Cohesive Element Method to Level Ice-sloping Structure Interactions

During the interactions between level ice and sloping offshore structures, crack initiations and propagations as well as the interactions between the resultant ice fragments occur. To simulate such complicated processes, the cohesive element method, which is capable of simulating dynamic fragmentation, becomes a potential numerical approach and has been applied to various kinds of offshore structures. One of the major challenges in applications of the cohesive element method is the mesh dependency or convergence issue for which remedies of random meshes and a random property field have been proposed in the context of concrete, ceramic, or glass fiber fracture problems. In this paper, random meshes based on Voronoi tessellations and a random ice property field following Weibull distributions were implemented into the numerical setup of the cohesive element method for level ice-sloping structure interactions to evaluate their performance in improving mesh convergence. Additionally, a new formulation based on added mass and hydrodynamic damping to capture the hydrodynamic effect of the fluid base was derived and utilized in the simulations. Based on a series of simulations, the time histories of the dynamic ice forces in the loading direction were compared with field data. It was found that an average Voronoi cell size close to the breaking length of the ice sheet yielded the best accuracy, since roughly all the cohesive interfaces near the structure failed in the simulations. This gives guidance in the determination of the average Voronoi cell size in the numerical setup according to empirical relationships between the breaking length and the ice thickness. Additionally, with the validated numerical model, the magnitude of the ice force in the transverse direction was found to be 30% of that in the loading direction, which serves as a preliminary method to determine the dynamic ice force in the transverse direction, facilitating the conceptual design of jacket structures with ice breaking cones.