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© 2011 ANSYS, Inc. September 7, 20111
Flow Assurance with ANSYS - II
Mohan Srinivasa, PhD
© 2011 ANSYS, Inc. September 7, 20112
Recap of Flow Assurance with ANSYS – I
Flow Assurance with ANSYS – II
• Vertical bubbly to slug transition
• Methanol flushing
• Wax formation
Future work
Conclusions
Agenda
© 2011 ANSYS, Inc. September 7, 20113
Flow assurance
Flow assurance
Slug flow
Hydrates
Wax
Erosion and
Corrosion
Gelrestart
Flow induced vibration
Sand transport
Gas lift
© 2011 ANSYS, Inc. September 7, 20114
Recap of Flow Assurance With ANSYS - I
© 2011 ANSYS, Inc. September 7, 20115
Heat transfer in single and multiphase flow pipe lines
• Reduce heat transfer losses to the environment
• Prevent conditions that lead to hydrate or wax formation
• Understand “cold spots” and remedies
Recap – Risk Avoidance Strategies
Experimental and CFD studies of heat transfer in an air-filled four-pipe tube bundle L. Liu, G. F. Hewitt, S. M. Richardson Taxy and Leberton, Use of CFD to study the impact of cold spots
on subsea insulation performance, 2004, OTC.
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Recap – Gas Liquid Flows
ANSYS Fluent predictions
International Journal of Multiphase Flow 32 (2006) 527–552
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Recap – Gas Lift
Experimental data from Bubble Size effect on the gas-lift technique PhD thesis of S´ebastien Christophe Laurent GUET
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Recap – Churn Flow Simulations
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Bubbly to Slug Transition
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A Priori identification of flow regimes
• Slip velocities between phases
• Determines the holdup
• Flow maps available only for simpleriser configurations
Frequency and severity of slugging
• Downstream equipment to buffet effectsdue to pressure surges and flow variations
• Fatigue, erosion and flow induced vibration
Modeling Vertical Flows
© 2011 ANSYS, Inc. September 7, 201111
Eulerian model with appropriate sub-models
• Account for bubble coalescence and breakup
• Use appropriate drag laws for various bubbles
Combination of models
• Eulerian model for dispersed flow regime (implicit)
• VOF model for slug flow regime (explicit)
• Switch models appropriately
• Multi-fluid VOF
Resolved bubbles simulation with VOF model
Possible Modeling Options
IncreasingComputationalCost
© 2011 ANSYS, Inc. September 7, 201112
Experimental data from Chemical Engineering Science 65 (2010) 3836—3848
Predict if fully developed flow will be bubbly or slugging
What do the various models predict?
• Accuracy and computational cost
Motivation
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Experimental Conditions
5 m
6 m67 mm
Cross-sectional
data collected.
Well mixed inlet
Bubble diameter = 5 mm
Air-water conditionsUsg = 0.56 m/sUsl = 0.25 m/s
Oil-air conditionsUsg = 0.56 m/sUsl = 0.25 m/s
Material Properties Water density=998 kg/m3, viscosity = 1 cPSurface tension 0.072 N/m
Material Properties Oildensity=900 kg/m3, viscosity = 5.25 cPSurface tension 0.05 N/m
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Eulerian Model with Population Balance
Air-water Air-oil Air-water Air-oil
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Holdup Predictions
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Flow Map Predictions
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Capable of modeling both regimes
Physics in the dispersed region
• Wall lubrication
• Sub-grid scale drag models based on predicted diameter
Physics in the stratified region
• Surface tension
• No-slip at the interface
Demarcation between the two regions identified based on the a transition air volume fraction
Population balance model
Multifluid VOF Model
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Sample Results
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Holdup Prediction for Multi-Fluid Model
© 2011 ANSYS, Inc. September 7, 201121
Methanol Flushing
© 2011 ANSYS, Inc. September 7, 201122
Methanol inhibits the formation of hydrates
During shut-in pipelines are flushed with methanol as a risk-avoidance strategy
Hydrate plugs could form more easily in under-inhibited systems than uninhibited systems! (Lee at al. 2003)
Need to predict methanol concentrations, and residual water accurately
Is methanol flushing adequate?
Methanol Flushing
© 2011 ANSYS, Inc. September 7, 201123
Problem Description
Hydrate inhibition of subsea jumpers during shut-in, T.Cagney and S.Hare, and S.J.Svedeman
Methanol mass flow inletPressure outlet(atmospheric pressure)
Oil, Water and Air
Find the distribution of methanol and water in the jumper after methanol is injected into the jumper
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Initial condition
Air
Oil
Water
Jumper Volume- 0.22m3As % jumper volume: Air- 33%, Oil-37%, Water-30%
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Methanol flushing: Animation
Methanol flow rate: 10 m3/hr
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Water volume as % of jumper volume Methanol volume as % of water+methanol in jumper
Comparison of Results with Experimental Data
Results comparison after one jumper volume of methanol flushing
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Wax Formation
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Inner pipe wall below cloud point temperature (or WAT)
Radial thermal and mass transfer gradients
• Convection and molecular diffusion in the laminar sub-layer
Aging of the wax layer
• Diffusion and counter diffusion of oil in the gel
Physics of Wax Formation
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Multiphase flow
Non-Newtonian rheology
• Fluid with a yield stress, and temperature dependent viscosity
Heat transfer including phase change
• Fluid mechanics at the wax-oil interface
Species transport
• Convective and diffusive
The Fluid Mechanics of Wax Formation
Huang et al. (2010), AIChE Journal, Vol 57, pp 841—851
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Wax as a species in oil phase
Will condense to a “Waxy” immobile phase
• Fixed velocity phase
Kinetics of deposition dependent on
• Concentration of wax in oil
• Temperature
Prototype for Modeling Wax Formation
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Review of various flow assurance models
Vertical bubbly to slug transition
Methanol flushing
Prototype model for wax formation
Contributors to this presentation
1. R Lubeena and Mohammad Ellyan
2. R Muralikrishnan
Conclusions
© 2011 ANSYS, Inc. September 7, 201132