Modelling Techniques and Case Study of Explicit 3D FE-Simulation of Ridge Loads Against an Offshore Structure

A ridge interaction with an offshore structure, Norstro¨msgrund lighthouse in the Gulf of Bothnia, was simulated by the finite element method (FEM) utilizing the explicit solution algorithm. The explicit method in a dynamic analysis enabled an efficient way to simulate the failure of a ridge. A shear-cap material model describing the ice rubble mechanical behaviour was developed for the 3-dimensional analysis and implemented into ABAQUS FEM-software as a user material subroutine. The FE-model described the ridge: the sail and keel consisting of ice rubble and the consolidated layer close to the waterline. A selected ridge-interaction event was simulated with a simplified geometry, based on measurements carried out in the STRICE project. The ridge field was idealized to have a constant thickness. The shape of a 3-dimensional structure was modelled using rigid elements. The phenomenological model can be utilized to gain an understanding of how the ridge fails during the interaction with an offshore structure, what the ridge loads are, and how they are distributed against the structure. A parametric study was carried out to study how the mechanical properties, i.e. the cohesion and friction angle of the rubble and the strength of the consolidated layer influence the forces and failure mechanisms.