31. Each network owner is required to prepare a safety
case, showing how it is safely managing the conveyance of gas.
The safety case has to be accepted by HSE before gas can be transported.
In response to the announcement of the sale, HSE reappraised its
safety case processes to ensure that they addressed the risks
of a fragmented industry structure. This reappraisal led HSE to
strengthen the importance of human factors, such as relevant experience,
and introduce an enhanced programme of audits to ensure that new
owners comply with their obligations.
32. Ofgem collects data from the network owners on
the number and duration of non-contractual interruptions. As of
June 2006, there had been six unplanned interruptions to the supply
of gas since the sales.
33. There are some 23,000 reported leaks a year from
iron pipes that were installed prior to 1997.
Cast iron pipes were used to transport gas until the 1970s when
they were replaced by ductile iron pipes. These were found to
fail unpredictably due to corrosion and were subsequently replaced
by polyethylene, a robust but flexible type of plastic.
34. HSE has approved an accelerated programme for
the annual replacement of iron pipes, which requires all pipes
designated 'at risk' to be replaced within 30 years. 'At risk'
pipes are those within 30 metres of buildings. The mains replacement
programme amounts to 102,000 kilometres of pipes and covers 39%
of the network (Figure 3).
Length of mains to be replaced under HSE's programme