THE 2.1km long outfall at Tremorfa in Cardiff is part of a Dwr Cymru Welsh Water's £250M scheme to provide a new sewer system for the Welsh capital.
Flows from smaller sewers will pass to a £150M treatment plant being built south east of Cardiff, which will feed the outfall, due for completion in 2002.
The project is the largest of 50 clean-up schemes planned for the Welsh coast and is the largest currently under construction in the UK. Installation of the outfall is being carried out by Dutch contractor Van Oord in conjunction with Hyder Consulting.
Pipeline construction involves onshore pre-assembly of the 36m long epoxy coated steel pipework, each made up of four separate sections. 2.1m diameter concrete cylinders, manufactured by Hepworth.
Each completed segment weighs 200t and is lowered on to the landward end of the pipeline using two 250t capacity cranes, working in tandem to minimise weld stresses. Once in position, the segment is welded and grouted into place.
As each new segment is added, the entire pipeline is winched further out into the sea by a specially designed barge, anchored 2.5km offshore.
The concrete cylinders provide balanced negative buoyancy, allowing the pipeline to be towed into position with minimum pulling force but with enough weight to anchor it in the sea bed trench once in position. The maximum force required to tow the complete pipe into position is just 80t.
The project was subject to severe environmental constraints as the area is a winter haven for wading birds. So to minimise disruption to the foreshore, the first 500m of pipeline is being laid in a 6m wide cofferdam.
To complicate matters further, contractors had to cope with an estuary with the world's second highest tidal flows and very low underwater visibility.