Modelling of ice-interaction with a sea channel marker in the Bothnian Bay

The Gulf of Bothnia freezes every winter. At times, the level ice thickness in the northern part reaches close to 1 m. In that sea area the ice usually introduces the most significant uncertainty for the design of offshore structures. Drifting level ice induces a dynamic load on the structure, which needs to be designed carefully to avoid harmful vibrations. Implementing an inclined element at the structure water line is a promising solution to mitigate ice loads and ice-induced vibrations. This presentation introduces the modelling of ice-interaction with a bottom-fixed sea channel marker, a slender monopile structure located by the 12.5 m deep waterway to Oulu, in the Bothnian Bay. The height of the structure is 12 m above the water line. The structure is implemented with a cone having a waterline diameter of 3 m at the mean water level. A structural model for the sea channel marker was created including the soil-structure interaction model. Dynamic ice loads were created by combining analytical models to determine the load peaks and previous full-scale and model-scale tests to determine the frequency of the load peaks. Case studies were carried out to study how the structural acceleration responses depend on the ice thickness and ice velocity. The kinematic responses were analysed to find out a correlation between the accelerations at the selected locations in the structure and the ice load. This information is crucial for finding optimal locations for acceleration sensors when planning the test set-up for full-scale measurements.