Abstract
The design of monopile foundations for offshore wind turbines is most often driven by fatigue. With the foundation price contributing to the total price of a turbine structure by more than 30%, wind farm operators seek to gain knowledge about the amount of consumed fatigue. Monitoring concepts are developed to uncover structural reserves coming from conservative designs in order to prolong the lifetime of a turbine. Amongst promising concepts is a wide array of methods using in-situ measurement data and extrapolating these results to desired locations below water surface and even seabed using models. The modal decomposition algorithm is used for this purpose. The algorithm obtains modal amplitudes from acceleration and strain measurements. In the subsequent expansion step these amplitudes are expanded to virtual measurements at arbitrary locations. The algorithm uses a reduced order model that can be obtained from either a FE model or measurements. In this work, operational modal analysis is applied to obtain the required stress and deflection shapes for optimal validation of the method. Furthermore, the measurements that are used as input for the algorithms are constrained to measurements from the dry part of the substructure. However, with subsoil measurement data available from a dedicated campaign, even validation for locations below mud-line is possible. After reconstructing strain history in arbitrary locations on the substructure, fatigue assessment over various environmental and operational conditions is carried out. The technique is found capable of estimating fatigue with high precision for locations above and below seabed.
Highlights
Accepted: 27 July 2021Offshore wind is a rapidly growing source of renewable energy and one of the key technologies to reach global climate goals
In order to learn about fatigue progress on critical locations of the turbine substructure, recorded measurements are extrapolated via virtual sensing technique modal decomposition and expansion (MDE)
This section describes the validation of Modal Decomposition and Expansion (MDE) accomplished by the use of acceleration measurements on the tower to reconstruct dynamic strain and strain measurements on the top of the transition piece (TP) (17 m LAT)
Summary
Accepted: 27 July 2021Offshore wind is a rapidly growing source of renewable energy and one of the key technologies to reach global climate goals. The substructure designs have become increasingly optimized, favoring the more cost-effective monopile This optimization implies, in particular for monopiles, that the as-designed fatigue life is a near perfect match with the intended operational life of 20 to 25 years. This makes monitoring of fatigue life progression a key component in any discussion on optimized maintenance or life time extension. Fatigue loading of offshore wind turbine monopile substructures is caused by the interaction between environmental loads and the structural dynamics of the offshore wind turbine itself [1] The design of these substructures is driven by this interplay of loads and structural dynamics. The first welds of the monopile beneath the seabed are most fatigue critical along with various welds of secondary steel on the transition piece
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