Fatigue of wind turbine concrete structures

Investigating the actual provisions for determination of fatigue loading and subsequently to develop optimised fatigue load determination models, e.g. by considering the statistical distribution of wind directions and by investigating the correlation between wind and wave loading for offshore structures. While this research mainly focuses on fatigue loading effects on structural elements in reinforced concrete of wind turbines; fatigue resistance of reinforced concrete (as investigated in task 2.1) will be adequately considered in the development of novel methods of examination of existing structures and fatigue design provisions for new structures.

Host institution

COWI, Kongens Lyngby, Denmark

ESR8

Joey Velarde

Local academic supervisor

Henrik Niedermayer (COWI)
hkn(at)cowi.dk
Tel: +45 5640 2135   

Academic co-supervisor

Prof. John Dalsgaard Sørensen (AAU)
jds@civil.aau.dk
Tel: +45 9940 8581

PhD enrolment

Aalborg University
PhD Director: Prof. John Dalsgaard Sørensen

Start date: 1st November 2016
Duration: 36 months

Synopsis

The main aim is to investigate and develop new methods for assessment of the fatigue load and resistance for reinforced concrete in gravity based offshore wind turbine foundations. The influence of various design driving effects for fatigue will be investigated in order to optimize the current design philosophy and minimize the total cost of energy.

Objectives

The objective of this work package is to study and improve modelling of design and cost driving aspects for gravity based foundation types of offshore wind turbines in order to achieve more cost-efficient solutions for future foundation designs.

Based on a review of the current methodology for assessment of fatigue loads for gravity based offshore wind turbine foundations, specific aspects with large optimization potential will be identified and analysed in more detail. It is very important to capture the various and complex interactions of the environment and individual subsystems within offshore wind turbines, e.g. leading to a pronounced aero-elastic behaviour. Adequate consideration of these interactions requires an integrated modelling approach for offshore wind turbines, their foundations and the environmental conditions. For the purpose of this project, an integrated offshore wind turbine and foundation model will initially be established by coupling of COWI's in-house tool IBDAS with an adequate aero-elastic simulation tool such as Flex5.

The design and load calculation tool will together with COWI's experience be used to establish a range of reference designs of gravity based foundations which are subsequently subjected to an identification of design and cost driving aspects.


The reference foundation designs, design methods and identified design and cost driving aspects are subsequently assessed with respect to their optimization potentials for more cost efficient design solutions. A selection of these will be subjected to more detailed investigations in order to achieve more cost-efficient gravity based foundations. Examples of such detailed investigations could include:

Improved modelling of fatigue wave loading

Improved modelling of combined wind turbine and wave loading, e.g. investigate length of time series, number of seeds for phase shift in wave load generation, wavelet discretization, resonant response

Impact of fully integrated analysis compared to non-coupled design


In combination with experience from previous gravity based foundations designed by COWI the most critical design load cases will be determined and investigated further. The further investigations will focus on the fatigue resistance and specifically the use of advanced non-linear damage material model for the verification of the concrete.

In most of today's projects the wind turbine suppliers provides the foundation designer with a summary of the loads in form of Markov matrixes, based on a load analysis assuming linear elastic material for the structure. The foundation is designed using linear damage accumulation according to Palmgren and Miner. This approach does consider neither the loading sequence nor the non-linear fatigue behaviour of the concrete. This simplification can lead to either conservative or unsafe design. Advanced material models can capture the stiffness and damage evolution in concrete under repeated loading. When these models are considered for the integrated load analysis, a more consistent design approach can be achieved.

The influence of using the advanced material models for assessment of the fatigue resistance for reinforced concrete compared to the traditional approach will be investigated for the developed reference foundation designs in order to assess the accuracy and optimization potential.

Tasks and methodology

  • State-of-the-art modelling and fatigue load analysis of offshore wind turbines and gravity based foundations.
  • State-of-the-art models for fatigue verification of reinforced concrete.

    Results

    • Methodology for accurately assessment of fatigue loads in gravity based. foundations for selected environmental loads and/or modelling assumptions
    • Methodology for accurately assessment of the fatigue damage in reinforced concrete.
    • Illustrative implementations for reference foundation designs.
    • Successfully defended PhD thesis.

    Dissemination

    • Three peer reviewed papers.
    • Two presentations at national/international conferences.
    • Presentations at workshops and for potential end-users.

    Secondments

      • AAU (Aalborg, Denmark)
        August, September & October 2017
        Fatigue reliability of concrete wind turbine and bridge elements
      • IFSTTAR (Marne-la-Vallée, France)
        April 2018
        Analysis of load distributions resulting of combined actions (wind and waves) and derivation of the stress cycle variations inducing fatigue
      • EPFL (Lausanne, Switzerland)
        November & December 2018
        Modelling of concrete strength subjected to fatigue load

      Research field

      Fatigue loading, fatigue resistance, gravity based foundation design

      Keywords

      Fatigue, reliability, concrete bridges, wind turbines

      Key publications

      • Grünberg, J. and Göhlmann, J. (2006), Damage calculation at a prestressed concrete tower for a Wind Energy Converter subjected to multi-stage fatigue loading (Schädigungsberechnung an einem Spannbetonschaft für eine Windenergieanlage unter mehrstufiger Ermüdung). Beton- und Stahlbetonbau, 101: 557–570.
      • Patrik Alexander Passon, Design of Offshore Wind Turbine Foundations, PhD-thesis, The Technical University of Denmark (DTU), 2015.