Energy losses during driving due to tapered section of monopile

Abstract

Efficient pile driving and accurate pile driving predictions are essential for the continued use of monopiles as an optimised foundation for the offshore wind energy sector, especially as the offshore wind industry continuously pushes limits of wind turbine and foundation size. For monopile foundation a crucial part of the structure from an installation perspective is the transition tapered section of the monopile between the top and bottom parts of the monopile and its interaction with the water column during pile driving. To evaluate the impact of this interaction on the driving energy, two monopile foundations installed at an offshore wind farm (OWF) site in the North Sea were instrumented and their installation was monitored in 2019. For the monitoring, optical fibre strain gauges and resistive accelerometers were placed at different levels above, below, and on to the tapered section of the monopiles. As an innovative part, all sensors were glued on the monopile shaft surface instead of the traditional welded/screwed approach. The driving energy loss between different sensor levels was calculated post installation. The measurement data were used to benchmark a series of finite element analyses (FEA) that were performed to identify and quantify the different factors affecting the energy loss.The study evaluates the effect of various parameters, such as wave reflections due impedance changes caused by diameter increase and wall thickness variation, bending of tapered section, and vertical acceleration of water due to interaction with the tapered section of the monopile. Measurements and the finite element analysis both reveal that a large part of the driving energy applied by the hammer is dissipated due to the above-mentioned factors with the water-pile interaction playing the most important role, resulting in a significant impact on the monopile installation.