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Marine special: Seabed surgery

Contractors had to overcome considerable logistical challenges to replace a section of damaged long sea outfall at Cleveleys on the Lancashire coast. Ed Owen reports.

The beaching of a ship will always make national headlines - a huge steel hulk lying helplessly on the sand makes for a wonderfully poignant photograph.

When the Riverdance ferry beached in severe storms in early 2008, its massive hull may have been responsible for more than just a headache for its owner.

United Utilities discovered shortly afterwards that one of its emergency stormwater outfalls, buried under the sand at around that point, was crushed. Whether the weight of the Riverdance was directly responsible or not is being settled in the courts, but nevertheless, the section of outfall needed to be replaced in a £2M operation.

Failing to repair the 1km carbon steel outfall would have allowed dirty water to keep seeping into the sand, polluting the beach. Contractor Volker Stevin was selected to fix the problem in July 2009.

It in turn approached NorthWest based consultant GHA Livigunn to design a solution to repair the 2.5m diameter section in the inter-tidal zone, some 350m from shore and 15m below the seabed.

Workers in Cleveley

Workers in Cleveley

Stormwater flows through the outfall from the Anchorsholme sewage works which serves Blackpool, Cleveleys and Fleetwood even after moderate rain. Flows can reach up to 2,000 litres per second at peak.

“The pipe was installed in 1984, and was thought to have a 60-year design life. When we dug down to the pipe, it was confirmed as damaged and a section needed to be replaced. It is a long pipe, extending around 1km from shore,” says GHA Livigunn’s lead consultant Noel Privat.

“It’s a plumbing job. We have to cut out the piece that is not working, put in a new piece, and then join it up,” he says.

Achieving this was a little trickier. A primary concern was that the untreated effluent from the sewage outfall had to be kept from the open water. The solution was to build a temporary cofferdam around the affected area and use divers to cut out a 20m section and then replace it.

But before work could begin the Riverdance had to be broken up and removed. Contractor Volker Stevin controlled operations from an 18m2 jack-up barge perched over the cofferdam.

The cofferdam itself is rectangular, 25m long by 7m wide and built from “spud” piles designed to be thicker where the lost pressure acts - in this case above seabed level.

“It’s a plumbing job. We have to cut out the piece that is not working and put in a new piece and then join it up”

Noel Privat

The piles are driven between 14m and 20m down - except where they are inserted above the outfall, where they are shorter. The 20m piles are at the corners of the cofferdam, where their extra depth adds stability.

Inside the cofferdam, excavations exposed the pipe which was then held in place by a series of steel frames built by the contractor and designed by oil and gas specialists at consultant Peter Fraenkel & Partners.

The frames allowed damaged and new pipe segments to be suspended from steel straps hanging from the frames, allowing them to be more accurately positioned.

While the section of pipe to be repaired is a small proportion of the outfall, the 2.5m diameter, 20m long damaged section is not insignificant. The replacement section was split into three - two 7.5m sections and a single 5m section to make transportation of the total 36t load easier. “They were huge,” says Privat.

The frames also provided a handy access point to the water. This was a significant safety boost as the work is potentially extremely dangerous, with divers working in near-zero visibility, and water pressure at 1.5 bar - equivalent to 15m of water. Cutting the damaged pipe was a tense moment, says Privat.

Logistics were a significant problem says Voker Stevin contracts manager Stuart Newby. “Every day was different, because of the tides and weather. We also needed two of everything because everything breaks down all the time - due to extreme conditions on this exposed stretch of sand.”

The coastline at Cleveleys

The coastline at Cleveleys

They also had to agree to strict daytime working hours between 6am and 10pm to keep local residents happy, so could not simply use the most favourable tides.

“This week there has been a 10.5m tide down to a 0.2m tide,” he said. In low tides it is possible to walk to the barge, but in very low water the barge is almost cut off, with water too shallow for a boat to float. In higher water a specially trained pilot can ferry workers to and from the platform.

But a very different problem quickly became apparent. When the works began last July, changing tides and the continuing use of the outfall had been taken into consideration. What they had not expected was the coldest winter for 30 years.

Initially, work was expected to last five months but the job was extended to eight as freezing conditions brought operations to a complete standstill over the winter.

When work finally began again, the damaged section of outfall was removed by a 70 tonne crane fixed to the jack-up barge.

Cleveley's outfall

Cleveley’s outfall

Once the damaged piece of pipe was removed, the diameter of the original pipe had to be checked for deformations so the new sections could be modified to fit snugly.

The new outfall sections were not welded or threaded but linked using rubber lined Teekay sleeves. These are semi circular gaskets which fit around the joint, sealing it.

The steel pipes are prevented from rusting by cathodic protection, so electrical connections were also made.
Some of the outfall remains exposed and has been painted by the divers using a special paint developed by the oil and gas industries.

“The setting process sets up an exothermic reaction between the steel and the paint, so the water in between evaporates, giving a perfect finish, despite painting underwater,” says Privat.

By early March the repair was complete, despite the winter delays, leaving the sand at Cleveleys clean in time for summer.

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