Author(s): D. M. Masterson; P. A. Spencer
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Abstract: Offshore structures experiencing time varying loads caused by a moving ice pack are subject to dynamic response. Such behaviour has been observed in many locations with varying consequences. These consequences have ranged from none in Cook Inlet to densification of the Molikpaq sand core in the Beaufort Sea (Jeffries and Wright, 1988) to failure of structures in the Gulf of Bohai and to violent reaction of lighthouses in the Baltic Sea. Experience from measurement and observations of these structures has shown that with proper definition of the ice pack mechanical characteristics and the load vs. time function, both caisson and multi-legged structures can be designed to function satisfactorily and to provide both good fatigue resistance and human habitability characteristics. Examination of hundreds of load vs. time traces from full scale structures confirms that the load-time function has the form of a sawtooth, whereby the load ramps up relatively slowly over a period of time and then drops suddenly when the ice strength is reached at the ice-structure interface. The period of the sawtooth tends to match the natural frequency of the structure however, it is observed to vary randomly about this frequency by about ±10 %. In addition, the amplitude of the sawtooth, which is nominally about 50 % of the applied load, varies randomly. The sawtooth shape and the random variation of the frequency and amplitude have very important consequences for the structure behaviour. Because of these constraints, classic harmonic resonance at zero damping can never be realized, although some dynamic amplification will result. The maximum dynamic amplification with zero system damping was observed to be less than 5. This paper uses measurements from structures in the Beaufort Sea, Cook Inlet, the Gulf of Bohai and the Sakhalin Shelf to illustrate and confirm the conclusions drawn.
Year: 2002