Ellllectrically Heated Flllowlines – ANSYS FLUENT … Heated Flllowlines – ANSYS FLUENT –Maxwell Coupled Solutions Ryan Magargle, Ph.D. DuraiDakshinamoorthy Ph D

l ll d l lElectrically Heated Flowlines – ANSYS FLUENT – Maxwell Coupled Solutions

Ryan Magargle, Ph.D.Durai Dakshinamoorthy Ph D

© 2011 ANSYS, Inc. September 12, 2011

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Durai Dakshinamoorthy, Ph.D.

Ansys, Inc.

Outline

Electrically Heated Flowlines: Overview

Design Challenges

Need for Simulation: ANSYS Approachpp Electromagnetic Simulation

Computational Fluid Dynamic Simulation

Results


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Prevent hydrate and wax formation Deepwater: Oil and gas exploration

Long tiebacks and operation shut down

Reliable alternative to chemical injection: Reliable alternative to chemical injection: environmentally friendly


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Hydrate Plug (courtesy: open literature)

Wax Deposition (http://pubs.acs.org/cen/coverstory/87/8738cover.html)

Design Challenges: Criteria

Prevent hydrate plug or wax deposition by i t i i th fl li t d imaintaining the flow lines at a design

temperature Deepwater production: May require Deepwater production: May require

continuous heating – Long lines

Maintain fluid temperature above design temperature – Short length

Raise the temperature of the fluid from ambient to above design temperature withinambient to above design temperature within specified time period


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Electrical heating options (3 examples) Pipe in Pipe: Closed current loop into one pipe

return on the other

Open Loop (DEH): Current loop into cable return in Open Loop (DEH): Current loop into cable, return in water and pipe

Induction: Three phase cable has no pipe return, induced currents only

Pipe in Pipe Open Loop (DEH) Induction


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Direct Indirect


Heating efficiency varies with each design Examples compared for 85W/m pipe heating

Efficiency measures ratio of useful loss in pipe to total losstotal loss

1200A flowline current49% Efficient



Pipe in Pipe Open Loop (DEH) Induction


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Pipe in Pipe Open Loop (DEH)

Efficiency

Induction

Design Challenges: Rating

Knowledge of supply current and Power Loss (heat generation) ‐

Electromagnetic field simulation

Knowledge of operating conditions and Knowledge of operating conditions and Thermal Loss – Computational fluid dynamics

simulation

Efficiently Design Coupled Approach is NeededEfficiently Design – Coupled Approach is Needed


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ANSYS Approach: Coupled Simulation

ANSYS offers solutions for both electromagnetic field simulations and CFD simulations.


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Electromagnetic Field Simulation

Simulation Goals

Examine Field Distribution

Determine Necessary Supply Current

Parameterize Loss vs Geometry

Compare Design Efficiencies

Determine Circuit Lumped Parameters


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Electromagnetics Field Simulation

Example: Field and loss distributions for l DEHopen loop DEH

85 W/m heating target

Copper Cable Core

Cable Dielectric

Cable SheathPipe insulation

Steel Pipe

Cable Sheath

Pipe Contents

Seawater 10” dia. pipe


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Field simulation input supply Unit current is applied to cable.

Equal and opposite units of current are applied to the pipe and water for the returnthe pipe and water for the return.

2D simulation of cross section is performed which does not include end effects at anodes.

cable

pipepipe


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water


Field simulation indicates how much current is needed to achieve desired power densityneeded to achieve desired power density.

Simulation determines 59 W/m/A2 in pipe

1200A rms is required for 85 W/m 1200A rms is required for 85 W/m

Current DensityMagnetic Flux LinesLoss in Pipe Current DensityMagnetic Flux Lines

62.50

75.00

87.50Loss vs. Current ANSOFT

Loss in Pipe

12.50

25.00

37.50

50.00

Pipe

Los

s


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0.00 500.00 1000.00Input Current [A]

0.00


Determine cable position effect on Loss

I d di t d i i i fl d it Increased distance decreases impinging flux density, which decreases AC losses.

Design cable position for performance, or characterize to Design cable position for performance, or characterize to understand non‐ideal placement effects.

100 00Pipe Loss vs Cable Position ANSOFT

Curve Info

85 00

90.00

95.00

100.00

[W/m

]

Curve InfoPipe Loss per Meter

Setup1 : LastAdaptiveFreq='60Hz' Phase='0deg'

70 00

75.00

80.00

85.00P

ipe

Loss


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8.00 10.00 12.00 14.00 16.00Cable Position [in]

65.00

70.00

Power Electronics Circuit

Simulation Goals

Determine necessary supply voltage

Size and rate passive electrical components

Build multiphase impedance balancing network

Improve supply power factor


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Determine necessary supply voltageC l l t t ’ l t i l i d Calculate system’s electrical impedance

Inductance, Resistance, and Capacitance are measured per unit lengthp g

cable Reduced Loop Representation

pipeR1 L1

C1

L2R2

0

water

0

How much voltage is needed to


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achieve necessary current?


Configure supply voltage connectionB l h l d th h Balance one‐phase load across three‐phase source

Correct load power factor from 0.3 closer to 0.9

P S T f d l k*

~

3PHASA * sin (2 * pi * f * t + PHI + phi_u)

PHI = 0° TWT

Power Source Transformer Load Balance Network*

0

R1 L1

C1

L2R2R3

R4

R5

~

~

PHI = -120°

PHI = -240°

TWT

TWTC2

L3

C3

0

0 R5 L3

Power Factor Correction DEH representation


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*Nysveen, A., et.al. ‘Direct Electrical Heating of Subsea Pipeslines – Technology Dwevelopment and Operating Experience’, IEEE Trans. Ind. App., Vol. 43, Jan. 2007


Balance the load to distribute current among phasesphases

R1 L1 L2R2R3~


PHI = 0°

PHI = -120°

TWT

TWT

0

0

C1R4

R5

~

~ PHI = -240° TWTC2

L3

C3

Three Phase Balanced LoadNo Load Balancing

500.00

1000.00

1500.00Three Phase Balanced Load ANSOFT

Curve InfoR3.I

TRR4.I

TRR5.I

TR

500.00

1000.00

1500.00No Load Balancing ANSOFT

Curve InfoR3.I

TRR4.I

TRR5.I

TR

-1000 00

-500.00

0.00

Y1 [A

]

-1000 00

-500.00

0.00

Y1 [A

]


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0.00 100.00 200.00 300.00 400.00Time [ms]

-1500.00

1000.00

0.00 100.00 200.00 300.00 400.00Time [ms]

-1500.00

1000.00


Add compensation capacitor to increase power factorfactor

R1 L1 L2R2R3~


PHI = 0°

PHI = -120°

TWT

TWT

0

0

C1R4

R5

~

~ PHI = -240° TWTC2

L3

C3

2000 00Current Supply with Compensation ANSOFT

500.00

1000.00

1500.00

2000.00 Curve InfoFrom Supply

TROn DEH Cable

TR

PF=0.99Power factor

-1000.00

-500.00

0.00

Y1 [A

]

Less current needed from supply

measures ratio of active power to active and reactive power


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0.00 100.00 200.00 300.00 400.00Time [ms]

-2000.00

-1500.00pp yreactive power


Verification: Track the active and reactive powers from the supply.

P f th S l

Once energized little reactive power is needed from the supply due to the balanced power factor correction.

10 00

12.50

15.00Simplorer_w_pf_comp1Power from the Supply ANSOFT

Curve InfoActive Pow er

TRReactive Pow er

TR

5.00

7.50

10.00

Y1 [m

eg]

-2.50

0.00

2.50 Active Power ~ 1.1 MW

Reactive Power ~ 0 VAR


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0.00 100.00 200.00 300.00 400.00Time [ms]

2.50

Once charged, reactive power is supplied by compensation capacitor

CFD Simulation Studies: Thermal Loss

Why CFD simulation?

Heat transfer in the flowline is due to Convection

External heat transfer – natural convection

B i d l i ti Boussinesq model approximation

2D Vs 3D modeling


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The Boussinesq Approximation

Boussinesq model assumes the fluid density is constant in all terms of the momentum

ti t th b d f tequation except the body force term.

gPUDtUD

02

0

I h b d f h fl id d i i

ρ0 , Constant (operating) density

DtVariable (local) density

In the body force term, the fluid density is linearized.

gTTg 000

The Boussinesq assumption is valid when density variations are small.Cannot be used with species transport or reacting flows

gTTg 000


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Cannot be used with species transport or reacting flows.

2D Vs 3D Model

2D model assumptions Deepwater operations – Long pipelines – No

end effect influence

Calc late fl id stead state temperat re Calculate fluid steady state temperature –Given current and ambient conditions

3D model 3D model Determine optimum power or length of the

flow line – Given flow conditions, current and ambient conditions

Shut down or start‐up operations


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CFD Solution Domain: 2D Model

Piggy back cable

Sea at ambient temperature

Pipe heat SourceThermal Insulation

Flowing Fluid ‐Not modeled


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CFD Flow Field: Velocity Field

Flow Due to Natural Convection ‐ Boussinesq approximation


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CFD Predicted Temperature Distribution

40 C >> 25 C (Hydrate Temperature)

For a given current and ambient conditions, steady state temperature of the fluid can be predicted


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state temperature of the fluid can be predicted

CFD Simulation Studies: 3D Modeling

Determine safer operating maximum flow line length Gi en flo conditions c rrent andlength ‐ Given flow conditions, current and ambient conditions

Considered a small segment of the flow line

Used Multi‐Fluid (VOF) approach to model the flow

The flowing fluid doesn’t use density based on Boussinesq approximationBoussinesq approximation

Fully developed flow profile is used @ the flow line inlet

Turbulent modeling is limited to the flow line –zone based selection


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CFD Simulation – 3D Domain

Pipe heat Source

Piggy back cable

Pipe heat Source

Flowing FluidSeaSea


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CFD Simulation – Velocity Vectors


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Temperature Gradient Along Pipe

Hydrate formation temperature = 25 C


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CFD Simulation Studies: 3D Modeling

Without electrical heating, the current scenario would have restricted the flow line to 0 5 Kmwould have restricted the flow line to 0.5 Km

By electrically heating the pipeline, the flow line could be extended to 3 Km for an optimum power Efficiently can limit the power for a given length

If power is not a restriction – the pipe could be maintained well above 25 °C and we could havemaintained well above 25 C and we could have design with no limit on the length


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Start‐up and Shut‐down Operations

Transient SimulationGiven Length, Power, and assumed all the lines are @ ambient conditionsthe lines are @ ambient conditions.


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Transient Heating of Pipe


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Conclusion

ANSYS coupled field solutions is useful for efficiently designing electrically heated flow linesdes g g e ect ca y eated o es

Electromagnetic Field Simulations

Examine field distribution and determine the supply currentcurrent

Compare design efficiencies

Configure power supply

C i l Fl id D i Si l i Computational Fluid Dynamic Simulations

Determine the required current to maintain a steady state temperature

Determine optimum power or length of the flow line –Given flow conditions, current and ambient conditions

Shut down or start‐up operations


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Natural Convection Phenomena

In natural convection, fluid motion is generated due to density difference (buoyancy) in the fluid caused by TT

dxxPP

(buoyancy) in the fluid caused by temperature gradients.

Body forces i ll i i l

wTT

yx x dyxyx

yx

Typically gravitational

Centrifugal (rotating machinery)

Coriolis (atmospheric and oceanic vorticalmotion)

P

yfTT

motion)

For this class of problems, flow and energy are strongly coupled

Forces acting on a fluid particle in natural

convection.


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are strongly coupled.

Documents

Ellllectrically Heated Flllowlines – ANSYS FLUENT … Heated Flllowlines – ANSYS FLUENT –Maxwell Coupled Solutions Ryan Magargle, Ph.D. DuraiDakshinamoorthy Ph D