Effect of Frequency Ratio on Dynamic Responses of Sand Under Wind Turbine Loading

Abstract:

The soil sample is subjected to a coupling effect of wind and wave action with various frequencies during the operation of offshore wind turbines. A series of tests were conducted to investigate the influence of the frequency ratio between wind and wave loads on sand using a multi-directional cyclic simple system （VDDCSS）. The effect of frequency ratio on the dynamic strain，pore water pressure，and dynamic strength of sand was analyzed. The test results show that the development of shear strain is affected by the bidirectional frequency coupling and the combination of x and y-direction stress caused by the frequency ratio. At a C_SR of 0.15， the bidirectional frequency coupling effect is greater than the stress combination effect. With the increase of the stress level （at a C_SR of 0.20 or 0.25）， both the frequency coupling effect and the stress combination effect are strengthened. At a low-stress level， the pore pressure curve of the failed specimen is "upward concave" and the frequency ratio has little effect on the pore pressure development of other specimens. As the stress level increases （at a C_SR of 0.20 or 0.25）， the final value of pore pressure u_max increases with increasing C_SR. Except for the samples corresponding to a f_r of 1.00 and 10.00， the dynamic strength of others soil samples decreases with the increase of f_r and C_SR. At a f_r of 5.00， the sample has a large strain，a high pore pressure value， and a low dynamic strength， which is related to the superposition effect of the two-way frequency coupling and stress combination.

Key words:cyclic simple shear;frequency ratio;dynamic strain;pore water pressure;dynamic strength