Impact of Forced Roll Motion on the Ice Resistance of Modern Icebreaking Bow Geometries

Following the development of low friction hull coatings and azimuthing propulsion for icebreaking vessels, the development of auxiliary systems for reducing ice resistance fell from focus of research. One of these systems is comprised of active heeling tanks which induce a forced roll motion on the icebreaker. Today it is not fully understood how effective or even useful such systems would be for the icebreaking performance in combination with a modern icebreaking hull form. In this study, the impact of active heeling systems on level ice resistance is investigated by performing ice model tests with an icebreaker representing the latest design generation. The level ice thickness used in the model tests corresponds to the maximum continuous icebreaking capability of the evaluated vessel in multi-year ice conditions. Additionally, a calculation method is developed to predict the impact of forced roll motion on the ice resistance. The calculated prediction is evaluated against the model-scale data. Finally, the effectiveness of the active heeling system is evaluated from an engineering perspective: does the active heeling system reduce the power demand, or would the same result be achievable by increasing the propulsion power accordingly. It was found that the roll motion impacts the ice resistance in level ice. The main influence in this regard lies with the tank volume and metacentric height of the icebreaker. Additionally, it was observed that an optimum heel angle dependent on the ice condition can be determined which is not necessarily the highest one achievable. The case study predicts a reduced power demand for a modern icebreaker hull form in harsh ice conditions.

Vortex Shedding From Hulls in Close Proximity With Relative Motion

The safe and efficient offloading of liquid natural gas (LNG) in a side-by-side configuration has emphasised the need to accurately quantify the hydrodynamic responses of floating bodies when located in very close proximity. A new series of computational results are presented for the forced roll of a hull in the presence of a second body representative of a floating FLNG facility. The vortex shedding phenomenon which provides a significant contribution to the damping of an isolated hull, is demonstrated to be modified by the presence of the second body. The total damping coefficients were found to be significantly reduced by approximately 50 percent for the very small gap widths considered. It is shown that in addition to the modification of the local shedding dynamics, the variation in the pressure field owing to the presence of the second body also contributes significantly to the reduction in the damping. The findings will be of particular interest to the offshore industry for improving and optimising offloading practices.

Experimental and Numerical Investigation of Ship Roll Damping With and Without Bilge Keels

This paper describes an experimental and numerical study on the effect of bilge keels on a specific offshore vessel. Forced roll and free decay experiments were carried out at MARINTEK in a joint project between VARD, DNV GL and MARINTEK during Nov. 2012. The 1:25 ship model was tested both with and without bilge keels. 19 forced roll periods were tested at 4 roll angles (2, 4, 6 and 10deg). Free decay tests were performed for initial roll angles 2, 4, 6, 10 and 15deg. Damping was extracted and compared with numerical simulations from a potential flow strip theory program (VERES) and a commercial CFD tool (StarCCM+). Both qualitative and quantitative agreement between the experimentally and numerically obtained damping parameters was documented. Discrepancies were identified and discussed. The three different methods of investigation gave together a basis for discussing the effect of bilge keels. From the present study, it is recommended that both experimental and numerical work in combination, for both forced roll and free decay tests, should be performed to assess the damping efficiency of bilge keels.

Analysis of the Physics of Bilge Keel Vortex Generation

A 2D forced roll oscillation experiment was performed to examine and characterize the physics of vortex generation from ship bilge keels. Measurements included ship model motions, normal force on the bilge keels, and flow field visualizations using Particle Image Velocimetry (PIV). Data was collected for a range of roll oscillation amplitudes, including large amplitudes where the bilge keels interacted with the free surface and for three roll oscillation frequencies. This paper presents analysis and description of the observed phenomena, and examines whether a functional relationship can be established between the force measured on the bilge keel and the flow field generated by the bilge keel. A qualitative discussion of the observed flow characteristics, including the vortex generation at the tip of the bilge keel, detachment, and interaction with the free-surface, is presented. In addition, the circulation in front and behind the bilge keel was calculated, analyzed, and compared to the measured bilge keel force. The analysis conducted was intended to provide improved understanding of the phenomena which occur during bilge keel vortex generation and aid in the development of improved roll damping and bilge keel force models for the prediction of ship motions.