Loadings on the Topside of a Gravity Based Offshore Structure Due to Drifting Ice Pieces by Extreme Waves

Abstract

The motion of the ice pieces of various sizes in the Northern Seas such as, in the Grand Bank area of Newfoundland and Labrador always imposes extra risks to the operations of the offshore fixed and floating structures. In stormy weather, the operation and safety of such offshore structures are greatly affected when heavy waves propagate over the region with drifting ice pieces. This can cause damages to the structures when the interaction between those ice pieces and structures occurs. The range of severity of such damages depends on the harshness of the wave actions and the diversity of the ice pieces. In this paper, small ice pieces of different sizes are used in the simulations of interactions with an offshore Concrete Gravity Based Structure (CGBS). The motions of the ice pieces of various geometries and their dynamic impacts on the structures are studied. The simulations are carried out using a commercial numerical solver to predict the trajectory of the ice pieces around the structures, possible interaction with the CGBS and their consequent loadings on different elements of the structure at interest. The outcomes of the simulations obtained from single and multiple realizations of several wave conditions for the vertical impact loads due to ice pieces colliding with the topside so far. The initial results show that an increase of the significant wave height increases the number of collisions with the structure significantly. The magnitudes of the ice impact forces on the topsides did not follow the same trend. In each data set, the average of the highest one- third of the impact forces increases with the increase of Significant Wave Height / Air Gap ratios. The effects of variations in wave realizations for sea states are reported in the paper. The simulations also show the importance and possibilities of incorporating loads due to a collision of the ice pieces with the topside in the design process of such structures in the ocean.