Estimation of hysteretic soil damping from laboratory measurements including strain accumulation
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- NGI Rapport 
The periodic forces due to wind, surface waves and swells on offshore installations such as offshore wind turbines lead to cyclic motion of the soil foundation. The damping properties of soils account for a part of the energy dissipation of the Offshore Wind Turbine (OWT) system. Understanding damping is therefore important with respect to the structural response and the cost-effective design of structures. Unfortunately, OWT foundation damping is sparsely studied, and the understanding of this parameter is therefore limited. The present results provide a step forwards towards a better understanding and interpretation of stiffness and damping parameters of soils, and focuses on basic concepts and on the effect of permanent strain accumulation in particular. Analyses of laboratory data have shown that the interpreted hysteretic soil damping becomes too large unless the permanent strain accumulation is filtered out. On the other hand, the strain accumulation contributes to hysteretic loss in addition to the material damping. The current study is a first step towards a better understanding and interpretation of damping parameters for soils, and focuses on basic concepts. A crucial next step would include methods for interpreting the damping correctly in models soilstructure interaction for realistic loads and structures, to clarify the importance of soil damping for offshore wind turbine foundations. Due to the industry’s need for simple and efficient models used for large scale, it is essential that the damping formulation is kept simple.