Roll damping prediction of a free floating barge

Abstract

Traditionally the problem of calculating the motion responses of a ship in a seaway has been formulated in frequency domain in terms of linear potential theory. By using the potential flow method in roll calculation, the fluid is assumed to be ideal, irrotational and viscous effects are neglected. Experiments have shown that the roll amplitude responses of rectangular bodies floating in beam waves are overestimated when calculated by potential flow method. This is largely attributable viscous effects [1]. For this reason seakeeping calculation methods introduce empirical factors to account for viscous effects. On the other hand, much of the nonlinear forces and moments experienced by ship in a seaway may be due to the viscous effects leading to flow separation and generation of vortices [2]. One approach to modelling flow separation and vortex shedding is to solve the Navier-Stokes equations. However, for moving bodies in the presence of a free surface at high Reynolds numbers (which implies the use of fine computational meshes) the software and hardware resources required, supposing the problem is even viable, are often so large as to be prohibitive. Another approach is to use methods based on vortex dynamics for modelling separated flows about bluff bodies. These methods were developed as a means of modelling high Reynolds number flows in which the vorticity is confined to small sub-domains of otherwise irrotational flows [2]. This work concerns development of a purely theoretical model for estimating the roll response of vessels that takes these effects into account. The objective of this thesis is to develop a model including viscous effects that can be used in seakeeping and survivability calculations. The idea being proposed is to match a local discrete vortex based method to a global model of a body floating with six degrees of freedom. A software is developed that can be bolted on to conventional seakeeping software so that the motions of sharp edged bodies floating in waves can be calculated without recourse to empirical methods. The theoretical approach to predict roll damping for a three-dimensional barge shaped floating vessel in the frequency domain is described here. The approach consists of matching a simple discrete vortex method (DVM) describing local separated flow, to an inviscid three-dimensional seakeeping code. Model tests have been carried out to validate the theoretical model and the associated add-on software. As demonstrated in this report, there is a good agreement between the model test RAO and the damped RAO indicating the theoretical method provides a good estimate of the viscous damping of the vessel due to vortex shedding from its edges. Although viscous damping in sway and heave motions is not as significant as for the roll for a barge the same methodology can be used to calculate viscous damping for both sway and heave as well. As tangential relative fluid velocities are used in this method the same final relative velocities can be used to calculate skin friction damping component. In this study skin friction damping is considered to be negligible and is ignored in the final calculated damped RAO.