The Use of Vibratory Driving of Foundation Piles for Offshore Applications

Paper

Pdf Summary

Offshore wind foundations are increasingly installed using vibratory hammers, particularly as monopiles grow larger. Vibratory driving can speed installation, reduce tool changes and pile stresses, and lower noise levels. Reliable prediction is essential to avoid premature refusal and to establish whether target penetration can be achieved.<br /><br />The paper describes simulation methods based on stress-wave analysis. Their soil models represent resistance with springs and dampers and account for soil fatigue, which temporarily reduces resistance near the pile during vibration. Soil strength generally recovers after driving; granular soils may even become denser. Prediction accuracy depends on representing these effects and the pile–hammer system appropriately.<br /><br />Experience from offshore projects shows that predictions are generally reliable in granular soils when based on site investigations. Stiff cohesive soils are more difficult, since cohesion can limit local soil weakening; predictions can be improved by calibrating models with measurements from test piles. The authors therefore advocate routine vibratory driving monitoring (VDM), including penetration, pile stresses and accelerations, hammer frequency and efficiency, and crane hook load. Comparing monitored performance with simulations enables model calibration and builds the data needed for better predictions.<br /><br />A Delft Offshore Turbine test-pile case study illustrates the approach. The pile was vibrated into dense sand and later extracted. Initial predictions identified difficult penetration in a dry sand layer, and monitoring confirmed the near-refusal there, although several measured parameters differed from predictions. A later offshore installation prediction matched results more closely and correctly forecast penetration to 17 m, while actual penetration was faster than predicted.<br /><br />Overall, vibratory pile driving is a viable foundation-installation method, but prediction and bearing-capacity assessment remain less established than for impact driving. Monitoring and continued analysis are needed, especially to improve confidence in cohesive soils.

Keywords

offshore wind foundations

vibratory hammers

monopile installation

vibratory driving

stress-wave analysis

soil fatigue

pile driving prediction

vibratory driving monitoring

granular soils

cohesive soils