Author(s): Michael Jefferies; Tuomo Karna; Sveinung Loset
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Abstract: This paper addresses interaction of vertical faced structures with moving sheet ice. Data had been obtained from narrow and compliant piles that we exposed to ice actions in Cook Inlet in late 1960’s. This data showed that the highest ice loads were obtained at a low ice velocity while ice was failing in a ´ratcheting´ mode. The same phenomenon was termed subsequently as ´phase-locked crushing´ when it was encountered with the much wider Molikpaq structure in the Beaufort Sea in the 1980’s. It has also been termed as ´intermittent crushing´ or ´alternating ductile-brittle crushing´ in reports on laboratory tests. The main features of the Molikpaq data are reviewed and a typical condition of ice-structure interaction studied in detail. The data shows ice load can increase when ice velocity decreases below 0.1 m/s. Under these conditions the data shows a transition from non-simultaneous failure into simultaneous crushing. The overall or ‘global’ load on the structure increase by about a third because of the synchronized ice crushing. The data obtained on the Molikpaq structure are compared with data obtained on the lighthouse Norstromsgund in the Baltic Sea. This structure is very stiff at the waterline. Comparisons between the two data sets support the conclusion that the maximum peak values of the ice load increase with the compliance of the structure, that is, with the waterline displacement. Waterline displacement appears to be the feedback mechanism synchronizing the usual non-simultaneous ice failure into a periodic, simultaneous ice crushing across the width of the structure.
Year: 2008