Study on the effects of radiation and diffraction forces on roll restoring arm with flared and tumblehome vessels

Abstract

The variation of roll restoring arm in waves is one of the critical factors for the occurrence of some dangerous stability failure modes, such as parametric rolling and pure loss of stability. This paper focuses on the accurate prediction of roll restoring arm in waves. Firstly, systematic partially restrained model tests of 75 experimental conditions at different heel angles, different forward speeds and different wave conditions are conducted by taking flared C11 containership under two drafts and ONR tumblehome hull as research objects. Secondly, detailed numerical simulation based on a three dimensional hybrid time domain panel methods are carried out, and the calculation results are benchmarked against model test data. Thirdly, the effects of different components of GZ are analyzed by comparing the numerical simulations with experimental results, in which the component induced by hydrostatic and Froude-Krylov forces (GZFK) are calculated by integrating pressure over the instantaneous wetted ship surface and the component induced by radiation and diffraction forces (GZRD) are calculated using the nonlinear body condition of the boundary-value problem. The investigations show that the conventional FK assumption in which GZRD is ignored can be employed for the calculation of roll restoring arm of zero speed in different wave directions, because GZRD is negligible at zero speed. Considering that there is no obvious improvement on the prediction accuracy in following waves with forward speed even though GZRD is taken into account, the FK assumption can also be used. The existence of GZRD is an indispensable part in head waves with forward speed, and the prediction accuracy of GZ can be drastically improved on the consideration of GZRD. Therefore, GZRD can be ignored for stability failure modes in following seas, while GZRD should be considered in head seas.