Ice-Induced Jacket Structure Vibrations in Bohai Sea

Abstract

Full-scale tests of ice-structure interaction were conducted on three multileg jacket platforms in Bohai Sea. Two of them were installed with ice-breaking cones on each of their four legs to reduce ice force and mitigate ice-induced vibration. The other is a three-leg platform with cylindrical piles without ice-breaking cones. Dynamic responses of these structures were recorded continuously, the variations in ice force were measured by load panels, and the failure processes of ice acting on cones and cylindrical pile were observed by video cameras. For the structure with ice-breaking cones, the ice sheet fails in bending, but the breaking frequency could coincide with the natural frequency of the structure at a fast ice speed and significant dynamic amplifying could be induced. For the cylindrical structure, two types of vibration are classed. At a high ice speed, the amplitude of structure response is uneven and ice fails with the brittle flanking model. At a medium ice speed, the event of steady-state vibration could take place. The explanation of ice-induced steady-state vibration is presented based on failure processes of ice within the ductile-brittle transition zone. Field observations reveal that sea ice can induce the offshore jacket structure to vibrate in resonance regardless of the fact that ice-breaking cones have been installed.