Flow Assurance with ANSYS - II · PDF fileFlow assurance Flow assurance Slug flow Hydrates Wax...

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Flow Assurance with ANSYS - II

Mohan Srinivasa, PhD

Mohan.Srinivasa@ansys.com

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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

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Flow assurance

Flow assurance

Slug flow

Hydrates

Wax

Erosion and

Corrosion

Gelrestart

Flow induced vibration

Sand transport

Gas lift

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Recap of Flow Assurance With ANSYS - I

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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

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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

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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

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Methanol Flushing

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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

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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

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