On the Application of Advanced Wave Analysis in Shallow Water Model Testing (Wave Splitting)

The HAWAI JIP originated from the observed differences between model tests and numerical simulations in shallow water. These differences were identified as being caused by spurious free waves and reflected waves in the model basin. Identifying these waves and incorporating them in the numerical simulations greatly improved the agreement between the model test results and the simulation results. Although the effect of spurious waves is also present in deep water, the effect is most profound in shallow water. The research therefore focused on shallow water, further refinement of wave identification, the background and theory behind spurious free waves, the effect of these waves on the motions of a LNG carrier and the occurrence of free waves in reality. In order to identify the different types of wave systems in a model basin a wave splitting (or separation) technique has been developed. This paper describes the current state-of-the-art of wave splitting and its limitations. Results are presented in the form of motion statistics of an LNG carrier that is moored in 15 m and 30 m water depth. The model tests results are compared with simulation results with and without the wave splitting methodology. INTRODUCTION One of the objectives of the HAWAI JIP is to better understand (low frequency) wave propagation in shallow water basins and the wave forces associated to these low frequency waves. This paper describes how the wave field can be split in to free and bound waves coming from both the incident (i.e. from the wave maker) and reflected (i.e. from the beach) wave direction. These separate wave systems were used to compute the response of the LNG carrier in the time domain. For low damped systems in shallow water the low frequency excitation can result in a large response. This is the case for surge motions of LNG carriers in shallow water. As shown in figure 1 these large motions directly relate to the mooring forces. The large LNG carrier motions that are observed in a model test basin can be (partly) related to spurious waves that are introduced by the fact that the wave maker does not meet the exact boundary conditions of the required water particle motions. Another reasons for the presence of these spurious waves can be the local shape of the basin bottom where free waves are generated in the shoaling process on an inclined bottom. Since the measured LNG carrier motions are susceptible to this type of basin effects they can not be used as design values in a straightforward manner. Wave splitting methodology was developed to identify and understand the wave systems and use numerical tools to translate the basin results back to design values for shallow water systems.