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/hr Typical 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 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 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 Hull VLCC Wing Tank Large number of internal 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 frames Corrugated 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