Session: 02-12-02 Structural Analysis and Optimization 2

Paper Number: 79907

79907 - Position Specific Monopile Fatigue Load in the Frequency Domain 

The scope of the following paper entails fatigue limit state analysis of offshore wind turbinessupported by monopile foundations. With a focus on current industry practice limited bytedious time simulations and cross-company workflow, a new approach for efficient fatiguedamage estimation in the frequency domain is proposed. In contrast to approaches availablein the literature, typically derived based on a single vibration mode and assuming a narrow-banded load spectrum, the present framework constitutes a more general approach. A toolbased on the proposed method holds the potential for fatigue load reduction relevant for further optimization of monopile design.Since the major part of fatigue limit state damage is commonly associated with wave loading,it is reasonable to focus on this without the need for a fully integrated aero-servo-elastic model. Instead, the wind contribution is assumed constant in-between turbine positions. Starting thewave load analysis from a JONSWAP spectrum, a transfer-function-based methodology isdeveloped for converting the spectrum into one of the dynamically induced stresses in the considered structure. The first transfer function uses the Morison equation to transfer the surfaceelevation of the waves into a wave-induced modal load. The structure’s frequency responsefunction is then used to transfer the modal load into modal displacements after which themodal displacements are summed to obtain the total displacements. The last step is to transferdisplacements into stresses which are done through Bernoulli beam theory. By an empiricalmethod, both stress ranges and corresponding applied cycles are obtained from the stress spectrum, leading to damage equivalent loads.By benchmarking the frequency domain loads against time domain loads, the frequency domain analysis has been shown to be applicable for position-specific load calculation where theassumption of a constant wind contribution was satisfied. The proposed method showed betteroverall accuracy compared to other frequency domain approaches for offshore wind foundations described in literature. Large potential for load reduction within the clusters is foundwhen using position-specific design loads rather than determining the loads conservativelyfrom the design positions. Best results are obtained for stand still conditions, as the powerproduction state is more sensitive to wind speed dependent aero-dynamic damping and controller settings. Despite the uncertainty of the power production state, a significant potentialof up to 30% load reduction has been found for typical site data during considered operationalstates, showing great optimization-potential by using the developed approach.

Presenting Author: Niels Christian Dahl Wood Thilsted

Authors:

Niels Christian Dahl Wood Thilsted
Jan Høgsberg Technical University of Denmark
Martin Bjerre Nielsen Wood Thilsted
Richard Thyssen Nørby Larsen Technical University of Denmark