A Study On The Mechanism Of "Secondary Load Cycle" Of Vertical Cylinders Under Steep Waves

Offshore structures can produce high-frequency vibrations under the action of extreme large waves,such as the “Ringing” phenomenon,which is generally believed to a high frequency response of the structure caused by resonant.Some experiments have found that when the structure's natural vibration frequency is far away from the region of third-order frequency of the wave,the high-frequency response of the structure could still be observed.This high-frequency response may be related to the “secondary load cycle” phenomenon.To further explain the mechanism of structural high-frequency response,the phenomenon of ‘secondary load cycle' was studied in this paper.In this paper,a physical experiment of force of vertical cylinders under regular waves was firstly performed.In the experiment,different wave parameters were considered,such as different wave periods and amplitudes,and the steepness of the wave steepness were from the medium to the extreme.In addition,the influence of the structural scale was also considered.Three cylinders with different radius: 0.05 m,0.075 m,and 0.125 m,were used.Relative to the length of the waves,the structure scales were small or medium-scale.Then,the experimental data and phenomena were analyzed.The influencing factors,critical conditions and frequency characteristics of the ‘secondary load cycle' phenomenon were studied.The results showed that the ‘secondary load cycle' phenomenon was mainly affected by the steepness of the wave.When the wave steepness(H/L)was greater than 0.095 tanh(kh)(where k is the wave steepness and h is the water depth),the structure could generate the ‘secondary load cycle' phenomena.When this phenomenon was relatively strong,it was usually accompanied by high frequency vibration of the structure.Spectral analysis also showed that when the ‘secondary load cycle' phenomenon occurred,the energy at the natural vibration frequency of the structure increased significantly.At the same time,combined with the experimental picture,the generation mechanism of the ‘secondary load cycle' phenomenon was analyzed.The analysis showed that the ‘secondary load cycle' phenomenon may be related to the wave run-up before the structure.When the waves passed through the structure,large wave run-up occurred at the front of the structure.After the wave peak propagated away from the structure,the wave run-up rapidly falls.The falling water body would squeeze the surrounding water bodies and structures and could generate a positive pressure on the structure.This may be the reason for the the surge of waves phenomenon.The steeper the wave steepness,the greater the high drop height,the more obvious the phenomenon.Finally,a numerical model was established to prove the mechanism obtained in the physical experiment.The results showed that the pressure on the front side of the cylinder was significantly increased when the ‘secondary load cycle' phenomenon occured due to the effect of high pressure of wave run-up falling,indicating that the effect of the wave run-up was the direct cause of the ‘secondary load cycle' phenomenon.Based on physical experiments and numerical simulations,this paper studied the influencing factors and mechanisms of the ‘secondary load cycle' phenomenon.The results of the study will help to understand the high-frequency response of marine engineering structures and provide assistance for structural design and safety assessment.