BAM delivers sub-sea electricity link to Europe

Near Grain in north eastern Kent BAM Nuttall is partway through a project that will see the UK plugged into mainland Europe. Paul Thompson  reports.

Part of Bam Nuttall's job is to construct twin transformer houses at Grain in Kent which feature two 25m-high, 45m-long reinforced concrete walls.

On a hot summer day the landscape on the Isle of Grain to the north east of the Medway towns of Strood, Rochester, Gillingham and Chatham is looking its brooding best.

The marshlands that inspired Victorian novelist Charles Dickens to set part of Great Expectations in the area have a way of making visitors feel somehow unwelcome, almost cut off from the outside world.

But near the small town of Grain contractor BAM Nuttall is working on a project that will do exactly the opposite and connect this section of the garden of England to mainland Europe. However, it is not the lure of the tourist Euro and a second tunnel to the continent that will link the two.

Undersea cable

In keeping with much of the power-related industry the latest link will be a giant 1000Mw capacity undersea extension cable running between two huge electricity converter stations, the first being built at Maasvlakte near Rotterdam in the Netherlands and the second here at Grain.

Contractor BAM Nuttall has taken the £23m deal to build the civils side of the UK facility under a joint venture with electrical engineering specialist Siemens which will carry out the electrical side of the work.

In a twist of fate BAM Nuttall's Dutch sister company BAM Civiel is taking care of the Maasvlakte station.

BAM Nuttall project manager Jay Moorhouse is the man charged with delivering the socket that will enable client BritNed - a joint venture between energy transmission companies National Grid and TenneT - to deliver its €600m sub-sea link.

Huge transformers

The transfer stations themselves each house six huge 240 tonne transformers that will convert the direct current electricity that passes along the 260km link between the Dutch and UK stations back into alternating current before it passes into the transmission networks on both sides of the North Sea.

"Alternating current loses power over distance so direct current is more efficient. Our job is to provide a station that can convert that back into alternating current for use on the network," explains Moorhouse.

And part of that job is to build the twin transformer houses which feature two 25m-high, 45m-long reinforced concrete walls, built in three 15m bays and connected by a two-storey, steel-framed and precast concrete-walled office and control block.

These houses will accommodate the twin cable and convert the power as it links from the bottom of the North Sea and into the UK network.

Towering over the rest of the site the two walls and the smaller dwarf blast walls that jut out from them at each bay, were slipformed by Austrian specialist Gleitbau Salzburg over a period of four weeks.

"Each of them took about a week to set up and then a week to pour," explains BritNed site civils manager Spencer Pepper. "Then it was a case of moving on to the next wall which took a similar time."

A set of hydraulic jacks set the slipform system on its way casting the 450mm-thick wall - reinforced using 16mm-20mm diameter rebar - at a rate of 150mm per hour.

The jacking rods were placed at 3m centres along the main wall, 14 in total with a further two jacking rods on each of the blast walls.

Concrete supplier Hanson Premix provided the C35 concrete for the walls from its depot at the Grain LNG (liquefied natural gas) terminal a few miles away and although it features plasticiser admixtures the recipe is a fairly standard concrete, admits Moorhouse.

"There was lots of testing carried out to make sure we got the concrete right before the pour.

"We did trials with Gleitbau Salzburg where we set up a pour and struck the concrete within six hours.

"Also we were fortunate that when we were pouring the walls during March and April the weather conditions remained constant.

"It doesn't matter whether it is hot or cold as long as the climate remains constant.

"It means you don't have to play with the mix to accommodate fluctuations," says Moorhouse.

Sensitive equipment

But it is the additions to the concrete reinforcing structure and the lengths that the team have to go to in an effort to fully insulate and protect the sensitive equipment from interference from any external electrical fields as well as protecting the concrete from disintegrating that set the project apart.

In the valve hall where the transformers will sit the team needs to build a Faraday Cage - an enclosure of continuously linked conducting material - which will protect them from external electrical fields.

This involves sitting a copper mesh on top of the rebar cage in the slab which then links on to the reinforcement in the wall.

Further links from the wall into the cladding enable the cage to be completed. And in the filter bed slabs the opposite is the case.

These have to be fully isolated because should any electricity discharge through the reinforcement steel the concrete could heat up, which causes it to crack and fail.

To minimise this possibility every connection in the rebar cage needed to be isolated using a rubber dowel bar sleeve and then tied using plastic cable ties.

"We did look at other options including glass fibre rebar but it was too expensive. At the moment it is almost five times the price of steel bar so it just didn't stack up," says Moorhouse.

And although the laborious task of isolating each connection meant the steel fixing for the filter beds took three weeks to complete there was no programme impact thanks to the civils side of the project being well on course for its December 2009 completion.

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Why Kent?

"There was a great deal of time and effort spent in making sure that we got the right location for the project," BritNed site manager Spencer Pepper explains.

"We had been looking for a site between Norfolk and Kent which satisfied our requirements for the scheme."

That wish list included the close proximity of an electricity sub-station to take the electricity out to the grid, an area to build the converter station with a minimal amount of local impact and also a site which boasted the geography and bathymetry to bring the cable to shore safely.

At Grain the twin cables are to be drawn through a 250mm diameter duct underneath a Site of Special Scientific Interest.

Areas to re-house the site's population of water voles have also been created and it is hoped that these will be able to receive the first of the voles later this year.

"They have been trapped and taken on holiday for a while until their new accommodation is ready for them," says Pepper.

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Fact File

• Scheme BritNed Subsea Link - UK Converter Station, Isle of Grain.
• Client BritNed Development - a 50:50 joint venture between National Grid and TenneT
• Contractor BAM Nuttall/Siemens joint venture
• Slipforming sub-contractor Gleitbau Salzburg
• Project value Overall project €600m.
• UK civils package £23m
• Contract type Bespoke Engineering, Procurement, Construction contract

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Pile placement in the Isle of Grain

While the location might be excellent from a strategic point of view the earth is not ideal for building large structures.

"The ground is not particularly favourable for us," says Jay Moorhouse, BAM Nuttall's project manager.

"It is a mixture of clayey alluvium and gravels."

Hence the installation of 1200, 500mm-diameter driven cast in situ piles reaching as much as 35m through the 1.3m pile mat and into the alluvial clay.

Prefabricated reinforcement cages from Staffordshire-based specialist Romtech supplement the C40 concrete mix pumped into the piles.

"It is a very good quality piling mat. 1.3m thick and produced using 80,000 tonnes of crushed granite brought by boat from Scotland by Bardon Aggregates," says Moorhouse.

The haul road is another area that has had to be rigidly designed.

Not your average site haul road thrown down with some leftover hardcore this has to be able to withstand loading from trucks weighing more than 240 tonnes as they deliver the transformers. It is more than 600mm thick and is strengthened by three layers of tensar geotextiles.