EERA DEEPWIND’2015
VERIFICATION OF THE SECOND-ORDER WAVE LOADS
ON THE OC4-SEMISUBMERSIBLE
Sebastien Gueydon
Maritime Research Institute of the Netherlands (MARIN)
BACKGROUND: FOLLOW-UP OMAE2014-23398
•
•
•
•
OC4 (NREL)
OC4-semi
Rigid body
LC2.2, only wave:
•
•
JONSWAP, γ = 2.87
Hs = 6 m, Tp = 10 s
• 2d order:
•
•
Excitation
Response
• Comparison of:
•
•
2
{DIFFRAC + aNySIM}
{WAMIT + FAST}
LC2.2 with 2d order excitation loads
Sebastien Gueydon
Tiago Duarte
Jason Jonkman
Ilmas Bayati
Antonio Sarmento
Maritime Research Institute of the Netherlands (MARIN)
EDP Innovacao, Instituto Superior Tecnico (IST)
National Renewable Energy (NREL)
Dipartimento di Meccanica, Politecnico di Milano
Instituto Superior Tecnico (IST)
COMPARISON FOR 1ST & 2D ORDER LOADS
FAST
aNySIM
0
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
1
0
-1
1000
Pitch (º)
1050
TenFair2 (kN)
Heave (m)
-5
1000
1050
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
5
0
-5
1000
TenFair1 (kN)
LC2.2-0deg-ALL
5
TenFair3 (kN)
Surge (m)
• LC2.2-ALL: All wave loads (1st and 2d)
1050
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
LC2.2-0deg-ALL
1200
FAST
aNySIM
1100
1000
1000
1050
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
1050
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
1050
1100
1150
1200
1250 1300
Time (s)
1350
1400
1450
1500
1400
1200
1000
1000
1200
1100
1000
1000
• aNySIM and FAST give very similar results
11
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
0.±5.7E3 kN
• All excitation forces in surge and tension in line 2
12
98± 240 kN
1160±64 kN
0.± 619 kN
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
0.±5.7E3 kN
• Wave 1st order surge excitation
13
98± 240 kN
1160±64 kN
0.± 619 kN
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
0.±5.7E3 kN
98± 240 kN
1160±64 kN
0.± 619 kN
• Wave 2d order difference frequency surge excitation
14
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
0.±5.7E3 kN
• Wave 2d order sum frequency surge excitation
15
98± 240 kN
1160±64 kN
0.± 619 kN
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
0.±5.7E3 kN
• Tensions follow the drift motion + wave motion
16
98± 240 kN
1160±64 kN
0.± 619 kN
STUDY OF 1ST & 2D ORDER LOADS
Effects of loads in surge on the tension
2d order sum freq can be neglected!
0.±5.7E3 kN
98± 240 kN
1160±64 kN
0.± 619 kN
• Tensions follow the drift motion + wave motion (little effects of sum freq.)
17
SPECTRAL ANALYSIS OF LC2.2 WITH 2D ORDER LOADS
10
PSD (kN2.s/rad)
10
10
10
10
10
10
LC2.2-0deg-ALL
10
F1x (kN)
F2Dx (kN)
F2Sx (kN)
TenFair2 (kN)
5
0
-5
-10
-15
-20
10
-2
10
-1
0
10
(rad/s)
10
1
10
2
• Tension in mooring line 2 at fairlead (Power Spectral Density)
19
10
PSD (kN2.s/rad)
10
10
10
10
10
10
LC2.2-0deg-ALL
10
F1x (kN)
F2Dx (kN)
F2Sx (kN)
TenFair2 (kN)
5
0
-5
-10
-15
-20
10
-2
10
-1
0
10
(rad/s)
• + 1st order wave loads in surge (PSD)
20
10
1
10
2
10
PSD (kN2.s/rad)
10
10
10
10
10
10
LC2.2-0deg-ALL
10
F1x (kN)
F2Dx (kN)
F2Sx (kN)
TenFair2 (kN)
5
0
-5
-10
-15
-20
10
-2
10
-1
0
10
(rad/s)
10
1
10
2
• + 2d order difference frequency wave loads in surge (PSD)
21
10
PSD (kN2.s/rad)
10
10
10
10
10
10
LC2.2-0deg-ALL
10
F1x (kN)
F2Dx (kN)
F2Sx (kN)
TenFair2 (kN)
5
0
-5
-10
-15
-20
10
-2
10
-1
0
10
(rad/s)
10
1
10
• + 2d order sum frequency wave loads in surge (PSD)
22
2
LC2.2-0deg-ALL
5
2
PSD (kN .s/rad)
10
F1x (kN)
F2Dx (kN)
F2Sx (kN)
TenFair2 (kN)
10
0
surge
10
-5
10
-2
10
-1
• + eigen frequency in surge
23
0
10
(rad/s)
10
1
10
2
LC2.2-0deg-ALL
5
2
PSD (kN .s/rad)
10
F1z (kN)
F2Dz (kN)
F2Sz (kN)
TenFair2 (kN)
10
0
heave
10
-5
10
-2
10
-1
0
10
(rad/s)
10
1
10
2
• Same graphic for heave => influence of 2d order diff freq
24
PSD (kN2.m2.s/rad)
10
10
10
10
LC2.2-0deg-ALL
15
10
5
0
pitch
10
-5
10
-2
10
-1
0
10
(rad/s)
10
1
10
2
• Same graphic for pitch => influence of 2d order diff freq
25
CONCLUSIONS OF OMAE2014-23398
Code comparison:
• Approach to compare the 2d order load (ex: OC4-semi, rigid)
• {WAMIT+FAST} ≈{DIFFRAC+aNySIM} for 1st & 2d order loads.
For the OC4-semi (rigid, WD=200 m, spread-moored):
• Sum-frequency 2d order loads can be neglected.
• Difference-frequency 2d order loads cause mooring line tension
variations.
• Difference-frequency 2d order loads have an effect on the heave
and pitch motion (when a full QTF is used).
26
FURTHER WORKS -> MOTIVATION FOR THIS PRESENTATION
Check against measurements:
• OC5 model-tests of UMaine & NREL at MARIN in 2013
• Do we see:
•
•
27
Surge drift?
Second-order response in heave and pitch?
DIFFERENCES BETWEEN OC4 & OC5 (MT)
OC4
Nacelle
OC5 as built
<<
Total mass
14072 t
14040
Pitch radius of inertia /G
28.27 m
33.38
Surge natural period
113.2 s
105.3 s
Heave natural period
17.0 s
17.3s
Pitch natural period
25.1 s
32.2 s
28
Eigen frequencies
(rad/s)
OC4
OC5 as built
Surge
0.05
0.06
Heave
0.37
0.36
Pitch
0.25
0.20
MODEL TEST
Operational wave
• JONSWAP
• Hs = 7.1 m
• Tp = 12.1 s
• Gamma = 2.2
No wind
29
S
u
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g
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P
i
t
c
h
H
e
a
v
e
MODEL TEST
• Focus on low-frequent wave
excitation
• Interested in frequencies
below 0.35 (rad/s)
30
S
u
r
g
e
P
i
t
c
h
H
e
a
v
e
COMPARISON WITH MODEL-TESTS
Power Spectrum Density of the surge motion
Resonance
peak caused
by 2d-order
1st-order wave response
peak
⇒ Low frequent (resonance) peak, underestimated in the
simulation
⇒ Wave response is very similar
31
COMPARISON WITH MODEL-TESTS
Power Spectrum Density of the heave motion
Resonance peak
caused by 1st-order
1st-order wave response
peak
⇒ Resonance peak caused mainly by 1st order wave excitation
⇒ Wave response and resonance peak slightly underestimated
32
COMPARISON WITH MODEL-TESTS
Power Spectrum Density of the pitch rotation
Resonance
peak caused
by 2d-order
1st-order wave response
peak
⇒ Low frequent (resonance) peak is present in the
measurements, and reproduced well by the simulation.
⇒ Wave response is very similar
33
CONCLUSIONS
• What’s observed in the simulation corresponds to model tests
•
Resonance response to second-order wave loads in:
Surge
Pitch
Recommendations for
OC4-semi:
• Full QTF
• 2d order wave loads:
•
•
34
Horizontal
Vertical
Surge resonance
response to 2d order
wave loads
(mooring)
Pitch resonance
response to 2d order
wave loads
THANK YOU!
Acknowledgment to UMaine
and DoE for making these
model-tests possible.
Blueweek in Delf (NL), April 20-22, 2015