Laing O’Rourke and its partners, Munters and Faber Maunsell have successfully completed a main cable de-humidification system on centre spans of the Severn Suspension Bridge for the Highways Agency. This is the first operational main cable de-humidification system in the UK. 

The project has involved wrapping the Severn Bridge main cable in polymer based material to create a homogenous seal and to install a de-humidification plant room within the bridge. The plant then drives dry air into the main cable to reduce the level of humidity in it and suspend or delay the progression of corrosion of the cable strands.

Technical details of the project

Cable Wrapping: The polymer based material supplied by D S Brown of North Baltimore, was spirally wound onto the main cable and heat shrunk in position creating a seal between each spirally wound section and a tight fit onto the main cable itself.  The process was devised by D S Brown who supplied the wrapping tool, called a Skewmaster, and the heating blanket.  To overcome the problem of heat transfer between the heating blanket and the irregular shape of the main cable, Laing O’Rourke specialist wrapping contractor BASE Structures, developed a heating blanket incorporated in an air cushion that, when pressurised, formed itself onto the surface of the main cable whatever the irregularity.  This was a great innovation by BASE Structures applying a simple solution to a technically difficult problem.

The sealing system was proof tested on an air test rig developed by Laing O’Rourke.  Perforated steel plate was rolled and formed to the diameter of the main cable, the wrapping was applied and the rig was pressurised, the results were excellent.

With the majority of the work at height above and alongside the busy M48, a simple solution for access was needed.  This was provided by ALPS the specialist working at height provider who supplied twin access cradles that span either side of the main cable.  The twin cradles were attached to fabricated trolley frames that run on the handstrand cables above the main cable.  With cable reelers installed at a higher position on the main cable the gantry assembly could be pulled up the main cable from one section to another.  The reeler is remotely controlled from the cradle omitting the need for any control or power cables in the cradle.

In addition, cable wrapping the end seals and longitudinal sealing was completed on the hanger cable band providing a completely sealed system.

De-humidification system: The system was designed and installed by humidity specialists Munters, working closely with Laing O’Rourke and the project consultants, Faber Maunsell to provide reliable plant and equipment for 24/7 operation and real time monitoring.

For injection into the cable the external wire wrapping was removed from around the main cable in three locations on both centre span main cables.  Wedges were inserted into the cable strands to open up a path for air flow, 16 Zinc wedges were installed in pairs around the main cable at each location.  Stainless steel sleeves were fabricated to a high standard by Jordan Engineering, normally associated with fabrications for the nuclear industry.  These fabricated sleeves formed a chamber around each of the locations where the main cable had been wedged open and provided an interface connection for the air flow pipework.

Air flow was provided from the plant room through the bridge structure and attached to the hanger cables up to the injection points in HDPE pipework, electro-fusion jointed and installed by Laing O’Rourke.

The plant room and dehumidification system was designed and commissioned by Munters. The plant room is inside the actual structure of the bridge and all the components, including the dehumidifier, had to be manufactured in sections to enable them to be lowered through the hatch, and then built inside the actual bridge.

There were 72 separate sections which took 6 men an entire day to transport them to the actual site within the bridge. It then took 3 men a further 5 days to construct the plant room, complete with the dehumidification system.

The bridge is a listed building and the necessary approval had to be sought to enable several holes to be drilled in the structure in order for the necessary ductwork to be installed for the dehumidification system.

The desiccant dehumidifier has various monitoring equipment linked into it and this records the relative humidity, the temperature and the air pressure going into the cables. It also records the relative humidity, the temperature and the velocity of the air coming out of the cables. This ensures that the dehumidification system is working to its optimum capacity.  The system has been designed to control the conditions at 20% relative humidity in the plant room, which is then blown up the cable, via the large fans within the plant room.