2010 sname roll motions of fpsos.ppt sname roll motions of fpsos.pdf · • fpso roll damping...
TRANSCRIPT
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Roll Motions of FPSOsRoll Motions of FPSOs
SNAME Texas SectionDecember 14, 2010
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'We thought we were gone' says cruise passenger
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Importance of Roll Motions
• Hard to predict accurately.• Roll motions have an effect on:
– Top sides foundations– Risers or their end fittings– Turret bearing loads– Efficiency of process equipment– Crew comfort/wellness– Helicopter operations– Loading/unloading of supply vessels– Shloshing loads in FLNG membrane tanks
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Basics of Roll Motion
• Resonant motion amplitude dominated by amount of damping at the natural frequency.
• Roll natural period of typical FPSO between 10-15s -> right in the range where wave spectrum has significant energy.
• Excitation Factors: • Damping Sources:– Relative Wave Angle– Separation Tn & Tp– Wave Spreading– Spectral Peakedness– Water Depth– Other
p g– Skin friction– Wave making– Eddy generation– Moorings & Risers– Internal waves in tanks– Other
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Effect of loading condition
• Wave Condition:– Hs = 6.7m– Tp = 11.4s– Direction = 270deg
• Loading Condition:Loading Condition:– Ballast, Tn = 11.7s– Full, Tn= 14.1s
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Effect of Loading Condition - cont’d
• Test Results:– Full, stdev = 1.5deg; mpm = 6.6deg– Ballast, stdev = 3.9deg; mpm = 13.3deg
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Effect of Bilge Keel Width
• Wave Condition:– Hs = 6.7m– Tp = 11.4s– Direction = 270deg
• Loading Condition:– Ballast, Roll natural period = 11.7s
• Caveats:– Very shallow draft condition– Damping from lower riser balcony
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Effect of Bilge Keel Width - cont’d
• Test Results:– 1.0m x 185m, stdev = 4.3deg; mpm = 14.5deg– 1.5m x 205m, stdev = 3.9deg; mpm = 13.3deg
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Effect of Bilge Keel Width - cont’d
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L T R A l iLong Term Response Analysis
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Long Terms vs Short Term Response Analysis
Short-term environmentDesign Storm
Short term response
Long-term environmente.g. Hindcast Database
Long term responseShort-term response analysis
Short-term Design Response
Long-term response analysis
Long-term Design Response
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Example Long Term Response Analysis
• Turret moored tanker in GOM• Hurricane hindcast database• FD Mooring Analysis using SPMsim• Resulting 100-year MPM Roll Amplitude• Response Based Design Criteria• Response Based Design Criteria
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Joint Distribution: Hs – Relative Wave Heading
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Joint Distribution: Roll – Relative Wave Heading
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Joint Distribution: Roll – Relative Wave Heading
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100-yr Cumulative Distribution of Roll
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MPM Roll from 100yr Storm vs 100yr Roll MPM
100yr Design Environment• Hs = 12.2m• Tp = 14.2s• Vw = 36.5m/s @ 30deg• Vc = 1.75m/s @ 45deg
MPM from 100yr storm
= 5.8deg@ g
100yr Roll Environment• Hs = 8.9m• Tp = 14.5s• Relative Heading = 135deg
MPM 100yr Roll = 9.9deg
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L l C diti th t ff t R ll M tiLocal Conditions that affect Roll Motions
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Seasonal swell direction Northwest Shelf, Australia
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Seasonal wind direction Northwest Shelf, Australia
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Relative Swell Heading, Northwest Shelf, July
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Wave and Current Rose, July, Offshore Ghana
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Brazil
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Example of CFD for Roll Motions
• 2D CFD using EOLETM by Principia• Comparison of 6 configurations• Forced oscillations at Tn = 14s with 5° and 10° amplitudes• Quadratic damping extracted from moment time series• Moment around center of roll integrated on 1m wide section: 2D
R l f li d i i 3D diff ti l i• Removal of linear damping using 3D diffraction analysis
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Configurations Studied
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Vortex Development induced by Appendages
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Estimated Roll RAOs for the various cases
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Effect of Mooring and Risers on Roll Motions
• Water Depth: 2140m• VLCC, Ballast Draft = 8.8m• Mooring Configuration:
– Spread mooring, 24 mooring lines– Turret mooring, 9 mooring lines
• Riser configuration:• Riser configuration: – 24 catenary risers– Total FZ ~ 5500 metric tons
• Beam Sea Condition 1-yr RP: – Hs = 4.5m– Tp = 9.9s
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Effect of Risers on Roll RAO
2.5
3.0
3.5
4.0
4.5
Mooring Lines + RisersMooring LinesNo Mooring Lines
deeg
/m)
Roll RAO - Effect of Mooring and Risers, 270deg
0
0.5
1.0
1.5
2.0
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21wave period (s)
Rol
l RAO
(d
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Effect of Riser Balcony Location
3.0
3.5
4.0
4.5
waves @ 90degwaves @ 270deg
eg/m
)
Roll RAO - Effect of Riser Porch Location
0
0.5
1.0
1.5
2.0
2.5
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21wave period (s)
Rol
l RAO
(dee
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Effect of Riser Balcony Location
1.25
1.50
1.75
2.00
27090
ng-o
ff (m
/m)
RAO of vertical motion at the riser hang-off point, amidships
0
0.25
0.50
0.75
1.00
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22wave period (s)
Z-m
otio
n at
rise
r ha
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Effect of Mooring Configuration
2.5
3.0
3.5
4.0
TurretSpread Mooring
g/m
)
Roll RAO, Turret vs Spread Mooring
0
0.5
1.0
1.5
2.0
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22wave period (s)
Rol
l RA
O (d
eg
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Second Order Roll Motions
Motions at Roll Natural Perioddriven by y
difference frequency moments
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Second Order Roll Motions
• Example– 170,000 DWT FPSO – Purpose Built– No bilge keels– Roll Period: Full = 25.2s; Ballast = 23.7s
• Test setup:– Horizontal mooring– Scale 1:60– 3-hr duration
• Simulation setup:– HOBEM 3D diffraction– Second order roll moment spectrum
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Separation between Roll period and wave period
20
24
28
32
36
40
2 5
3.0
3.5
4.0
4.5
5.0
Roll RAOWave Spectrum
m2 s) (d
eg/m
)
0
4
8
12
16
20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00
0.5
1.0
1.5
2.0
2.5
wave frequency (rad/s)
S (m
Rol
l RA
O
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Second order Roll Moment Spectrum
2.E+08
2.E+08
3.E+08
)^2*
sec)
3.0
3.5
4.0
4.5
5.0
K4)
SMx:
Roll RAOs
0.E+00
5.E+07
1.E+08
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Difference Frequency
SMx
((t
ons*
m)
0.0
0.5
1.0
1.5
2.0
2.5
|X4|
/(F4/
K
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Second Order Roll Motions – Simulation vs Test
Full Draft Roll Motions (deg)Mean Max Min Stdev
Simulation WF 0.0 1.4 -1.4 0.4DF -0.7 10.2 -12.2 3.2Total -0.7 11.6 -13.6 3.2
Test Total 0.4 9.8 -11.4 2.4
Ballast Draft Roll Motions (deg)Mean Max Min Stdev
Simulation WF 0.0 2.1 -2.1 0.6DF -0.5 10.3 -11.2 3.1Total -0.5 12.4 -13.3 3.1
Test Total -0.3 11.0 -11.9 2.5
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Concluding Remarks
• Accurate Roll predictions still difficult• Bilge keels are very effective in reducing roll• For SPMs LTRA is necessary to find extreme roll• Determine critical sea state and bilge keel before model test• Bilge/ bilge keel configurations can be compared using CFD• Bilge/ bilge keel configurations can be compared using CFD• If roll period is long don’t forget about second order roll
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Thank you.
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References
• FPSO roll damping prediction from CFD and 2D and 3D model tests investigations, ISOPE 2004 Toulon.
• On Second Order Roll Motions of Ships, OMAE2003-37022• Extreme Responses of Turret Moored Tankers, OTC 12147• FPSO Roll JIP MARINFPSO Roll JIP, MARIN