CFD-based Hydrodynamic Performance Analysis Of Floating Wind-turbine Platform

Offshore wind power in China still has many technical problems to be solved because its short-time development.Compared with coastal wind resources,the space of offshore is vaster,and the wind speed is higher and more stable.Offshore wind turbine foundation evolved from offshore oil platform,however,in the process of design,the influence of the load characteristics of wind,waves well as the ocean currents should be considered.In addition,in the deep water more than 500m,it is also necessary to adopt floating platform to reduce the cost.The development of floating platform in high seas used for offshore wind power absorption is only in its infancy,in such case,the application is less.The floating platform widely used in oil and gas industry,because of its large volume and good stability,are commonly calculated by using AQWA,SESAM,HydroSTAR softwares which are based upon the potential flow theory.That has been able to satisfy the design requirements.The movement of floating wind turbine platform,which has smaller size,leads to the change of the wind speed at wind turbine disk.It will affect the output power of the device significantly and in consequence more accurate stability is needed.Due to the floating platform is smaller and easier to satisfy the CFD calculation conditions,the prediction of motion in the wave based on viscous theory is more accurate,which can provide more accurate calculation for the whole coupling hydrodynamic numerical model.This work will use the CFD technology to establish numerical tank for the hydrodynamic performance calculation of triangle and square semi-submersible platform,and discuss the effect of heaving plate on the performance of floating platform.Based on the theory of viscous flow,the fluid flow is simulated,fluid motion equation is solved,the stress of the floating body and motion are simulated.Based on the waveform and the validation of computational domain length and grid,determine appropriate calculation model.This paper will discuss numerical calculation method of the motion of floater in regular waves by a single truncated vertical cylinder using CFD technology.Two types of semi-submersible platform in regular waves are numerically calculated using two computing softwares which are based upon the potential and viscous flow theories respectively.The influence of viscosity and incidence wave directions are considered.The pressure and vorticity contours of the moving platforms under the resonant frequency are discussed.The precision of the viscous flow is much higher than the potential flow by comparing the maximum response amplitude at resonant frequency with two calculation methods.In addition,in the comprehensive comparison of two platforms with the aspect of movement RAOs,the stability of triangular platform is better.Based on the study of vorticity field for the considered two types of platforms with different scales of heaving plates,the influence of platform motion with viscous effect and different diameters of heaving plate were discussed.Research results have shown that near the peak of RAO,the calculation results in viscous flow are obviously better than those in potential flow.But within the scope of the left and right sides of RAO peak,the calculation results in viscous flow are close to those in potential flow.It has been proved from the flow field characteristics at the bottom of the cylinder that two platforms with heave plate can increase the stability of platform.In comprehensive comparison of the movement RAOs of the two platforms,the stability of square platform is better.