Vibratory Driving of a Monopile at a North Sea Site-Paper

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The paper examines the vibratory installation of a 4.7 m diameter, 40.5 m long monopile at a North Sea platform. The target penetration was 31.5 m below the seabed. The site consisted mainly of dense to very dense sands, with a thin silt layer.<br /><br />Installation predictions using the Hypervib-1 method identified a risk of early refusal if the hammer operated below its nominal frequency of 23.3 Hz. During installation, a hydraulic power-pack failure limited the vibrohammer’s frequency to approximately 16–19 Hz. The pile reached only 25 m before vibratory driving stopped; installation was later completed with an impact hammer.<br /><br />Monitoring included pile accelerations and strains, as well as hydraulic oil flow and pressure. The measurements showed that penetration slowed as soil resistance increased, while vibration amplitudes fell and higher-frequency harmonics became more pronounced near refusal. Measured forces transferred to the pile varied substantially with depth and soil resistance, underscoring the difficulty of predicting the actual driving force.<br /><br />The post-installation analysis found that the lower-than-expected operating frequency was likely the main cause of early refusal. Other possible influences included soil damping, uncertainty in force transfer along the elastic pile, and crane hook force. Hypervib-1 gave reasonably accurate, slightly conservative refusal predictions when measured pile forces were used, but predictions based on nominal hammer forces substantially overestimated drivability.<br /><br />The authors conclude that installation monitoring is essential for understanding performance and refining future predictions. Uncertainty in the forces transmitted to the pile and their amplification along its length remains a major limitation in vibratory drivability assessment.

Keywords

monopile installation

vibratory driving

North Sea platform

early refusal

dense sand

Hypervib-1

pile monitoring

hydraulic power-pack failure

drivability prediction

impact hammer