Ridge keel surcharge effect during an interaction with a cylindrical offshore structure

In freezing sea areas, the major load for offshore structures is usually caused by drifting sea ice ridges, which consist of a sail above the water and a keel beneath the waterline. In first-year ridges, the keel consists of an upper refrozen or consolidated layer and unconsolidated rubble below. The major load contribution of an ice ridge is presented by the consolidated layer and the underlying keel. Dolgopolov et al. (1975) published an analytical model for calculating the ridge load on a vertical pier. The model, adopted from soil mechanics, is based on passive earth pressure theory. It provides a ridge keel load for a vertically faced structure in an easy way using a limited number of input parameters. Due to its simplicity, the model has been adopted by ISO 19906 (2019). When the ridge keel interacts with the vertically faced structure, part of the rubble - broken during the interaction - starts to accumulate in front of the structure. This means that the ice contact area against the structure increases. Dolgopolov et al. (1975) proposed to apply an increase in the keel thickness instead of the intact thickness of the ridge keel. This phenomenon is often called a surcharge effect. In the original Dolgopolov model, the surcharge effect was considered by increasing the design keel thickness. The lower bound is equal to the original keel thickness, and the upper bound is equal to the original keel thickness plus half the diameter of the structure. However, this effect was not adopted by the ISO standard because it would result in extremely high ridge loads on very wide structures (80 m) (Kärna and Nykänen, 2004). For typical Baltic structures (mostly less than 10 m in width), the surcharge may be important, and non-conservative design loads can be estimated without it. In this paper, we have carried out numerical simulations demonstrating both the keel deformations (surcharge) and the additional contact load regarding the surcharge. The simulations confirm that ice accumulates roughly as suggested by Dolgopolov et al. (1975), but the increase in horizontal force caused by the surcharge was minimal because the ridge material was weakened when broken (substantial softening).