Modern methods for investigating the stability of a pitching floating platform wind turbine

Lennie, Matthew; Marten, David; Pechlivanoglou, George; Nayeri, Christian Navid; Paschereit, Christian Oliver

doi:https://doi.org/10.5194/wes-2-671-2017

Articles | Volume 2, issue 2

https://doi.org/10.5194/wes-2-671-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The Science of Making Torque from Wind (TORQUE) 2016

https://doi.org/10.5194/wes-2-671-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 2, issue 2

Research article

|

22 Dec 2017

Research article |

| 22 Dec 2017

Modern methods for investigating the stability of a pitching floating platform wind turbine

Matthew Lennie, David Marten, George Pechlivanoglou, Christian Navid Nayeri, and Christian Oliver Paschereit

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Final revised paper (published on 22 Dec 2017)
Preprint (discussion started on 21 Dec 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Revisions', Anonymous Referee #1, 24 Jan 2017
RC2: 'referee comment', Anonymous Referee #2, 22 Mar 2017
AC1: 'Response to Reviewers', Matthew Lennie, 05 Apr 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (10 Apr 2017) by Carlo L. Bottasso

AR by Matthew Lennie on behalf of the Authors (21 Apr 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 May 2017) by Carlo L. Bottasso

RR by Anonymous Referee #1 (17 May 2017)

RR by Vasilis A. Riziotis (03 Aug 2017)

Suggestions for revision or reasons for rejection

The paper deals with stability investigations of floating turbines. In the first part of the paper the in-house free wake lifting line code LLFVW is evaluated against predictions of models of varying fidelity (including BEM and CFD option). In the second part of the paper stability characterization of a translating floating turbine, based on imposed kinematics is performed.
Validation indicates that the code performs well in situations of platform pitching motion. The main conclusion from stability characterization is that damping of the turbine surge mode can be significantly affected by the phase of the pitching motion.
To the reviewer opinion the paper can be considered for publication after some points are clarified and some revision is made according to the discussion below:
1) The main requirement for publishing a torque paper to WES special issue is that the paper is extended or modified by at least 30%. The changes made in the paper concern section 4 which has been significantly extended by describing damping characterization methods employed in the paper. However to my opinion section 4.1 presents standard methods and it could be significantly shortened to the level that only supports derivations of 4.2. For example the link between work of aerodynamic forces and damping is well known concept and there are several references on that to list. I’m afraid, only new methodologies could be presented in such an extended way while a brief description of all the rest with reference to previous developments would be sufficient. To my opinion the effort should be placed in presenting more damping results on different motion cases. For example although the title of the paper refers to pitching floating platforms the stability analysis has been only performed for translating platforms (btw. The title should be changed to stability of floating platform if no pitching cases are included).
2) There are some issues in relation to Hilbert method. In derivations of section 4.2, and in particular in equation 42, the force is expressed as a periodic function with a frequency equal to the frequency of X variation. Therefore, the force is a time function and varies with the modal frequency. It is not made clear whether the force can also depend on eg. the frequency of rotation (then Tu=Tu(t)). This point should be explained because then it is clarified that the method can be used not only for non-linear damping identification but also for periodic systems. Is that the case? Because only if this is the case combined platform motion and yawed conditions (periodicity w.r.t rotational speed) can be considered.
Actually we have also employed the same method for non-linear damping characterization some years ago, see,
Riziotis, V.A., Voutsinas, S.G., (2006) “Advanced aeroelastic modelling of complete wind turbine configurations in view of assessing stability characteristics,” Proceedings of the EWEC ‘06, Scientific Track, Athens, Greece, February 27 – March 2.
Based on the work of,
Simon, M. and Tomlinson, G.R., “Use of the Hilbert Transform in Modal Analysis of Linear and Non-linear Structures”, Journal of Sound and Vibration, 96, 421-36, (1984).
and I’ m a little puzzled by equations 46 and 47. If you read the second paper you will see that in case of viscous damping (linear, constant coefficient system- this a case of a linear aeroelastic system) ACt(t) of equation 47 should be a pure exponential function and of course the exponent is related to the modal damping. Isn’t that the case for equation 42 if Tu is constant and if so why then Ksi appears to be periodic (sinusoidal variation)? I would expect Ksi to be periodic only in case of non-linearity or periodic input/coefficients of the aeroelastic equation.
3) The last comment (you will see it also in the attached pdf) is that, in a servo-aero-elastic system the phase difference between the different dofs of motion is determined through the mode shapes. In the example presented in section 5, which simulates a floating turbine with pitch control, the phase difference between pitch and surge motion is selected arbitrarily in order to highlight effect on damping. How can somebody be sure about the phase characteristics of the dofs if you do not know the mode shape and then how could you know what the mode shape looks like if you don't have a linearized model of at least the dynamic system? Therefore the analysis presented cannot be disconnected from the linearization of the dynamics as stated in the introduction section. It is still a linear analysis w.r.t to dynamics although non-linear aerodynamic affects can be accounted for without needing to linearize aerodynamics.

Editorial changes/modifications and specific comments are discussed in the accompanying pdf.

Referee Report: PDF

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ED: Reconsider after major revisions (06 Aug 2017) by Carlo L. Bottasso

AR by Matthew Lennie on behalf of the Authors (18 Sep 2017)

ED: Referee Nomination & Report Request started (23 Sep 2017) by Carlo L. Bottasso

RR by Vasilis A. Riziotis (21 Oct 2017)

ED: Publish as is (30 Oct 2017) by Carlo L. Bottasso

ED: Publish as is (01 Nov 2017) by Gerard J.W. van Bussel (Chief editor)

AR by Matthew Lennie on behalf of the Authors (09 Nov 2017) Manuscript

Short summary

Floating platform wind turbines present a challenge for engineers to simulate. This paper explores some better methods for simulating the aerodynamics of wind turbines as they move about on a floating platform. We also derived a new way of investigating whether the aerodynamics of the wind turbine rotor help it stay stable.