WAVE-INDUCED LOADS ON A CATAMARAN HULL WITH FORWARD SPEED D. Sen, Indian Institute of Technology Kharagpur, INDIA A. Negi, Indian Register of Shipping, Mumbai, INDIA ABSTRACT In this paper, we compute and study motions and loads on a catamaran hull with forward speed, based on a strip theory method in which the sectional hydrodynamic properties are determined using a source- distribution or so-called Frank close-fit method. Two alternative schemes for this double-hull problem have been used for the computations of the sectional properties. Comparative results with available experimental data on the sectional hydrodynamic properties and ship motions are shown, followed by presentation and study of various modes of important structural loads on a catamaran hull. Results are discussed in comparison with available experimental data. It is found that while results follow the general trend with the results reported in some of the available studies, wide scatter still exists among the results. The two schemes used here also do not produce consistent results over the speed range. This suggests that further work is necessary before reliable tool for wave-induced loads on a catamaran hull can be made available. 1. INTRODUCTION Since 1970, there has been an increasing interest in applying the twin hull or catamaran concept to vessels for special services as, e.g., ferries, oceanographic research, rescue, offshore support and naval operations in shallow water. Although, most of these are designed as oceangoing vessels, compared to monohulls relatively less is known on their behavior at sea including wave-induced dynamic loads. Among all the environmental loads that a ship encounters in extreme weather from wind, current and waves, the dynamic wave-induced load is the most significant and has a crucial role in ship hull structure design. Wave-induced loads can be divided into different frequency ranges depending on the dynamic behavior and flexibility of the ship structure. The low frequency loads are second order wave exciting forces inducing slowly varying rigid body motions. Typical responses are large motions, i.e. drift, in horizontal plane of moored offshore structures. Typical wave frequency responses are the rigid body motions and accelerations. Loads are due to hydrodynamic pressures around the hull of the marine structure that induce local and global loads. The global loads are the shear forces and bending moments when the hull girder can be assumed to behave as a rigid beam. The high frequency loads are impact type hydrodynamic pressure loads where the structural dynamic is important. The high frequency loads are, for example, springing and whipping loads, which induce dynamic and vibrating responses on the hull structures. Mono hulls experience vertical and horizontal shear forces and bending moments and torsional moment depending on the ship heading, frequency of waves and speed. For usual mono-hull structures with long and slender geometries, an important wave induced response is the vertical wave bending moment (VBM) that induces global hull girder stresses. For monohulls, VBM is therefore of prime concern to classification societies as it serves as an initial safety check for the overall structure. For catamarans, another important structural problem is the design of the cross-structure which connects the two hulls. It is thus necessary to predict the loads on this part of the structure. This requires evaluation of other types of bending moments such as transverse bending moment or prying moment and pitch connecting moment, in addition to the usual vertical and horizontal bending moments in the longitudinal plane. Broadly there are two approaches in evaluating wave-induced loads on the hull girder. In the first, namely evolutionary approach, the wave-induced loads specified in the rules issued by classification societies have evolved gradually through trial, Author's Copy ASRANet 2014