The dynamic ice force and structure response of vertical and conical structures are discussed based on field measurements in Bohai Bay. It is found that significant ice-induced vibrations can occur on both kinds of structures. Adding a cone can avoid a more intense and harmful steady-state vibration. Full-scale tests have been conducted on a monopod structure before and after adding an ice-breaking cone. The effectiveness of mitigating ice-induced vibrations through adding an ice-breaking cone is evaluated based on test data.

INTRODUCTION

When ice sheets run through fixed offshore structures, the icebreaking process causes dynamic ice force and induces structural vibrations. Ice-induced vibrations of many structures in cold regions have been observed—for example, oil drilling platforms in Alaska's Cook Inlet (Peyton, 1968; Blenkarn, 1970), lighthouses in Bothnia Bay (Engelbrektson, 1977) and jacket oil platforms in the Bohai Sea (Yue and Bi, 2000). When ice acts on vertical structures, the resulting ice-crushing failure can produce the largest horizontal load and cause strong steady-state vibration. Because the physical process of ice crushing failure is very complicated, there have not been a proper mechanical explanation and a practical prediction model for ice induced vibrations on vertical structures. Steady-state vibrations were observed on the vertical structures in the Bohai Sea, and they threatened the structural performance and production facility. Pipeline fracture and flange loosening took place under steady vibrations, which are the most serious condition induced by ice actions. Adding ice-breaking cones at the water level is one of the methods to resolve ice-induced vibrations. Several ice resistant structures were designed to be of a conical shape at the water level—for example, piers of the Confederation Bridge in the Southern Gulf of the St. Lawrence (Brown, 1997), offshore wind turbine foundations in Denmark (Määttänen, 1996) and oil platforms in China's Bohai Bay.

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