MEDWAY COUNCIL COMAH OFF-SITE EMERGENCY PLAN ...

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ABRIDGED - Medway Council Off-site Emergency Plan – National Grid Grain LNG Ltd. without serious injury’. Escape distance is specific to people, and so is not considered any further with respect to effects on the environment. Secondary fires distance is defined as ‘the distance at which secondary fires may be initiated by piloted ignition in buildings made of combustible materials such as wood’. The threshold level for piloted ignition of wood is 12 kW/m 2 . Above this secondary fires will occur if the receiving material is irradiated for a sufficient length of time. Spontaneous combustion of grasses occurs with thermal radiation levels of approximately 15kW/m 2 for approximately 15 minutes. Therefore, some vegetation within this hazard range could be expected to ignite. 15.5.1 A secondary fire could present a more significant effect on the vegetation present on the Medway or Thames Marshes Ramsar, SPA and SSSI Sites. Marsh communities are usually resistant to burning, except in periods of exceptional drought, because they are so wet. However, if the fire was severe enough to damage the marshland, there is a potential for natural succession to give rise to the development of a slightly altered habitat. Following an incident an investigation would be undertaken to assess whether a MATTE had occurred. Grain Pit is beyond the secondary fires hazard range, and is therefore unlikely to be harmed by thermal radiation from the terminal. 15.5.2 The effects of fire on grassland and agricultural land, and the definition of a MATTE have been discussed in previous paragraphs in 12.4.1, and the same conclusions apply, although a considerable area may be affected, rapid post-fire regeneration is expected. 15.5.3 It is not considered possible that a secondary fire would cause the soil off-site to be heated to such a high temperature to cause sufficient damage to constitute a MATTE. Similarly, the SAMs and Grade One listed buildings are beyond the secondary fires hazard range from the terminal and are unlikely to be damaged. 15.5.4 The potential effects of fire on species have already been discussed in the section on flammable gas clouds, and it is considered that the same conditions would apply to a secondary fire. A post-incident investigation involving consultation with the appropriate statutory conservation organisation would be carried out to ascertain whether a MATTE had occurred. 16 Other Environmental Consequences of a Major Accident (extract from National Grid Grain LNG’s COMAH Safety Report) 16.1 Gas Release - The release of gases produced by combustion of natural gas (carbon dioxide, carbon monoxide, nitrogen oxides, and hydrocarbons) to the atmosphere from an ignition scenario is not considered to be a significant environmental effect, compared to the national releases of these gases. 16.1.1 Release of natural gas without subsequent ignition would result in the emission of methane to the atmosphere. Methane is a significant greenhouse gas, with a global warming potential 21 times greater than carbon dioxide. However, any release of methane from the National Grid Grain LNG Storage site would not contribute significantly to global warming. In terms of annual emissions of methane from the UK, which are around 3,800,000 tonnes, release of the total site inventory would be insignificant. June 2011 Page 34 of 52
ABRIDGED - Medway Council Off-site Emergency Plan – National Grid Grain LNG Ltd. 16.2 Gasholder Water Overflow – Addition of excess top-up to the above ground, water sealed, low-pressure gasholder on site could result in overflow of holder water. Holder water can contain concentrations of various chemicals, which could be potentially damaging in the environment. However, a study has concluded that a release of gasholder water on any scale would not harm the environment. 16.3 Diesel Release – The release of diesel from the site is considered to be extremely unlikely, as diesel would have to escape a bund and two interceptor pits in series. However if it were to be released, it would either form pools or soak into the ground, or float on the surface of water. The extent of any environmental damage is highly dependent on the quantity released, and it should be borne in mind that if any diesel left the site it would only represent a small fraction of the total inventory. 16.4 Firewater Runoff – Firewater is the term given to the water used to extinguish fires. In some cases the fire service may add surfactant to water used for extinguishing fires. This is unlikely to be used in a gas fire, but may be used to extinguish a fire involving liquid hydrocarbons. Surfactant may mobilise contaminants present in the soil, but the risk at the site is not considered to be significant. The effect of firewater without surfactant is not likely to be different from the effect of prolonged heavy rain. 16.5 Firewater Reservoir – The firewater reservoir on site is treated with algaecides to control fish and algae. Although commercial algaecides are normally used these are not considered to be harmful to the environment under the terms of the COMAH Regulations. 16.6 Conclusions – Under current criteria it is very difficult to predict whether a fire or diesel release at the National Grid Grain LNG Terminal could be classed as a major accident to the environment due to the number of variables involved. However the major accident scenarios (contained in the site’s COMAH safety report) that could lead to off-site effects have very low frequencies, the highest being that of a major earthquake. June 2011 Page 35 of 52
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