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Experiments by Invited Researchers

 

Model – Scale Experiments of Accidental Collision of Ice Masses with a Floating Structure

Project acronym: HyIV-Aalto-02
Name of Group Leader: Ekaterina Kim
User-Project Title: Model – Scale Experiments of Accidental Collision of Ice Masses with a Floating Structure
Facility: Ice Tank
Proceedings TA Project: FINDINGS AND LESSONS LEARNED FROM IMPLEMENTING ICE-STRUCTURE IMPACT TESTS IN WATER AND IN AIR
Data Management Report: Report

User-Project Objectives

Objectives of the laboratory-scale experiments are the following: (i) Provide data for the validation and calibration or further development of the existing analytical and numerical models of glacial ice masses-structure collision; (ii) Gain knowledge about the possible consequences of accidental collisions of ice masses with a structure; (iii) Gain experience on physical modeling of glacial ice masses-structure collisions where both ice and structure undergo deformations. We believe that laboratory experiments of accidental collisions with ice masses (a collision scenario, where both the ice and the structure can undergo large deformations) are essential to verify current methods (assumptions) for integrated analysis of accidental ice impacts. To our knowledge, the tests conducted are the first laboratory experiments that focus on deformations of both the ice and the structure.

Short description of the work carried out

Satisfactory integrated behavior of the ice mass and the floating structure was achieved. After the impact, plastic deformations of the plates were registered as well as some ice crushing/splitting etc. All necessary data (i.e., ice mass, shape, towline force, carriage position, strains at impact, accelerations and angular rates, etc.) were successfully recorded and stored for later post-processing (see Project Data Storage Report). The aforementioned objectives of the laboratory-scale experiments were achieved. Difficulties encountered: i) Ice manufacturing process: Growing of an ice block free of cracks (defects) and internal unfrozen water pockets was challenging, since the ice, growing from the surface, was acting as an isolation layer slowing down the freezing process. This resulted in an unfrozen water- pocket within the ice mass. Due to this water-pocket, an internal pressure was building within the ice. This, in turn, caused internal ice cracking upon future freezing. A rigid mould, used for ice manufacturing, could promote rise of internal pressure. ii) Experimental setup: It was difficult to achieve a ‘perfect’ central impact of the ice mass into a purpose build target (e.g., control the position of the ice mass at velocity of 2 m/s). Since the pressure mapping system was placed only on the part of the impacted target it was difficult to ensure whether the registered pressure corresponds to 100% of the impact load. iii) Other. It was impossible to synchronize all measurements. For a given size and shape of the ice mass, it was difficult to increase initial kinetic energy of the ice without changing its failure mode upon the contact with steel plate. During post processing of the results it was difficult to separate hydrodynamic effects (i.e., effects related to towing the ice mass into a purpose-build target) from actual impact forces. Publications: Kim, E., Storheim, M., von Bock und Polach, R. and Amdahl, J. (2012) DESIGN AND MODELLING OF ACCIDENTAL SHIP COLLISIONS WITH ICE MASSES AT LABORATORY-SCALE, 31st International Conference on Ocean Offshore and Arctic Engineering, 1-6 July, Brazil, Paper OMAE2012-83544

Highlights of important research results

Ice-mass manufacturing technique that was used during HYDRALAB experiments resulted in the following ice characteristics: i) Produced ice mass is completely frozen. ii) Uniaxial compressive strength of prismatic ice specimens sampled from a large ice block is 0.89±0.32 MPa. (The ice samples were loaded at constant speed at ambient temperature of ~0oC.) Obtained ice strength is similar to that of warm freshwater ice at temperatures between -5 oC and -1oC. iii) Grain size of the ice mass is dominated by the microstructure of the crushed ice that was used to produce this ice mass. (In given experiments the grain size is ~2 -5 mm.) The test program addressed the question of whether 1-tonne ice mass impacting the stiffened panel at 2 m/s could lead to significant plate deformations. The data indicate that integrated behavior of the ice mass and the structure was achieved for certain plate thicknesses. During the collision event both the impacted panel and the ice were dissipating collision energy: plate - by deforming plastically and ice - by local crushing, splitting etc. The appearance of the damage zone of ice was similar to those found in literature. Maximum plate deflection measured was 12 mm. From the limited number of drop tests it was observed that confinement of the impacting ice and local ice shape at the contact zone significantly influences the impact scenario. The program has provided an invaluable data for validating analytical or numerical models for integrated analysis of the crushing and deformation of ice and a steel structure for accidental ice impacts.

Publications
Ekaterina Kim, Martin Storheim, R?diger von Bock und Polach, and J?rgen Amdahl Design and modelling of accidental ship collisions with ice masses at laboratory scale , 2012 Kim, E., Storheim, M., Bock und Polach, R., & Amdahl, J. (2012). Design and modelling of accidental ship collisions with ice masses at laboratory scale. In Proceedings of the 31st International Conference on Ocean, Offshore and Arctic Engineering. Paper OMAE2012-83544.
E. Kim, M. Storheim, J. Amdahl, S. L?set, R. von Bock und Polach Drop tests of ice blocks on stiffened panels with different structural flexibility , 2013

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