Fatigue assessment of offshore wind turbines on monopile foundations using multi‐band modal expansion

Abstract

Offshore wind turbines (OWTs) are subjected to both quasi-static loads originating from variations in the thrust force and dynamic loads linked to turbulence, waves and turbine dynamics. Both types of loads contribute to fatigue life progression and thus define the turbine's age. As a structural health monitoring solution, one could thus directly measure the stress history at fatigue critical locations. However, for OWTs on monopile foundations some fatigue critical locations are located below the seabed. Installing strain sensors at these hotspots is therefore impossible for existing wind turbines. This measurement restriction is overcome by reconstructing the full-field response of the structure based on the limited number of accelerometers and strain sensors (installed at a few easily accessible locations) and a calibrated finite element model of the system. The system model uses a multi-band modal expansion approach constituted of the quasi-static and dynamic contributions. These contributions are superimposed to reconstruct the stress history at all degrees of freedom of the finite element model, and the subsequent assess fatigue life consumption at all fatigue hot spots of the OWT. In this paper, the proposed virtual sensing technique is validated by predicting the stresses in the transition piece with 12 days of consecutive measurements from an operational OWT. The data set contains both variations in environmental and operating conditions as well as extreme events. Finally, a full-field strain assessment in the tower and foundation system of the OWT is demonstrated.

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

“…In monopiles, this leads to an increase in the bending moment in the structure below the seabed, which is typically the critical fatigue location for monopile-supported OWTs. 28 Figure 4A shows the change in bending moment in the monopile for a combination of 1MN notional static wind and wave loads. Due to scour, the peak bending moment increases and its location shifts downward.…”

“…In summary, scour has been shown to influence the fatigue life (FL) of OWTs, and the effect of scour and backfilling on long‐term damage in a parked wind turbine has been researched . Due to the complexities associated with the fatigue analysis of OWTs, simplified methods have been proposed for this analysis . However, the quantification of the scour influence on the FL of an operational wind turbine and specific simplified methods for scour effects on fatigue analysis remain outstanding.…”

“…17,18,23 Due to the complexities associated with the fatigue analysis of OWTs, simplified methods have been proposed for this analysis. [24][25][26][27][28][29] However, the quantification of the scour influence on the FL of an operational wind turbine and specific simplified methods for scour effects on fatigue analysis remain outstanding. This paper investigates systematically the influence of scour on a reference 5-MW OWT, using finite element (FE) time-domain dynamic decoupled simulations.…”

Modeling of quasi-static thrust load of wind turbines based on 1 s SCADA data

“…Moreover, as fatigue is a design driver for current offshore wind farms, fatigue analysis of existing wind turbines can optimize future design. Currently, fatigue assessments of support structures are often based on measurements of the load history (Loraux and Brühwiler, 2016;Iliopoulos et al, 2017;Schedat et al, 2016;Ziegler et al, 2017). Most of them imply continuous strain measurements at accessible locations.…”

“…However, for several reasons accelerometers are preferred over strain gauges, although they are not suited to measuring quasi-static loads. In the research presented by Iliopoulos et al (2017), the strain gauges are thus crucial to capture the quasi-static part of the loading. The research presented in this paper aims to replace the use of strain gauges for the estimation of quasistatic loads.…”