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Experiments by Invited Researchers

 

Deciphering of ice induced vibrations

Project acronym: HyIV-HSVA-02
Name of Group Leader: Prof. Mauri Määttänen
User-Project Title: Deciphering of ice induced vibrations
Facility: ARCTECLAB
Proceedings TA Project: DECIPHERING ICE INDUCED VIBRATIONS - DIIV
Data Management Report: Report

User-Project Objectives

The physical background is not well understood for the origin of severe vibrations in offshore structures while moving ice is crushing. The objectives of the DIIV project were to excite ice-induced vibrations in a thoroughly instrumented model structure installed in the Large Ice Model Basin ARCTECLAB at HSVA. The model scale ratio target was 8 …10. The main interest is the net energy input from ice to structure during one vibration cycle. Continuous and violent natural frequency lock-in vibrations will develop if the input energy exceeds the energy consumed in various damping processes during a vibration cycle. The varied parameters were model ice thickness and strength, model structure natural modes and frequencies, and different ice crushing cylinder diameters and friction values. A novel approach was to furnish the test pile with an actuator that provided variable sinusoidal movement to the ice crushing cylinder. This technique that has been used earlier in vortex shedding vibration studies is expected to give more direct ice-structure interaction parameters data. The measurement results together with determined parameters in dynamic ice structure interaction process will be used to validate theoretical ice-structure dynamic interaction models, and thereafter for more reliable offshore structures design.

Short description of the work carried out

A total of six ice sheets were tested. Ice thickness varied from 40 to 90 mm, and ice velocity from 0 to 350 mm/s. The structural configuration was changed to give natural frequencies from 7.5 to 30 Hz. This was accomplished by changing both the tuning masses and support stiffness of the test pile. Also the ice crushing cylinder diameter was varied: 100, 220 and 400 mm. The cylinder surface roughness could be changed in order to verify friction effects. Various velocity dependent dynamic ice load histories were measured at different structural configurations. All the three in literature defined ice-structure interaction scenarios were captured: intermittent, frequency lock-in and random. The most wanted - frequency lock-in vibration - sustained only for short times, e.g. 10 to 30 vibration cycles. The test structure behaved mostly according to design expectations. The change of a structural configuration was straightforward and could be changed in a short time. The only problem was a too brittle frictional coating that peeled off during a test. Another difficulty was the servo-controlled actuator. As the sinusoidal movement direction changed the servo control, that was giving signal to push against the ice, should have changed instantaneously to pull in the direction of constant ice movement while maintaining the unaltered pushing load. During the last tests - without ice field softening with time - there was not chance to tune the control loop. Hence some of the controlled movement data has uncontrolled slippage in position. The model ice properties that are normally tuned for ship model tests - with ice failure in bending - required much cooler testing environment in order to maintain sufficient crushing strength. Data points harvesting for an ice crushing strength data plot was started in order to predict the dependence on model ice crushing strength based on routine ice test basin temperature, ice bending and uniaxial compressive strength test samples.

Highlights of important research results

For the first time in ice induced vibration tests dynamic calibration and frequency response function method was solely used to solve the true ice load. Slow constant acceleration ice velocity increase method was used to find out velocity dependence. This was further refined in constant velocity tests with forced ice crushing cylinder movement. The analysis of measurement results has just started with general overview. The thorough analysis will be carried out as a part of different arctic technology MSc and PhD studies within three years. The measured data set includes a comprehensive variety of different structural natural mode combinations, ice thickness and velocities. The visual overview of the data plots indicates that the measurement system has functioned as expected and has given good quality data. The relatively short appearances of frequency lock-in vibrations indicate that the energy input to the structure has been close to the same level as the dissipated energy in structural damping and ice crushing. The quality of data gives promise that the ice parameters that control energy input and dissipation during ice crushing can be analyzed and recognized. The HSVA model ice loading rate dependence on uniaxial compressive strength at actual test temperature will be determined from the ice samples that were stored during the tests. Then numerical dynamic ice-structure interaction models can be tested and refined.

Publications
M??tt?nen, M., L?set, S., Metrikine, A., Evers, K-U., Hendrikse, H., L?n?y, C., Metrikin, I., Skjerve-Nord, T., and Sukhorukov, S.Novel ice induced vibration testing in a large-scale facility: Deciphering Ice Induced Vibrations, Part 1 , 2012 Proceedings 21st IAHR International Symposium on Ice ?Ice Research for a Sustainable Environment?, Dalian, China, June 11 to 15, 2012
Sjerve-Nord, T., and M??tt?nen, M.Modal Analysis in Ice-structure Interaction: Deciphering Ice Induced Vibrations (DIIV), Part 2 , 2012 Proceedings 21st IAHR International Symposium on Ice ?Ice Research for a Sustainable Environment?, Dalian, China, June 11 to 15, 2012
Hendrikse, H., Metrikine, A., and Evers, K.-U.A method to measure the added mass and added damping in dynamic ice-structure interaction: Deciphering Ice Induced Vibrations, Part 3 , 2012 Proceedings 21st IAHR International Symposium on Ice ?Ice Research for a Sustainable Environment?, Dalian, China, June 11 to 15, 2012
Nord, T.Frequency domain force identification in ice-structure interaction , 2013 Proceedings of the 22nd International Conference on Port and Ocean Engineering under Arctic Conditions (POAC?13), June 9-13. Espoo, Finland, Paper ID: POAC13-080

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