Dynamic Response Of Monopile Supported Offshore Wind Turbine Under Different Scour Depths

Offshore monopiles are generally subjected to numerous cycles of wind,wave and current loads along with other dynamic loads and earthquake loads and so it is difficult to guarantee dynamic stability of overall structure.Combined action of wind and waves create scour phenomenon at pile bottom.Over the years various researchers have studied effects of scour on different parameters of offshore wind turbine foundations and have found that scour causes dynamic instability of structure and changes in bearing capacity thereby creating higher design requirements.Available research on dynamic response of monopiles under scour is relatively rare.In order to mitigate consequences of scour over last few years various methods and systems were developed for protecting offshore structures.Most common remedial measures used in offshore structures against scour problems are scour protections.But scour protections in turn are affected by scour resulting in not serving their purpose.Another approach studied by various research is structural control which reduces loads on offshore wind turbine by affecting response of structure.Tuned mass dampers are one of the structural control devices for mitigating effect of resonance on structures which are most commonly used in buildings and bridges in seismic intensive areas and are proven to be very useful.Providing tuned mass dampers in offshore wind turbines can reduce fatigue damage due to scour,and thereby increase feasibility of foundation.Available research on use of tuned mass dampers in wind turbine structures is very few.Existing research has not focused on effect of tuned mass damper on dynamic response of structure in case of scour conditions.So,based on existing research first chapter focuses on establishing the objective of research which is to study and analyze dynamic response of offshore wind turbine supported by monopile foundation with tuned mass damper under different scour depth.For the purpose of study a 4 mega-watt offshore wind turbine supported by monopile foundation in H1#Binhai North offshore wind farm project of Jiangsu province is taken into consideration.Data of project site which includes hydrological and geological characteristics of site,monopile and wind turbine characteristics,design standards and control criteria obtained from feasibility study stage report of wind farm area are explained in detail under second chapter.Using the data of project site explained under second chapter,based on Det Norske Veritas and site specific conditions,extreme working condition of structure is considered as wave force caused by H_1%wave height and stratified tidal force during 50-year return period.Third chapter focuses on wave and scour theory along with calculation of wave load for the selected extreme working condition,scour depth and scour pit diameter calculation using different formulae developed by various research.In the research by Dai Hao,MATLAB is used to generate velocity,acceleration potential functions for Linear?Airy?wave theory for similar conditions of this paper.Also wave load is calculated for Airy wave theory using Morison's equation.Scour depth and scour pit diameter are then calculated using different local scour formulae developed by different research for tides alone,currents alone,waves alone and combined wave and current conditions.Representative scour depths are then selected in obtained range of scour depth from different formulae.There are various software that can be used for finite element modeling and analysis.ABAQUS software is used to obtain system natural frequency under no scour,no elongation coefficient and no tuned mass damper.Fourth chapter focuses on finite element software and process of modeling and analysis along with calculation of tuned mass damper properties like mass,damping and stiffness coefficient.Calculated range of scour depth along with different elongation coefficients ranging from 0 to 3 is considered for analysis.A full scale finite element model is developed where Lumped mass model developed by Penzien is used to represent whole wind turbine system.Drucker-Prager constitutive model of ABAQUS is used to simulate main characteristics of soil.Lanczos method is used to obtain frequency and mode shapes.ABAQUS is then used to carry out Transient dynamic analysis?Time history analysis?.Tuned mass damper located in nacelle and operating in fore-aft direction is adopted for which frequency is same as that of frequency of primary structure under no scour,no elongation coefficient.Den Hartog's formula is then used to calculate mass ratio and optimal damping ratio.For calculated mass ratio properties of tuned mass damper like mass,damping coefficient and stiffness coefficient are obtained.Two different mass ratios are considered for studying dynamic response of wind turbine under varying scour depths.Frequency and mode shapes obtained from finite element analysis are analyzed in fifth chapter to present some meaningful conclusions in sixth chapter.