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Introduction to free spans
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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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Why free spans?
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Uneven seabed
Reinertsen Engineering
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Seabed scouring
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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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Local imperfection
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Free span terminology
Span shoulder
Gap / Span height
Span length
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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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Fracture surface
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Free spans must be taken seriously
Non-stationary (developing) spans represent a particular challenge
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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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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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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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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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