Proceedings of 32nd International Conference on Offshore Mechanics and Arctic Engineering OMAE’13 June 9-14, 2013, Nantes, France OMAE2013-10690 NUMERICAL STUDY OF PARAMETRIC ROLL ON A FISHING VESSEL Marilena Greco NTNU/CeSOS/AMOS, Trondheim, Norway CNR-INSEAN, The Italian Model Basin, Rome, Italy Email: marilena.greco@ntnu.no;marilena.greco@cnr.it Claudio Lugni CNR-INSEAN, The Italian Ship Model Basin, Rome, Italy CeSOS/AMOS, Trondheim, Norway Email: claudio.lugni@cnr.it ABSTRACT Present research activity examines numerically the occur- rence of parametric roll on a fishing vessel interacting with reg- ular head-sea waves. The adopted solver is an efficient 3-D nu- merical Domain-Decomposition strategy for the seakeeping of a 6-dof vessel without and with small forward speed and possibly subjected to bottom-slamming and water-on-deck events. Here, the vessel has been assumed at rest and the excitation frequency is varied in the first parametric resonance zone and occurrence and features of the instability are examined in terms of nonlin- earities of the incident waves and roll natural-to-incident wave frequency ratio. The analysis is performed both fully within the potential-flow theory and examining the effect on the parametric resonance of the viscous correction to the roll damping obtained from free-decay 3D model tests on the same ship. A system of four cables, horizontal in the mean configuration, will be used experimentally to limit the horizontal vessel motions. Here the numerical solver is used to analyze the influence of cable stiff- ness and of cable configuration on the vessel behavior and to help the design of the physical set up. The vessel has deep draft and high mean freeboard, these aspects work against the occur- rence of bottom slamming and water-on-deck events. Without forward speed, no bottom slamming phenomena were observed while limited number of water-on-deck events with small amount of shipped liquid was recorded for the highest-frequency incident waves with largest steepnesses, among those causing parametric roll. INTRODUCTION Parametric roll involves resonant roll motion with instability behavior which may result in significant amplification of the roll motion (see e.g. [1]). The occurrence is connected with the time variation of the stability properties of the vessel due to changes in the restoring moment, and so of the transverse metacentric height GM, caused by high enough heave and pitch motions, as well as interaction with incident waves. The phenomenon is supported by large vertical ship motions, incident waves sufficiently aligned with the vessel longitudinal axis and not changing much in time in terms of period and amplitude. Another important factor is the roll damping, the lower it is the higher the possibility to have parametric roll will be. Finally there are ratios between the roll natural frequency and the excitation frequency, say ω 4n /ω e , for which the instability occurs more easily. Considering an uncou- pled Mathieu-type instability analysis for the roll motion, this gives ω 4n /ω e = 0.5,1, 1.5, and so on, as more dangerous fre- quency ratios. This phenomenon is important for large vessels like container ships, as well as for small fishing vessels. In the former case the main issue is damage and loss of containers and cargo, while in the latter case the risk of capsizing is critical. Here the case of a fishing vessel is analyzed numerically to identify the relevant physical and geometrical parameters in- volved and also to help the design of 3D model tests to analyze the phenomenon for a vessel free to oscillate in heave, pitch and roll, and able to oscillate also in other degrees of freedom, e.g. in surge, through the use of a cable system. Since almost regu- lar incident waves propagating along the ship longitudinal axis are more of concern, the case of head-sea regular waves is inves- tigated. In the next section the used numerical solver is briefly 1 Copyright c  2013 by ASME