ABSTRACT

Full scale tests were conducted on a monopod structure before and after an icebreaking cone was added. The effectiveness of mitigating ice induced vibration through adding ice-breaking cone was evaluated based on test data. It was found that significant ice induced vibration can occur on both of vertical and conical structures, but that vertical structures suffer from more intense and harmful steady state vibration. The dynamic ice force and structure response of vertical and conical structure were discussed based on test data.

INTRODUCTION

The concept of reducing ice force by adding ice-breaking cone to vertical structure has been proved by the theoretical and experimental work (Wessels.E and Kato.K.,1988; Koh Izumiyama, 1991). Though the details of the effects on ice induced vibration are not yet completely understood. Persistent steady vibration will occur on vertical structures under certain ice condition, and could induce serious accident. Icebreaking cones have been added, while strong vibrations are still persisting on some structures, such as the jacket oil platform in the Bohai Bay. Meanwhile conical and inclined structures have some other problems, such as increase the structure width at the waterline complicated fabrication and the danger to vessels. After the ice-breaking cones were first installed on the vertical legs of jacket structures in the Bohai Bay since 1988, the cones have been widely used. Field measurement systems for studying the ice force and structural response to ice motion have been installed on seven jacket structures, some with cones and some without. Jacket structure JZ9- 3WHPE is a monopod structure, and full scale tests were conducted on the structure before and after ice-breaking cone added. This paper first demonstrates the theoretical explanation for ice induced structure vibration, and relates it to the form of dynamic ice force history.

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