physical and numerical modelling of mooring system for ... · • ansys aqwa allows to accurately...

Aalborg, 3rd SDWED Symposium

SDWED WP2 – Moorings Barbara Zanuttigh, Elisa Angelelli, Luca Martinelli and Francesco Ferri

Physical and numerical modelling of mooring system for floating Wave

Energy Converters

03-06-2014

Structural Design of Wave Energy Devices – w

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WP2 Aim

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The objectives of WP2 are:

• to analyse possible mooring systems for WECs;

• to develop knowledge and methodologies to be able to calculate mooring response;

• to give estimates on lifetimes related to the designs.


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Motivation

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January 2009

November 2011


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Requirements of moorings

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• Extreme waves: survivability

• Extreme loads

• Reliable design of moorings

• Operational wave climate: power production

• Device optimisation

• Farm layout


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Approach to WEC mooring design

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Choice of mooring system

type

General layout

shape and n° of lines

Cable Composition

• Compliance • material

Anchor positioning Verification of

behaviour • Station keeping • Rigidity • Loads

Verification of line and

anchor resistance

DESIGN PHASE VERIFICATION PHASE


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Aims

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• Investigation of the mooring system typologies on the power production

• Investigation of the pre-tension level of the spread mooring system on: – the power production

– the forces acting on mooring lines

– the device motions


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The device

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Methodology

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• Physical tests • Models • Laboratory configurations • Wave States • Experimental result

• Numerical simulation • Set-up • Motion & Mooring


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Physical tests

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2 experimental campaigns: •First

• Scale 1:30, h=0.70m • Deep wave basin @AAU • AIM: mooring system typologies

•Second • Scale 1:60, h=0.45m • Shallow wave basin @AAU • AIM: effects of pre-tension level of the

spread mooring


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First campaign – 1:30 scale

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SPREAD MOORING SYSTEM

CALM MOORING SYSTEM


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POWER TAKE-OFF SYSTEM


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Irregular Waves (IR)

WS HS [m] TP [s] LP [m]

1 0.067 1.05 1.67

2 0.067 1.19 2.02

3 0.100 1.05 1.73

4 0.100 1.19 2.10

5 0.100 1.43 2.66

6 0.100 1.94 4.28

7 0.133 1.43 2.88

8 0.133 1.94 4.28

9 0.167 1.43 2.88

10 0.167 1.94 4.28


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Effects of mooring on the power production

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Effects of mooring on the device efficiency

03-06-2014 B. Zanuttigh et. al., “Effects of mooring systems on the performance of a wave activated body energy converter”, Renewable Energy 57 (2013) 422-431


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Second campaign – 1:60 scale

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SPREAD MOORING SYSTEM

PTO

Load Cell (Front Right)

Front MTi

Back MTi


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• Device geometry: Long 0.95m Wide 0.38m

• Asymmetric spread mooring system


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Lc Lc

3 Mooring pre-tension levels:

LC= length of the chain lying on the seabed at the rest. Progressively reduced from the 80% to the 65% and finally to the 50% of the total chain length (average pre-tension of 0.6-1.0-1.6 N respectively).


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Optimisation of PTO rigidity

03-06-2014

P = F*v r1=7cm r2=9cm r3=11cm r4=13cm r5=15cm r6=17cm

PTO

r4


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Wave attacks

03-06-2014

Irregular Waves (IR)

WS Hs [m] Tp [s] l/LP

1 0.033 0.72 1.21

2 0.033 0.90 0.79

3 0.050 0.96 0.70

4 0.050 1.08 0.58

5 0.067 1.27 0.45

6 0.083 1.45 0.38


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Effects of mooring on the power production

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Lc 80%→Lc 65% P -6%

Lc 80%→Lc 50%

P -16%


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Forces on the moorings

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Forces on the mooring lines

WS Hs [m] Tp [s] l/LP 1 0.033 0.72 1.21

2 0.033 0.90 0.79

3 0.050 0.96 0.70

4 0.050 1.08 0.58

5 0.067 1.27 0.45

6 0.083 1.45 0.38


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Effects of mooring on the device motions Translations


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Forces/displacements


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ANSYS – AQWATM

WorkBench

AQWA-LINE

AQWA-LIBRIUM

AQWA-FER

AQWA-DRIFT

AQWA-NAUT

Hydrodynamic Diffraction

Hydrodynamic Time Response


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Numerical model


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Numerical and experimental device


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Comparison of loads

• Front chains 20 25 30 35 40

-0.3

-0.2

-0.1

0

0.1

0.2

time (s)

tens

ion

(N)

DEXA Mooring load, test: Onda4p80

(ch24+ch25)/2Cable 3


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Model limitations

• The model reproduces only long-crested waves (1D).

• Deep water only.

• Inaccurate representation of the PTO rigidity.

• A short time history of wave elevation in a specified point may be defined: the duration of the time history in the file is 7200s.

• Wave direction may not be modified during the sequence .

• Reflection and transmissions are only computed based on the initial geometry under water, for waves of small amplitude.

• Wave drift loads are not exactly analysed.

• The geometry is limited by the size of the mesh and by the tolerance dimensions.


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New mooring scheme


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Loads on mooring lines

WS Hs (m) Tp (s)

4 3 8.4

5 4 9.8

6 5 11.2

7 8 13.1

8 8 14.0


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Effects of wave obliquity

• F1/100 for incoming waves HS=4m, TP=9.8s


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Conclusions

03-06-2014

• The device power production is mooring dependent.

– The optimal PTO rigidity depends on the mooring scheme and on the level of pretension of the mooring lines.

– A CALM scheme leads to greater power production and device efficiency than a SPREAD mooring scheme.

– for a given PTO rigidity, the variation of the mooring pre-tension level –from a slack (LC=80%) to a taut (LC=50%) configuration– leads to a decrease of the power production by 16%.

• The loads on the moorings increase with increasing HS, and show a modest dependence on l/Lp. In general, these forces on mooring system are well below typical working condition in all LC.


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Conclusions

03-06-2014

• ANSYS AQWA allows to accurately tests many different mooring schemes provided that these are examined

– in deep water and

– under long-crested waves.

• Numerical and experimental results differ also due to the PTO representation.

• Necessary to wave obliquity into account when designing mooring schemes.


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Thank you

03-06-2014

Authors gratefully acknowledged the support of the European Commission through MARINET (www.fp7-marinet.eu) and MERMAID project (www.mermaidproject.eu) and the support of the Danish Council for Strategic Research through SDWED project (www.sdwed.civil.aau.dk).

Barbara Zanuttigh, Elisa Angelelli, Luca Martinelli and Francesco Ferri

http://www.fp7-marinet.eu/

http://www.mermaidproject.eu/

http://www.sdwed.civil.aau.dk/

Funded by

The International Research Alliance

03-06-2014

physical and numerical modelling of mooring system for ... · • ansys aqwa allows to accurately...

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