Natural gas production a vulnerable operation

By Carl Chairul

JAKARTA (JP): Natural gas is a very volatile and flammable fluid. It occurs in underground reservoirs at high pressure and extremely high temperatures. To turn it into liquefied natural gas (LNG) at atmospheric pressure and a temperature of minus 1,610 degrees Celsius is no piece of cake. The first drop of Indonesian LNG was commercially produced on July 5, 1977, in Bontang, that is more than five years after the discovery of the Badak gas field where the gas was derived. The time span shows how complicated a natural gas operation is.

Like its "brother" crude oil, natural gas is referred to as hydrocarbons, as it is composed of hydrogen and carbon atoms. Methane (CH4) is the chief component of natural gas that is liquefied to make what is known as LNG. Other main constituents are ethane (C2H6), propane (C3H8) and butane (C4H10).

Handling this kind of substance requires special care. A small mistake could result in a disaster. But that doesn't mean that everybody working in a gas producing company is exposed to risks. The operation is fully protected and the people are secured to the extent that nobody is endangered as long as all safety requirements are met and all regulations are adhered to.

The production and liquefaction of natural gas is a complex operation commonly consisting of two phases -- field operation and plant operation. From the bowels of the earth, the produced natural gas comes with various contaminants such as water, sand, mud, sulfur, CO2, etc. In some fields like Badak, Bekapai and Handil, natural gas is also entrained in crude oil; therefore it is called associated gas.

The field processing is aimed at separating the gas from the contaminants. The first stage of separation occurs in a series of vessels called separators. This is where water, sand and mud, if any, are disposed of. Separation is not enough to dry the gas; therefore, it must be treated further in a dehydration process. At this stage, the natural gas is ready to be used as fuel and as the raw material of a fertilizer plant.

For the purpose of export, however, the gas needs to be liquefied in a liquefaction plant consisting of several trains. A train is a complete set of equipment used for a certain purpose. One LNG train is designed to liquefy approximately 350 MMSCFD (million standard cubic feet per day) of natural gas.

Prior to the liquefaction, the gas still has to go through a series of processes to get rid of all impurities such as sulfur, CO2 and heavier hydrocarbons. In the precooling process, the remaining moisture and heavier hydrocarbons are removed. This is possible as each component responds to pressure and temperature differently. Water turns into ice at zero degrees Celsius. Propane and butane liquefy when they are cooled to 00 at the pressure of 100 psi (pounds per square inch), or to minus 430 degrees Celsius at atmospheric pressure, while methane will not liquefy until the temperature reaches minus 1,610 degrees Celsius.

Liquefaction, the main part of the process, takes place in a huge cryogenic tower, which is also called the main heat exchanger. (Cryogenic is the term used in relation to the process or production of a substance at a very low temperature. Liquefying natural gas is a cryogenic process.)

In the main heat exchanger, precooled gas and liquefaction refrigerant flow in two parallel tubing circuits. Each circuit is made up of aluminum tubing, designed in such a way that the cold evaporating refrigerant cools the natural gas and, ultimately, turn it into liquid. It takes 400 miles of tubing and huge compressors driven by gas turbines to turn the gas into liquid at a temperature of minus 1,610 degrees Celsius. In this transformation, the volume of the gas is reduced to 600th of its original volume.

Coming out of the cryogenic tower, the liquefied gas is piped through heavily insulated pipe to the storage tanks, waiting for shipment.

How safe is it?

Pressure and fire are the two main "killers" in the petroleum industry, although an extremely cold substance like LNG at minus 1,610 degrees Celsius is also life threatening. Let's take a look at the well site, from which raw natural gas comes out of the ground at high pressure. All you can see from a well is a set of valves and spools called a Christmas tree. From here, natural gas flows to the processing site through a string of high pressure flowline. This flowline could burst if the pressure is higher than what it was designed to withstand. It may also break due to a traffic accident, an earthquake or a bomb. In any case, the gas will be vented into the atmosphere, causing a hazard to people and the environment. Once the gas catches fire, you have an inferno.

But this can be prevented, thanks to the protective device called a hi-lo shutdown valve (SDV) installed on the Christmas tree. If the pressure exceeds its set point, the SDV will close the well automatically and stop the flow into the flowline. The same thing will happen if the pressure in the flowline suddenly drops due to flowline rupture.

Entering the field processing plant, the flow of gas from several flowlines is combined into a manifold. Again, at this manifold, another SDV is installed. Any hazard that occurs in the plant will trip the SDV and stop the flow of gas. This will cause pressure to build in both the manifold and in the flowlines. But, the SDVs at the Christmas trees will take care of the pressure build-up by closing the flow from the well.

Each plant is equipped with both a fire and overpressure protection system. The vessels in the plant are equipped with relief valves that, in case of overpressure, will vent the gas through the flare stack where it is safely burned off.

As for fire protection, the plant is secured with a system that is so sensitive that any unusual source of heat or smoke can be detected. Any irregularity will automatically activate the protection system in a logical pattern, starting with an alarm to alert the operators, a shutdown to stop the operation and the activation of a fire extinguishing system such as water, dry chemicals, etc. In the same manner, the fire brigade standing by around the clock will take all necessary measures to fight the fire.

Besides the protective systems, there are so many regulations and procedures created to ensure the safety of the plant and the people. First of all, nobody is allowed to enter the plant area without permission. Even if you are granted permission for entry, you have to go through Pentagon-type security procedures. You are definitely not allowed to bring matches or lighters into the plant, let alone to smoke. And driving a car in the plant with gasoline fuel is prohibited.

So, as far as safety and security are concerned, it can be said that natural gas production operations are definitely safe. Only one snag: the system is not specifically designed to protect against bombs, bullets or sabotage.