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The Rough offshore gas storage field, located approximately 29 kilometres off the east coast of Yorkshire is the UK’s largest facility for the storage of gas.

The field is designed to meet peak winter demand by injecting gas supplied from Centrica Storage customers via the Easington onshore terminal into the Rough Field Reservoir approximately 300 mts below the seabed. This stored gas is then available to be produced back into the National Transmission System at rates of up to 45 million cubic metres per day – equivalent to approximately 10% of the total gas supplied in the UK during the coldest winter day.

The principal constituent of the natural gas stored at Rough Offshore is methane. The reservoir used to store the gas also contains a light liquid hydrocarbon known as condensate. Condensate is removed from the gas stream at the onshore terminal at Easington and is distributed by an onshore pipeline to be processed and used as an industrial fuel. Natural gas, due to its chemical composition, ease of use and high energy efficiency makes it the cleanest of all fossil fuels, generating almost 40% less carbon than hard coal for the same amount of energy supplied. The use of natural gas over other fossil fuels such as coal or oil therefore results in a decrease in greenhouse gas emissions per unit of energy.

The nature of the business means that the product (natural gas) is not produced “raw” from this facility but is brought in from the national grid, stored, and sent back to the national grid. It can be appreciated therefore that there is little scope for reduction in raw material usage, however recycle/reuse does take place in a number of areas and will be detailed below.

Atmospheric emissions

The Rough operation is characterised by a number of sources of atmospheric emissions typical of offshore gas developments. Main Power generation and gas compression turbines are run on fuel gas, which is derived from the gas storage reservoir during production & standby and from injected gas during injection, whilst emergency back-up generators and firepumps are diesel driven. Combustion emissions from gas fuelled turbines include carbon dioxide, oxides of nitrogen, carbon monoxide and unburnt hydrocarbons, while the combustion of diesel on site also gives rise to emissions of sulphur dioxide. Combustion emission levels from gas turbines are linked to the amount of energy required for normal process operations as well as the efficient operation of these machines. Emissions from diesel plant are minimal due to the use of low sulphur diesel and being emergency use only.

Natural Gas (methane) emissions primarily occur from venting during normal plant operations and fugitive emissions from valves & pipe flanges etc. It should also be noted that depressurising the plant by venting to atmosphere during emergency situations is an important aspect of safe operations. Therefore, although this can be seen as a significant environmental impact it is recognised that there is no alternative, safety takes priority over the environment in this situation. Robust operating and maintenance procedures reduce the likelihood of emergency releases.

Aqueous discharges

Aqueous discharges are principally associated with the small amounts of oil in water produced during normal production operations. In addition there are discharges of sewage, cleaning and maintenance chemicals, anti-biofouling chemicals and service water from the accommodation facilities. In terms of volume, the seawater used by the cooling water systems is by far the biggest during operation. However, as this system works on the basis of direct flow through with the addition of heat, the effect on the marine environment is considered small.

Energy usage

Most of the energy consumed by the field is used to drive the gas re-injection compressors and power generation turbines. The energy required for gas re-injection generally increases as the reservoir becomes full because the reservoir pressure increases. It is not possible to compare annual consumption because the reservoir is filled to different levels each year. This of course makes it problematic to target improvements in energy usage as there is no consistent baseline to work to. Nevertheless, reduction in energy usage is one of the current environmental objectives.

Re-use/Recycle

Waste oil and condensate contaminated with water is returned to shore and for recycling. Methanol is injected onshore to prevent hydrates from forming in the process pipework. It is recovered from the produced water and re-used. Fluorescent light fittings, scrap metal, waste paper, cardboard etc are sent onshore for recycle/re-use.

Unplanned releases

Emissions and discharges for unplanned events e.g. oil spills, have been identified and characterised and the magnitude and significance of their effects evaluated. This category includes oil spills from platform topsides, i.e. during diesel bunkering from supply vessel, tank rupture, bulk chemical spills, pipeline failure and plant failures leading to emergency venting to atmosphere. An Emergency oil spill contingency plan is in place to deal with unplanned aqueous releases.