Experimental wave effect on vertical relative motion

Abstract

Ship motions are influenced by the sea state. Conventionally the responses are calculated in the frequency domain. This method, however, is valid only for narrow band spectra. As the seaway becomes more nonlinear, the ship motions cannot be readily predicted using the spectral method. Experiments conducted by Dalzell, have shown that the Response Amplitude Operator (RAO) decreased with increasing sea state or non linearity. Conventionally in the shipbuilding industry, the ship motions are studied by the linear RAOs and the energy density spectrum of the seaway. This method does not take into consideration any non linearities in the system. These are ignored and the ship seaway system is modeled linearly. The following work analyzes ship motions in the conventional linear approach and compares it to time domain simulations using the technique outlined in the work, viz. UNIOM (Universal Nonlinear Input Output Method). Time domain simulation of the SL-7 container ship hull is carried out. A comparison of the most probable peak value of the different modes of motion indicates that the linear theory tends to overpredict.