BAC Corrosion Control has been appointed by Pierse McAlpine Tideway Consortium to design, supervise installation and commission an impressed current cathodic protection system for the submarine pipeline being constructed as part of the £200 million Dublin Bay Project. This is an integrated scheme to provide for the collection and treatment of wastewater from the entire Dublin City urban drainage catchment area.
The submarine pipeline will run from a new pumping station at Sutton, on the north bank of Dublin Bay to Ringsend on the western edge of the bay. It involves the construction of approximately 10.5 kilometres of 1422 mm diameter steel pumping main pipeline to be laid under the seabed across Dublin Bay in inter-tidal and marine areas from the Sutton Foreshore to the inlet of the proposed, upgraded wastewater treatment works at Ringsend. The pipeline will cross under the navigation channel of Dublin Port, have a 90 degree turn and will include a section of about 325 metres on land adjacent to the Ringsend treament works.
The pipeline comprises API 5L grade X42 steel pipe with fusion bonded epoxy and polyurethane wrapping on the outside. Over this 160 mm of grade 40/20 concrete weight coat has been applied to give the pipe minimal negative buoyancy for laying on the seabed. In the water each metre of pipe weighs 75kg. Strings of nearly 400 metres long are assembled on land at Sutton, rolled on to the launchway and pulled across the seabed by a giant barge. To allow for deeper draught vessels in future, the trench in which the pipe lays has been cut 25 metres deeper under the shipping channel.
The BAC system will protect the pipe from corrosion throughout its lifetime using an impressed current system, initially rated at a maximum output of 30 volts and 50 amps DC. Impressed current cathodic protection systems employ inert (non-galvanic) anodes with an external source of DC power to impress a current from anode to cathode (pipe) through the seabed. System design is such that the spread of protection can be up to 50km or more for well-wrapped pipe under submerged conditions.
Anodes are commonly used in groups and buried in a low resistance backfill called a groundbed. ICCP systems are considered to provide the most efficient cathodic protection for long distance lines. ICCP systems used on pipeline installations, have monitoring points at regular intervals along their length, and these can be linked into a telemetry scheme allowing pipe protection levels to be checked from a central monitoring station.
The majority of BAC’s equipment including the electrical power will be located at Sutton as will the anode groundbed. There will be points along the whole of the route for monitoring and most of these will be readily accessible at low water. Near the deep water channel a test cable point from the pipeline is exposed on the surface, being terminated on a marker buoy.
To minimise possible disturbance to wildfowl wintering in the bay, the pipe must be laid by the end of September. The BAC ICCP system is due for completion in November. The new Sutton pumping station is scheduled to begin moving the wastewater through the pipeline to Ringsend in the Spring of 2002.