PhD project Freddy Madsen Advanced modelling of floating wind turbine response

Advanced modelling of floating wind turbine response

Floating wind energy is currently in transition from single demonstration projects to industrialized utilization, but while floating wind power technology is a recognized growth field, the breakthrough into an industrialized business is still to be achieved.

Background

The PhD study lies within the FloatStep project funded by Innovation Fund Denmark, led by DTU and involving Siemens Gemesa, Danish Hydraulic Institute, Stiesdal Offshore Technologies, Stromning and University of Western Australia. FloatStep will run for 40 months and will focus on bringing cost competitive floating wind one step closer to commercial exploitation.
The present PhD project contributes to the scientific model development and experimental analysis in FloatStep. It will take basis in a state‐of‐the‐art full aero‐servo‐hydro‐elastic model of the TetraSpar DTU 10MW floating wind turbine in HAWC2 [1].

Project objectives

This numerical model will serve as an important tool in the three following study field blocks of the PhD project:

  • Characterization of extreme events
    This part of the project takes basis in existing experimental data, measured by the candidate at DHI in 2017. The data set consists of response measurements for a scaled TetraSpar 10MW setup in waves‐only and combined wind and wave conditions. The measurements involve low‐frequency response from nonlinear subharmonic wave forcing, and as a first activity the wave‐only data will be analysed with respect to these responses by use of the four‐phase harmonic separation method [2]. Similar analysis has been made in the research group for wave loads on monopiles. For the present study, a challenge is related to the explicit second‐order dependence of the low‐frequency forcing to the linear input spectrum. It is thus part of the research to connect the data‐driven observations to an approximate stochastic model. Here forward modelling with the HAWC2 code utilizing full quadratic force transfer will be used in combination with NewWave‐type [3] input conditions.
    As a next step, the response in combined wind and waves will be analysed, following the same methods of ensemble‐averaging. The analysis includes the combined effect from the rotational motion of the rotor, active control and aerodynamic damping. The focus is the extreme events, which will be characterized to answer the simple question ‘what causes the large responses’. Again, by aid of a stochastic input‐description, the response levels can be linked to exceedance probabilities for the given wind‐wave conditions.
    A final step, will be a validation study based on the full‐scale TetraSpar Demonstrator data in combination with a HAWC2 model for this setup.
  • Test and validation of improved control methods
    The second part of the project, will test an improved control method by physical test of the scaled floating wind turbine model. It is known that reducing the pitch motion of the floating wind turbine will lead to load reduction, and thus potentially to a lifetime extension. The lab controller is upgraded, so that it takes the floater and nacelle acceleration into account, implemented in the physical model of the DTU 10MW floating wind turbine and tested in different operational conditions and configurations at DHI. The state‐of‐the-art aero‐servo‐hydro‐elastic model (HAWC2) is then validated against the model tests.
    As a second step, the option of improving the control approach with Model Predictive Control (MPC) will be considered. Based on the results from a pilot investigation, it will be decided whether to include it in the model scaled turbine during the experiments.
  • Validation of elastic deformation
    In the third research element, a recently developed method for hydrodynamic response of floaters with structural flexibility will be validated by a combined numerical and physical model study. A reliable description of the floater flexibility is important in the design models, since the global natural frequencies are affected by it. The model builds on the generalized deflection mode option in the wave-structure solver WAMIT and has been coupled into the HAWC2 model. Model tests will be carried out by DHI in collaboration with the candidate. They will focus on a generic geometry with a single flexible degree of freedom for a generic floating body. The body will be tested in both a fixed and floating configuration with varied stiffness to allow for different types of hydro‐elastic resonance.

Perspective

The expected outcomes of the PhD project will be:

  • The outcome of the extreme event characterization will be 1) a rational description of the load input and response for extreme events for the TetraSpar floater, 2) a validated method for predicting this event type in relation to the stochastic input space for wind and waves, 3) the ability to characterize such extreme events for other floater‐turbine configurations and 4) a validation study of the developed model based on full-scale data.
  • The model scaled controller is upgraded, so that it takes the floater and nacelle acceleration into account, implemented in the physical model of the DTU 10MW floating wind turbine and tested in different operational conditions and configurations at DHI. The HAWC2 model is then validated against the model tests.
  • The validation of elastic deformation will be reported in a journal paper as a follow up to the method paper of [4]. This will include test setup and validation study of inclusion of floating elasticity in dynamic calculations, implemented in HAWC2.

Contact

Freddy Johannes Madsen
PhD student
DTU Wind Energy