Scour influence on the fatigue life of operational monopile‐supported offshore wind turbines

Abstract

Offshore wind turbines supported on monopiles are an important source for renewable energy. Their fatigue life is governed by the environmental loads and in the dynamic behavior, depending on the support stiffness and thus soil‐structure interaction. The effects of scour on the short‐term and long‐term responses of the NREL 5‐MW wind turbine under operational conditions have been analyzed by using a finite element beam model with Winkler springs to model soil‐structure interaction. It was found that due to scour, the modal properties of the wind turbine do not change significantly. However, the maximum bending moment in the monopile increases, leading to a significant reduction in fatigue life. Backfilling the scour hole can recover the fatigue life, depending mostly on the depth after backfilling. An approximate fatigue analysis method is proposed, based on the full time‐domain analysis for 1 scour depth, predicting with good accuracy the fatigue life for different scour depths from the quasi‐static changes in the bending moment.

Riprap Scour Protection for Monopiles in Offshore Wind Farms

“…If this is not addressed in a suitable way and just in time, the scour phenomenon can cause operational unavailability, changes in the natural frequencies of the soil-structure system [12], fatigue life reduction [13], and even in extreme cases the collapse of the structure [14]. There is a significant need to advance in this field and therefore, it is very important to carry out some researches about scour.…”

Local Scour around a Monopile Foundation for Offshore Wind Turbines and Scour Effects on Structural Responses

“…Note that time-varying stiffness/damping properties of the soil due to long-term cyclic loading [93,94] and scour depth variation due to backfilling are not accounted for. Rezaei et al [95] presented a study of scour and backfilling effects on the FL of a 5 MW monopile supported OWT, using quasistatic, modal, and time-domain fatigue analyses. It was demonstrated that a scour depth of 1.5D could result in a 45% reduction of the FL of the wind turbine structure.…”

Modelling wind turbine tower-rotor interaction through an aerodynamic damping matrix

“…For structural designers it is convenient to characterise the aerodynamic damping using a linear damping ratio associated with the first vibrational mode of the tower, as this allows the dynamic response of the system to be calculated rapidly, e.g. for fatigue analyses [10]. Specific studies of the aerodynamic damping have generated experimental or analytical modal damping ratios that capture this source of dissipation [11].…”

“…For OWTs, the aerodynamic damping values quoted below were obtained by subtracting an assumed underlying damping of 2% from the measured total damping [15]. For onshore wind turbines, the total damping is a closer approximation of the aerodynamic damping [10] as foundation damping is low. Hansen et al [16] estimated the aerodynamic damping in a 2.75 MW operating OWT using two experimental methods.…”