An explanation for the Molikpaq May 12, 1986 event

The ice crushing induced vibration and so-called lock-in behavior exhibited by the Molikpaq structure during a test case event that occurred on May 12, 1986 has been explained in terms of ice spalling, and associated sawtooth load and displacement patterns, and resonance of the coupled structure–ice system where the mass and spring constant associated with the ice sheet are variable ‘effective’ quantities that depend on the ice sheet speed. Spalling at the ice edge contact zone plays a key role since the spalling frequency is directly proportional to the speed of the ice sheet towards the structure. Two modes of spalling are described for this particular event. One mode, called system resonant capped (SRC) spalling, causes loading and displacement that is similar to classic high-amplitude sawtooth loading and displacement and occurs at spalling frequencies in the vicinity of, or less than, the resonant frequency of the structure–ice system. In the present case the SRC spalling appears to be at the system resonant frequency. The other spalling mode is low-displacement–amplitude erratic spalling that is characterized by temporal unevenness and unevenness in the magnitude of consecutive displacement sawteeth. This type of spalling occurs at an average spalling frequency of about 4Hz, which is substantially higher than the SRC spalling frequency range. For the analysis rough estimates for the variation of the effective mass and effective spring constant of the ice sheet throughout a range of spalling frequency were determined numerically. This treatment predicts that, for ice with similar properties as the May 12 event at least, sawtooth loading of the structure that causes structural oscillations is inevitable when an expansive ice sheet crushes against it. Potential means of significantly reducing the amplitude of the load and displacement oscillations by influencing fundamental spalling characteristics using blade-like spall initiators installed on the structure faces at the ice level are discussed. ► Resonance is a key factor in the Molikpaq lock-in behavior. ► The ice spalling frequency is directly proportional to the ice sheet speed. ► The ice effective mass and effective spring constant vary with spalling frequency. ► Two modes of ice spalling behavior can explain episodes of lock-in and non lock-in.