Session: 06-10-01 Offshore Industry: Structures and Design I

Paper Number: 124291

124291 - Full-Scale Measurements of Offshore Monopile Foundations and Numerical Benchmark With Current Best Practices. 

In offshore wind, the modelled structural dynamics used in design do not fully describe the reality of the built structures. For monopile supported turbines previous work, by the authors and others, shows an underestimation of eigenfrequencies during design. As fatigue and lifetime of a foundation is determined using these eigenfrequencies, better models that more accurately predict reality are needed. Past research has indicated that a better understanding of the soil structure interaction is a crucial factor in reducing the observed mismatch. But also, sea state and uncertainties concerning the wind turbine itself add to the problem. 

In this regard the paper presents a unique dataset of eigenfrequency measurements taken at three monopile foundation locations, two with sandy topsoil layers and one with a clay topsoil layer, within the same windfarm in the Belgian North Sea. The three datasets span a timeframe of around six months, during which a rock armor scour protection was installed, and the structures consisted of monopiles capped by a transition piece. This intermediate state of construction means all eigenfrequency uncertainties related to the wind turbine properties can be eliminated, so the focus can be on the effect of soil structure interactions and variability due to sea state. 

Analyzing these measured eigenfrequencies we can see correlations with the different sea states, specifically with tidal level and wave height and period. In addition, the moment of scour protection installation is detectable by a consistent increase in natural eigenfrequency at all three locations. This indicates an increase in the stiffness of the foundation. The effect of increased stiffness is less pronounced at the clay location compared to the sandy ones. A detailed numerical finite element model, based on design documentation and best practices, is first used to quantify the current ability to match the real-world observations. In addition, the primary factors needed to improve the modelling of the foundation response are identified. These results can later be used in future work to propose improvements to the modelling of offshore monopile foundations. 

Presenting Author: Kristof Winkler Vrije Universiteit Brussel, OWI-Lab

Presenting Author Biography: Phd student at Vrije Universiteit Brussel working for OWI-Lab. Main focus of research is modelling offshore windturbines with monopile foundations and trying to help resolve the discrepancies between current design models and real life measurements.

Authors:

Kristof Winkler Vrije Universiteit Brussel, OWI-Lab
Carlos Sastre Jurado Vrije Universiteit Brussel, OWI-Lab
Bruno Stuyts Ugent, Geotechnical Labratory
Wout Weijtjens Vrije Universiteit Brussel, OWI-Lab
Christof Devriendt Vrije Universiteit Brussel, OWI-Lab