Earlier investigations have indicated that proper prediction of nonlinear responses due to nonlinear waves is important for ship safety in extreme seas. Nonlinearities may increase significantly ship responses in steep sea-states, particularly when rogue waves are present. So far most of the model tests in rogue waves have been performed without ship forward speed. Therefore the effect of forward speed on ship responses is still not sufficiently investigated. The present study is addressing this topic by taking an LNG tanker as an example. Ship responses of the LNG tanker with/without forward speed are studied with both model test and numerical simulations. The model test is carried out in the sea-keeping tank of the Technical University of Berlin (TUB), and numerical simulations are performed using the 3D Panel code. The statistical uncertainty of the ship responses due to different time spans is investigated also. The study shows that ship forward speed increases the ship motions and hogging moment. The change of sagging moment due to the forward speed is very small at slow speed. After the ship forward speed is increased to a certain value, the sagging moment increases significantly with ship speed.

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