Numerical simulations of ice loads on conical wind turbine foundation in the eastern Baltic Sea

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School of Engineering | Master's thesis
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vii + 61 s. + liitt. 9
The interaction of sea ice with offshore structures is an important engineering concern in ice infested areas. This thesis deals with the interaction analysis between level ice and fixed, conical foundation of wind turbines using the explicit finite element code LS-DYNA. The aim of the thesis is to implement a numerical Baltic Sea ice model for ice-structure interaction analysis and to study the effects of the structures slope angles and of ice velocity. The thesis focuses on numerical modelling of ice and its failure mechanisms and on developing contact formulation between the ice and structure as well as for the contact between the ice fragments. The ice is treated as an isotropic, brittle material described by separate stress-strain curves for compression and tension. The failure of the ice is defined by a pressure criterion. When the state of pressure in an ice element exceeds a certain value, the element erodes but its mass will remain active in the simulation. The buoyancy force is accounted for by defining the restoring force for ice nodes that depends on gravity, the specific weight of water and the effective area. In this thesis, ice loads on conical structures were calculated considering sea ice conditions in the eastern Baltic Sea, including the mean level ice thickness and approaching ice velocities. In addition, the effect of the slope angles of wind mills foundation was studied. Numerically obtained ice forces were compared to analytical methods for calculating the ice sheet forces on conical structure as presented by the rules of classification societies. The results of numerical simulations indicate lower maximum interaction forces than suggested by the analytical methods. The deviation increased as the sloping angle of the conical structure increased. However, considering the disregarded nonlinearities and the conservative character of analytical methods, higher force levels can be expected in analytical methods. On the other hand, simulations showed that the ice velocity has an effect on ice loads. Ice forces on the structure become higher compared to the analytical results when the ice exceeded a velocity of 0.3 m/s.
Kujala, Pentti
Thesis advisor
Tabri, Kristjan
Ehlers, Sören
ice loads, finite element method, flexural failure