Since late of 1980"s, several jacket offshore oil platforms were deployed in north part of Bohai Sea of China, where sea water freezes every winter. These platforms were designed mainly considering the extreme static ice force. Instance vibrations induced by moving ice were found on some of the platform in every winter. Field monitoring of ice induced vibrations and full scale measurements of ice force were carried, which lead conclusions that for the jacket structures which satisfy to resist maximum ice force may exist risks caused by ice induced vibrations. The dynamic behavior of ice resist jacket structure and potential failure modes provoked by ice induced vibrations are discussed in present paper.
It is known that the offshore oil platforms designed in ice infested area need to stand not only maximum ice forces but also dynamic ice which may result in harmful vibrations. The phenomenon of ice-induced structural vibrations has been noticed and studied since early of 1960s(Peyton 1968). In the 1970s, the lighthouse in the Gulf of Bothnia encountered ice-induced vibration and collapsed as a result of moving ice (Engelbrekston, 1977). In the 1980's the ground base of caisson named Moligpac in Beaufort Sea was liquated because of ice-induced vibrations (Wright and Timco, 1994). Since the late 1980's, the problem of ice-induced vibrations was noticed after the first platform was established in Liaodong Bay of Bohai Sea (Yue and Bi, 2001), and observations in the past several winters showed that the ice-induced periodic load caused the crews' discomfort, some damage of pipes and facilities on the platforms. It seems that slender structures are more liable to subject ice induced vibrations as their natural frequency may close to or even coincide with the prominent frequency of ice forces. It has demonstrated, by field and laboratory tests, that the sheet ice may create cyclic force during cutting through the fixed structures.