Probabilistic force calculations for structures in ice-covered seas

Abstract

Ice ridges are expected to govern the design of conical structures in the arctic offshore against sliding forces and to affect the design of gravity or pile structures against overturning moments. Given that the mechanics of sea ice and the degree of consolidation and failure mode of ice ridges against a particular structure geometry are specified, there will still be uncertainty as to the size, shape and number of ridges that may impact a structure placed in these areas. The situation is analogous to designing against storm waves. The concern in both cases is with the probability of exceedance (risk) of rare events that approach the design strength of a surface-piercing structure founded on the seafloor. This paper describes a procedure for calculating the probability distribution of ice forces on a structure. A Monte Carlo approach is taken, together with ice mechanics, to convert information on structure geometry, structure location, and ice conditions at the location into a probability distribution for the force that will be imposed on the structure over a specified time period. The probability calculation is comprehensive in the sense that it can accommodate information on ice movement, ridge spacing, ridge geometry, etc. A comprehensive calculation gives the best estimate of risk under the assumptions made, and provides a framework for making prompt use of additional data as it is gathered. The data gathering effort itself can be focused by determining those environmental parameters to which the design ice force is most sensitive at acceptable risk levels.