ИСТИНА

The problem of the determination of ice loads (or actions in ISO terminology) on offshore structures is essentially more difficult, from the methodological point of view, as compared with other environmental factors, such as wind, waves, currents. An engineer needs much more elaborated probabilistic approach for the ice case to determine the design values of ice loads, which yearly exceedance levels are set usually as 0,01 and lower. Reasons for this are well known: first, floating ice is diverse in shape, ice feature morphology is complicated and a number of ice structure parameters are not available for direct measuring at that, and second, in most cases ice load is a straightforward result of ice failure, a process that due to its physical nature has a very high level of internal uncertainty. In fact, it is reasonable to treat ice failure as a structural failure of a multi-element system, and in conditions when the individual elements have a non-trivial properties. The lecture, first, presents some basic ideas on probability aspects of multi-element systems failure that is closely related to the general subject of the reliability of structures. Then, with the help of a number of simple probabilistic models, the cases of level ice, ridges and icebergs impacts on offshore structures are presented. Certain attention i s given to a discussion on the overall reliability of the procedure of design ice load determination, which is closely connected with the accuracy of the probability distribution functions describing ice parameters and the adequacy of load equations.