Detecting critical scour developments at monopile foundations under operating conditions

Early warning systems for critical conditions at offshore wind turbines are needed to reduce maintenance costs and avoid catastrophic failures. Monitoring of the critical scour development at monopile foundations is commonly done with cost-intensive scour depth measurements. The scour condition is regarded critical when the depth exceeds the maximum allowed design scour depth during normal operation or due to a severe storm. This practice can lead to high maintenance costs and potentially unnecessary maintenance activities such as refilling of the scour hole or reconstruction of the scour protection. Instead, the exploitation of the structural reserves of fatigue driven monopile foundation designs stemming from design assumption versus real site conditions is suggested. Damage accumulation is highly influenced by the time behaviour of the transient scouring and real soil properties. This paper elaborates on novel low cost monitoring methods to detect when a scour development is truly critical when taking site conditions into account. A combination of fatigue monitoring and natural frequency supervision is proposed for critical scour identification in the framework of an early warning system.