In the design of ships and offshore structures, it is often desirable to assess the effects of environmental forces such as wind and waves on the vessel prior to its construction. Hence, several computational methods have been developed to predict the seakeeping performance of a prototype vessel in the design stage. Many of the commonly used methods are limited in their applicability to either vessel geometry or vessel operating conditions. The time-domain ship seakeeping simulation code, MOTSIM, has recently been extended for use with multi-waterplane vessels such as semi-submersibles and catamarans. As a further extension, the MOTSIM solver was modified to allow simulation of two vessels connected by a mechanical constraint such as an Articulated Tug Barge (ATB) Unit. -- Some validation studies were carried out to validate the modifications to the MOTSIM code. Model test data for a triangular semi-submersible platform was compared against simulated results. Comparison between the experiment and simulations was generally good except for very low wave frequencies which was likely due to wave reflection in the model basin. -- Similarly, simulations were performed for an ATB unit. Comparison of the connection loads and relative motion between the vessels appears quite reasonable for the limited set of simulations completed. Instability of the constraint algorithm caused a reduction in the number of simulations included in this study. -- The results presented indicate a strong potential for the application of the MOTSIM seakeeping code to problems involving multiple waterplane vessels or multiple vessels in proximity. However, further validation work is needed to confirm the accuracy of the code for more general vessel geometries.
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