Velocity profile simulation for natural gas flow underneath waterbody following a full-bore rupture of an offshore pipeline E.O. ObaniJesu and E.O. Omidiora PROFESSIONAL PAPER This work develops a model based on principle of conservation of momentum to predict natural gas flow pattern in waterbody following an accidental release through a full bore rupture (FBR) from a submerged pipeline. The model was discretized using Finite Difference Method; Crank-Nicholson numerical technique was applied to simulate it while MATLAB 7 was used to simulate the resulting algorithm. Solutions to the model are generated at various mesh points in the computational domain to show the flow pattern at various points within the waterbody both vertically and horizontally. This model gives a good representation of the flow pattern when compared with the existing similar models, thus, the model is useful for the Accident Response Planning Unit (ARPU) in case of such disaster. Key words: natural gas, pipeline, velocity profile, full bore rupture, ARPU INTRODUCTION Natural gas is a naturally occurring mixture of simple hy- drocarbons and non-hydrocarbons that exists as a gas at ordinary temperature and pressure (Maddox and Can- non, 1998). 9 The gas consists principally of methane (CH 4 ) and ethane (C 2 H 6 ) with functional amounts of pro- pane (C 3 H 8 ) and butane (C 4 H 10 ). In addition to hydrocar- bon components, raw natural gas contains a varying amount of non-hydrocarbon contaminants or diluents, such as hydrogen sulfide (H 2 S), carbon dioxide (CO 2 ), ni- trogen (N 2 ) and helium (He). Its composition varies with origin, type genesis and location of the deposit, geological structure of the region and other factors. 18 The gas is used as domestic and industrial fuel, raw material for the synthesis of methanols, formaldehyde and other chemical compounds 5 and for thermal genera- tion purpose. 12 It is also used in air conditioning systems for domestic cooling; it is an important base ingredient for plastic, fertilizer and anti-freeze. It is used in metal preheating (mostly for iron and steel), drying and dehumidification, glass melting and food processing. Natural gas is continuously transported through a com- plex network of pipelines designed to quickly and effi- ciently transport the gas from its origin to areas of high demand under high pressure and temperature of about 113 K. 11 For offshore continuous transportation of the gas, pipes are laid in trenches dug on the floor of the wa- ter body after which the pipe is fitted with a concrete cas- ing to ensure that it stays at the bottom of the water. Another form of offshore continuous transportation of the gas is by allowing water to suspend the pipe-length through buoyancy. 14 However, these pipes are subject to rupture through corrosion, assembly errors, manufac- turing defects, improper maintenance, fastener failure, design errors, improper material and improper heat treatment, casting discontinuities, fluctuation in operat- ing conditions and inadequate environmental protection and control. 13,10,15 Following this failure, the conveyed fluid escapes into the waterbody to disturb its composi- tion and biomass. Release of pollutants such as hydro- carbons and hydrogen sulfide to waterbody leads to mass mortality of many organisms, including fish and benthic molluscs 18 , renders the water unfit for human consumption 21 , hydrate formation and release of Volatile Organic Compounds (VOCs) to the atmosphere. To minimize these problems, numerous models have been developed to study the velocity profile of the gas from the point of discharge in order to accurately repre- sent the real world situation by means of iterative proce- dure. 2,23,15 Obanijesu and Mosobalaje 15 developed a similar model on concentration profile. This work devel- ops a model to predict the velocity profile of the gas in- side waterbody following such an accident based on the principle of conservation of momentum. The developed model was discretized using Finite Difference Method (FDM) while Crank-Nicholson numerical technique was applied to simulate the resulting algorithm. Simulating such scenario is important in understand- ing the behavior of such fluid in a stratified or unstratified waterbody. It also makes the prediction of the extent of pollution possible in terms of concentration and area size. Also, the direction of flow of the polluting fluid in the receiving waterbody could be easily predicted. Also when new elements are introduced into a system, this study can be used to anticipate bottlenecks or other problems that may arise in the behavior of such system. It can be used to experiment new situations about which we have a little or no information so as to prepare for the aftermath. 610 NAFTA 60 (11) 610-614 (2009)