Experimental study of wave-driven impact of sea ice floes on a circular cylinder

摘要

The impact of isolated sea ice floes with offshore structures is a significant environmental hazard for Arctic offshore operations. Most attention to date has focused on the impact of glacial icebergs, and very large drifting ice floes with offshore structures. There appears to be a lack of data in the case of isolated, wave-forced floe-structure interactions. To address this an experimental investigation was conducted to identify impact characteristics of floes of various shapes and sizes with a single circular cylinder. A wide selection of regular and irregular wave conditions were examined and the floe kinematics and impact characteristics determined. In regular waves, the results showed floe kinematic heave and surge responses was unaffected by the presence of the structure at distances of x/D ～ ≥ 10. At x/D ≤ 10 a slight increase in heave response was observed. In the same region, surge was markedly reduced regardless of whether there was an eventual impact. Drift velocity appeared to be the main control on whether the floe would impact (if drift velocity was high enough) or become trapped in the lee of the cylinder and be deflected to one side before impact. 3D analysis of impacts showed that two broad types of impact occurred; a relatively head-on impact and a more side-on impact. Head-on impacts were dominated by linear kinetic energy, while side on and secondary impacts exhibited increased rotational kinetic energy. In irregular waves, the impacts were found to occur at any point in the wave cycle, whereas the impacts in regular waves tended to occur at the point of maximum surge near the crest of the wave. The influence of λ/L_c and H/λ on impact occurrence and characteristics was investigated. While H/λ did not have a significant effect on impact occurrence, it appeared that the lower the λ the greater the chance of impact. The influence of L_c and shape in irregular waves indicated larger L_c and cross-sectional area of the floe relative to the cylinder increased impact occurrence.