K. Chow University of Hertfordshire Fluid Mechanics Research Group

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

2. 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.

3. Deck-mounted fan 14,000 m³/hr Typical COT – 24,000m³ Gas Freeing Process [1]

4. Gas Freeing Process [2]

5. 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

6. 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

7. Shortcomings 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 Internal tank geometry has changed, especially with newer double-hulled tanks Not a lot of work done towards this area of tanker operations

8. 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

9. 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

10. Case 1 Typical Single Hull VLCC Wing Tank Large number of internal web-frames 22,500 m³ volume 1,860,456 Cells

11. Modelling Process – Computational Model

12. Case 2 Newer double-hulled wing tank Lower web without transverse 8,512 m³ volume 840,956 Cells

13. Modelling Process – Computational Model

14. Case 3 Smaller chemical/oil tank No intrusive frames Corrugated tank sides 2,592 m³ volume 652,190 Cells

15. Modelling Process – Computational Model

16. Modelling Process - Idealisations 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 Heat transfer and temperature effects assumed to be negligible CAD Model of balanced accuracy and detail is constructed

17. 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

18. Case 1

19. Case 3a

20. Case 3b

21. Case 2b

22. Case 2a

23. 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

24. 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

28. 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