Comparison of Ice Load Calculation Algorithms for First-Year Ridges

In offshore areas affected by dynamic sea ice, ridges usually represent the most extreme feature for exerting a load on an offshore structure. First-year ridges have a strong consolidated layer at the waterline, underlain by a loose accumulation of ice blocks that can extend 10 or even 20 m below the surface. The physical and mechanical properties or ridges are not well known, nor are the actual modes of failure. Nevertheless, a number of algorithms have been developed for the calculation of forces generated on an offshore structure by a firstyear ridge. One set of algorithms is used for the relatively solid consolidated layer and another for the keel and sail. The algorithms described in the literature for keel failures are mostly derived from soil mechanics applications; i.e., the keel is assumed to have soil-like properties. These calculation algorithms assume some failure mode for the keel, which generally can be categorized as global or local. Twelve different algorithms for calculating loads generated by keels have been identified for examination in this paper. For the purpose of the present study, these algorithms are evaluated for cases for which there are full-scale field data. Five cases of ridges failing against the offshore structure \u201cMolikpaq\u201d are presented and used for validating the calculation models. The results of the calculation algorithms are compared with each other and the full-scale data. The individual algorithms are each critiqued, identifying elements that make them more or less suitable for calculating forces for the Molikpaq cases. A common set of realistic ridge properties is used for the calculations to provide a fair basis for comparing the algorithms. In spite of these common properties, the failure load predictions vary greatly, with differences in predicted loads by factor of 20 for the consolidated layer and 7 for the keel. Future research directions for ridge forces are discussed.NRC publication: Ye