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

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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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First campaign – 1:30 scale

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POWER TAKE-OFF SYSTEM

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First campaign – 1:30 scale

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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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Second campaign – 1:60 scale

03-06-2014

• Device geometry: Long 0.95m Wide 0.38m

• Asymmetric spread mooring system

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Second campaign – 1:60 scale

03-06-2014

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

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

PTO

r4

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Wave attacks

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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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03-06-2014

ANSYS – AQWATM

WorkBench

AQWA-LINE

AQWA-LIBRIUM

AQWA-FER

AQWA-DRIFT

AQWA-NAUT

Hydrodynamic Diffraction

Hydrodynamic Time Response

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03-06-2014

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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New mooring scheme

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03-06-2014

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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03-06-2014

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

Funded by

The International Research Alliance

03-06-2014