The effects of wind load direction change on offshore wind turbine monopile response

Monopile is the most common type of foundation for onshore and offshore wind turbines. Wind load alternately applies in different directions during the life cycle of the wind turbine. In this paper, the influence of alternating change of wind load direction on the response of wind turbine piles is studied. The functional performance of soil-pile system is simulated with a bounding surface soil behavior model presented by Dafaliad-Manzari that is implemented in FLAC3D software. The softening and hardening effects of soils due to cyclic load patterns is incorporated in the model in a convenient manner. The results show that alternative change in the direction of loading results in the decrease in the maximum horizontal displacement and rotation up to 16% in comparison with the uni-directional loading mode. This difference in vertical displacement is about 100%. Residual displacement and rotation in the horizontal direction also decreases 13% and 18%, respectively. Alternating change of loading causes upward moves of the monopile; the pile moves upward from its original level. In general, the change of loading direction causes a significant change in the pile response, although the values of this difference vary in different parameters and for altered loading modes.