Global ice ridge ramming loads based on full scale data and specific energy approach

This thesis analyses the loads that occurred during an ice ridge ramming experiment with the icebreaker Oden. Sea ice ridges are formed due to breaking and deformation of the ice cover. Wind, current, thermal expansion and Coriolis forces induce compression and shear forces onto level ice which can break the ice into rubble. The blocks of ice rubble are pushed together, forming a wall of broken ice in hydrostatic equilibrium. This wall of broken ice forced up by pressure is defined as an ice ridge. In general ice ridges are long, nonsymmetrical, curvilinear features with a wide variability of sizes and shapes. In Arctic regions, sea ice ridges are often used to calculate the design load in the absence of icebergs. Ice ridges also play a major role in icebreaker efficiency, since an icebreaker might need several ‘rams’ to break through an ice ridge. Ice ridge actions on icebreakers are not completely understood. Complex ice behavior under rapidly applied stress, and the complex geometries of the bodies in contact makes it a challenging research topic. The dynamic behavior of the vessel during the ramming can be used to make an estimate of the ice loads that occurred. This thesis analyses the ice load that occurred during a ridge ramming experiment that was performed with icebreaker Oden during the ODEN AT research cruise project in 2013. To advance our understanding in the global ice ridge ramming loads, twomodels were developed: 1) a simulationmodel using the Specific Energy Absorption (SEA) of mechanical crushing of ice to calculate the global ice loads, 2) a load identification model using full-scale data to determine the global ice loads. The simulation model was developed to enhance the understanding of relevant physical phenomena and parameters. During this process, specific energy principles of crushing of ice were identified as a promising although relatively unknown method for impact dynamics into ice. The Specific Energy Absorption (SEA) of mechanical crushing of ice is defined as the energy per unit mass of crushed ice, necessary to turn solid ice into crushed (pulverized) material. Besides the SEA value, the penetration velocity, density of ice, and volume of crushed ice, are required to calculate the ice load. A contact model was developed to determine the load location and direction on the hull. The icebreaker Oden is represented by a nonlinear mass-damper-spring system. Maneuvering theory is applied, which means that the hydrodynamic variables are estimated at one frequency of oscillation. In the simulation model, a known thrust force is applied on the vessel, making it move forward in open water, and then penetrate the ice ridge. The simulation model calculates the ice loads and vessel’s motions (i.e. accelerations, velocity, and displacement). The load identification model combines the Kalman filter and a joint input-state estimate algorithm to estimate the state- and excitation vector from acceleration, velocity and displacement data in 3DOF (i.e. surge, heave, pitch). The joint input-state estimate algorithm combines measured data with an estimate of the state of the system in a way that minimizes the error. The full-scale data analyzed in this thesis includes a profile of amulti-year ice ridge, vessel characteristics, acceleration data from a motion reference unit (MRU), GPS data, and propulsion data. From the results of the load identification model, we conclude that the current combination of model and data does not provide sufficient information to estimate the global ridge ramming loads with high reliability. The main reasons for this are the absence of additional MRU(s), the low sample frequency of the MRU, the data uncertainty, and the simplified hydromechanics. However, the suggested approach to calculate the global ice loads is reliable as long as the data is valid. This is verified by recalculating the ice loads from the data (i.e. motions), generated by the simulation model. Results indicate that the specific energy approach can be used to simulate an impact of a vessel into an ice ridge, under assumption that the ice fails purely due to crushing. This assumption is only valid during the beginning of the impact, as other failure modes often start to dominate as the penetration of the vessel into the ridge increases.Mechanical, Maritime and Materials EngineeringOffshore and Dredging Engineerin