Eurotunnel monitors water 40m under the seabed
16 Mar 2007
The Channel Tunnel is 50km long, with the 39km undersea section making it the longest undersea tunnel in the world. Eurotunnel continuously monitors natural groundwater collected around the tunnels before pumps it to the coast for discharge into the sea.
The Eurotunnel system consists of three separate tunnels: two rail tunnels through which the trains travel, and a central service tunnel. This “safe haven” is used for maintenance and evacuation, and is linked to the rail tunnels every 375 metres. On average, the tunnels lie 40 metres below the seabed of the English Channel.
The service tunnel is maintained at a higher air pressure and thus remains free from smoke and fumes in the event of an incident. It provides access to a wide range of assets that ensure safe and efficient operation of the tunnel. This includes the water monitoring systems, the pumps and pipelines.
Water that seeps down to the tunnels is a mixture of groundwater and seawater. It is collected at six drainage stations and is continuously monitored to protect the enormous pumps (capable of almost 1000 m3/hr) and pipes from corrosion and ensure that water discharged to the sea is not environmentally harmful.
The previous monitoring system suffered from blockages in the small pipes that passed water to the sensors. Hach Lange and Eurotunnel engineers therefore decided that a flow-though holding tank would resolve potential problems with blockages; large bore pipes could be employed and sediment could be removed easily.
Each of the six flow-through tanks contains sensors for conductivity, turbidity, dissolved oxygen, pH, Redox and temperature and data is transferred to a PLC that is programmed to raise alarms when pre-specified conditions occur.
If an alarm is raised all water is immediately passed to an underground storage sump and remains in quarantine until tested and passed as fit to be allowed into the pipeline. The monitoring system returns to normal once water quality levels leave the alarm condition.
The new system is more reliable and requires less maintenance and is easier to operate, said Kevin Rivers, senior M&E technician at Eurotunnel. “The water quality monitoring sensors are connected to SC100 controllers which are ‘plug and play’ – all you have to do is tap the serial number into the controller and it starts to monitor correctly automatically.
The reliability of the new system means that we no longer experience false alarms, which is a major benefit; there are strict procedures in place before an M&E team can enter the tunnel to investigate an alarm and coupled with the amount of time it takes to drive to the monitoring equipment, false alarms are very costly.”
According to Rivers, the new system requires about one quarter of the maintenance that was previously necessary, in part because the new dissolved oxygen sensor employs an optical monitoring technology that does not require recalibration – we simply change the sensor cap every year.
“These installations have been successful because the technological requirements identified by the Eurotunnel team coincided with the development of new monitoring equipment,” said Hach Lange, project manager Clive Murren. “The instrumentation has been deployed in a customised monitoring system, designed specifically to overcome the problems that had been previously experienced.”