Probabilistic Three-Dimensional Model of an offshore Monopile Foundation: reliability based approach

When wind turbines are to be installed offshore, expensive geotechnical in-situ tests are carried out at the location of each turbine and only a quantile value (typically the 5% quantile) of the measured strength parameters is used as design parameter , e.g., the 5% quantile value of the undrained shear strength of the soil. Typically, measurement, statistical and model uncertainties are not taken into account in code - based, deterministic design. Hence, current methodology based design may be expensive, but the reliability of the foundation is unknown. Instead, a reliability - based design process based on stochastic analysis of the soil parameters is proposed to obtain an efficient design with known reliability and smaller costs for tests and construction. In this study a monopile foundation in undrained, over -consolidated clay is considered as an example. A three –dimensional (3D) finite -element model is established and a stochastic model for the undrained shear strength of the soil is proposed using random field theory. The Mohr – Coulomb constitutive model is used to model the soil behavior. Reliability indices of the monopile are obtained through an advanced reliability method and a probabilistic procedure is proposed regarding the 3D design of monopile foundations.