Based on most recent estimates, roughly 4% of the man-made atmospheric emissions in the UK are due to the offshore production of oil and gas.
Overall, atmospheric emissions can result from a number of sources on an offshore installation which may include: flare systems, vents, diesel engine and gas turbine exhausts, and leakage of gases from tanks, pipework, refrigeration systems and other such sources.
These atmospheric emissions generally fall into one of five categories from the standpoint of environmental impacts. These categories are described in the following sections. Where comparisons are made with UK wide emissions, the data is based on the UK National Air Quality Information Archive.
Greenhouse gases such as carbon dioxide (CO2) and methane (CH4).
The UK government through its commitments to the UNFCCC and more recently the Kyoto Protocol has made commitments to reduce emissions of these gases. In 1997 the UK offshore industry emitted 22.7 M metric tonnes of CO2, roughly 4% of the UK's overall emissions. Fig. 7 indicates the various sources of CO2 emitted by the industry in 1997:
The UK offshore industry's overall response to climate change has already been addressed in an earlier section of this report. One of the two more significant sources of emissions from the offshore industry (flaring) is discussed in more detail later in this section.
The extent of emissions from the other significant emission source (gas consumption) is a function of the maturity of the industry. As more and more oil and gas is produced, the pressure in the underground reservoirs continues to decline. This pressure must either be maintained (often by injecting seawater into the reservoir), or be compensated for by the installation of large pumps and compressors to help transport the oil and gas ashore. Both options require an increasing amount of energy to be produced on the platform. The most convenient source of energy is from gas produced from the reservoir and burned as fuel.
The oil and gas industry has collaborated on a number of initiatives associated with improvements in the efficiency of power generation and use. One of these is a joint programme between IPIECA (the International Petroleum Industry Environmental Conservation Association) and UNEP (the UN Environment Programme) which resulted in the publication of a guideline document: Climate Change and Energy Efficiency in Industry.
 With regard to the other major Greenhouse gas - CH4, the offshore industry emitted just under 0.1M metric tonnes in 1997. This amounts to 2.5% of UK wide emissions of CH4.
CH4 emissions from the offshore industry have shown a slow but continuing decline in recent years as can be seen in Fig. 8.
Ozone Depleting Substances
Gases such as CFC's (Chloroflurocarbons) damage the naturally occurring ozone layer in the high levels of the atmosphere, and result in the breakdown of the atmospheric shield against solar ultra-violet radiation. The UK government is a signatory to the international treaty which aims to phase out the use of these gases (the Montreal Protocol). The oil companies along with other industries comply with the government's ozone-depleting chemical phase out plans. From the standpoint of the offshore oil industry, the implications of these plans relate to the gradual phase-out of certain types of Refrigerants and fire-fighting equipment. In order to assist oil companies develop their phase out plans, UKOOA has produced a number of ozone related technical guidelines (Ref. 5).
Photochemical Smog
Gases such as Oxides of Nitrogen (NOx) and VOC's (Volatile Organic Compounds) react at ground level to form photo-chemical smog. Smog is largely an issue of local air quality, which particularly impacts the urban environment. Smog is mostly associated with automobile use. Emissions from oil terminals and other such facilities only contribute some 2% of UK photochemical smog precursors.
Despite this relatively small contribution, the UK industry is investing in the development and installation of equipment to reduce VOC emissions in its near shore operations and at onshore terminals. As a result, 1997 VOC emissions were down 14% on 1996 levels.
Sulphur Dioxide
Gases such as sulphur dioxide (SO2) react with water to cause acid rain, a regional pollutant. In a European context, acid rain is a particular consequence of the burning of high sulphur fuels in the industrial parts of Europe. Most recent estimates indicate that onshore industrial sources contribute 24% to UK wide emissions of SO2.
North Sea oil generally has a very low sulphur content and only in a very few cases are sulphur scrubbing plants required on offshore installations. In 1997, only 0.5% of the UK's SO2 emissions came from the offshore oil and gas industry.
Carbon Monoxide
Carbon Monoxide (CO) is a pollutant that impacts local air quality in a similar way to those photo-chemical smog precursors described above.
CO is a product of incomplete combustion. In the offshore environment, this usually relates to the operation of diesel engines and gas turbines. The less efficient the combustion process, the more CO is formed. However, the impact of CO pollution is localised since the gas rapidly converts to CO2 in the atmosphere.
Local CO impacts are mostly associated with vehicle use. Based on most recent estimates, the offshore oil and gas industry contributes less that 1% of UK wide CO emissions.
Gas Flaring
In virtually every situation where an oil field is being developed some natural gas will also be produced. In the majority of cases where this occurs it will be necessary to have a flare lit at all times for safety reasons. This is necessary to ensure that when a facility starts production or during periods when plant fails there is a mechanism for removing the gas from the platform as quickly and as safely as possible to protect the workforce on the installation.
The hydrocarbon gas that is contained within oil reservoirs, called associate gas, is produced as a by-product of oil being brought to the surface. In many cases it will be possible to export this gas to market but in others this will not be practical. When it cannot be supplied commercially to customers or re-injected back into the underground oil reservoir there is very little that can be done but to flare any gas not used by power generation equipment; fortunately this happens in relatively few cases and the volumes involved are relatively small.
The decision about whether associated gas can be conserved or not is often complex. In remote North Sea operations the decision is usually base on whether there is sufficient gas to economically justify installing a pipeline to deliver the gas to one of a number of gas-gathering pipelines. Some gas gathering pipelines are essentially full whilst others may not be able to accommodate the type of gas the reservoir would produce. A further complication is the amount of associated gas varies not only from field to field, but through the life of the field. For re-injection to be an option, the underground formations must be of the right kind of geology to accept the gas.
Ultimately, the decision on how to deal with associated gas is a mix of commercial, technical and safety matters. It is consequently influenced both by the UK fiscal regime and the available market for gas.
Even if gas can be exported or re-injected some flaring may be necessary in oil and gas production operations to ensure safety.
A complete flare system consists of the flare stack or boom and also of pipes which collect the gasses to be flared. The flare tip at the end of the stack or boom is designed to assist entrainment of air into the flare and so improve burn efficiency. Seals installed in the stack prevent flashback of the flame, and a vessel at the base of the stack removes and conserves any liquids from the gas passing to the flare. Depending on the design, there may be one or more flares required at a production location. Plates 1 & 2 show typical onshore and offshore flaring arrangements.
A flare is normally visible and generates both noise and heat. In the process of flaring, the burned gas generates mainly water vapour and carbon dioxide. Efficient combustion in the flame depends on achieving good mixing between the fuel gas and air, and on the absence of liquids. Low-pressure pipe flares are not intended to handle liquids and do not perform efficiently when hydrocarbon liquids are released into the flare system.
The percentage combustion efficiency of a well designed and operated flare is in the high nineties. Recent work by the US Environmental Protection agency has shown that combustion efficiencies are often greater than 98% (Ref. 2).
Calculations of combustion efficiency often ignore carbon emitted as soot or smoke, because unless it is very smoky, the solid carbon emitted is likely to be insignificant. In fact, measurements made on propane flares have shown that soot accounted for a decrease of less than 0.5% in the combustion efficiency.
| 
Environmental Report 1998 Index
