response-based metocean criteria for optimisingdesign and operation of fpsos
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Shell Exploration & Production
Cop
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Response-based Metocean Criteria for Optimising Design and Operation of FPSOs
Hermione van Zutphen
Marios Christou
Kevin Ewans
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Outline
• Metocean design conditions & LSM
• Description of the metocean environment
• Response analysis
• Extremes, short term and long term variability
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Why metocean is so important
• Meteorology and Oceanography
• Understanding the environment
– Extremes:
• Winter storms
• Tropical cyclones (hurricanes, typhoons)
• Currents
– Operational
• Operability of processing equipment and offloading
• Wind waves and swells
• Internal currents
• VIV
• Squalls
• Tides
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Metocean conditions for FPSO design
• Independent extremes (guidelines)
– For example DNV:
• For spread-moored systems, loading from wind, waves and current in the same direction
• At least head, quartering and beam load directions as well as in-line conditions for symmetrical anchor patterns
• Weather-vaning: include directional spread of wind, waves and current
• Without site specific data: colinear and non-colinear environment
– API: extreme independent with associated conditions (API)
• Response based conditions
– Based on extreme response
– Long-term environmental dataset (30 years)
– Vessel model
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Response Based Analysis
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
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Environment “perspective” Response “perspective”
WindWavesCurrentDirectionality
Simple Jacket
WindWavesCurrentDirectionality Governing response
10-4Structural
Model
WindWaves (sea /swell)CurrentDirectionality!
Turret-moored floating system
Governing response
10-4
Governing response
10-4
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Shell’s method for response based analysis: LSMAvailable for:
• Fixed structures
• Ship-shaped structures, either turret- or spread-moored
• Semi-submersibles
• TLPs
• Pipelines
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Metocean Data
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
The easy bit for the metocean engineer!
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Ocean Environment Description
Requires a long-term database of:
Waves• wave height, period, and direction or• directional wave spectrum or• wind-sea and swell
Winds• speed and direction• wind spectrum• wind profile
Currents• current speed and direction• current profile
For 15 years of 3 hour intervals: 44070 records!
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Waves
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What is a sea state?
• Random superposition of regular waves
– Many amplitudes
– Many frequencies
– Many directions
• Statistical representation
– Hs: significant wave height
• Wind seas & swells
– Tp: peak period
– Direction
– … spectrum
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Frequency-direction wave spectrum: wave systems
Wind sea
Swell
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Wind
• Steady wind + wind gusts
• Statistical description of random nature of winds
– API wind spectrum
10-3
10-2
10-1
100
100
101
102
103
f (Hz)
S(f
,z)
(m2/s
)
API wind spectrum
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Currents
• Periods ranging from seconds to days
• Geostrophic currents and Ekman Transports
• Wind-induced currents
• Density-driven currents
• Tidal currents
• Deepwater currents
Courtesy of Dr. Cort Cooper - Chevron
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Offshore System Model
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
The easy bit for the floating structures
engineer
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Floater model• Hydrodynamics
• Hydrostatic coefficients• First order wave forces• Second order wave forces• Viscous damping (incl. roll)• Wind and current drag
• Wind and current loading • Relevant areas for wind and current coefficients• Wind and current coefficients / forces
• Mooring / Tendons and risers• Horizontal restoring force - deflection curve• Number of lines and orientation• Line length and orientation• Tendon and riser mass and stiffness
• Hull inertia
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Responses
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
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Floaters: Solving equations of motion
• Frequency domain – mostly linear
• Time domain – slow with sampling variability
• Probability domain – Spectral Response Surface method
• Equations of motion are transformed into probability domain
• Probabilities of response to a sea state (most probable maximum)
• Fast and includes non-linearity in forcing
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Spectral Response Surface Method
• Basic variables for surface elevation and wind gust:
– Stochastic variables characterised by a normal Gaussian distribution N(0,1)
– Normalized by standard deviation
– Normal random variables of unit-variance and mean zero + their Hilbert transforms (to include phase information)
• Response equations in terms of standard normal variables solved by a FORM–type (First Order Reliability Method) analysis
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All methods begin by treating the ocean surface as the sum of many frequency components. Then………..
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xi
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xi x lin diffraction
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xi x lin diffraction x dynamics
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xi x lin diffraction x dynamics
responsei
Sum over all components
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xj xk O(2) diff dynamics
xi x lin diffraction x dynamics
responsejk
Sum over all components
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Frequency domain xi ~ i
xj xk O(2) diff dynamics
xi x lin diffraction x dynamics
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Frequency domain xi ~ i
Time domain
xj xk O(2) diff dynamics
xi x lin diffraction x dynamics
-1
0
1
0 12 24
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Frequency domain xi ~ i
Time domain
Probability domain
xj xk O(2) diff dynamics
xi x lin diffraction x dynamics
-1
0
1
0 12 24
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Which responses are calculated?
FPSOs
• Offsets (any and specified direction)
• Accelerations
• Hull girder bending moment
• Heave, Roll, Pitch, Yaw
• Heave at turret, Heave at bow
• Green water elevation relative to bow
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Extremes
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
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Responses per Storm - Storm Generation
• Correlation between successive sea states
• Uncertainty in the extreme wave of a sea state
• Highest maximum wave in a storm not necessarily associated with peak significant wave height
• Assumption of independence of sea states avoided by using storms as independent events
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s (m
)
1
6
5
4
3
2
7
80% peak of storm
threshold 1 m
Definition of a storm
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Evaluation of extreme response statistics
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Short term variability
0 200 400 600 800 1000 1200-3
-2
-1
0
1
2
3Realisation # 1
Wave h
eig
ht
(m) Hmax = 4.7 m
0 200 400 600 800 1000 1200-3
-2
-1
0
1
2
3Realisation # 2
Wave h
eig
ht
(m) Hmax = 5.1 m
0 200 400 600 800 1000 1200-3
-2
-1
0
1
2
3Realisation # 3
Wave h
eig
ht
(m)
Time (s)
Hmax = 3.9 m
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Short term variability
0 1 2 3 4 5 6 7 8 9 100
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Wave height (m)
Non-e
xceedance p
robabili
ty
H
Hmax
Hmax - density
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Long term variability
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19990
2
4
6
8
Hs (
m)
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19992
4
6
8
10
12
Mean p
eriod (
s)
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Long term variability
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19990
2
4
6
8
Hs (
m)
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19992
4
6
8
10
12
Mean p
eriod (
s)
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Storm history
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19990
2
4
6
8
Hs (
m)
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19992
4
6
8
10
12
Mean p
eriod (
s)
15-Jul-1998 17-Jul-1998 19-Jul-19981.5
2
2.5
3
3.5
4
4.5
5
Hs (
m)
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0 5 10 150
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Wave height (m)N
on-e
xceedance p
robabili
ty
15-Jul-1998 17-Jul-1998 19-Jul-19981.5
2
2.5
3
3.5
4
4.5
5
Hs (
m)
HmaxMPs
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01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19990
2
4
6
8
Hs (
m)
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19992
4
6
8
10
12
Mean p
eriod (
s)
Hmps3Hmps4
Hmps10
…HmpsN
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Exceedance distribution of storm
0.01
0.1
1
0 2 4 6 8
Hmp
Q(H
mp
)
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0.01
0.1
1
0 2 4 6 8
Hmp
Q(H
mp
)
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0.01
0.1
1
0 2 4 6 8
HmpQ
(Hm
p)
Q(Hmps|n-years)Q(Hmps|100-years)
Hmps100
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Design Conditions
Metocean Environment
N-year values for responses
Operational Behaviour
Design Cases for N-year response
ResponsesExtreme
Value Analysis on Responses
Response Based Metocean Design
Conditions
Offshore System
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Floaters
• Not so straightforward for floating systems
• Manual process
• Good understanding required:
– Metocean environment
– Structure response
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Heave design condition
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Sensitivity study
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