ATOFINA - High Pressure Pipeline Rupture (Final)

7/30/2019 ATOFINA - High Pressure Pipeline Rupture (Final)

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Simulation of

High Pressure

Pipeline Rupture with

Aspen Dynamics ®

Claudine Boisson

Process Simulation Team

[email protected]



Simulation of Pipeline Rupture with Aspen Dynamics2

Outline

Introduction

Objective and methodology

Building up the modelTheoretical analysis

Simulation results

Conclusion

1

2

3

4

5

6




1. Introduction: Pipelines

Pipelines are widely used to transporthazardous fluids (crude oil, natural gas,petroleum products, etc.)

Overall length > 750,000 km in North America

> 130,000 km in EuropeGood safety record: pipelines comparefavorably with other transportation methods*

* According to the US National Transportation Safety Board andCONCAWE (the oil companies’ European organization for environment, health and safety)




1. Introduction: Safety issues

But a single pipeline accident can cause acatastrophic disaster and cost millions of dollars.

For both environmental and economic

reasons, companies need to assess thedamage an accident would cause (fire,explosion, toxic release).

For high-pressure pipelines, full rupture is

often the “worst case” scenario. Simulation of pipeline rupture is a key issue

regarding safety and risk assessment.




1. Introduction: Pipeline rupture

What happens when a pipeline carryingliquefied hydrocarbons breaks down?

Inside pressure suddenly decreases in theneighborhood of the opening:

Superheated liquid flashes 2-phase critical flow is established Temperature decreases

A sharp vaporization front propagates within thepipeline, away from the point of failure.

Block valves isolate the ruptured segment.

The remaining fluid is released until P = Pout (effect of gravity on the liquid phase if the pipe isinclined).




1. Introduction: Simulation tools

Simulation of high-pressure pipeline rupture isparticularly difficult (highly transient nature, stiff term, two-phase heat and mass transfer, nonthermal equilibrium, …).

Experimental data are scarce.

Few codes exist (commercial tools are evenscarcer).

To get reasonably accurate results, one has touse complex codes (such as ProFES, META-HEM, BLOWDOWN) - which still sometimesperform poorly.




1. Positioning of Aspen

Dynamics

Aspen Dynamics is an easy-to-use tool for

dynamic simulation of process plants,

Not dedicated to pipeline depressurization

simulation,But has built-in models for pipe, valve and

pipeline.

Is Aspen Dynamics able to simulate a pipelinerupture and accurately predict the released

fluid flowrate and composition?




Outline

Introduction



Simulation results

Conclusion

1

2

3

4

5

6




2. Objective and methodology

Objective: ASSESS ASPEN DYNAMICS’ CAPABILITIESTO SIMULATE A HIGH PRESSURE PIPELINERUPTURE

No field data directly available. Basic approach:

Can Aspen Dynamics perform the calculations?

How simplified is the model? How accurate are the results?

(Accuracy estimated through theoretical evaluationof the equations used)




2. Methodology

Choose a test case.

Build up a model with Aspen Dynamics.

Perform the calculations (if the calculations

fail, simplify the model).

Theoretical analysis of:

simplifications

validity of the equations used impact of numerical methods

Evaluation of the accuracy of the results.




2. Test case

Horizontal 8-in diameter pipeline

Supercritical ethylene (100 bar abs/ 15°C/ 100,000 kg/h)

Block valve every 14.5 km

Closes in 40s when P < 65 bar absFull-bore rupture of pipeline in the middle of a 14.5 km

segment

C2H4

14.5km

PC

PT

PC

PT




Outline

Introduction


Building up the model Theoretical analysis

Simulation results

Conclusion

1

2

3

4

5

6




3. Building up the model:

Steady-state

110 bar abs

15 °C 93 bar abs,

13 °C Aspen Plus 11.1

Single component: ethylene

Property method: PSRK

2 “Pipe” blocks (8 in , 14.5 and 7.25 km length)

Ball valve (8 in

)Flowrate: 100 t/h

Model exported as a pressure-driven dynamicsimulation





Dynamic

Aspen Dynamics 11.1

Pressure driven model: P1 and P3 fixed.

Simulation of the rupture: outlet pressure (P3) set

from 93 bar abs to 1 bar abs.The valve is closed when P2 < 65 bar abs.

110 bar abs

P1

93 bar abs

1 bar abs

P3 P2





Dynamic

Calculation options

(had to be carefully adjusted to obtain convergence) :

Global property mode : Rigorous

Maximum number of Fortran errors : 500Non linear solver : Newton (max. iterations : 1000)

Integrator : Variable Step Implicit Euler (Gear andRK4 failed)

Discretization interval : 10 segments in the first pipeblock, 50 in the second

Tolerances : 0.0001




3. Restrictions imposed by

the software

Property method:

Calculation failure with BWRS. Had to use PSRK (cubic

equation of state), which is less accurate:- for liquid molar volume and enthalpy

- near the critical region

Number of segments in the pipe block :

systematic run failure with more than 50segments




Outline

Introduction



Simulation results

Conclusion

1

2

3

4

5

6




4. Closure relationships

Thermodynamics: Thermal & phase equilibrium: wrong (Tgas Tliquid)

but applicable to long pipelines (>100m)

Cubic equation of state shortcoming near thecritical region

Friction force (momentum equation): Various correlations available

Heat transfer:

Constant fluid-wall heat transfer coefficient:inaccurate (coefficient very different for gas andliquid overall coefficient changes with time)

Interfacial heat transfer: neglected




4. Numerical methods

Pipe model: Finite Difference Method Numerical diffusion

Long CPU time and convergence problems

(Ex : 10 segments in the second pipe : 30 min50 segments in the second pipe : 3h100 segments in the second pipe : no convergence)

Due to :

Fixed grid : discretization into a large number of segments large number of equations

Supercritical property calculations non-linear solver not robust enough (frequent run failures)

Time increment and stiff equations




4. Estimation of results

accuracy

Some good points: No assumption of isentropic or isenthalpic

decompression

Continuous phase change along the pipe length

using rigorous flashes Modeling of two-phase choked flow

Major source of inaccuracy: NumericalMethods

Convergence possible only with a reduced number of segments

Finite Difference Method Numerical diffusion Sharp fronts are smoothed




4. Estimation of results

accuracy

Other inaccuracy factors

PRSK: C2H4 density is ~15% too low (deviation fromNIST* values in the simulated conditions)

Flowrate is underestimated (~5%) and pressuredrop overestimated (up to 10%)

Global heat transfer coefficient : no accurate valueexists. Temperature can be over- or underestimated,depending of the chosen approximation.

Empirical correlations for friction factor dependant onflow regime maps: 2 sources of inaccuracy

* National Institute of Standards and Technology




Outline

Introduction



Simulation results

Conclusion

1

2

3

4

5

6




5. Simulation results

Ruptured pipe inlet and outlet pressure

0

20

40

60

80

100

120

0 10 20 30 40 50

Time (min)

P r e s s u r e ( b a r

a b s ) Outlet

InletValve closed

Pipeline rupture

Choked flow





Ruptured pipe inlet and outlet flowrate

0

100

200

300

400500

600

700

0 10 20 30 40 50

Time (min)

F l o w r a t e ( t / h

r )

Outlet

InletValve closed

Pipeline rupture





Ruptured pipe inlet and outlet temperature

-120

-100

-80

-60

-40

-20

0

20

0 10 20 30 40 50

Time (min)

T e m p e r a t u r e (

° C )

Outlet

Inlet

Valve closed

Pipeline rupture

End of

choking




Outline

Introduction



Simulation results

Conclusion

1

2

3

4

5

6






6. Conclusion (cont’d)

Suggested path forward: focus on

numerical methods improvement

Method of Characteristics (curved

characteristics):

Discontinuities are propagated with little

numerical diffusion

Variable grid instead of fixed grid

Documents

ATOFINA - High Pressure Pipeline Rupture (Final)