Offshore Wind Turbine Foundation Design including Soil Damping Models
Arash Hemmati, Mahdi Khorasanchi, Nigel Barltrop
University of Strathclude, Glasgow, UK
Offshore wind turbine foundations are subject to complex dynamic loading from multiple sources including wind, wave and current and they are highly prone to fatigue damages. Therefore proper consideration of damping plays a crucial role and can reduce fatigue damages substantially. Hence it is imperative for designers to estimate and model the damping accurately in the foundation design. The total damping is often defined as a superposition of the damping sources despite the non-linear nature. Modelling of soil damping is complex and its effect on the offshore wind foundations has not yet been thoroughly investigated. Due to the lack of commonly available full-scale measurements, soil damping is usually being modelled as a contribution to global proportional or Rayleigh damping in industry and academia. However, this soil damping model does not represent the physical behaviour. In this paper, the possibility of OWT foundation by modelling soil damping as dashpots along the pile is investigated. It is concluded that OWT design can be optimised by soil damping dashpots.
In this paper, soil damping is modelled by means of dashpots with damping parameters associated with the soil stiffness and mode shapes. The effect of using the viscous dashpots representing soil damping is investigated. A number of design load cases including operational, parked, start-up and emergency stop are used in time integrated analysis. As the significance of soil damping is highly dependent on frequencies of the system and loading, the study is performed for various natural frequencies and loading frequencies. A statistical analysis is performed on the responses.
Results show that the significant impact of soil damping is experienced in the base moments and the design load cases in non-operational conditions have more potential for load reduction and ultimately fatigue damage optimisation. This is due to the lack of aerodynamic damping in these load cases. Also, it is shown that modelling soil damping can optimise the foundation in the vicinity of the mudline as in this area soil is more strained and soil damping plays an important role in load reduction.
It can be concluded that soil damping modelled by dashpots may provide some additional damping when it is combined with structural damping accounted by Rayleigh damping and therefore it can be used to optimise the design of offshore wind turbine foundations. It is shown that soil damping is underestimated in the design of OWT foundations as it is usually being included in total structural damping.
The paper gives an overview of optimisation potential of soil damping modelled by means of dashpots. This offers an understanding of the effects of soil damping on OWT foundations design and can enable the foundation designers to calibrate the commonly used soil damping ratio that is currently being used in total structural damping in the wind industry. This calibration may provide potential optimisation of the design and can lead to lower Levelized Cost of Energy (LCOE).