• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.297-313
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• 2020

An Arctic Spar is characterized by its conical shape near the waterline. In this case, the nonlinear effects from its irregular hull shape would be significant if there is either a large amplitude floater motion or steep wave conditions. Therefore, in this paper, the nonlinear effects of an Arctic Spar are numerically investigated by introducing a weakly nonlinear time-domain model that considers the time dependent hydrostatic restoring stiffness and Froude-Krylov forces. Through numerical simulations under multiple regular and irregular wave conditions, the nonlinear behavior of the Arctic Spar is clearly observed, but it is not shown in the linear analysis. In particular, it is found that the nonlinear Froude-Krylov force plays an important role when the wave frequency is close to the heave natural frequency. In addition, the nonlinear hydrostatic restoring stiffness causes the structure's unstable motion at a half of heave natural period.

• Ocean Systems Engineering
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• v.6 no.4
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• pp.345-362
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• 2016

This study aims to develop the numerical method to estimate level ice impact load and investigate the dynamic interaction between an arctic Spar with sloped surface and drifting level ice. When the level ice approaches the downward sloped structure, the interaction can be decomposed into three sequential phases: the breaking phase, when ice contacts the structure and is bent by bending moment; the rotating phase, when the broken ice is submerged and rotated underneath the structure; and the sliding phase, when the submerged broken ice becomes parallel to the sloping surface causing buoyancy-induced fictional forces. In each phase, the analytical formulas are constructed to account for the relevant physics and the results are compared to other existing methods or standards. The time-dependent ice load is coupled with hull-riser-mooring coupled dynamic analysis program. Then, the fully coupled program is applied to a moored arctic Spar with sloped surface with drifting level ice. The occurrence of dynamic resonance between ice load and spar motion causing large mooring tension is demonstrated.

• Ocean Systems Engineering
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• v.11 no.4
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• pp.313-330
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• 2021

In this study, the dynamic interaction between an Arctic Spar and drifting level ice is examined in time domain using the newly developed ice-hull-mooring coupled dynamics program. The in-house program, CHARM3D, which is the hull-riser-mooring coupled dynamic simulator is extended by coupling with the open-source discrete element method (DEM) simulator, LIGGGHTS. In the LIGGGHTS module, the parallel-bonding method is implemented to model the level ice using an assembly of multiple bonded spherical particles. As a case study, a spread-moored Artic Spar platform, whose hull surface near waterline is the inverted conical shape, is chosen. To determine the breaking-related DEM parameter (the critical bonding strength), the four-point numerical bending test is used. A series of numerical simulations is systematically performed under the various ice conditions including ice drift velocity, flexural strength, and thickness. Then, the effects of these parameters on the ice force, platform motions, and mooring tensions are discussed. The simulations reveal various features of dynamic interactions between the drifting ice and moored platform for various ice conditions including the novel synchronous resonance at low ice speed. The newly developed simulator is promising and can repeatedly be used for the future design and analysis including ice-floater-mooring coupled dynamics.