Ice-induced structural vibration mainly depends on the two coefficients of dynamic ice force: amplitude and period. According to the field measurement results obtained on two jacket structures installed with composite upward and downward ice-breaking cones in the Bohai Sea, the ice-induced structural vibrations vary with the ice acting position on the cone because cone shape (upward cone or downward cone and width) is one of the dominant influencing factors of the ice force period and sea ice breaking length. With the increase in the cone diameter, the ice failure mode on the upward cone was transformed from the wedge failure mode (with the long sea ice breaking length) into the plate failure mode (with the short sea ice breaking length). The plate failure mode was observed on the downward cone. When the small cone was selected, the significant difference in the dynamic ice force period was observed between upward and downward cones. When the wide cone was selected, the difference was not significant.

Introduction

When ice sheets pass through a fixed offshore structure, continuous ice failure produces dynamic ice loads, which lead to the structural vibration called ice-induced vibration. Since ice-induced vibrations were observed on oil platforms in Alaska's Cook Inlet (Peyton, 1968; Blenkarn, 1970), Bothnian Bay Lighthouse in the north of Europe (Engelbrektson, 1977), and other offshore structures in marine ice areas, ice-induced vibrations of offshore structures have been extensively studied. At present, serious ice-induced vibrations are also observed on the oil platforms in Liaodong Bay of the Bohai Sea. When ice loads act on vertical structures, ice loads mainly lead to crushing ice failure, which can produce the largest horizontal ice load compared to other ice failure modes. Yue and Bi (2000) installed measurement systems on vertical offshore structures in the Bohai Sea and observed the most serious ice-induced steady vibrations under a certain ice velocity. The accidents of pipeline rupture and loosening flange occurred on offshore platforms because of ice-induced structural vibrations. It is necessary to obtain a proper way to eliminate or reduce ice-induced vibrations.

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