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; none of the pipework or ancillary equipment is contained. The risk of oil being lost from ancillary equipment and pipework is high; the Control of Pollution (Oil Storage) Regulations 2001 recognises this fact and require that tanks have all ancillary equipment such as sight tubes, taps and valves retained within a secondary containment system.