The link below is what your looking for:
http://www.hmrc.gov.uk/manuals/eimanual/EIM33103.htm
The effect of this provision is that a structure that satisfies the definition of offshore installation will remain within the definition unless it has permanently ceased to be used as an offshore installation with no prospect of resuming such use and it has been put to an entirely new use.
Example
The following example illustrates how the legislation works. On 6 April 2007, a mobile drilling rig in port in Rotterdam obtained a contract for work in the Dutch sector of the North Sea from 1 May. It left Rotterdam on 30 April and carried out exploration drilling for gas until 31 July. On completion of its contract, it returned to Rotterdam and remained idle until 31 December. On 1 January 2008, it obtained a new drilling contract in the Gulf of Mexico and left Rotterdam on 1 February arriving in Mexican waters on 28 February where it drilled for oil between 1 March and 5 April.
The rig was an offshore installation throughout 2007-08. At any one time, it was either in use, to be used or it had been used for “exploration with a view to exploiting mineral resources by means of a well”. This includes all periods in transit between locations and the periods when the rig was not used. Therefore, earnings attributable to any duties performed on the rig in 2007-08 are not eligible for SED.
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