challenges in the hydrodynamics modelling of wave energy · pdf filechallenges in the...

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)

http://dx.doi.org/10.3390/en9070506



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.”




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).




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




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

[email protected]

[email protected]



mailto:[email protected]

mailto:[email protected]

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