Experimental study on kinematics of sea ice floes in regular waves

Abstract

Ice floes in severe Arctic seas can gain significant kinetic energy. Such a fast moving floe presents a significant impact threat to offshore structures. Most attention to date has focused on glacial icebergs, which are now reasonably well understood; there appears a lack of knowledge in the case of isolated sea ice floes. To address this an experimental investigation of the relationships of the motion response of floes to the floe and wave characteristics was conducted. Of greatest importance to impact force calculations are the heave and surge motions, as well as the velocity and acceleration of a floe in waves. The tested variables included a wide range of regular waves with a variety of floe model shapes and sizes. The results showed that scale effect, floe size, floe orientation and surface roughness did not affect the heave and surge motion, and the motion observed was notably different to glacial icebergs. Wavelength affected motion the most; all models displayed fluid particle-like motion paths at λ between 3.3 and 5 times their characteristic lengths and the majority of scatter between different floe geometries in motion response was confined λ/Lc<8. Floe thickness also had a dramatic effect on heave and surge, with thicker models experiencing significant resonances. Drift velocity matched Stokes drift quite well, though the larger models appeared to show greater than predicted drift velocities. Maximum velocities did not generally exceed particle velocity, but remained >0.7 VP for surge and >0.8 VP for heave at λ/Lc≥5. Such high velocities at shorter λ suggest that a more conservative design approach may be necessary. An extrapolation of these velocities leads to a conclusion that prototype ice floes can obtain kinetic energies of 106J in certain conditions.