Response-based Metocean Criteria for OptimisingDesign and Operation of FPSOs

Shell Exploration & Production

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Response-based Metocean Criteria for Optimising Design and Operation of FPSOs

Hermione van Zutphen

Marios Christou

Kevin Ewans


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

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


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


Shell’s method for response based analysis: LSMAvailable for:

• Fixed structures

• Ship-shaped structures, either turret- or spread-moored

• Semi-submersibles

• TLPs

• Pipelines


Metocean Data





ResponsesExtreme



Conditions

Offshore System

The easy bit for the metocean engineer!


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

Wind sea

Swell


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


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


Offshore System Model





ResponsesExtreme



Conditions

Offshore System

The easy bit for the floating structures

engineer


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





ResponsesExtreme



Conditions

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





Time domain



-1

0

1

0 12 24



Time domain

Probability domain



-1

0

1

0 12 24


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





ResponsesExtreme



Conditions

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

Shell Exploration & ProductionH

s (m

)

1

6

5

4

3

2

7

80% peak of storm

threshold 1 m

Definition of a storm


Evaluation of extreme response statistics


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


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


Long term variability

01-Jul-1998 01-Sep-1998 01-Nov-1998 01-Jan-1999 01-Mar-19990

2

4

6

8

Hs (

m)


4

6

8

10

12

Mean p

eriod (

s)


Long term variability


2

4

6

8

Hs (

m)


4

6

8

10

12

Mean p

eriod (

s)


Storm history


2

4

6

8

Hs (

m)


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)


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



2

4

6

8

Hs (

m)


4

6

8

10

12

Mean p

eriod (

s)

Hmps3Hmps4

Hmps10

…HmpsN


Exceedance distribution of storm

0.01

0.1

1

0 2 4 6 8

Hmp

Q(H

mp

)


0.01

0.1

1

0 2 4 6 8

Hmp

Q(H

mp

)


0.01

0.1

1

0 2 4 6 8

HmpQ

(Hm

p)

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

Hmps100


Design Conditions





ResponsesExtreme



Conditions

Offshore System


Floaters

• Not so straightforward for floating systems

• Manual process

• Good understanding required:

– Metocean environment

– Structure response



Heave design condition


Sensitivity study

Date post:	21-May-2015
Category:	Business
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Response-based Metocean Criteria for OptimisingDesign and Operation of FPSOs

Business