MSCA-COFUND-CLEAR-Doc-PhD Position#CD22-37: Offshore wind turbine monopile foundation: Experimental and numerical study of soil-structure interaction

Offshore wind turbines (OWT) form one of the principal solutions adopted to ensure the cleanliness, sustainability, and renewability of energy sources. Therefore, this field is growing exponentially, manifested by the increase of the dimensions and capacities of these structures. Besides, the environmental loading acting on these structures equally increases, imposing particular attention on the design of the foundation system. Due to economic and practical reasons, monopile foundations are commonly adopted. The form of these monopiles depends on wind turbines growth, moving to a smaller length-to-diameter ratio and then to more rigid behavior. The behavior of this kind of monopiles under lateral cyclic loading still involves some knowledge gaps. Recently, several physical models have been developed to fulfill this purpose, leading to propose a conservative design method. Due to the problem complexity (soil-structure interaction, dynamic aspect, soil liquefaction, scouring, . . .) and despite the progress achieved in this field, conflicting findings are still encountered, depending on the considered approach to the problem.

The geotechnical team of the Navier laboratory is working on offshore wind turbine’s foundation since 2014 with the PhD thesis of Laura Kerner. This thesis deals with the evolution of the first natural frequency of an offshore wind turbine. In order to avoid any resonance phenomenon, a precise evaluation of the natural frequency of the wind turbine after its installation but also its evolution during the operation of the turbine are the two main issues considered. An experimental work was developed with two 1g physical models installed in sand. Procedures were set up for static and cyclic tests on these models.

This first study was followed by the PhD thesis of Hussein Bakri from 2018 to 2021. Based on Kerner’s work, new scaled models were developed with a set of scaling laws under laboratory gravity focused on the soil non-linearity, the soil-structure interaction and the dynamic response of the system. Finite element simulations were carried out to verify the dynamic similarity between the scale models and the prototype for the first mode of vibration. The sand behavior at different stress levels is investigated in order to determine the sand state required to simulate the prototype sand state at the laboratory scale. A constitutive model based on Hoek-Brown failure criteria is developed to simulate the sand behavior at different stress levels. Moreover, cyclic strain-controlled tests with different excitation frequencies are performed, allowing to validate the testing procedure.

These two studies allowed to build original and robust procedures to test offshore wind turbine’s scale models submitted to static or cyclic loading. Some questions remain and need further research. A cyclic test program is suggested in the work of Bakri in order to constitute a database to adopt or propose empirical laws predicting the displacement/rotation accumulation and the foundation stiffness evolution as a function of the cycles number and the conditions of the studied case.

This PhD study entitled Offshore wind turbine monopile foundation: experimental and numerical study of soil-structure interaction has two main goals:

-Study of offshore wind turbine’s scale model subjected to cyclic loading. This study will include the improvement of the cyclic test procedure with a load-controlled setup. A parametric study can be considered: loading characteristics, sand density index, monopile rigidity, etc. The subsidence cone formation around the monopile should be also evaluated.

-Study and development of empirical laws predicting the displacement/rotation accumulation and the foundation stiffness evolution as a function of the cycles number. Simulation of permanent strains/displacements after first load cycle (as a basis for the usual cyclic extrapolation methods) should be considered. One possible approach is a 3D FE analysis of laterally loaded monopile and soil continuum with hypoplastic constitutive model using Plaxis. This work will be based on the cyclic test program conducted in Navier laboratory but also on the database provided by the BAM. The BAM has been dealing with several topics on the foundations for offshore wind parks for the past 15 years. Therefore, the BAM has conducted physical model tests in reduced scale but also large-scale field tests in an open-air facility in Horstwalde.

This multidisciplinary study will be supervised by experts in experimental geotechnical test, structural tests and/or numerical study.