Study Of Local Scour Characteristics And Protection Of Solidified Soil At Offshore Wind Turbine Monopile Foundation

In the marine environment,the existence of pile foundation changes the original flow state,leading to local scour around it.This affects the bearing capacity of pile foundation and the safety of turbine.Different from the traditional bridge piers,the local scour mechanism of the offshore wind turbine monopile foundation is relatively complex in combined waves and current along with more influence factors,leading to the differences in the existing local scour depth prediction methods and obvious deficiencies in the scour protection measures.In this paper,local scour problems at monopile foundation are studied systematically in single current,single waves and combined waves and current through model flume experiment.Based on dimensional analysis priciple and group method of data handling(GMDH),empirical formulas are proposed respectively for prediciting local scour depth in combined waves and current.The scouring characteristics,the effect of scour protection and scour repair are studied for solidified soils of mucky clays.Through model flume experiment,the local scour problems at monopile foundation in different flow structures are studied systematically.The maximum value of local scour depth around the pile occurs at±45°in front of the pile under single current and combined waves and current,it occurs at±45°in back of the pile in single waves comparatively.The minimum value occurs at the back of the pile in all working conditions.The local scour pit pattern around the pile in combined waves and current is similar to that in single current,but the maximum local scour depth in combined waves and current is smaller than that in single current.The local scour depth S/D at small-diameter pile is larger than that at large-diameter pile on the whole.The local scour depth is mainly controlled by the flow velocity and increases with the increase of it in combined waves and current,while it is less affected by wave height.The S/D increases with the increase of KC number and velocity ratio U_cw,but the rate of increase slowed down gradually.In the condition that KC number is small,the S/D increases rapidly with the increase of U_cw,and the local scour depth may also increase rapidly with a smaller flow velocity.In the condition that U_cw is large(U_cw?0.6),a larger local scour depth likely forms with a smaller KC number.In the condition that U_cwis small(0.4<U_cw?0.6),S/D is greatly affected by KC number.Based on the dimensional analysis principle,grouh method of data handling(GMDH)and experimental data,empirical formulas are proposed respectively for predicting local scour depth,which consider the main controlling factors in combined waves and current.Combined with the experimental data and the field monitoring data of Rudong offshore wind power plant in Jiangsu Province,the performance of the predictive equations of local scour depth were compared to verify the reasonability and effectivity of the proposed formulars,including Chinese standard formula,DNV formula,HEC-18 formula,Rudolph formula,Raaijmakers formula and the prediction formulas in this paper.The calculation accuracy of the proposed formula 1 is highest,the relative error,the average absolute relative error,the variance and the normalized variance are all smaller than those of the other formulas.The relative error of the proposed formula 2 is smaller,but special attention should be paid to the actual engineering application because the predicted values are generally less than the measured values.The predicted values of the proposed formula 1 is larger than the measured values on the whole,indicating that the accuracy and security of it are both superior to formular 2.Therefore,the poposed formula 1 is more suitable for predicting local scour depth at offshore wind turbine monopile foundation.The scouring characteristics,the effect of scour protection and scour repair are studied for solidified soils of mucky clays through scouring characteristic experiment and model flume experiment.Based on the results,the critical shear stress is the same in different conditions when solidified soils are not cured,that increases with the increase of curing time and cement ratio.But the growth rate decreases with the increase of curing time,while increases with the increase of cement ratio.The erosion rate is positively correlated with the critical shear stress,the relationship between them is approximately exponential.The growth rate of erosion rate with the critical shear stress is greatly affected by curing time and cement ratio.The range of erodibility coefficient and critical shear stress of solidified soil are quite different from those of cohesive soil,the quantitative function relationship between the two is established.The local scour pit around the pile is approximately circular on the plane,and its plane size is mainly controlled by the flow velocity,while the wave height has little effect on it.The changes of the plane size of scour pit with the flow conditions are the same as S/D.The damage of solidified soil itself and the"secondary scour"phenomenon are the mainly damage forms when it is used as scour protection measure,while the repair effect mainly depends on the"secondary scour"when it is used for scour repair,as the solidified soil itself will not be damaged for good integrity.The damage pattern and the effect of scour protection and scour repair of large-diameter pile and small-diameter pile are similar,the effect mainly depends on the range of solidified soil.It can achieve a better effect when the range of solidified soil greater than the plane size of the scour pit without protection,and that is 3D for small-diameter pile while 2D for large-diameter pile.