Sammendrag
Wind energy is one of the widely used renewable energy sources in today’s world. During the last decade, offshore wind energy has grown into a large green energy supplier in Europe due to its advantages over other energy sources. Wind energy is generated by wind turbines which are supported by its foundation. Monopile is one of the widely adopted supporting structures in sand for offshore wind turbines (OWT). These structures are exposed to not only monotonic loading but also cyclic loading conditions. The cyclic strength of structures is generally lower than its monotonic strength. Extensive research and development are required in design and analysis under cyclic loading. Hokksund sand is a type of sand used for most of the research conducted at the NTNU. This type of sand undergoes change in stiffness and strength degradation. The extent of changes in properties of the sand and its behavior under cyclic loading conditions are not properly known.
SANISAND-MS constitutive model is contemporarily employed to understand the behavior of sand under cyclic loading conditions. The documented results from triaxial tests already performed at the NTNU laboratory were used to calibrate the monotonic parameters of the SANISAND-MS model. The calibration results indicate that the relationship between stress and strain from numerical model aligns well with the experimental values. To further investigate this behavior, a monopile foundation was modelled in PLAXIS 3D to simulate the behavior of sand under drained cyclic loading conditions. Semi-empirical method of pile design has several shortcomings compared to numerical simulations. The results suggest that the monotonic strength of the monopile is well above its cyclic strength under various cyclic loading conditions. Additionally, the stiffness of the monopile increases while its strength degrades under cyclic loading. The findings indicate that the rotational criteria of pile failure are more conservative than displacement failure criteria. The sensitivity analysis shows that ratcheting parameter (μo) is particularly sensitive among the cyclic parameters of SANISAND-MS model.
This study recommends conducting cyclic triaxial test for Hokksund sand for proper calibration of cyclic parameters of SANISAND-MS model. Field test or laboratory test of monopile under cyclic loading is suggested to be conducted to verify the results from the numerical model.
