When it comes to costs the devil is in the engineering detail
In his report to the Department for Transport on electrification discussed in the previous item, Professor McNaughton examines a number of the technical issues identified as increasing costs of current schemes. He provides some cost-saving proposals.
DEAD SECTIONS
Prof McNaughton is critical of proposals to run an electrically ‘dead’ section of contact wire under structures with restricted clearance to avoid engineering work. This is ‘unlikely to be a sensible solution in other than very specific circumstances’. And the report cautions that the whole life operational consequences of such extended neutral sections ‘will usually be disproportionate to the initial benefits’.
There was a successful application at one bridge on the Paisley Canal scheme in Scotland. This electrification has been described as a £28 million scheme delivered for £12 million.
However, neutral sections represent a discontinuity in the contact wire. The transition to the reduced wire height under a low bridge or tunnel will involve a gradient in the contact wire from the standard height on each side.
At Steventon Bridge on the Great Western, the wire remains live but the gradient imposes a 60mph Permanent Speed Restriction. This is because the pantograph following the descending wire increases the force on the contact wire and this force is dependent on speed.
Subjecting neutral sections with in-line insulators to these increased forces would be ‘undesirable and a risk to performance’, notes the report. Reducing the gradient to reduce the force would extend the length of the dead section.
Only where providing clearances would be prohibitively expensive, as at Paisley Canal, should neutral sections be adopted.
As for true discontinuous electrification, where trains run under their own power between electrified sections, this may be appropriate in ‘limited low-performance areas or where hybrid electric and self-powered trains have already been acquired’. However, Prof McNaughton emphasises that given affordable costs, continuous electrification of the mainstream main line network should be the norm.
CLEARANCES
New standards for vertical electrical clearances at overbridges or at stations have been blamed for cost escalation. Prof McNaughton notes that in some recent schemes the differences in cost between providing ‘desirable’ or either ‘reduced’ or ‘special reduced’ clearances to the underside of bridges have been ‘significant’. The risk-based evidence necessary to allow deviation from the ‘desirable’ values ‘appears to have become misunderstood’. Alternatively, project schedules did not allow for the time and effort at the design stage, including preliminary surveys, needed to make a valid risk-based case for adoption of location-specific reduced clearances.
Where it can be shown that there is a significant risk of flashover surge, relatively cheap surge arrestors should be the first choice before adopting the cost and disruption of structure reconstruction. The report notes that recent schemes in Scotland have shown that soundly-evidenced, risk-based clearance values can be agreed with the safety regulator.
Reflecting this experience, the report recommends the adoption of ‘reduced’ or even ‘special reduced’ clearances, combined with a risk assessment including use of conductive flashover assemblies and fitting surge arrestor equipment if necessary.
Prof McNaughton quotes the ‘compelling’ example at Cardiff, where an intersection bridge has been developed by GWEP with a ‘special reduced’ clearance of only 20mm. He concludes ‘it seems likely the only locations there could be any advantage in using an electrically-isolated section would be to avoid altering or rebuilding parapets on a Grade 1 or 2a listed bridge structure’. The innovative approach adopted for this bridge won a prize in the 2018 Railway Industry Innovation Awards for consultancy firm Andromeda Engineering, working with Network Rail, GLS Coatings (provider of the insulated coating) and surge arrestor supplier Siemens (p63, August 2018 issue).
TIME PRESSURE
‘Network Rail attempted to keep (electrification) costs down by saying that the new specification (for parapets) is really high and asking whether it could risk-assess things in order to get a derogation and therefore not to have to comply. Last year, when … the cost issues became really clear, it became obvious to the ScotRail Alliance and to me that if we did not comply with the standards the Office of Rail and Road would not sign off the line to go live; if we had continued to debate the matter, we would have got late into the programme and built the railway only for the ORR to say that we could not run anything on it. Around March I said to the project team, “Stop the debate and move to the new standard. The railway will have hundreds of years of life into the future, so fix this now”.’ Phil Verster, then Managing Director, ScotRail Alliance, giving evidence on the Edinburgh Glasgow Improvement Programme, September 2016
PARAPETS
On the subject of bridge parapets, where, in Scotland, the decision to increase the height retrospectively to the new standard caused delay and extra costs, the report again argues for risk-based measures. The required height had been raised from 1.5m to 1.8m. GWEP has similarly complied with the new specification.
Acknowledging that there may be specific locations where this increase is a material issue, Prof McNaughton argues that it may be acceptable to adopt a lower height on a risk-assessed basis. Examples include where the public cannot get right up to the parapet or where there is public no right of way.
Alternatives to raising the parapet where full compliance with the 1.8m height is disproportionately expensive include fitting a grille on the outside face of a bridge to catch objects thrown over the parapet. This is another example of a theme running through the report: time spent on developing an Acceptance Plan, including the risk-based assessment of the specific location, saves money.
SELFIE STICK SHOCK
Extensively covered in this column has been the impact on electrification costs on ORR’s adoption of revised values of safe clearance from public locations to live equipment. The increase was from 2.75m to 3.5m.
This was attributed to the need to recognise new risks, for example, passengers with selfie-sticks or metal curtain rods and workers carrying metal scaffolding. The former Chief Engineer notes caustically ‘It is difficult to see how the practice of carrying scaffolding has changed over the years nor that this specific change of clearance would fully mitigate against gross misbehaviour or training failure of staff with access to railway sites necessary for a safety risk to occur’.
In practice, argues the report, the 3.5m clearance height is more about contact with a train’s live pantograph, rather than the contact wire itself. The specific risk is of someone misusing a long selfie-stick, or carrying a scaffold pole inappropriately, immediately beside the pantograph of a stationary train or when standing virtually on the edge of a platform during the passage of a through train.
‘It seems to me that improbabilities are being compounded and therefore the basis for a national risk-based challenge (to the increased clearance) may have potential’, Prof McNaughton notes, adding that pantographs with insulated horns may solve this problem.
HOPS AND PILES
As the GWEP installation began, readers began reporting piled foundations for the OHLE masts protruding several metres above the ground. As reported previously, this was the result of a flawed method of calculating pile depth, instead of using the established ORE method (so 20th century, you know). As a result some piles had to be driven in two stages to achieve the necessary depth, with the High Output Plant System (HOPS) train having to return after a second pile had been welded to the first section.
For those used to British Rail electrification schemes, pile depths of 11 metres seemed ludicrous. Indeed, one reader commented that in his day job they were the size used for cruise liner moorings.
Analysis by Southampton University has now provided the ‘compelling evidence’ to support the ‘empirically sound’ ORE method. Prof McNaughton notes that the ‘limit state geotechnical approaches’ adopted initially ‘seem to have been provoked by concern about professional indemnity more than practical engineering’.
Thanks to the work at Southampton, future schemes will revert to traditional piling depths of under 4.5m. The University work also suggests that with more experience it may be possible to reduce the depth nearer to the 2.5m used on the East Coast electrification, where masts are still standing after three decades.
Shorter piles will also maximise the performance of the HOPS. This 21st century works train was designed before the GWEP overhead line electrification and was specified to handle pile diameters and depths produced by the ORE method.
Prof McNaughton notes that to fulfil its purpose HOPS will need to be re-engineered, including the production control systems that were de-scoped from its design. The report instances an automatic alignment system to drive masts truly vertically ‘rather than having to rely on the manual control and eye of an operator normally working in the dark
NUFF SAID
Network Rail’s Series 1 OHLE was developed against a very high business specification for the Great Western. It is well designed for those routes which will require sustained speeds in excess of 125mph whilst withstanding very high side-wind loading to avoid operational restrictions from prevailing (in GB normally westerly) gales or gusting caused by topographical factors such as when crossing exposed valleys. It is unclear when such a scheme will next come forward with these needs.’ Prof Andrew McNaughton
