Comparison of Self Weight Penetration Prediction Methods for Large Diameter Monopiles in North Sea Soils

Abstract

A significant number of large diameter monopiles (>6 m diameter) have been installed as foundations for offshore wind turbine generators (WTG) across Northern European waters over the past decade, where the self-weight penetration (SWP) during the installation process was recorded. The prediction of the SWP for offshore foundations, such as piles and skirted foundations, is an important installation design task. The prediction of the SWP is subject to great uncertainty, resulting in a large range between lower and upper bound SWP predictions. The methods available to predict the SWP range from simple empirical methods that derive the penetration resistance from cone penetration test (CPT) measurements using skirt and tip resistance coefficients to more advanced methods, based on bearing capacity theory requiring soil strength parameters as input. For the empirical methods, the prediction accuracy is directly dependent on the data used to establish suitable resistance coefficients. Investigating different soil conditions and foundation dimensions (scale effects), therefore, extends and improves the applicability of the empirical SWP prediction methods. This paper will use a portion of the recorded SWP data to benchmark existing SWP prediction methods and to establish improved resistance coefficients for the empirical method using a Monte Carlo type approach. For the Westermost offshore wind farm assessed in this study, the findings show that the existing SWP prediction methods generally under predict the observed SWP. The results of the Monte Carlo type analysis show that the new resistance coefficients are generally lower than the generic resistance coefficients provided by DNV (1992). The results also show that the tip resistance in coarse grained materials and the shaft resistance in fine grained have the largest influence on the accurate prediction of the SWP.