HYWEC (Hydrodynamics of Wave Energy Converters) Workshop at BCAM April 3-7, 2017

Challenges in the hydrodynamics
modelling of wave energy converters
Vincenzo Nava
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Image from the website of OPERA H2020 project,
courtesy of EVE
Screenshot from Compass
Youtube Video

Outline
Motivation
Top Five Challenges in Modelling the Hydrodynamics of WECs
2 ▌
Four Lessons Learnt

Why Harnessing Energy From Oceans?
3 ▌
Credits
the data originate from the ECMWF WAM model archive and are calibrated and corrected by Fugro OCEANOR against a global buoy
and Topex satellite altimeter database).

How can we get Energy From Oceans?
4 ▌
Souce(s)
Figure inspired from
Peñalba, M., Ringwood, J. V., A Review of Wave-to-Wire Models for Wave Energy Converters, Energies 2016, 9(7), 506;
doi:10.3390/en9070506
WAVE
RESOURCE
ELECTRIC
GENERATOR
WAVE
ENERGY
CONVERTER
POWER
TAKE OFF
POWER
CONVERSION
GRID
ABSORPTION
FPTO
x, x, x T
ω
i
V
Pout
TRANSMISSION GENERATION CONDITIONING
Oscillating Water Column (OWC)
Overtopping Devices
Oscillating Bodies
(Point Absorbers & Surge Devices)

A more precise modelling of the
hydrodynamics will improve:
Why numerical models are needed to simulate the FSI?
5 ▌
POWER
PRODUCTION
FORCES,
DISPLACEMENTS,
STRESSES AND
DEFORMATIONS
OVERTOPPING
OPTIMISATION OF
SHAPES, LOSSES,
ETC…
WAVE PROPAGATION
ENVIRONMENTAL
IMPACTS
ECONOMIC
ASSESSMENTS
SURVIVABILITY
IMPACTS ON
THE
ENVIROMENT

Top FIVE Challenges
1.
2.
3.
4.
5. WAVE PROPAGATION

Reduction of Energy – Using TMA spectra
7 ▌
Source
A. Sinha, D. Karmakar, and C. Guedes Soares, “Shallow water effects on wave energy converters with hydraulic power take-off system,”
Int. J. Ocean Clim. Syst., vol. 7, no. 3, pp. 108–117, 2016.

Modeling devices with Mike 21
8 ▌
Source
E. Angelelli and B. Zanuttigh, “A farm of wave activated bodies for coastal protection purposes,” Coast. Eng. Proc., vol. 1, no. 33, p. 68,
2012.
M. Folley et al., “A review of numerical modelling of wave energy converter arrays,” in ASME 2012 31st International Conference on
Ocean, Offshore and Arctic Engineering, 2012, pp. 535–545.
“Boussinesq models are not capable of
modelling the hydrodynamics of a moving
device.
They may be used to model device
characteristics, such as wave transmission
reflection and absorption.
If radiation characteristics are known, these
may be included by use of an internal
generation line, although this may become
cumbersome when more than 1 WEC is
considered and, for this reason, their use
warrants caution.”

Top FIVE Challenges
1.
2.
3.
4. VALIDATION OF MODELS
5. WAVE PROPAGATION

How validate and verify models?
10 ▌
• Code-to-code verification
EXAMPLE: Task 10 – OES – Methodology
 Modeling case studies, as for example a heaving sphere (decay tests, regular
waves tests, irregular wave tests)
 Review the models
o Model a WEC
o Compare with existing test data

How validate and verify models?
11 ▌
• Experimental Data
Source
Percher, A., Kofoed, J.P., Handbook of Ocean Wave Energy, 2016, Springer

12 ▌
Performance of WECs will normally be scaled using Froude similitude.
Tests with large scales are recommended when scale effects are envisaged.
Important factors in energy conversion that are not addressed by standard scaling
procedures include, but are not limited to, the effects listed below:
• The power output of devices utilising a pneumatic power take off is related to the
compressibility of the air, which is dictated by atmospheric pressure and the
absolute temperature of atmosphere. Therefore, the stiffness of the air “spring”
will not be scaled correctly using Froude similarity if geometric similarity is
maintained.
• In small - scale model tests, viscous damping and in particular damping associated
with vortex shedding from sharp edges cannot be scaled appropriately with
Froude similarity and may be overestimated.
Source
ITTC – Recommended Guidelines, Wave Energy Converter Model Test Experiments, 2014
Weber, J.,2007,“Representation of non-linear aero-thermodynamic effects during small scale physical modelling of OWC WECs”,
Proceedings,7th European Wave and Tidal Energy Conference(EWTEC 2007), Porto,Portugal
Importance of scale – Scaling Law

Importance of scale – Instrumentation and Indirect Measurements
13 ▌
Source
C. Ruzzo, G. Failla, M. Collu, V. Nava, V. Fiamma, and F. Arena, “Operational Modal Analysis of a Spar-Type Floating Platform Using
Frequency Domain Decomposition Method,” Energies, vol. 9, no. 11, p. 870, Oct. 2016.
Measurements of:
• Displacements / Velocities / Accelerations
• Line Tensions
• Wave Characteristics
• Overtopping
Use of indirect measurements,
especially with data in sea.
Development of algorithms for
indirect measurements, also from
other sectors. For example, FDD for
modal characteristics (from the
Structural health monitoring
Sector).

Top FIVE Challenges
1.
2.
3. MODELING ARRAY EFFECTS
5. WAVE PROPAGATION

Why arrays?
15 ▌
Deploying arrays of
ocean energy devices
would increase the
profitability of the
investment
Image courtesy of

How to model array effects?
16 ▌
Source
H. Day et al., “Hydrodynamic modelling of marine renewable energy devices: A state of the art review,” Ocean Eng., vol. 108, pp. 46–
69, Nov. 2015.
M. Folley et al., “A review of numerical modelling of wave energy converter arrays,” in ASME 2012 31st International Conference on
Ocean, Offshore and Arctic Engineering, 2012, pp. 535–545.
•Point-absorber method
(diameters smaller than wavelenghts and
interdistances - diffraction of radiated and
diffracted waves from each device by others in
the array is neglected)
•Plane-wave method
(spacing between axisymmetric absorbers is
large compared to the incident wavelength –
diffraction is taken into account)
•Multiple Scattering Method
(radiation and diffraction taken into account as
succession of distinct scattering events)
•Direct Matrix Method
(the amplitudes of all scattered waves
simultaneously without the need for
iteration, using a single body solution)
SPH

Some results
17 ▌
Personal Note: when fully designing an
array of heaving point
absorbers, most probably
the layout would depend
on other subsystems
rather than on the
hydrodynamic impacts
Source
Peñalba, M., Touzon, I., Lopez Mendia, J., Nava, V., A numerical study on the hydrodynamic impact of device slenderness and array size
in wave energy farms in realistic wave climates, 2017, under review

Some results
18 ▌
Source
E. Renzi, A. Abdolali, G. Bellotti, and F. Dias, “Wave-power absorption from a finite array of oscillating wave surge converters,” Renew.
Energy, vol. 63, pp. 55–68, Mar. 2014.

Development of Tools
19 ▌
DTOcean is the acronym for an European FP7
project and it stands for
Optimal Design Tools for Ocean Energy Arrays
Hydrodynamics
modeled using semi-
analytical methods
BEM NEMOH + DMM

Top FIVE Challenges
1.
2. VISCOUS EFFECTS AND EXTREME LOADS
5. WAVE PROPAGATION

Sloshing
21 ▌
Source
Y. Zhang, Q.-P. Zou, and D. Greaves, “Air–water two-phase flow modelling of hydrodynamic performance of an oscillating water column
device,” Renew. Energy, vol. 41, pp. 159–170, May 2012.
It is difficult to extract
energy from sloshing
frequency, so they should be
avoided.

Vortex Formation
22 ▌
Source
Y. Zhang, Q.-P. Zou, and D. Greaves, “Air–water two-phase flow modelling of hydrodynamic performance of an oscillating water column
device,” Renew. Energy, vol. 41, pp. 159–170, May 2012.
The formation of the vortex in front of and
behind the front wall of the air chamber and
evolution of the air vortex can be seen
clearly which is a good indicator of energy
loss.

Vortex Induced Vibration / Motions
23 ▌
Source
Z.-S. Chen and W.-J. Kim, “Numerical investigation of vortex shedding and vortex-induced vibration for flexible riser models,” Int. J. Nav.
Archit. Ocean Eng., vol. 2, no. 2, pp. 112–118, Jun. 2010.
If the structure is flexible or flexibly mounted,
these vortices may cause vibrations, leading to
stresses and fatigue damage. This motion of the
body influences, in turn, the vortex formation
process, establishing a feedback mechanism that
may lead to stable or unstable dynamic equilibria.
On the hull:
- They increase the global loads
- They may increase the offset (drag effect)
On the moorign lines:
- They may affect fatigue life of the line
The disagreement between numerical and
experimental observations attributes much to the
underestimation of multimodal vibration state by
numerical method.

Vortex Induced Vibration / Motions
24 ▌
Source
M. S. Triantafyllou, R. Bourguet, J. Dahl, and Y. Modarres-Sadeghi, “Vortex-Induced Vibrations,” in Springer Handbook of Ocean
Engineering, Springer, 2016, pp. 819–850.
On the hull:
- They increase the global loads
- They may increase the offset (drag effect)
On the moorign lines:
- They may affect fatigue life of the line
Higher harmonic components of strain and
acceleration are significant. Fatigue damage
calculations, by taking into account the higher
harmonic components, give fatigue damage
values of up to one order of magnitude larger
than those calculated based on only first-
harmonic signals.

Extreme Wave Loads
25 ▌
Source
J. Westphalen, “Extreme wave loading on offshore wave energy devices using cfd,” 2011.

Top FIVE Challenges
1. NONLINEARITIES AND COUPLING
2. VISCOUS EFFECTS AND EXTREME LOADS
5. WAVE PROPAGATION

Mooring Lines Effects
27 ▌
Source
S.-H. Yang, J. W. Ringsberg, E. Johnson, Z. Hu, and J. Palm, “A comparison of coupled and de-coupled simulation procedures for the
fatigue analysis of wave energy converter mooring lines,” Ocean Eng., vol. 117, pp. 332–345, May 2016.

Parametric Roll – Mathieu/Hill Instability
28 ▌
Source
K. Tarrant and C. Meskell, “Investigation on parametrically excited motions of point absorbers in regular waves,” Ocean Eng., vol. 111,
pp. 67–81, Jan. 2016.

Parametric Roll – Mathieu/Hill Instability
29 ▌
Source
H. Mao and H. Yang, “Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves,” Int. J. Nav.
Archit. Ocean Eng., vol. 8, no. 1, pp. 13–21, Jan. 2016.
The phenomenon happens in real seas: which are numerical tools to learn more
about its occurrence?

Lessons Learnt
1) Describing the problem properly
and identifying the outcomes
Problem statement stage is important, in order to focus on the right
physics to study.

Lessons Learnt
2) Importance of using the appropriate
numerical model for solving the
problem
Risks:
a. Unneeded
computational
burden
b. Not answering to the
questions

Lessons Learnt
3) Interpreting the results of the
experimental / numerical study in
order to “bark at the right tree”
The results of a good
experimental campaign or
numerical test can be jeopardised
by a wrong understanding of
what’s going on.

Lessons Learnt
4) All the problems are important, but
some problems are more important
than others
Developers have identified
different priorities for the
problems to be solved through
their experience.
Moreover, sometimes we look
at a problem from a wrong
perspective.

THANKS FOR YOUR ATTENTION
Vincenzo Nava
vincenzo.nava@tecnalia.com
vnava@bcamath.org

HYWEC (Hydrodynamics of Wave Energy Converters) Workshop at BCAM April 3-7, 2017

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