One of the greatest challenges of developing the arctic regions is the harsh environmental conditions caused by the presence of ice. When offshore structures interact with ice, significant structural motion is induced by the ice load, which influences the overall structural robustness. In this study, three different numerical tools are developed to investigate the ice-structure interaction using three scenarios. First, the fixed-type monopile offshore wind turbine in level ice is studied. Because the crushing is the dominant failure mode of level ice against a cylindrical structure, a mechanical ice-crushing model is developed to estimate the ice force on the structure in time domain. The model is further implemented into the wind turbine analysis software, FAST, developed by National Renewable Energy Laboratory (NREL). Second, the floating offshore platform, Artic Spar, in level ice is investigated by employing an analytical method. Artic Spar is characterized by the inverted cone-shaped hull near the waterline so that level ice can fail when bending. The fully coupled floater-riser-mooring dynamic analysis program, CHARM3D, is extended by implementing the analytical ice-bending model. Third, the numerical software for the interaction between level ice and an arbitrary-shaped floating offshore structure is developed by coupling two software programs, LIGGGHTS and CHARM3D. Based on the discrete element method, level ice is modelled as an assembly of multiple spherical particles, and the bonding parallel method is employed to consider the interaction force among the bonded particles. Throughout the newly developed numerical simulation tools, the ice load on different offshore structures is numerically estimated, and the corresponding structural performances are systematically investigated. In addition to these three ice-structure interactions, the nonlinear behavior or Arctic Spar is investigated in time domain. To capture the nonlinearity of platform motions, a nonlinear time-domain simulation tool considering the nonlinear hydro-restoring coefficient and nonlinear Froude-Krylov force is developed by extending CHARM3D with a body-nonlinear method. The heave-to-pitch and heave-to-heave Mathieu instabilities of Arctic Spar are also investigated in both regular and irregular waves.