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Oil Storage - Double Walled Tanks [Hardfacts]


Pollution from the storage of oil and related substances is a significant environmental problem. There are around many incidents involving spillage of oil each year. The majority are caused by oil leaking from tanks, often during delivery/filling of the tank. For several years fuel and oil have accounted for around 12% of all pollution incidents in England and Wales.

The Control of Pollution (Oil Storage) (England) Regulations 2001 have helped improve the control of oil storage and reduce the number of incidents. These Regulations have helped stop these incidents by requiring tank owners to provide a secondary containment facility, such as a bund sump or drip tray to prevent oil escaping into the water environment.

Note: These Regulations only apply to England - they do not apply in Wales or Northern Ireland. Similar legislation for Scotland and for Northern Ireland is in place.

Double Skinned Tanks

The terms "double skinned", "integrally bunded", "twin walled" and "bunded" plastic and steel tanks are often used in relation to oil storage. 

A Double Skinned Tank is just that - a primary tank with another "skin" placed around it with a very small gap (interstitial space) between the two; but the pipework or ancillary equipment such as filling points and delivery hoses/nozzles is positioned outside of the outer skin.

The risk of oil being lost from ancillary equipment and pipework is high; the Control of Pollution (Oil Storage) (England) Regulations 2001 recognise this fact and requires that tanks with external ancillary equipment such as sight tubes, taps and valves are retained within a secondary containment system, generally called a bund.

A double skinned tank does not provide protection against spillages arising from over-filling or failure of a sight glass or from a leaking hose or filling valve/nozzle.

Double skinned tanks are not compliant with the Control of Pollution (Oil Storage) (England) Regulations 2001 unless additional secondary containment is provided for the tank and ancillary equipment, such as an in-situ bund.

However, double skinned tanks are recommended for underground storage of oil/petrol etc because the interstitial space between the tank skins can be monitored for leaks. Underground installations should also have twin walled, non-corrosive pipework specified. See PPG27 for more information.

Proprietary Tank Systems

Proprietary Tank Systems come in a huge range of designs and are produced by many different manufacturers who may make certain claims about the environmental performance of their products.

Proprietary Tank System or just Tank System is the preferred generic term for tanks often referred to as integrally bunded or twin walled, to prevent the confusion described above. Some tanks systems may have adequate secondary containment to comply with the Oil Storage Regulation requirements (see definition of primary and secondary containment in CIRIA Report RP163) but whilst some may be descirbed as "high specification primary containers" and would therefore be non-compliant without additional containment such as a bund.

To comply with the Control of Pollution (Oil Storage) (England) Regulations 2001 tank systems must have some form of secondary containment:

  • to prevent incident from overfilling
  • to minimise damage from third party interference
  • to contain leaks from primary containers or ancillary equipment

Tank Systems should also be protected from impact damage by suitable physical barriers or if possible re-positioned. If well designed, manufactured, sited, installed, used and maintained correctly Proprietary Tank Systems can be just as effective as a conventional in-situ bunded tank.

The Environment Agency's Pollution Prevention Guidance Note 2 - Above Ground Oil Storage Tanks gives some basic principles about the points to look out for when considering a tank system. Commercially available products can be assessed against the check list.

References and Resources

For England: The Environment Agency

For Northern Ireland: The Department of the Environment

For Scotland: Scottish Environment Protection Agency SEPA

  • Guidance Note on the Control of Pollution (Oil Storage) (England) Regulations 2001  - free leaflet and flowchart
  •  PPG 2 Above ground oil storage
  •  PPG 26 Storage and handling of drums and Intermediate Bulk Containers - Joint Environment Agencies/CIRIA Guidelines

Joint Environment Agencies/CIRIA Guidelines 

Available from DEFRA -

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Please Note: This document contains general information and guidance and is not and should not be relied on as specific advice. The document may not cover every risk, exposure or hazard that may arise and Aviva recommend that you obtain specific advice relevant to the circumstances. AVIVA accepts no responsibility or liability towards any person who may rely upon this document.

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