Ice induced vibrations of flexible offshore structures: The effect of load randomness, high ice velocities and higher structural modes

In this study a phenomenological model is proposed of Ice Induced Vibrations (IIV) and an attempt is made to answer the following questions: (i) can IIV occur at high ice sheet velocities? (ii) what are the conditions for IIV to occur at a higher natural frequency of the structure? (iii) can an initially aperiodic (you probably mean aperiodic, which is the opposite) ice loading cause IIV? The model of crushing level ice presented in this paper may be referred to as a strip model, implying that the ice is represented by a sequence of strips parallel to the ice velocity vector. The dynamic behavior of each strip is described based on the idea by Matlock that has recently been improved by Huang and Liu. A further improvement introduced in this paper is that the load produced by each strip is randomized. The attractive elements of the strip model are that it reproduces both the descending character of the strength-indentation rate diagram and contains an inherent, though implicit, periodicity in loading on a rigid structure. The structure that undergoes IIV is mimicked by a generalized beam with coordinate dependent parameters that allow for tuning of both the modal spectrum and modal damping. Using the proposed model it is shown that an initially aperiodic ice loading can trigger IIV both at low and high ice velocities. IIV at higher modes can be predicted only if the modal damping of those modes is small relative to that of the lower modes.