High precision maintenance dredging on the Thames

Strong tidal currents in the mouth of the Thames leave heavy deposits of silt material, posing potential dangers to freight traffic on the waterway. This maintenance dredging application is critical to ensure deep water is always available at the lowest tides to the approaches and the berthing facility to the freight terminals on the gateway to the Thames on the north side. Working for GPS Marine and mounted on their self propelled dredge pontoon ‘Arnaud Regis’, this heavy duty long reach excavator is the largest of its type in the UK and is fitted with an electronic dig system linked to a satellite based GPS. This set up has been used successfully on other prestigious projects on the Thames, including Westminster Bridge scour protection, jetty works adjacent to the Millennium Wheel and de-silting operations to the Thames Barrier. The system is thus proven to have a high degree of accuracy.

This computerised system is calibrated on the machine boom, dipper arm and bucket components, linking to a reference point on the machine superstructure. In turn this is referenced to GPS coordinates using a base station and computer on board the Arnaud Regis. Inputs to the machine hydraulics and the connection of a tide gauge enables real time monitoring of the underwater excavation via a 3D computer display on board the Arnaud Regis and a screen in the cab of the excavator. This allows the operator to effectively ‘see’ underwater, controlling the angle of the bucket and achieving an accurate profile on the river bed.

The data is logged on the computer, producing an as-built picture of the dredged area. At depths around 20m, the long reach excavator can remove up to 10 tons of material in one bucket, which required the spudlegs on the pontoon to be extended and the deck strengthened.

Working in a harsh environment such as the high tidal range and strong currents in the Thames estuary, it is crucial that the correct depth is achieved without error, whilst maintaining a high degree of safety. Marine contractors GPS Marine recognise the importance of the right equipment, a skilled crew and a very competent long reach operator to deliver maximum precision with high regard for safety and the environment.