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Floating glacial ice features of various sizes pose great threats to the structural integrity of offshore structures in arctic regions. Potential collisions with large icebergs should generally be avoided through proper ice management, but smaller glacial ice features such as bergy bits or growlers (with a characteristic water line < 15 m) may be overlooked by radar in high sea states, and are likely to hit the platform accidentally. This paper evaluates the load carrying capacity of a semi-submersible platform to accidental ice impacts. The shared energy approach is adopted as done in ship collision analysis, assuming collision scenarios with rigid platform-deformable ice on the one hand, and rigid ice-deformable structures on the other hand. Both ice and the platform should deform and dissipate energy under the same force level. For structural analysis, non-linear finite element code LS-DYNA is used. An alternative approach is the simplified analytical method, which allows for fast and reasonable prediction of structural damage. Simplified expressions for stiffened panels under ice pressure patch loading are introduced. The resulting resistance curves compare reasonably with LS-DYNA simulations.

The initial geometric imperfection is recognized as substantially influential on the buckling behaviour and ultimate strength of stiffened plated structures. Conventionally, the initial geometric imperfection is defined in a deterministic manner, such as a hungry-horse mode, ARE mode or critical buckling mode with characteristic magnitudes. However, due to the uncertainty in manufacturing and in-service effects, the initial geometric imperfection should be better described as a random field. This paper aims to assess the effects of the stochastic nature of geometric imperfection on the ultimate strength of stiffened panel in compression. A probabilistically-based imperfection model developed by spectral representation method is applied to an orthogonally stiffened panel. A Monte-Carlo Simulation enhanced by a Latin-Hypercube Scheme (LHS) is utilized to sample the initial geometric imperfection. A series of nonlinear finite element analyses are completed to calculate the ultimate compressive strength of the case study model. With reference to the deterministic representations, the impact of the stochastic imperfection model is discussed. The present work may provide useful insights for the reliability-based ship structural assessment regarding the uncertainty due to geometric imperfection.