Identification of strain-induced effective magnetisation in the presence of pile displacement

Abstract

The demand for renewable energy has grown over the past decade and is projected to accelerate further. This rapid growth has led to an increase in offshore wind deployment and to the installation of increasingly large steel monopiles to account for fatigue damage during installation. Conventional contact-based sensors, such as strain gauges and accelerometers, are often not feasible offshore due to harsh installation conditions and the risk of sensor damage. To address this challenge, a novel non-contact sensor system has been developed that uses magnetomechanical effects to infer strain and an optical sensor to measure velocity. Recent full-scale hammer tests showed that this system can successfully capture strain and velocity during impact piling, allowing for comparison with the industry-standard Pile Driving Analyzer. However, accurate interpretation of magnetometer signals requires understanding the influence of various magnetic disturbances. This work investigates key interference sources arising during offshore pile driving, including the magnetic stray fields of adjacent ferromagnetic components, specifically the outer pile and the hammer-sleeve assembly, as well as the effects of the gradual rotation of the latter. Methods to separate strain-induced magnetization from these other magnetic contributions are explored to enhance signal fidelity. This is achieved through a combination of theoretical calculations and experimental measurements. By examining how these interferences influence strain measurements, this study enhances measurement reliability and advances non-contact monitoring technologies for offshore monopile installations.