vedeld introduction.pdf

7/30/2019 Vedeld Introduction.pdf

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Introduction to free spans


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Slide 2

Content

Why do we have free spans?

Do free spans represent a problem?

Example of failures Basis for DNV RP-F105


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Slide 3

Why free spans?


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Uneven seabed

Reinertsen Engineering


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Slide 5

Seabed scouring


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Slide 6

Other reasons

Temporary spans at pipelinecrossings

Tie-in to subsea structures, platforms

etc. Other eroding processes than

scouring (e.g. sand waves)

Pock marks


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Slide 7

Local imperfection


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Slide 8

Free span terminology

Span shoulder

Gap / Span height

Span length


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Slide 9

Are free spans a problem?


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VIV - a real problem?

A few cases


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Cook Inlet Area


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Subsea Pipelines in Cook Inlet

Aging pipelines (installed late 60ties)

Significant corrosion in some oil lines

Strong tidal current Seabed conditions are very dynamic

Scouring produces span gaps of less than a foot

14 failures due to VIV during 1965-1976!

Annual inspection for free spans (side scan sonar) Spans longer than 50 and 1 gap intervened to avoid VIV damage/failure


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Slide 14

Fracture surface


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Slide 15

Free spans must be taken seriously

Non-stationary (developing) spans represent a particular challenge


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Slide 16

Costs related to failure

Mobilisation for repair several million US$

Offshore time 0.5-1 million US$/day

Loss of income example Langeled gas export 15-20 million US$/day

Environmental costs/aspects

Loss of reputation

Less regularity gas market

Cost may be huge!


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Problem areas


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Free Span Assessment - Multidiscipline

Environmental conditions

- Flow conditions from combined wave and current

- Local topography

Loading Mechanism- Vortex Induced Vibration (in-line & cross-flow)

- Direct wave loads & Proximity Effects

Structural Response- Soil-pipe interaction

- Non-linearities (geometrical, static/dynamic properties)

Acceptance criteria- SN-approach (weld, defects, )


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Slide 20

Basis for DNV-RP-F105

VIV Models based on experience fromR&D projects & pipeline design:

MULTISPAN Project (1994-1996)- Response Model for In-line VIV

- On-set criteria for cross-flow

- Reliability based calibration

GUDESP PROJ ECT (1989-1994)- Cross-flow Response model

- Effect of Waves

Research projects- SVS full scale test

- MASPUS lab test

DHI/Statoil study Ormen Lange

- VIV testing multi-span & multi-mode

- Soil damping study

Allows for state-of-the-art fatigueanalyses

Links in-line VIV and wave loads

Allows cross-flow vibrations Safety philosophy in compliance

with DNV-OS-F101

Introduces consistent link between

analysis models and safety factor(s)

Applied in numerous projects in

- North Sea

- Persian Gulf

- South East Asia

- GOM

- WA


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Uniqueness ref. API RP 1111 (1999)


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Calculation Tool

Free span assessment complex

Require detailed knowledge in several disciplines:

- hydrodynamics, VIV and load models

- environmental conditions, long-term statistics- fatigue calculations

- structural response incl. geotechnical aspects

DNV-RP-F105 still complex (and difficult?) to use

Need for a calculation tool to:

- make it easier to apply the RP

- enable a cost-efficient span assessment


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Slide 23

FatFree Main SheetFatFree Main Sheet

12.06.2006

Vers. 10.0

DNV version Expiry date: 31.12.2007 Release Note

Project: Date: 12.06.2006 Calculations by

No Wave Case References: verification of version Verified by

h [m] 300 fo(in-line) 0,773 struc 0,000 m1 3

L [m] 40 fo(cr-flow) 0,798 soil (in-line) 0,000 m2 3 1,00

e [m] 2,69 Ain (in-line) 446 soil (cr-flow) 0,000 Log(C1) 11,222 k 1,00

d [m] 0 Acr(cr-flow) 461 h,RM 0,000 Log(C2) 11,222 f,IL(inline) 1,00

pipe 0,0 max 940 KS(in-line) 0,00 logNsw 8,00 f,CF(cr-flow) 1,00

D[m] 0,612 /D 0,24 KS(cr-flow) 0,00 S0 [MPa] 0,00 S 1,00

L/D 65 Seff/PE -0,23 KV 2,105E+07 SCF 1,00 on,IL 1,10

KL 1,592E+07 on,CF 1,00

KV,S 5,300E+05 R 1,00

kc 0,33 Heff [N] 2,00E+05 Ds 0,5000 0,30 steel 7850

fcn (MPa) 45 p [bar] 105 tsteel 0,0132 [oC

-1] 1,17E-05 concrete 2240

T [

o

C] 0 tconcrete 0,0500 E [N/m

2

] 2,07E+11 coating 1300tcoating 0,0060 CD(current) 1,00 cont 153

RESULTS

In-line (Response Model) 1,09E+03 yrs

Cross-Flow 1,00E+06 yrs Peak Von Mises Peak Von Mises

x(1 year) 0,0 158,2 x(1 year) 7,2 135,2In-line (Force Model) - yrs x(10 year) 0,0 158,2 x(10 year) 16,7 141,4In-line (Combined) - yrs x(100 year) 0,0 158,2 x(100 year) 26,1 148,6

Current Modelling

Code

Directionality

Current

Current Sheet Name

Calculation options

Return Period Values

FATFREE IS READY

FATIGUE ANALYSIS OF FREE SPANNING PIPELINESMuthu Chezhian ([email protected])

Olav Fyrileiv ([email protected])

Programmed by DNV Deep Water Technology

Kim Mrk ([email protected] )FATFREE

Safety FactorsSoil PropertiesResponse Data SN-Curves

Densities [kg/m3]

DYNAMIC STRESS [MPa]

Wave Modelling

FATIGUE LIFE

STRUCTURAL MODELLING

Constants

InlineCross-flow

Pipe Dimensions [m]Coating data

Free Span Scenario

Wave Sheet Name

Wave-template

Functional Loads

Flat sea-bed RP-F105 Span User DefinedF1 (free corrosion)

CALCULATE

UPDATE SHEET

PRINT RESULTSSPAN RUNS

USER HELP

OPTIONS

No Wave

Discrete - C dir.

Uc Histogram

RP-F105

Automatic Generated

Damage distribution vs direction

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 20 40 60 80 100

RM (In-Line)FM (In-Line)Cross-FlowComb.(In-Line)

pdf for omnidirectional current

0,0

1,0

2,0

3,0

4,0

5,0

0,0 0,2 0,4 0,6 0,8 1,0

RM(cross-flow)*4

RM(inline)*10

velocity

User DefinedSingle-mode

Well defined


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Slide 24

Summary

Free spans due to:- Uneven seabed

- Scouring

-Other seabed erosion phenomena

- Tie-in / crossings

Free spans may cause failure

Large costs associated with failures

Free spans require multi-discipline assessment

Basis for DNV RP-F105


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Slide 25
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vedeld introduction.pdf

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