36 months
From 01.11.2016 to 31.10.2019

Aalborg University (Denmark)
PhD director: Prof. John Dalsgaard Sørensen (AAU)

The defence is scheduled for Friday 6 March 2020, at Aalborg University - Copenhagen campus, Denmark.

The members thesis committee are:

• Prof. Lars Damkilde, Dept. of BUILT Environment, AAU (Chairman)
• Prof. Michael Muskulus, NTNU, Norway
• Prof. Henrik Bredmose, DTU Wind Energy, Denmark
• Prof. John Dalsgaard Sørensen, Dept. of BUILT Environment, AAU (PhD director)
• Claus Kramhøft, Chief Specialist, COWI, Aarhus C (Supervisor)

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 details. 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.

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

• AAU (Aalborg, Denmark)
  August, September & October 2017
  Fatigue reliability of concrete wind turbine and bridge elements.

• EPFL (Lausanne, Switzerland)
  From November 2018 to January 2019
  Modelling of concrete strength subjected to fatigue load.

• Grünberg J. and Göhlmann J.
  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, 2006

• Patrik Alexander Passon
  Design of Offshore Wind Turbine Foundations
  PhD thesis, The Technical University of Denmark (DTU), 2015

• Velarde, J., Mankar, A., Kramhøft, C., & Sørensen, J. D.
  Probabilistic calibration of fatigue safety factors for offshore wind turbine concrete structures
  Engineering Structures, Volume 222, 2020 

• Velarde, J., Kramhøft, C., Sørensen, J. D., & Zorzi G.
  Fatigue reliability of large monopiles for offshore wind turbines
  International Journal of Fatigue, Volume 134, 2020

• Velarde J., Vanem E., Kramhøft C., Sørensen J.D.
  Probabilistic analysis of offshore wind turbines under extreme resonant response: application of environmental contour method
  Applied Ocean Research, Volume 93, December 2019

• Velarde J., Mankar A., Kramhøft C., Sørensen J.D.
  Uncertainty Modeling and Fatigue Reliability Assessment of Offshore Wind Turbine Concrete Structures
  International Journal of Offshore and Polar Engineering (IJOPE), Volume 29, Issue 2, June 2019, pp.165-171

• Velarde J., Kramhøft C., Sørensen J.D.
  Global sensitivity analysis of offshore wind turbine foundation fatigue loads
  Renewable Energy, Volume 140, March 2019, Pages 177-189

Probabilistic Design of Offshore Wind Turbine Support Structures