Soil damping is usually modeled as a contribution to global proportional or Rayleigh damping in industry and academia. However, this soil damping model does not represent the physical behavior. In this paper, soil damping for offshore wind turbine foundation is modeled by means of dashpots with damping parameters associated with the soil stiffness. The case study used in this paper is a 5MW NREL wind turbine resting on a jacket foundation as the effect of soil damping in this type of foundation is more significant due to its structural nature and number of piles. A number of dashpots for each pile are used with six degrees of freedom to represent soil damping.
First, free decay vibrations are assesed for different soil damping levels. Next, the aerodynamic loading is applied at the interface level and the integrated analysis is performed in dynamic mode for stochastic loading. Displacements at top of the foundation, base shear and base moment forces for a number of design load cases are evaluated to assess the effect of quantifying soil damping with dashpots. Also, fatigue damages of different hotspots are assessed and compared with the damages from models with no soil damping. It can be concluded that soil damping modeled by dashpots may provide some additional damping and it is more realistic. It leads to load reductions especially mudline moment, with the maximum of 20% for the peak values. Therefore, this type of soil damping modeling can be used to optimize the design of offshore wind turbine especially for the critical hotspots near the mudline.
