GASIN - An Extract from GASIN Knowledge Base on Gas Industry Facilities and Safety Concerns

An Extract from GASIN Knowledge Base on Gas Industry Facilities and Safety Concerns

Tuesday, 01 March 2016 00:00
Published in Bulletin - Edition 2
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By GASIN Reporter

The direction of Federal Government of Nigeria (FGN) towards gas, according to the Nigerian Gas Master Plan (GMP), is to fully exploit the potential in gas for accelerated economic development, in pursuit of the 10% GDP growth aspiration with a concurrent focus on a viable domestic, regional and other export markets. The GMP shows that three (3) central gas gathering and processing facilities (CPF) will be located in West Delta (Warri/Forcados area), Obiafu (North of Port Harcourt), which is one of GASIN's target communities, and in Akwa-Ibom/Calabar area. In these CPFs, wet gas will be treated, liquefied petroleum gas extracted and lean gas exported into transmission system (NNPC, 2008). 

The processes involved in the exploitation of natural gas are enormous, which is why a basic or preliminary understanding of the issues associated with this sector cannot be over-emphasized by the stakeholders. Already, the various gas fields have been identified, some of which have been barricaded by the gas companies. This, most importantly, determined the siting of the CPFs. The transportation of natural gas by pipeline during the exploration and utilization etiquettes is categorized, an understanding of which clearly depicts the facilities, installations and safety issues, which should be of primary concern around the gas fields and CPFs. Gas production, transmission and distribution pipelines are the three common categories of pipelines used in the transportation of natural gas. 

Gas production pipelines, also sometimes called gathering or flow lines, are those pipelines connecting the gas producing field wells to gas processing plants that process or treat the gas to meet transmission pipeline quality specifications. In most gas processing plants, liquid is removed and the gas dried to substantially reduce corrosion potential. Additional treatment of the gas may be required to remove certain higher risk contaminants, such as H2S, if present in sufficient quantities, to prevent problems on transmission pipelines. 

Because most production gas can contain the multiple phases of solid, liquid (water and hydrocarbons), and gas, production pipelines must be able to withstand additional “reactive forces,” both internal loading stresses (i.e., slugging) and chemical, not encountered on transmission or distribution pipelines. 

The corrosiveness and toxicity of the gas is dependent on the specific gas composition and there are usually very limited, if any, regulatory restrictions on many contaminants that can seriously chemically attack, impact the pipeline, or create other problems on their release. Depending on their pressures, production pipelines can fail as either leaks or ruptures. 

Gas transmission pipelines are those pipelines that move or transport conditioned or treated natural gas, meeting various quality specifications, from the gas fields or processing plants to the lower pressure distribution systems. Transmission pipelines tend to be larger in size, moderate to moderately high in pressure, and traverse long distances as their primary purpose is to move large volumes of non-reactive gas as economically as possible. Usually such main arterial pipelines consist of one large diameter pipeline, though multiple main pipelines can be run in parallel (called looping) to increase capacity. 

Along the transmission pipelines are compressor stations to re-pressure the gas as it moves down the system. Transmission pipelines operate under published quality specifications requiring that the gas carried be non-reactive and non-corrosive to the pipeline. Transmission pipelines are operated as a single-phase, gas, mainly composed of methane and other minor components (i.e., ethane, propane) and inert gases (e.g., nitrogen, carbon dioxide). Odorant with a very distinctive smell is usually added to the gas in transmission pipelines to aid in the identification of possible gas leaks from these systems, though not all countries require odorant on all transmission pipelines. 

Gas distribution pipelines consist of a network of lower-pressure gas pipelines usually taking gas from transmission pipelines, at various points down the system, through pressure reducing (i.e. pressure-relieving)/metering stations that drop the gas pressure from the transmission system pressure to the much lower pressure distribution system. 

Distribution pipeline systems consist of a grid of larger diameter pipes called mains, and smaller diameter service lines that run from the mains to connect directly to homes or businesses. Because distribution systems are in close proximity to large concentrations of people, they are designed and operated at much lower pressures (usually much lower than 14 Bar, or 200 psig) than production or transmission systems. Newer modern distribution pipelines are made of steel or plastic while older networks may be cast, or wrought iron, or other metals such as copper. Because of their much lower pressures, distribution pipelines fail as leaks rather than ruptures. Odorant is added to the gas in distribution pipelines to aid in the identification of possible gas leaks, both in the distribution system piping and in the much lower pressure home piping, where available. 

On the other hand is Gas Processing/Treatment Plant or Gas Terminal. Typically, along a production pipeline is a processing plant that contains equipment to process or treat gas gathered directly from field producing wells, permitting the natural gas to meet quality specifications for transmission pipelines. Depending on its capacity, a processing facility may accept more than one production pipeline. Processing facilities are usually located on or near gas production fields particularly if the gas is especially reactive. Presently, there is a gas processing facility operated by Nigerian-Agip Oil Company (NAOC) in Ebocha, which is also in the same axis where the FGN has cited a gas CPF according to the GMP. These two communities are among GASIN's target communities. So, it is obvious that some, if not all of the types of natural gas pipelines explained above traverse some communities around Ebocha. 

In addition, the handling, transportation treatment or processing of natural gas go with a good number of health, safety and environmental (HSE) issues ranging from environmental pollution from leakages, venting, flaring and liquid/solid discharges to other health and social issues. Natural gas is not found pure as it is often associated with other elements and compounds. Thus, the composition of produced gas varies and the composition factors are especially important on production gas pipelines as composition can seriously impact the operability of a pipeline, especially the pipeline's integrity. Critical composition issues include the wet gas versus dry Gas factor and gas components other than methane. 

Concerning the Wet Gas Versus Dry Gas, it is extremely unusual for gas produced from a gas field to be in a dry state. The presence of water is almost always assured. Gas containing water is classified as “wet gas” and brings with it certain risks to a pipeline operation. Water is required for internal corrosion on pipelines to occur. In addition, water or other liquid slugs can seriously change loading stresses on a pipeline. Slug catchers, which are large catch vessels installed to trap liquids, are placed along production gas pipelines. The settlement of water in low points in production pipelines can also serve to concentrate and accelerate selective internal corrosion attacks that can occur much faster than general corrosion. As a result, over emphasis on a general corrosion allowance to protect a pipeline can be ineffective at preventing pipeline failure from selective rapid corrosion attack, especially on production pipelines most at risk from such occurrences. 

Components other than methane in produced gas can also have serious impacts on production pipelines. Carbon dioxide and certain sulphur compounds (e.g., CO2, H2S) in the presence of water can lead to acid attack and internal corrosion. Heavier components, such as propane, butane and heavier (C5+) will also tend to form liquids and periodically drop out along the pipeline adding to loading stresses associated with liquid slugging. The stated design components of a gas pipeline and gas composition may not necessarily be compatible as the gas field ages, or if a new gas field is brought on line and tied into the same production pipeline. These changes can affect the internal corrosion rate as well as the internal corrosion potential on the pipe. Thus, adherence to internationally-accepted standards is one of the key monitoring tools GASIN uses in favour of the stakeholders to advocate for preventive measures against the causes of pipeline ruptures and gas leaks.

Thus, liquid contaminants, consisting of hydrocarbon and water must be removed from the wet production gas as such liquids not only add to corrosion potential but also create the internal loading stresses on pipelines that can be quite high, especially when these accumulated liquids are driven by the high pressures from the gas field and production pipeline. In addition, unique contaminants such as excess CO2 or H2S must be treated if they are present in appreciable quantities that might affect transmission or distribution pipeline systems or customer safety. 

Cold venting and excessive flaring in a gas processing facility are of major concern. Cold venting is the release of gas, usually methane, out of a gas processing plant via vent stacks to atmosphere in such a manner that it is not burned. The theory is that the lighter-than-air gases rise up into the atmosphere. While most vented gas streams are mainly methane, which is lighter than air, serious safety concerns appear when heavier-than-air components or toxic chemicals start to show up in the gas stream than might be vented. 

 

Cold venting can be very dangerous, not only for the neighbouring population but also for the plant personnel. Depending on the composition of the material in the gas stream, especially if a plant is located on a site in proximity to people, dispersion can send heavier-than-air gas components to ground level with tragic results like health issues due to exposures. Cold venting is usually the by-product of remote oil field design, but an over-focus on capital reduction for gas field development (i.e., to boost rate of return) can drive a company to select cold venting over wiser alternatives that require additional equipment.

Cold venting should not make sense in a world where energy prices are increasing, but it can still occur because of the economics and investment philosophies of particular companies. Failure to properly restrict the option of cold venting should be regarded as a serious deficiency and prevented in any modern processing plant design and approval. Several responsible governments and world agencies have incorporated practices to discourage cold venting in their energy field development. It is in GASIN's interest to advocate for the enforcement of such practices by the Nigerian government.

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