Abstract

Since the first monopiles were manufactured for offshore wind turbine foundations, the effect of entrapped water during impact pile driving (also referred to as water hammering) has been discussed within the industry. It is acknowledged that the water movement within the pile during driving will impact the installation, potentially increasing the blow count and driving induced fatigue damage to the pile. In this paper, previous studies on the subject will be investigated and further sensitivity studies presented based on a detailed foundation design for an offshore wind farm to be erected in Europe during 2021, ultimately to arrive at recommendations for future foundation designs.

INTRODUCTION

The effect of entrapped water during impact pile driving installation, also referred to as water hammering, has been discussed in the industry since the first monopiles were manufactured. It is acknowledged that the water moving up and down within the open-ended pile during installation will impact pile driving, essentially by damping the energy transferred through the pile, potentially increasing the blow count and impacting the driving induced fatigue damage to the pile. Unfortunately, limited work is published on this matter, and the available literature does not give firm recommendations about how to treat this issue, which has resulted in a continuous debate.

In this paper, previous studies on the subject will be investigated and further sensitivity studies will be presented based on a detailed foundation design for an offshore wind farm (OWF) to be erected in Europe during 2021, ultimately to arrive at recommendations for how to treat this topic in future monopile foundation designs. The derivation of the energy loss due to the damping of the entrapped water will be estimated, and the impact of this loss will be evaluated for several cases with varying pile geometry, soil conditions and driving resistance.

This study is conducted using the final foundation design and hammer assemble as provided for the OWF project. The foundation concept is ø7.0 m monopiles installed by impact pile driving in primarily dense, sometimes calcareous, sandy soil conditions, in some cases mixed with clays and rock material. Variations in input parameters for the driveability studies are considered, such as the soil resistance during driving (SRD) incl. estimates of quake and damping and the in-situ variation of the encountered soil conditions based on available CPT data.

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