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Thursday, 29 September 2016
09:00 - 10:30 Loads and fatigue
Turbine technology  
Onshore      Offshore    

Room: Hall G2

In this session, participants will hear about the latest developments in wind turbine loading and system behaviour. Presentations will cover a variety of topics - from modelling approaches to turbine control. The research work presented brings new approaches which will help the industry increase the reliability of turbines and reduce installation costs, especially offshore. Presented results will rely on real-world measurements and operational data.

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Learning objectives

  •      Understand how to realise dynamic model validation;
  •      Get into an approach to describe wind-wave-correlation for simplified but still accurate loads simulation;
  •      Learn about the effect of tuned mass damper on offshore wind turbine (OWT) tower loads;
  •      Learn about load reduction by means of individual pitch control based on simulation results and measurements.
Michael Muskulus, Associate Professor and Head of the Offshore Wind Turbine Technology Group, Norwegian University of Science and Technology (NTNU), Norway
Fabian Vorpahl, Leading Expert Offshore Engineering Tower & Substructure, Senvion, Germany


Narasimhan Sampath Kumar Atkins Limited, United Kingdom
Narasimhan Sampath Kumar (1) F Trevor Hodgson (1) Irina Cortizo (1)
(1) ATKINS LIMITED, Glasgow, United Kingdom

Presenter's biography

Biographies are supplied directly by presenters at WindEurope Summit 2016 and are published here unedited

Dr Narasimhan Sampathkumar has 16+ years of Structural Engineering experience, especially in advanced Finite Element methods and holds Doctorate from Southampton University, Southampton, UK. His research concerned with Three dimensional geometrical and material nonlinear FE analysis of adhesively bonded joints for marine structures. For the past 9 years he has been involved with design of wind turbine structural components and since 2012 design of foundations for Offshore Wind turbine has been his area of focus. His recent projects included conceptual and detailed design of substructures like Jacket and Monopile. His area of interest is dynamic response between wind turbine and offshore substructures with keen interest and has been implementing various method of coupled analysis on different projects.


Approach to wind wave correlation in coupled analysis of offshore WTG substructures


Offshore wind turbines need to be designed to withstand the expected environmental conditions throughout their design life. One of the requirements of the analysis process is to provide a simulation of the complete system including the tower and foundation. It is also necessary to simulate the combined effect of wind and wave loading on the structure throughout its life time. This combination of wind and wave loading is often simplified to enable practical analysis of the complete structure. However, it is an engineering challenge to undertake such simplification without loss of accuracy. A methods to achieve this is explained herein.


It is current practice to perform structural analysis using time domain procedures, which can be very time consuming. Simplification of wind/wave relationships is important to reduce total analysis times to practical levels. The approach adopted herein makes use of spectral analysis techniques to derive a calibrated and simplified wave climate to be used in the more lengthy time domain simulations.

Main body of abstract

Traditionally, the various wind conditions are analysed in detail, covering expected magnitudes and directions and defining their probability of occurrence over the wind turbine design life. However, the superposition of wave loading is also important, not just its varying magnitude but also its alignment / misalignment relative to the wind. Wind / wave misalignment is particularly significant for likely governing fatigue conditions during power production, when aerodynamic damping differs with direction relative to the turbine axis.

To achieve practical analysis times, it is normal to simplify the wave model by specification of sea states as a function of wind speed. Directionality of the waves is typically represented by discretized misalignment relative to the wind. This approach inherently results in the probability of wave magnitude and direction being subservient to the wind probability. This might not accurately reflect the real wave climate of the site and consequently, the wave component of fatigue damage may be incorrect.

An approach to deriving coupled wind/wave conditions is to develop relationships between wind and wave based on site specific hindcast data. A technique has been developed for calibrating the above simplified relationships using spectral fatigue analysis methods. This calibration of simplified wave height, period and direction is against site specific scatter tables and derives percentages of exceedance for polynomial relationships between the variables. Alternatively, a factored mean fit on the scatter tables may also be used to obtain relationships between the variables.

Results are presented showing calibrated wind-wave data relationships. The sensitivity of the results to the type of fit used is also explored and presented herein.


The resultant calibrated wave fatigue conditions are then suitable for combination with wind conditions and will yield more accurate combined fatigue lives. Alternatively, the approach may be used to simplify the number of wind-wave combinations, without loss of accuracy. The herein presented calibration technique enables a more efficient analysis process yielding accurate results. It will consequently have a positive impact on the overall wind farm design planning and programme and in particular, it will benefit the design of offshore substructures.

Learning objectives
The information provided in this paper will provide sufficient evidence to demonstrate the calibration principles and will show the results of the applied technique when used in a commercial wind farm project. This will benefit the industry by allowing robust structural designs of the foundation of wind turbines under combined wind and wave loading effects.