Pipeline Walk

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Managing Unidirectional Movements (Walk) of HPHT Submarine Flowlines During Startup HPHT Submarine Flowlines During Startup Heating and Shutdown Cooling IOPF2010-1003 Presenter/Author; Gautam Chaudhury Company; INTECSEA (Worley Parsons Group) IOPF2010-1003-Chaudhury IOPF 2010 1

Transcript of Pipeline Walk

Page 1: Pipeline Walk

Managing Unidirectional Movements (Walk) of HPHT Submarine Flowlines During StartupHPHT Submarine Flowlines During Startup

Heating and Shutdown Cooling IOPF2010-1003

• Presenter/Author; Gautam Chaudhury• Company; INTECSEA (Worley Parsons Group)

IOPF2010-1003-Chaudhury

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Overview of PresentationBackground of Pipeline WalkBackground of Pipeline Walk

- Expansion and contraction of pipelines resting on seabed- Examine parameters governing pipeline walkingp g g p p g- Sources responsible for walking and consequences- Explain mechanism of walk by ratcheting

Determine magnitude of walk and Manage- Derive magnitude of walk from unbalanced forces

D i it d f lk f th l di t h ti- Derive magnitude of walk from thermal gradient heating- Provide procedure for managing walk efficiently

Verification and ConclusionsVerification and Conclusions- Verification by FEA- Major conclusions and use

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Expansion and Contraction of Pipeline on Seabed

• Pipelines expand and contract due to changes in pressure and temperature subject to resistance from soiltemperature subject to resistance from soil

• For short pipelines, each operation forms a virtual anchor (VA) near or at the middle and the pipe ends move in and outp p

• Under symmetric condition net movement is zero• Under asymmetric conditions;

-The VA location is different during heating and cooling-This results in unidirectional end displacements (Walking)

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Parameters governing pipeline walkingParameters governing pipeline walking

• Virtual Anchor; An apparent fixity point (At proximity to middle); pp y p ( p y )• Full cyclic constraint; No displacement for a portion of length in

the middle• Fully mobilized (Short); Axial displacements occur over the full

length. Mathematically the condition is• Where is defined as the driving force as

LFfr /∆<F∆• Where, is defined as the driving force asF∆

tAEAiPePiF ∆+−−=∆ ...).21.().( αν

Fully Constrained

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Fully Mobilized

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Sources responsible for walking and consequences

• A unique combination of asymmetric load, soil frictional q y ,resistance, and temperature or pressure

• Asymmetric load originates from unbalanced end tension, b d l d th l di t ( hi h i l t)seabed slope, and thermal gradient (which is always present)

• Therefore, for short pipelines there is potential for walking• Magnitude of walk/cycle is small but accumulation may be high• Magnitude of walk/cycle is small but accumulation may be high• Overstressing of end connectors. (High risk)• Loss of SCR tension. (Low risk)( )• Increased load in a lateral buckle (Moderate risk)• Route curve pull out (Moderate risk)

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Mechanism of walk by ratcheting

A t f hift i t l h ff t• Asymmetry from any source shifts virtual anchor off center• This is because pipe expansion and contraction is non uniform• Unbalanced force is generated between virtual anchors• Unbalanced force is generated between virtual anchors• Schematic diagram shows movement of pipe between anchors

in the same direction.

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Magnitude of walk from unbalanced forces(Assumed Uniform Heating)

S ti f i t l h (VA) d lk• Separation of virtual anchors (VA) produces walk• Magnitude of walk = Driving strain between VA minus total

resistance in strain of the pipeline. ].[ frLVW ∆resistance in strain of the pipeline. ].

.[AEfVsWc −∆= ε

).2/().(1 frTefrLl +=))..(.(

ASinSwfrLVsWc θε −

−∆=).2/().(2 frTefrLl −=

).

(AE

frTeVs /=

frSinLSwVs /θ frSinLSwVs /.. θ=

µ.Swfr =

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Magnitude of walk from thermal gradient heatingSome basic Observations

• The heating process is asymmetric (Transient) but end result is as if symmetric (Final hot mid point). This is because≈VA

- Final along the pipe length is small- Soil resistance forms VA (not proportional to pipe movement)

• Cold VA is always at the middle (Cooling is Symmetric)

t.∆

• Cold VA is always at the middle (Cooling is Symmetric)• Direction of walk is always from hot VA to cold VA

90

50

60

70

80

90

ure

(Deg

C)

0

10

20

30

40

Tem

pera

tu

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0 100 200 300 400 500 600 700 800 900

Element From Hot End

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Magnitude of walk from thermal gradient heatingSet Up EquationSet Up Equation

• Consider the pipeline as several segments of small lengths ‘dl’• Heating occurs from one segment to the next (hot end to cold)• At each step VA is at the middle of the respective total length• This leads to an equivalent VA at the hot side (walk = > cold)• The equation for net magnitude of walk can be expressed as

600

800dlfrlLlfAE

WcL

].2

.[1 2/

∫−

−= θ

0

200

400

600Fo

rce

(KN)

AE 2. 0

-800

-600

-400

-2000 100 200 300 400 500 600 700 800 900

Axi

al F

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800

Node Number

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Magnitude of walk from thermal gradient heatingSolve Equation

• Integrating and Using boundary condition Wc=0 when fr=0

)5.1(8

2

frfffr

AELWc −= θ

θ• The derivative of the equation is• Equating this to zero maximum walk is when

..8 fAE θ).31(

θffr

dfrdWc

−=

3/θffr =q g• ABAQUS analysis with finite mobilization distance• Applying ABAQUS based boundary condition

8/.3 θffr =

2

• Following two conditions were also reported by M. Carr et all

)34(

..8

2

frfffr

AELWc −= θ

θFollowing two conditions were also reported by M. Carr et all

• For walk to occur• Magnitude of walk is maximum when

frf .5.1>θ8/.3 θffr=

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Managing walk efficiently• The aim is to arrest or reduce walk cost effectivelyThe aim is to arrest or reduce walk cost effectively• Past work suggested correction force Fc = = L.fr• Fc is independent of magnitude of walk. Leads to a situation

LSw..µ

p gwhere Fc is highest when walking magnitude is lowest

• Walk means mitigation force required is smallTh t h i

]...[AEfrLVsWc −∆= ε

• There are two choices;Increase fr to make the pipeline fully constrainedCorrect Vs by end restraints (Most efficient) active or passiveCorrect Vs by end restraints (Most efficient) active or passive

• Thermal gradient case (Active Fc);WcVseq

AELfr

Vseqavg ..2

.−

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2/.. LQcavg αε = frVseqFc .=

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Verification by FEA (ABAQUS)Pipe/Soil ModelPipe/Soil Model

- Fully mobilized pipe (4Km long, EA=3.94e6KN, Sw=1KN/m)- Soil friction linearly mobilized at 0.01m then kept constant- Soil friction coefficient = variable (0.1 to 1)

Cases examinedCases examined- 100KN end tension uniform heating - Seabed slope 1.4325 Deg (Eqv to 100KN)

Ct 080=∆Ct 080=∆

- Thermal gradient of 33.6 C/Km ( = 1.551KN/m)

• Mitigation forces were tested by employing end springs and

θf

• Mitigation forces were tested by employing end springs and they were found to match well with the predictions

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Verification by FEA (ABAQUS)

0 4

0.5

0.6

0.7

0.8

0.9

(m/C

ycle

)

Prediction Eqn=9ABAQUS FEA

0 4

0.5

0.6

0.7

0.8

0.9

(m/C

ycle

)

Prediction Eqn=10ABAQUS FEA

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1

Wal

k (

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1

Wal

k (

Case 1 Case 2Soil Friction Force (KN/m) Soil Friction Force (KN/m)

0.14

0.16

0.06

0.08

0.1

0.12

Wal

k (m

/Cyc

le)

Prediction Eqn=17

ABAQUS FEA

0

0.02

0.04

0 0.2 0.4 0.6 0.8 1

Soil Friction Force (KN/m)

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Case 3

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Major conclusions and use

• Short HPHT pipelines have high potential for walk• Accumulation of walk over the field life may pose risk

• Proposed tools provide accurate walk and mitigation forces• Managing walk by end restraint method is most cost effective• Managing walk by end restraint method is most cost effective

• True physical model is more complex and case dependent p y p p• Primary uncertainties are friction coefficients, mob-distance,

and gradient heating, specially for theoretical predictions• Analytical tools are given for preliminary screening,

understanding the parametric influences, and planning for final FEA design check.

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Thank You for Your AttentionThank You for Your Attention

• Future Contact:Future Contact:• Gautam Chaudhury

T l 281 925 2443• Tel; 281 925 [email protected]• INTECSEA/Worley Parsons Group

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