Study Of Mechanism And Theoretical Model Of Ice-induced Vibrations Of Narrow Structures

The interaction between sea ice and narrow marine structures (such as jacket platforms, jack-up platforms, bridge piers, bases of wind turbines, and lighthouses) results in dynamic ice forces. Severe ice-induced vibrations of structures arise in certain conditions. Ice-induced vibrations of jacket platforms may cause subsequent consequences such as structural fatigue, damage of pipelines and facilities, and crew discomfort. However, the mechanism of the formation of dynamic ice forces is still open to debate, and the design codes for narrow and compliant ice-resistant structures are still not satisfactory. To guide the design of ice-resistant platforms and guarantee the safety of jacket platforms in the Bohai Sea, it is necessary to investigate the mechanism and theoretical model of ice-induced vibrations of narrow structures.Based on the field test data and model test data analysis, this paper investigated the failure process and mechanism of the ice sheet loading vertical jachet platforms, analyzed the influence of the gear teeth on ice force and developed the spectrum model for dynamic crushing ice forces on jack-up legs with gear teeth in the Bohai Sea, constructed dimensionless parameters of ice-induced vibrations of conical jacket platforms, and established an ice-induced vibration prediction model for new conical jacket platforms which are in the design stage in the Bohai Sea.Related studies of this paper are listed as follows:(1)"Low level ice force" based on non-simultaneous crushing failure has been proved to be inapplicable to narrow and compliant vertical structures, based on the field test data analysis. According to the analysis of sea ice crushing failure process, the mechanism for simultaneous crushing failure of sea ice has been pointed out to be the "full contact" between the ice sheet and the vertical structure.(2) The physical mechanism of ice-induced self-excited vibrations of narrow and compliant vertical structures has been explained based on the analysis of failure behavior of sea ice in interaction with the structure. It has been pointed out that the linkage of the wing cracks in sea ice at the starting point of the unloading phase is the reason why the swinging back of the structure triggers the unloading phase.(3) Based on the stress field analysis of the ice sheet, the failure mechanism of the ice sheet loading at the junction of the positive and inverted cones has been explained:the tensile stress and the shear stress propagates the cracks in the ice sheet, leading to the upward or downward spalling of the ice sheet and the subsequent downward or upward bending failure of the ice sheet. The mitigation effect of the ice-breaking cones while the ice sheet is loading at the junction of the positive and inverted cones has been proved using field test data. (4) Based on the model test data analysis, a method for calculating the crushing ice force on jack-up legs with gear teeth has been proposed:the crushing ice force can be calculated using the "effective diameter" of the jack-up leg and the crushing ice force formula based on simultaneous crushing failure. The spectrum model for dynamic crushing ice forces on jack-up legs with gear teeth in the Bohai Sea has been developed:the auto-PSD of dynamic crushing ice forces can be gained by "adding" the auto-PSD and cross-PSD of local dynamic crushing ice forces based on the spectrum model for dynamic crushing ice forces on cylindrical legs in the Bohai Sea.(5) Based on the process and physical parameter analysis of ice-induced vibrations of conical jacket platforms, the dimensionless acceleration of the structure and and the dimensionless velocity of the ice sheet vnd have been constructed, furthermore, the dimensionless parameter for ice-induced vibrations of conical jacket platforms CAⅡⅤ has been constructed. According to the analysis of the model test data from Canadian Hydraulics Centre of the National Research Council of Canada (NRC), the trend of CAⅡⅤ and the discrepancy source of CAⅡⅤ have been pointed out:CAⅡⅤ is constant when all structure parameters and ice parameters except ice velocity are constant, and variations of CAⅡⅤ result from variations of structure parameters or ice parameters except ice velocity.(6) Based on the dimensionless parameter for ice-induced vibrations of conical jacket platforms CAⅡⅤ, using the simplified single degree of freedom system of conical jacket platforms, and the ice force spectrum model and the random ice force model for narrow cones in the Bohai Sea, the acceleration response prediction model for new conical jacket platforms which are in the design stage in the Bohai Sea has been established. Meanwhile, the framework of establishing the prediction model in other ice-infested waters has been developed.