Three Dimensional Time Domain Approach And Application For Ship Motions

With urgent need of oil and gas Resources, the field of naval architecture and ocean engineering has achieved continuous development. More and more new types of ship and ocean structure are developed. The calculation of ship motions and wave loads in the waves is in need more and more.So it is very important to seek a reasonable and practical prediction method of ship motions and wave loads.At present, frequency domain method is applied to simulate motions of ship and ocean structures in the waves frequently. However, frequency domain theory is only efficient to treat the steady state problem. As for time domain theory, in principle, it can treat the fully non-linear problem and simulate motions of arbitrary bodies. In addition, with the appearance of large-capacity and high-speed computer, it is feasible that time domain theory is applied for numerical simulation of ship motions and wave loads.So it will become the inevitable direction of development of naval architecture and ocean engineering hydrodynamics that 3D time domain method is applied for numerical simulation of wave-body interactions. Conclusively, 3D time domain numerical simulation is very important both in theory and engineering application.How to evaluate time-domain Green function and its gradients efficiently is the key problem to analyze ship hydrodynamics in time domain. In this paper, based on the Bessel function, an Ordinary Differential Equation (ODE) was derived for time-domain Green function and its gradients in this paper. A new efficient calculation method based on solving ODE is proposed. It has been demonstrated by the numerical calculation that this method can improve the precision of the time-domain Green function. Numerical research indicates it is efficient to slove the hydrodynamic problems.In this paper, a 3D time domain program based on time-domain Green function has been developed to simulate the interaction between waves and floating bodies without forward speed. The radiation and the diffraction problems of a Wigley-hull-form ship were discussed, and the hydrodynamic coefficients, wave forces and motion amplitudes were computed to verify the accuracy and reliability of the 3D time domain program. The numerical results agree well with the experimental and reference results.In this paper, a 3D time domain method is developed to investigate the gap influence on the wave forces for three-dimensional multiple floating structures and obtained some important conclusions as follows: (1) The numerical computations have proved the existence of the sharp peak force response on each floating body at some special resonant wave numbers. The resonant wave numbers are also proved around kL=nπ(n=1, 2,…,∞) with a corresponding frequency shift. (2) Special hydrodynamic resonance due to small gaps between multiple floating structures on wave forces is examined. Strong and complicate hydrodynamic interactions between the floating bodies are observed (3) The strong hydrodynamic interaction feature is practically significant for the design of module structures and the links (connection) in whole the floating body system.In this paper, a 3D time domain program based on time-domain Green function has been further developed to simulate the interaction between waves and ships with forward speed. Considering the effects of forward speed and waterline integral, the radiation and the diffraction problems of a Wigley-hull-form ship at different forward speed were discussed, and the hydrodynamic coefficients, wave forces and motion amplitudes were computed to verify the accuracy and reliability of the 3D time domain program. The numerical results agree well with the experimental and reference results. It will also lay a foundation for further engineering application.