and many of the traditional methods have problems in producing accurate results for certain cases due to assumptions and neglected physical effects in the underlying mathematical model. The overall objective of this project is the development and implementation of a fully nonlinear unsteady three-dimensional boundary element method for ship motion analysis, applicable to problems with a high degree of nonlinearities while still being suitable for practical use. The method is capable of predicting wave induced loads and motions with actual forward speed, taking into account implicitly all kinds of nonlinearities, i.e. higher and lower order frequency components, hull shape above the calm water level and self interaction between the forward speed flow field as well as radiated and diffracted waves. This thesis presents the mathematical model, the numerical method and numerical results obtained from the application of the method to a number of relevant test cases. Results from the application of the method are provided, including the propagation of a regular long-crested wave, the wave resistance problem in calm water and the radiation and diffraction problem in regular and irregular head sea. The results are limited to heave and pitch motions and are compared with analytical or experimental data, numerical results from other methods or, in some cases, both. The method has proved to be able to capture strong nonlinear effects in heave and pitch motions and its feasibility as a qualified analysis tool for such cases has been demonstrated.