A new time domain Rankine panel method for simulations involving multiple bodies with large relative displacements

•Multiple vessels undergoing large relative displacements in waves are investigated.•A new time domain Rankine panel method is developed to handle the problem.•Results are compared with a set of dedicated captive model tests.•Transversal forces/moments present different amplitudes for mirror body positions.•Numerical method was able to reproduce the experimental records very well.

Many important offshore operations involve multiple vessels whose relative positions change considerably in the course of the operational procedures. When the action of sea waves on the dynamics of these vessels is concerned, an accurate modeling of the hydrodynamic problem is not an easy task. The use of frequency-domain panel codes is compromised by the variations of the bodies’ relative positions and may lead to inconsistent results. A time-domain approach is clearly more appropriate, but even in this case the representation of the moving bodies requires a re-meshing procedure and considerable computational efforts. This paper addresses the problem of wave interaction with multiple bodies with large relative displacement by integrating a re-meshing procedure to a time-domain Rankine panel code. The mathematical algorithms adopted for generating the new free-surface grids and performing the interpolation of wave elevation and velocity potential are described in detail. The paper also presents a set of dedicated captive model tests performed in a wave basin as a means of verifying the performance of the numerical method. Tests involved models with simple geometries that performed slow prescribed drift motions in the presence of incoming regular waves. Hydrodynamic forces were measured on one of the models and different levels of transient interaction effects could be reached by varying the frequency ratios between waves and body motions. The good performance of the method is then attested by directly comparing the hydrodynamic forces predicted by the computational code with those measured during the model tests. Moreover, it will be shown that the numerical model was able to capture some interesting Doppler effects that will be discussed together with other validation results.