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|Title:||Dynamic ice-structure interaction during continuous crushing|
|Authors:||United States. Army. European Research Office.|
University of Oulu.
Määttänen, M. (Mauri)
|Publisher:||Cold Regions Research and Engineering Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||CRREL report ; 83-5.|
Abstract: This report presents the results of dynamic ice-structure interaction model tests conducted at the CRREL Ice Engineering Facility. A flexible, single-pile, bottom-founded offshore structure was simulated by a test pile with about a one-to-ten scale ratio. Urea (instead of sodium chloride) was used as dopant to scale down the ice properties, resulting in good model ice properties. Six ice fields were frozen and 18 tests carried out. In all cases distinctive dynamic icestructure interaction vibrations appeared, from which abundant data were collected. In tests with linear ice velocity sweep, sawtooth-shaped ice force fluctuations occurred first. With increasing velocity the natural modes of the test pile were excited, and shifts from one mode to another occurred. The maximum ice force values appeared mostly with low loading rates but high forces appeared randomly at high ice velocities. As a general trend ice force maximums, averages and standard deviations decreased with increasing ice velocities. The aspect ratio effect of the ice force in continuous crushing follows the same dependence as in static loadings. The frequency of observed ice forces is strongly dominated by the natural modes of the structure. Dynamically unstable natural modes tend to make the developing ice force frequencies the same as the natural frequencies. Otherwise the resulting frequency depends directly on structural stiffness and ice velocity and inversely on the ice force range. During vibrations the displacement rates of the structure overcome the velocity of ice, making low loading rates and hence high ice forces possible. During crushing, ice induces both positive and negative damping. The latter easily becomes so high that the pile becomes dynamically unstable and is the origin for ice-induced vibrations. As the negative damping effect prevails only during a part of a vibration cycle, the overall state is stable and steady state limit cycles develop. These measured results verify earlier theoretical predictions and confirm a basis for safer design against dynamic ice forces.
|Appears in Collections:||CRREL Report|