Shell Exploration & Production: Response-Based Metocean Criteria for Optimising Design and Operation of FPSOs

Shell Exploration & Production

Response-based Metocean Criteria for
Optimising Design and Operation of FPSOs
Copyright: Shell Exploration & Production Ltd.

Hermione van Zutphen
Marios Christou
File Title

Kevin Ewans
9/7/2009


Outline

• Metocean design conditions & LSM
• Description of the metocean environment
• Response analysis
• Extremes, short term and long term variability


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


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


Response Based Analysis

Metocean Environment
Extreme Response Based
Responses Value Metocean Design
Analysis on Conditions
Responses
Offshore System

Operational N-year values for Design Cases for
Behaviour responses N-year response


Environment “perspective” Response “perspective”

Wind
Waves Structural
Current Model 10-4
Directionality Governing response

Wind
Waves 10-4
Current
Directionality Governing response
Simple Jacket

Wind
Waves (sea /swell)
Current 10-4
Directionality! Governing response
Turret-moored
floating system


Shell’s method for response based
analysis: LSM
Available for:
• Fixed structures
• Ship-shaped structures, either turret- or spread-moored
• Semi-submersibles
• TLPs

• Pipelines


Metocean Data The easy bit for the
metocean engineer!

Responses
Offshore System



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!


Waves


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


Frequency-direction wave spectrum:
wave systems

Swell

Wind sea


Wind

• Steady wind + wind gusts
• Statistical description of random nature of winds
– API wind spectrum

3
API wind spectrum
10

2
10
S(f,z) (m2/s)

1
10

0
10
-3 -2 -1 0
10 10 10 10
f (Hz)


Currents

• Periods ranging from seconds to days

Courtesy of Dr. Cort Cooper - Chevron

• Geostrophic currents and Ekman Transports
• Wind-induced currents
• Density-driven currents
• Tidal currents
• Deepwater currents


The easy bit for the
Offshore System Modelstructures
floating
engineer

Responses
Offshore System



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


Responses

Responses
Offshore System



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


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


All methods begin by treating
the ocean surface as the sum of
many frequency components.
Then………..


xi


xi x lin diffraction


xi x lin diffraction x dynamics



responsei

Sum over all components



xj  xk  O(2) diff  dynamics

responsejk
Sum over all components




Frequency domain xi ~ i





1

Time domain
0

0 12 24

-
1





1

Time domain
0

0 12 24

-
1

Probability domain


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


Extremes

Responses
Offshore System



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


7
Definition of a storm

6

80% peak of storm
5
Hs (m)

4

3

2

threshold 1 m
1


Evaluation of extreme response statistics


Short term variability
Realisation # 1
3
Hmax = 4.7 m
Wave height (m)

2
1
0
-1
-2
-3
0 200 400 600 800 1000 1200

Realisation # 2
3
Hmax = 5.1 m
Wave height (m)

2
1
0
-1
-2
-3
0 200 400 600 800 1000 1200
Realisation # 3
3
Hmax = 3.9 m
Wave height (m)

2
1
0
-1
-2
-3
0 200 400 600 800 1000 1200
Time (s)


Short term variability
1
H
0.9 Hmax
Hmax - density

0.8

0.7
Non-exceedance probability

0.6

0.5

0.4

0.3

0.2

0.1

0
0 1 2 3 4 5 6 7 8 9 10
Wave height (m)


Long term variability
8

6
Hs (m)

4

2

0
01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-1999

12

10
Mean period (s)

8

6

4

2


Storm history
5
8

4.5
6
Hs (m)

4
4

3.5
2

Hs (m)
0 3

2.5
12

10 2
Mean period (s)

8
1.5
15-Jul-1998 17-Jul-1998 19-Jul-1998
6

4

2


1

0.9

0.8

Non-exceedance probability
0.7

0.6

0.5
5
0.4

4.5 0.3

0.2
4 HmaxMPs
0.1
3.5 0
Hs (m)

0 5 10 15
Wave height (m)
3

2.5

2

1.5
15-Jul-1998 17-Jul-1998 19-Jul-1998


Hmps10
8 HmpsN

6
Hmps 3
Hmps4
…
Hs (m)

4

2

0

12

10
Mean period (s)

8

6

4

2


Exceedance distribution of storm
Hmp
1
0 2 4 6 8
Q(Hmp)

0.1

0.01


Hmp
1
0 2 4 6 8
Q(Hmp)

0.1

0.01


Hmp Hmps100
1
0 2 4 6 8
Q(Hmp)

0.1

Q(Hmps|n-years)
Q(Hmps|100-years)
0.01


Design Conditions

Responses
Offshore System



Floaters

• Not so straightforward for floating systems
• Manual process
• Good understanding required:
– Metocean environment
– Structure response


Heave design condition


Sensitivity study

Shell Exploration & Production: Response-Based Metocean Criteria for Optimising Design and Operation of FPSOs

Shell Exploration & Production: Response-Based Metocean Criteria for Optimising Design and Operation of FPSOs

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