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FPS RISER DESIGN

Dr Hugh Howells2H Offshore Inc

OMAE, FPS in Harsh Environments WorkshopNewfoundland Hotel, St Johns, July 1999

Development Targets

• West Africa 600-2500m• Gulf of Mexico 1500-2500m• Brazil 900-2000m• Voring Basin 800-1500m• West of Shetland 750-1500m• Others

FPS Riser Design Challenges

• Deep water• Weight, collapse

• Severe currents• VIV fatigue, suppression devices• large offsets

• Severe waves• large motions• fatigue damage

• Insulation• Needed to prevent hydrate formation• Increased drag

Riser Types

• Vertically Tensioned– Spar, TLP, DDF

• Flexible (up to 1000m, max 10 inch)– Simple and wave catenaries

• Rigid Catenary (400m+, up to 30 inches)– Steel, titanium– Simple, wave and bottom weighted

• Free Standing– Hybrid bundles

Flexible Riser Applications

Internal Diameter (inches)4 8 12 1 6 20 24 2 8 32

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200

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1600

Flexibles

REQUIREMENTS FOR FUTUREDEVELOPMENTS

FLEXIBLES

Export Trunk LinesExport Feeder LinesProduction LinesService Lines

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4 6 8 10 12 14 16 18 20 22 24 26 28 30

FPS Riser Applications

Flexible Risers

Hybrid Steel Catenary

Diameter Inches

Wat

er D

epth

(m)

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2

Rigid Catenary Risers

Simple Catenary

Plan Length 0.75-1.5 Depth

Mean Top Angle 10-25 degrees

TDP Touch Down Point

Flex Joint/Elbow and WeightAssembly

Vertical Section

Tether (3 Off)Horizontal Section

Pile Foundation

Lazy Wave Catenary

Simple SCR Features• Extension to Pipeline• Low Cost• Mild environments• Small vessel motions• Small vessel offsets• Large TDP motions• TDP trenching

800m 20 inch TLP SCR West of Shetlands

Vessel Motions

-20.0

-10.0

0.0

10.0

20.0

-20.0 -10.0 0.0 10.0

GoM WoS

TLP

SEMI

FPSO

KEY:

SURGE (m)20.0

TDP Buckling Wave Catenary Riser Features

• Buoyant Arch• Higher Cost• Large vessel motions• Small TDP motions• Harsh environments• Large vessel offsets• Complex installation

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3

1200m 10 inch WaveCatenary West of Shetlands

Bottom Weighted Riser

Flex Joint/Elbow and Weight

Assembly

Vertical Section

Tether (3 Off)Horizontal Section

Pile Foundation

Hybrid Risers

• Vertical Bundle of Steel Pipes• Syntactic and Air Can• Offset or Non-Offset• Flexible Jumpers• 350m-3000m Water Depth• FPSO, Barge, Semi

Offset and Non-Offset Hybrid Riser Arrangements

Hybrid Risers

• Vertical Bundle of Steel Pipes• Syntactic and Air Can• Offset or Non-Offset• Flexible Jumpers• 350m-3000m Water Depth• FPSO, Barge, Semi

Hybrid Riser Cross Section

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Hybrid Riser Construction Global Riser Analysis

• Highly Dynamic & Non-Linear Response• Time Domain FE Analysis

– Flexcom, Riflex, ABAQUS

• 3 Dimensional Structure and Loading• Numerous Load Cases• Highly Iterative• Analysis Intensive• Uncertainties in seabed interaction

Simple SCR Extreme Stresses28IN Simple Catenary Riser

Extreme Hurricane Static/Dynamic von Mises stresses

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0 500 1000 1500 2000 2500 3000 3500

Distance Along Riser From Vessel (m)

von

Mis

es/Y

ield

Mod 1 Far Ex Mod 2 Far Ex Mod 3 Far Ex

Mod 1 Far Static Mod 2 Far Static Mod 3 Far Static

Wave Catenary ExtremeStresses

28IN Buoyant Wave RiserExtreme Hurricane Dynamic and Static von Mises stresses

Optimised Configuration

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0.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0 4000.0 4500.0Distance Along Riser From Vessel (m)

von

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es/Y

ield

Near Extreme Far Extreme Near Static Zero Static Far Static

Simple SCR First Order Fatigue

28in Simple Catenary RiserFIRST ORDER FATIGUE LIFE

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Distance from Vessel (m)

Unf

acto

red

Fat

igue

Life

(ye

ars)

Dir 1 Dir 2 Dir 3 Dir 4 Dir 5 Dir 6 Dir 7 Dir 8 Min Life

Wave Catenary Fatigue28in Buoyant Wave Catenary Riser

FIRST ORDER FATIGUE LIFE

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10

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100000

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Distance from Vessel (m)

Unf

acto

red

Fatig

ue L

ife (

year

s)

Dir 1 Dir 2 Dir 3 Dir 4 Dir 5 Dir 6 Dir 7 Dir 8

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ACDSee GIF Image

Vortex Induced Vibration

• High frequency stress reversals• High fatigue damage rates• Analysis using SHEAR7 and VIVA• Many uncertainties in analysis methods

Catenary Riser VIV Fatigue28in Simple Catenary Riser

VIV DAMAGE - TRANSVERSE CURRENTS

1E-21

1E-19

1E-17

1E-15

1E-13

1E-11

1E-09

1E-07

1E-05

0.001

0.1

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Location Along Riser from TDP (x/L)

Fat

igue

Dam

age

(Per

Yea

r)

Bin 1 Bin 3 Bin 6 Bin 9 Bin 12 Bin 15

Bin 18 Bin 21 Bin 25 Total

VIV Suppression

• Strakes– Higher drag loading– Higher stresses

– Higher pipe wall thickness– Complex installation– Higher cost

• Fairings– better response, more expensive

• How much suppression is needed?

VIV Analysis Uncertainties

• Changes in incidence angle along length• Changes in structural properties• Changes in diameter• Non-monotonic current distribution• Current direction variation with depth• Seabed interaction• Strake design and effectiveness• Vessel motions and riser tension variation• Wave loading

Installation Issues

• Weather windows - increased time, current• Tension - limitation for large dia. lines• Tow-out - fatigue damage• VIV suppression - effect on method• Reeled pipe - residual stresses

Reeled Installation

• SCR Installation by Reel Vessel• High Levels of Plastic Deformation (2%)• Effect on Fatigue Performance ??

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Thermal Insulation

• Wax and hydrate prevention• External coatings• Pipe in Pipe - feasible as SCR?• Heating (electrical or water circulation)• Low weight in water• Increased drag diameter• Degraded response and worse VIV• Increased cost

Riser Design Developments

• STRIDE JIP– Steel Risers in Deepwater Environments

• VIV response - tank and open water tests• Riser-seabed interaction, testing and

analysis• Materials - fatigue of girth welds• Installation - fatigue performance of

reeled pipe

Conclusions (1/2)

• FPS Riser Technology Developing- NotMature

• Wide Range of SCR Applications– 500m-3000m– 4 - 30” diameter– Mild - Harsh environments– TLP - FPSO

• Low Cost Potential

Conclusions (2/2)

• VIV understanding developing• Reeled installation may be feasible• Thermal insulation very important• TDP and effects of trenching ??• Risers a key FPS technology

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