Seakeeping Analysis and Hull Optimisation on Wet Deck Slamming of Fast Ferry Catamarans

Abstract

The present research performed a design exploration of fast ferry catamarans with hard chined demihulls sailing in irregular head waves, to analyse the influence of the demihull shape (without appendages) on wet deck slamming, and identify the geometrical properties and their trends to reduce it. Wet deck slamming is a phenomenon known to cause passenger discomfort and seasickness, often a source of delay in the ferry's schedule and can even cause irreversible damages and endanger the crew. The research is carried out with numerical methods, where two main programs are used, namely the strip theory-based program for non-linear motions of high speed crafts FASTSHIP, and the design optimisation, parameter estimation and sensitivity analysis software, DAKOTA. To set up the experimental analysis, design and analysis of computer experiments (DACE) methods are used. The use of DACE involved the application of an in-house built parametric design tool for mass generation of hulls, coupled with a sampling-based method for the exploration of the design space. Because DACE methods require a large amount of data points (of the order of tens of thousands), it is deemed unfeasible to perform all the analysis with FASTSHIP. Therefore, a surrogate model is built instead, with the global approximation-based method known as Kriging. By considering the surrogate model in the study, mass production and evaluation of different hull forms were achieved, which led towards a brute force surrogate-based optimisation approach. Such approach enabled an in-depth exploration of both sub-optimal and Pareto-optimal regions of the multi-objective domain, thus not only showing the hull characteristics of the best hulls but also their trend of change from the sub-optimal to the Pareto-optimal region. Additionally, an alternative approach based on evolutionary algorithm is coupled to the surrogate model for a more direct and straightforward optimisation process, namely the multi-objective genetic algorithm (MOGA). Both approaches converged towards the same region, thus reinforcing the reached solution. The outcome of the research established a valid framework for performing hull design explorations on catamarans while addressing wet deck slamming. Furthermore, it showed that the demihulls should adopt bulkier forms. However, a trade-off existed between the various considered objective functions, hence the final decision should be based upon the user's criteria.