- 2 Feb 2021
Date received: 18 Nov 2020
Date responded: 11 Dec 2020
Can you please confirm if Transport Scotland has looked at the possibility of installing heated wires to the cables of the Queensferry Crossing to prevent the build-up of ice?
Can you also please confirm if Transport Scotland has looked at measures used in other countries that have experienced the build-up of ice on bridge cables, such as in Canada and/or Scandinavian countries?
If so, can you provided details of what system was looked at and the outcome – i.e. was it decided not to proceed with an alternative measure to fitting ice sensors on the Queensferry Crossing?
As the information you have requested is 'environmental information' for the purposes of the Environmental Information (Scotland) Regulations 2004 (EIRs), we are required to deal with your request under those Regulations. We are applying the exemption at section 39(2) of the Freedom of Information (Scotland) Act 2002 (FOISA), so that we do not also have to deal with your request under FOISA.
This exemption is subject to the 'public interest test'. Therefore, taking account of all the circumstances of this case, we have considered if the public interest in disclosing the information outweighs the public interest in applying the exemption. We have found that, on balance, the public interest lies in favour of upholding the exemption, because there is no public interest in dealing with the same request under two different regimes. This is essentially a technical point and has no material effect on the outcome of your request.
Q. Can you please confirm if Transport Scotland has looked at the possibility of installing heated wires to the cables of the Queensferry Crossing to prevent the build-up of ice?
A. The possibility of installing heated wires to the cables of the Queensferry Crossing, as a method of preventing the build-up of ice has been studied as part of the wide range of options initially considered. However the electrical power demand of such a system would be very high. The efficiency of such a system is unknown at this scale and the maintenance challenges to keep a complex heating system working with no reasonable access are significant. Trials of targeted heating systems have been undertaken by other bridge operators, but none have proven sufficiently efficient or reliable to warrant a full deployment.
Q. Can you also please confirm if Transport Scotland has looked at measures used in other countries that have experienced the build-up of ice on bridge cables, such as in Canada and/or Scandinavian countries?
A. Transport Scotland has reviewed measures used by other bridge operators around the world. We regularly liaise with and share learning with other bridge owners and operators. However, every bridge has different design details and operates in a different climate, so while we can learn from experiences elsewhere, there is no off-the-shelf solution that is suitable for the Queensferry Crossing – any such solution will be bespoke.
Q. If so, can you provided details of what system was looked at and the outcome – i.e. was it decided not to proceed with an alternative measure to fitting ice sensors on the Queensferry Crossing?
The recently installed ice sensors allow the bridge operator to monitor the four weather conditions that we know can cause ice accretion when they converge within specific parameters. This helps identify periods of high risk and provide the bridge operator with advance notice to mobilise patrols of the bridge to identify any ice accretion on site and if required close the bridge to protect public safety.
In terms of preventative measures, we continue to investigate solutions. The general consensus emerging from a review of measures on other bridges indicates that:
- Whilst many methods of prevention and removal have been studied, tested and deployed, no single method or technology has been found to be completely successful,
- Most bridges have devised a monitoring and forecasting system which seeks to give advance warning of the risk of ice forming,
- Most bridges, having not yet identified a completely successful prevention or removal methodology, and instead close the bridge and wait for the ice to fall.
Our previous Operating Company, Amey, reviewed a range of existing and under development ice prevention/removal technologies that could potentially be implemented on the Queensferry Crossing, taking into account experience from similar bridges in other countries in identifying potential viable options.
Systems that have previously been, or that could potentially be adapted and tested for use in isolation or as part of a strategy, on major bridge structures like the Queensferry Crossing include:
- Retaining or controlling ice build-up: modifying the shape of the stays to enable them to hold onto the ice until it can be removed safely,
- Thermal de-icing: warming the stays, or the ice-substrate interface, to prevent ice accumulation or to remove accumulated ice,
- Sheath coatings: different chemical coatings applied to the stay to repel water and prevent ice accretion,
- Mechanical vibration: mechanically induced vibration can be used to break the bond of the ice to the stay to control the removal of ice from the stay,
- Mechanical de-icing: mechanical release chain collars that are used during closures to remove built up ice from the cables,
- Expulsive de-icing systems: which includes pneumatic expulsive (inflatable boot), and electrical expulsive de-icing systems to remove ice from the sheath,
- Other de-icing mechanisms: including ultra-sonic de-icing to motivate high frequency vibrations, and high frequency microwave de-icing.
Whilst it has not yet been confirmed that any of the options considered will actually work on the Queensferry Crossing, the options identified as having the most promise and therefore meriting future/additional research and development work specific to the Queensferry Crossing include:
- Cable sheath surface modifications to retain and control the accreted ice.
- Mechanical vibration systems – targeted vibration systems.
- Dynamic actuation – ultrasonic de-icing.
These proposals are currently being reviewed by our new Operating Company, BEAR Scotland, and the most appropriate options will be considered for further investigations, testing and development.
The Scottish Government is committed to publishing all information released in response to Freedom of Information requests. View all FOI responses at http://www.gov.scot/foi-responses.
Please quote the FOI reference
Central Enquiry Unit
Phone: 0300 244 4000
The Scottish Government
St Andrews House