The Regulation,Control And Evaluation Of Dynamic Response For Offshore Wind Turbine Foundations

The regulation,control and evaluation of the dynamic response for offshore wind turbine foundations under complex loadings are key difficult points in the design and operation period.There are three major problems for the development of offshore wind power in China: poor geologic conditions of seabed,complex loading environment,and strict design requirements.Hence,this thesis aims to conduct the researches in response to these problems.A new type foundation is invented,and the dynamic response is revealed.A vibration evaluation method is established.Finally,engineering control measures are recommended.The main novel achievements are as follows:1.A new combined foundation for offshore wind turbine is invented,which contains a pile and a caisson.Compared with monopile foundation,the combined foundation can increase the capacity of the wind turbine without increasing the pile length and diameter,or reduce the pile diameter and length for the same capacity,thus reducing construction difficulty and foundation cost.The combined foundation also has the advantages in high load bearing capacity,small mud surface displacement,small inclination angle,antierosion and anti-liquefaction.2.The dynamic response of offshore wind turbine foundation under loads coupling of wind,wave and earthquake is revealed.The influence of load coupling effect is studied.Separately,under wind and earthquake coupling,inertial interaction of the structure and foundation dominates.Under wind and wave coupling,seabed liquefaction occurs and leads to the foundation deformation to accumulate.Under wind,wave and earthquake coupling,all the effect containing the inertial interaction of the structure and foundation,pile soil interaction,and seabed liquefaction induced by earthquake and wave occur.The response differences of different foundations such as rigid pile,elastic medium-long pile,and new combined foundation under loads are analysed.The new combined foundation has large stiffness,so it can significantly reduce the system vibration amplitude and residual deformation.Its upper part translates while lower part bends.Hence both the displacement at mud surface and the foundation inclination are small.3.The calculating formula for the system frequency design is obtained.In the method,the variation of cross-section geometry of the wind turbine tower is considered.So the inertia moment and distribution quality of the tower changing with diameter are characterized.The foundation rotational stiffness induced by pile-soil interaction is also added.The optimal design of the structural parameters is analysed.When tower length exceeds 100 m,and tower wall thickness exceeds 0.1m,there is only marginal change in frequency.Larger pile diameter can help to reduce the effect of bad soil condition.And there is a best diameter which should be studied in design.For the fundamental frequency of wind turbines based upon large diameter monopiles,an optimal pile length exists.4.Furthermore,a practical algorithm method for analysing the system frequency deviation during the operating period is formed.It can effectively evaluate the frequency deviation caused by the effect of cyclic creep of the soil around pile,seabed scour and liquefaction,seabed slope slide induced by wind,wave and earthquake.Engineering control measures for improving the dynamic resistance of offshore wind farms are recommended.