CFD Modelling of Gas Freeing of VLCCs K. Chow University of Hertfordshire Fluid Mechanics Research Group 2006 European PHOENICS User Meeting.

CFD Modelling of Gas Freeing of VLCCs

K. Chow

University of Hertfordshire

Fluid Mechanics Research Group

2006 European PHOENICS User Meeting

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.

What is Gas Freeing?

Deck-mounted fan

14,000 m³/hrTypical COT – 24,000m³

Gas Freeing Process [1]

Gas Freeing Process [2]

Every year, there are a number of potentially fatal accidents due to insufficient or poorly managed gas freeing

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

Safety

Gas freeing is still a time-intensive process

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

Legislation

Existing legislation passed in the mid 70’s; 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

Not a lot of work done towards this area of tanker operations

Internal tank geometry has changed, especially with newer double-hulled tanks

Shortcomings

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

Current Work

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

Simulation Description

22,500 m³ volume

1,860,456 Cells

Typical Single HullVLCC Wing Tank

Large number ofinternal web-frames

Case 1

Modelling Process – Computational Model

8,512 m³ volume

840,956 Cells

Newer double-hulled wing tank

Lower web without transverse

Case 2

Modelling Process – Computational Model

2,592 m³ volume

652,190 Cells

Smaller chemical/oil tank

No intrusive framesCorrugated tank

sides

Case 3

Modelling Process – Computational Model

Gas flow is at relatively low velocities; M<0.3, 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

CAD Model of balanced accuracy and detail is constructed

Heat transfer and temperature effects assumed to be negligible

Modelling Process - Idealisations

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

Turbulence Modelling

Case 1

Case 3a

Case 3b

Case 2b

Case 2a

Internal tank geometry is very important;

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

• 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

Initial (Steady State) Results

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

Examine different situations with a view to increasing efficiency of gas freeing

Investigate the effects of stratified layers upon jet impingement both in near and far-field to the impingement zone

Future Work

• 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

Conclusions