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Taywood puts its foot on t e gas

CONCRETE - On the edge of the Pembrokeshire Coast National Park, contractor Taylor Woodrow is surging through a project to build fi ve gas containment tanks.Paul Thompson reports

SIMON Jenks, Taylor Woodrow project manager, is looking tired as he sits at his desk in the project office at the South Hook Liquefied Natural Gas Terminal near Milford Haven.

He and the rest of the 400 -st rong work force at the site is halfway through a 14-day stretch of non-stop concrete pouring as the first of five massive liquefied natural gas containment tanks rises out of the south Wales bedrock.

'It is a demanding schedule, ' Mr Jenks concedes, 'but one that we are confident we can keep to.' It will be no easy task. Building f ive 94 m external diameter, 43 m-high prestressed concrete gas tanks will give the Taylor Woodrow team a few sleepless nights ? in fact they will be working through the nights for a fortnight during the construction of each tank.

There are two 200 men shifts working 13-hour stints in order to hit the 14-day target, which will see the final tank wall stand 31 m high. Normally these kind of storage tanks are jump-formed but by slip-forming them Taylor Woodrow aims to slash months of the overall construction time.

'We will be slip-forming the walls up to 31 m then eventually we will hit the final height when the domed concrete roof gets placed , ' says Mr Jenks.

The huge site, perched on the edge of the Pembrokeshire Coast National Park, is one of several devoted to the oil and gas industries in the area. Most of the big oil companies have some representation in the locality, making good use of the deepwater anchorage provided by Milford Haven, which can accommodate the largest of supertankers.

Some of these vessels, specially built to transport liquefied natural gas from fields in the Middle East, will bring their cargo in to dock at the finished LNG terminal and unload directly into the tanks.

A few hundred metres away from the dock and deep below the smattering of site offices that sit on the rim of a huge crater cleaved from the site, workers scurry around the first of the gas tanks that is coming out of the ground.

When finished, a prestressed concrete outer shell will shroud a double-skinned and insulated steel liner tank that is welded to steel embedment plates set within the concrete skin.

More than 160 of the 12 mm-deep, 120 mmwide steel embedment bands will run vertically around the tank. Another two run horizontally.

'They are not there to offer any structural support, ' says Mr Jenks, 'merely to act as fixing points for the steel liner.' The slip-form system has been developed specifically for the production of these sort of concrete containers by Austrian-based specialist Gleitbau. Taylor Woodrow has worked with Gleitbau on previous projects and knew its level of expertise. Taylor Woodrow called the company in to help design forms that would ensure that steel embedments in the concrete would actually sit f lush with the inner face of the concrete.

'In the past, slip-form shutters have always been slightly splayed. That means that concrete gets in behind the embedments and they end up recessed into the finished face. Gleitbau developed a system that ensures that these embedments are pulled f lush with the inner face of the concrete so it is easy to weld the steel tank into posit ion, ' says Mr Jenks.

The system was trialled at Gleitbau's Salzburg base but the Taylor Woodrow team also wanted to test and perfect the method on site. A fullscale test wall was built to help iron out any problems there may have been with the system.

'As it was it all went very well but we felt it was important to let everyone have a go with the system so they all knew their various roles and how it would work, ' says Mr Jenks.

The test wall sits close to the concrete batching plant set up on the project to help feed the appetite of the slip-forms. Not only does it ensure the continuity of supply but having a dedicated concrete plant also helps the team tailor the mix as and when climatic conditions alter.

'You can only finish slip-forming once, so we need absolute control over the mix, ' says Mr Jenks.

Wind is the major problem for another of the three project managers on the scheme, Mike Finlay. He is behind the concrete m ix and keeps a close eye on the forecast before altering the design if the weather requires it.

'Wind is the real killer for us, ' says Mr Finlay, 'It means we have to retard the mix because we can't work safely under high winds.' The recipe for the mix includes sea dredged sand, limestone dust, cement and ground granulated blast-furnace slag, plus 'a few sweet ies', says Mr Finlay.

Like all good cooks he is unwilling to unveil the secrets of the recipe but admits that he uses a super plasticizer normally used in f loor screeds to help improve the concrete's pumping characteristics.

'It gives a good cohesiveness, improves the pumpability and the final finish is good too, ' he says.

He is also the engineer who coined the ph rase 'porridge prams' to describe the 16 mobile concrete hoppers that are constantly pushed anti-clockwise around the tank by an army of workers.

Each bar row holds 0.25 cu m of concrete that is dispensed from the top of the concrete pump at an abutment on each quadrant of the tank. Half of the load is directed into pouring the abutment itself, the other half into a hopper, which in turn will load the barrows used to form the main tank.

'We then have four poker men working away at the concrete, ' says Graeme Bowen, the third in the triumvirate of Taylor Woodrow project managers on site.

Mr Bowen explains that to keep cont rol of the progression of the job the 700 mm-wide walls are built up in a series of layers, each around 200 mm deep. These layers normally take around 1.5 hours to pour and each shift averages some 1,200 mm of formed wall.

'It's a very good system, ' he says. 'Once a layer has been finished at the buttress then it means the layer is finished on the main wall.' Once the shell has been completed the concrete tanks will be prestressed using horizontal hoop tendons made up of 19 st rands of 15.7 mm-diameter wire that are stressed to a 4,241 kN capacity. These will be passed through 100 mm-diameter ducts as will the vertical tendons, which are produced using 12 strands of the 15.7 mm-diameter wire and are stressed to a maximum of 2,678 kN.

The prestressed walls of the tank sit on top of a heavily reinforced and heated concrete base.

The base boasts a 1 m-thick ground bearing slab at its outer edges dropping down to 400 cm thick in the centre. It is heated to stop the cold liquefied gas creating a layer of permanently frozen ground, akin to Alaskan permafrost zones, in the west Wales count ryside.

Factor in the huge costs involved in this massive feat of civil engineering ? the overall cost of the project is more than £7.2 billion, although Taylor Woodrow is unwilling to discuss how much it is asking for its part in the scheme ? and the project team needs to hit some major targets before the Christmas shutdown.

At least three of the tanks must be completed before the holiday period to allow main contractor CB&I John Brown to get in and fix the steel lining and roof to the tanks before Taylor Woodrow is back to cast the concrete roof.

But this is not enough for Mr Jenks and the other project managers.

He says: 'We definitely have to have three of them ready by Christmas but we are aim ing to get fou r done.' That goal shouldn't really be a problem. With the first tank under their belts everyone on site knows exactly how to keep the project running like clockwork and after all, practice makes perfect.