冰激结构频率锁定振动的发生机理及简单分析方法

冰激结构频率锁定振动是冰区海洋工程结构的危险工况.对频率锁定振动过程的传统机理解释没有体现这一过程的全部物理特征,导致现有的分析方法无法准确分析这一问题.本文基于对现场测量结果的分析,提出了一种海冰韧性损伤--破碎过程与结构振动耦合导致频率锁定振动的机理.该机理认为,海冰在直立结构频率锁定振动过程中发生韧性损伤-破碎行...

MECHANISM AND SIMPLE ANALYSIS METHOD OF ICE INDUCED FREQUENCY LOCK-IN VIBRATION OF OFFSHORE STRUCTURES

Ice induced frequency lock-in vibration of offshore structures has been recognized as a serious load condition in ice regions. The traditional mechanism cannot fully explain all the physical phenomena in the frequency lock-in vibration, nor can a reasonable analysis method be obtained. Based on the analysis of full scale measurement data, this paper proposes a new mechanism of frequency lock-in vibration caused by the coupling of sea ice ductile damage-collapse failure and structural vibration. It is believed that the ductile damage-collapse failure of sea ice occurs during the frequency lock-in vibration of vertical structures, which is coupled with the phase of structural motion, resulting in frequency lock-in vibration. Saw-teeth-shape ice load is caused by the action between sea ice and the structure. The action process can be divided into loading and unloading stages, the loading stage takes about 3 times the time of the unloading stage. In the loading stage, the structure moves against ice from equilibrium position to its maximum negative amplitude and then moves back together with ice at the same direction to its maximum positive amplitude. Cracks are formed in the contact part of the sea ice and the structure, but not collapse. The sea ice undergoes ductile damage at this stage. In the unloading stage, the structure moves in the opposite direction to the sea ice, swinging back from the maximum amplitude to its equilibrium position. The sudden increase of strain rate leads to the accelerated propagation and instable fracture of cracks. The sea ice with ductile damage collapses at this stage. Based on the new mechanism explanation, this paper presents a simple analysis method of ice induced frequency lock-in vibration of offshore structures. It is considered that the sea ice break length is the key parameter of frequency lock-in vibration. In the ideal situation, the sea ice break length is about 2.2 times of the vibration amplitude at the waterline of the structure. When the ice velocity is close to the ratio of sea ice break length to the natural vibration period of the structure, frequency lock-in vibration will occur. This method has guiding significance for evaluating the occurrence probability of frequency lock-in vibration and fatigue damage of offshore structures.