Ice Induced Vibrations - Observations of a Full Scale Lock-in Event Available to Purchase

A full scale event with frequency locked-in ice loads is presented in this work. Data collected at the Norströmsgrund lighthouse in Gulf of Bothnia during the STRICE project consists of several events with lock-in like ice loads. The 180 seconds long event presented in this work occurred 30 March 2003 and was one of the harshest during the entire measuring program 1999–2003. Physical conditions related to ice thickness, drift speed and ice temperature have been reported earlier to play a key role in the occurrence of lock-in like ice loads. This study reports some new features related to ice loads and response. Overall 190 cycles at the fundamental mode was found. The peak load represents an effective pressure of 1.3 MPa with an average load amplitude of 58 % of the peak load. Focus in the study was the triggering of locked-in ice loads. It was found that spatial synchronization of local loads was the reason for triggering. The synchronization could be caused by different physical phenomena where the drift speed, ice thickness and ice temperature is of substantial importance. As well, temporal fluctuations of loads were studied within the event. Together with oscillations at the structural fundamental frequency, a process fluctuating at a 5 sec period was found. However, no clear explanation for the 5sec build-ups was found. At last the end of the event was studied. Stop in lock-in like ice loading was in this case, limited by an in-plane collapse and further flooding of the ice sheet.