Abstract

AbstractThe cost of offshore wind energy can be reduced by incorporating control strategies to reduce the support structures' load effects into the structural design process. While effective in reducing the cost of support structures, load‐reducing controls produce potentially costly side effects in other wind turbine components and subsystems. This paper proposes a methodology to mitigate these side effects at the wind farm level. The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures. The safety margins are generally closely correlated with the modal properties (natural frequencies, damping ratios). This admits the possibility of using modal identification techniques to reassess the structural safety after installing and commissioning the wind farm. Since design standards require conservative design margins, the post‐installation safety assessment is likely to reveal better than expected structural safety performance. Thus, if load‐reducing controls have been adopted in the structural design process, it is likely permissible to reduce the use of these during actual operation. Here, the probabilistic outcome of such a two‐stage controls adaptation is analyzed. The analysis considers the structural design of a 10 MW monopile offshore wind turbine under uncertainty in the site‐specific soil conditions. Two control strategies are considered in separate analyses: (a) tower feedback control to increase the support structure's fatigue life and (b) peak shaving to increase the support structure's serviceability capacity. The results show that a post‐installation adaptation can reduce the farm‐level side‐effects of load‐reducing controls by up to an order of magnitude.

Highlights

  • Support structures are a major contributor to the cost of offshore wind energy

  • A number of control strategies are available for reducing load effects in offshore wind turbine (OWT) support structures.[2,3,4,5,6,7]

  • The p-y curves are derived from field tests performed on flexible piles with length to diameter ratios well above those of OWT monopile foundations. 11,13 while the p-y curves include corrections for cyclic loading, these are calibrated based on field data collected from piles subjected to a low number of loading cycles compared with the cycle count over the service life of an OWT.[11,13]

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Summary

INTRODUCTION

Support structures are a major contributor to the cost of offshore wind energy. Favored for their cost-efficiency, monopile foundations are prevalent in the European offshore wind industry.[1]. The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures.[8,9,10,11,12,13,14] In particular, uncertainty in the support structures' fundamental natural frequency presents a challenge in estimating their fatigue life. For the majority of turbines, this would lead to a win-win situation where load-reducing controls have been used to reduce the support structure's cost without having to be used in actual operation, at least not to their fullest potential.

BACKGROUND
GEOTECHNICAL MODELS
Geotechnical uncertainties and random field modeling
Random field model
Soil-structure interaction model
SIMULATION SETUP
Simulation model
Fatigue life analysis
Design Situation
Serviceability analysis
SELECTING CHARACTERISTIC SOIL PROPERTIES
TWO-STAGE ADAPTATION OF A CONTROL STRATEGY FOR FATIGUE LOAD REDUCTION
First-stage controls adaptation
Second-stage controls adaptation
Probabilistic outcome of the two-stage controls adaptation
TWO-STAGE ADAPTATION OF A CONTROL STRATEGY FOR SERVICEABILITY LOAD REDUCTION
Findings
CONCLUSIONS

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