K. Chow

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CFD Modelling of Gas Freeing of VLCCs
2006 European PHOENICS User Meeting

• K. Chow
• University of Hertfordshire
• Fluid Mechanics Research Group

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What is Gas Freeing?
Gas Freeing is the removal of unwanted gas (such
as VOCs, inert gases), usually performed by
mixing ventilation
A deck-mounted fan is used to blow air into the
tank other vents are opened to allow the gas/air
mixture inside the tank to escape.
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Gas Freeing Process 1
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Gas Freeing Process 2
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Safety
In the past, poor gas freeing lead to a series of
oil tanker explosions, resulting in fatalities
and total loss of the vessel
Legislation passed in the mid 70s (ISGOTT, SOLAS)
greatly reduced the likelihood of gas tank
explosions
Every year, there are a number of potentially
fatal accidents due to insufficient or poorly
managed gas freeing
Gas freeing is still a time-intensive process
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Legislation
SOLAS

• Vents not less than 10m between each other, or
  other air intakes to enclosed spaces
• Gas Outlet velocity not less than 30m/s at a
  height of 2m above the deck

ISGOTT

• Tank is considered gas-free when concentration
  levels are below 40 of the lower flammability
  limits (LFL)
• For cold work and entry into tank, gas
  concentration levels must be below 1 LFL
  concentration of oxygen and other toxic gases
  must be constantly checked

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Shortcomings
Existing legislation passed in the mid 70s
tanker and sizes have increased greatly since then
Current methods and practices are also based on
smaller vessels, scaled up for larger ships
Effects of tank structural geometry on the gas
freeing process is not entirely understood
Internal tank geometry has changed, especially
with newer double-hulled tanks
Not a lot of work done towards this area of
tanker operations
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Current Work
To simulate and examine the flow field inside a
crude oil tank during the gas freeing process
To understand the physical mechanisms that drive
the mixing ventilation process by jet mixing
To investigate the effects of geometry upon the
efficiency and time for gas freeing
Ultimately, to improve the methodologies of gas
freeing to devise new procedures if necessary,
and to examine new equipment that can improve the
quality and reduce the time taken to gas free a
tank
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Simulation Description
3 different geometries of tanks of varying sizes
used to create 5 simulations
Simulations were solved for steady state results
In initial work, velocity field is examined for
regions of weak and strong circulation
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Case 1
Typical Single Hull VLCC Wing Tank Large number
of internal web-frames
22,500 m³ volume
1,860,456 Cells
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Modelling Process Computational Model
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Case 2
Newer double-hulled wing tank Lower web without
transverse
8,512 m³ volume
840,956 Cells
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Modelling Process Computational Model
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Case 3
Smaller chemical/oil tank No intrusive
frames Corrugated tank sides
2,592 m³ volume
652,190 Cells
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Modelling Process Computational Model
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Modelling Process - Idealisations
Gas flow is at relatively low velocities Mlt0.3,
therefore incompressible
Initial studies involved a single fluid single
phase flow later studies will examine multiple
gas species
For initial studies, turbulence represented by
K-Epsilon model
Heat transfer and temperature effects assumed to
be negligible
CAD Model of balanced accuracy and detail is
constructed
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Turbulence Modelling
Balance between simulation run-time and accuracy
2-equation standard K-Epsilon model utilised

• Behaviour, accuracy and performance is well known

• Not as empirical as other models

• Constants have wide applicability with limited
  reduction in accuracy

• Balance between accuracy and simulation run-time

• Better convergence behaviour than RNG

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Case 1
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Case 3a
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Case 3b
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Case 2b
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Case 2a
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Initial (Steady State) Results
Internal tank geometry is very important

• Geometry at floor level affects the spread of
  the jet impingement
• region
• Geometry above floor level (deck transverses,
  cross ties) affect
• the spread of the jet

Heavy ground-level partitioning causes jet flow
to be restricted to between-web spaces
Air jet creates constant patterns of circulation
inside tank leading to re-entrainment of mixed
air but poor mixing in low velocity regions
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Future Work
Perform time-dependant analyses to examine the
interaction of the air jet on the unwanted gases
during the simulated gas-freeing operation
Examine applicability of more accurate turbulence
models (e.g. RSM, LES) and accuracy of jet
prediction
Investigate the effects of stratified layers upon
jet impingement both in near and far-field to the
impingement zone
Examine different situations with a view to
increasing efficiency of gas freeing
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Conclusions

• Initial studies on VLCC tanks undergoing gas
  freeing have been conducted

• Current operations leave scope for improvements
  in flow optimisation
• and fan design

• Discharge into heavily framed floor greatly
  reduces spreading of jet at floor
• level

• Discharge into non-obstructed floor regions
  result in much stronger
• recirculation patterns

• Ceiling-mounted transverse structures cause
  reduction in cross-
• sectional spreading of jet