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Managing Unidirectional Movements (Walk) of 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

Overview of PresentationBackground of Pipeline Walk - Expansion and contraction of pipelines resting on seabed - Examine parameters g p governing p p g pipeline walking 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 magnitude of walk f Derive it d f lk from th thermal gradient h ti l di t heating - Provide procedure for managing walk efficiently Verification and Conclusions - Verification by FEA j - Major conclusions and use2

IOPF 2010

Expansion and Contraction of Pipeline on Seabed Pipelines expand and contract due to changes in pressure and temperature 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 out pp 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)


IOPF 2010

Parameters governing pipeline walking Virtual Anchor; An apparent fixity p ; pp y point ( p (At proximity to middle) 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 fr < F / L Where F is defined as the driving force as Where,

F = (Pi Pe).Ai.(1 2. ) + E.A..tFully Constrained Fully Mobilized


IOPF 2010

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, seabed slope, and th b d l d thermal gradient ( hi h i always present) l di t (which is l t) Therefore, for short pipelines there is potential for walking 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)5

IOPF 2010

Mechanism of walk by ratcheting Asymmetry from any source shifts virtual anchor off center A t f hift i t l h ff t This is because pipe expansion and contraction is non uniform Unbalanced force is generated between virtual anchors Schematic diagram shows movement of pipe between anchors in the same direction.


IOPF 2010

Magnitude of walk from unbalanced forces (Assumed Uniform Heating) S Separation of virtual anchors (VA) produces walk ti f it l h d lk Magnitude of walk = Driving strain between VA minus total L. fr f resistance in strain of the pipeline. Wc Vs [ W = V .[ ]E.A

l1 = ( L . fr + Te ) /( 2 . fr )l 2 = ( L . fr Te ) /( 2 . fr )

L.( fr Sw.Sin ) Wc = Vs.( ( ) E.A

Vs = Te / fr Vs = Sw.L.Sin / frfr = Sw .7

IOPF 2010

Magnitude of walk from thermal gradient heatingSome basic Observations The heating process is asymmetric (Transient) but end result is as if symmetric (Final hot VA mid point). This is because - Final .t 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) Direction of walk is always from hot VA to cold VA90 80 Tem peratu (Deg C) ure 70 60 50 40 30 20 10 0 0 100 200 300 400 500 600 700 800 900 Element From Hot End


IOPF 2010

Magnitude of walk from thermal gradient heating Set 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 asL/2

1 Wc = E.A


Ll [ f .l fr ]. dl 2Axial Force (KN) F

800 600 400 200 0 -200 -400 -600 -800 800 Node Number 0 100 200 300 400 500 600 700 800 900


IOPF 2010

Magnitude of walk from thermal gradient heatingSolve Equation Integrating and Using boundary condition Wc=0 when fr=0 L2 fr Wc = ( f 1 .5 fr ) 8 .E . A f 3 . fr dWc ) = (1 dfr f The derivative of the equation is Equating this to zero maximum walk is when fr = f / 3 q g ABAQUS analysis with finite mobilization distance fr = 3. f / 8 Applying ABAQUS based boundary condition 4 L2 fr Wc = ( f fr ) 8.E. A f 3 Following two conditions were also reported by M. Carr et all For walk to occur f > 1.5. fr Magnitude of walk is maximum when fr = 3. f / 810

IOPF 2010

Managing walk efficiently The aim is to arrest or reduce walk cost effectively Past work suggested correction force Fc = .Sw.L = L.fr Fc is independent of magnitude of walk. Leads to a situation p g where Fc is highest when walking magnitude is lowest L. fr Walk Wc=Vs.[ ] means mitigation force required is small E.A Th There are t two choices; h i Increase fr to make the pipeline fully constrained Correct Vs by end restraints (Most efficient) active or passive Thermal gradient case (Active Fc);Vseq = Wc


fr . L 2 .E . A

avg = .Qc.L / 211

Fc = Vseq. frIOPF 2010

Verification by FEA (ABAQUS)Pipe/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 examined - 100KN end tension uniform heating t = 80 0 C - Seabed slope 1.4325 Deg (Eqv to 100KN) t = 80 0 C - Thermal gradient of 33.6 C/Km ( f = 1.551KN/m) Mitigation forces were tested by employing end springs and they were found to match well with the predictions


IOPF 2010

Verification by FEA (ABAQUS)0.9 0.8 0.7 Walk (m/Cycle) ( 0.6 0.5 0.4 04 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 Soil Friction Force (KN/m) Prediction Eqn=9 ABAQUS FEA Walk (m/Cycle) ( 0.9 0.8 0.7 0.6 0.5 0.4 04 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 Soil Friction Force (KN/m) Prediction Eqn=10 ABAQUS FEA

Case 1Walk (m/Cycle)

0.16 0.14 0.12 0.1

Case 2Prediction Eqn=17


ABAQUS FEA0.06 0.04 0.02 0 0 0.2 0.4 0.6 0.8 1 Soil Friction Force (KN/m )

Case 313

IOPF 2010

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 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 g FEA design check.14

IOPF 2010

Thank You for Your Attention Future Contact: Gautam Chaudhury T l 281 925 2443 Tel; gautam.chaudhury@intecsea.com INTECSEA/Worley Parsons Group


IOPF 2010