Wave-induced cross-structure loads are presented for a SWATH (Small Waterplane Area Twin Hull) vessel whose hull is made up of simple geometrical shapes. The hydrodynamic problem is solved using the higher-order boundary element method and the generalized modes approach available in the radiation-diffraction program WAMIT (Lee and Newman, 2001). The difference between cross-structure loads computed in earth-fixed coordinates and the same loads computed in body-fixed coordinates is illustrated. In the absence of any damping from viscous effects and separation, the computed free surface elevation in the gap between the hulls exhibits large sloshing at resonant frequencies. The effect of sloshing resonance on the computed loads is shown. A technique proposed by Newman (2004) to simulate the additional damping and thereby obtain more realistic predictions of free surface elevations and cross-structure loads is demonstrated.
In modern ship design practice, theoretically determined loads are often used along with finite element methods to assess the global and local ship structure response. For example, the dynamic loading approach (DLA) established by the American Bureau of Shipping (ABS) provides a ‘design by iterative analysis’ framework to complement the traditional semi-empirical, rule-based design (Liu, Spencer, Itoh, Kawachi and Shigematsu, 1992). For SWATH (Small Waterplane Area Twin Hull) vessels, the ABS guide (1999) defines the load cases that typically need to be considered. These, shown schematically in Fig. 1, all stem from the differential loading on the two hulls.
It is possible to compute these differential loads by solving the hydrodynamic problem for the six rigid modes of the SWATH vessel in the usual manner, and then integrating the resulting pressure over the submerged surface of each hull separately to get individual hull forces. However, integration of panel pressures in a post-processor requires the handling of large data files and is cumbersome.
