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GOOD MIXER

Five huge concrete tanks, similar in size to the Albert Hall, have been built to store liquified natural gas (LNG) at the South Hook terminal in Pembrokeshire.

From a distance there is no way of gauging the sheer scale of the new £3bn liquefied natural gas (LNG) facility being built in South Hook near Milford Haven in South Wales, but it soon becomes apparent when you get up close to the massive storage tanks. There are five of them in all, each measuring 98m in diameter and 40m in height – similar in size to the Albert Hall. Each will eventually store 150,000m3 of liquid gas at a temperature of -167°C when the facility is up and running.

Taylor Woodrow arrived on site at the South Hook project in April 2005, having won a contract for all the civils work from the scheme's overall contractor Chicago Bridge & Iron (CB&I). "When we were tendering this work we looked at various ways of building these tanks and thought it was going to be inevitable that they would have to be done using jump form," explains Taylor Woodrow's engineering manager Mike Finlay. "Understandably, the client wants the facility in production as soon as possible, and speed of construction is critical to that, but jump form would have meant a long programme."

Finlay and his team knew that slipforming would be a quicker option, but were aware that it had not been done on a project like this, where the internal face of each tank contains 164 vertical "embedments" cast flush to the concrete. The "embedments" are needed for fixing the steel lining that fits inside each tank.

Usually slipforming is used when there are few – if any – special features to be accommodated in the concrete, as the process requires the concrete to be poured continuously as the formwork rises. However, Taylor Woodrow and specialist contractor Gleitbau developed a slipforming system that enabled the "embedments" to be formed which, says Finlay, saved at least six months on the programme compared with the jump form alternative.

"Once we had decided to slipform, one of the most critical elements was the concrete," he says. "Given that the quality of the product that comes out of the bottom of the slipform is nearly all due to the quality of the concrete that goes into the top – as well as the workmanship – we spent lot of time developing the mixes."

Taylor Woodrow was given some requirements for the mix designs – minimum cement content, water/cement ratio, required strength and slump – but was left to design the detail itself. While the requirements are different for the bases, walls and roof, the contractor opted for limestone aggregate – which gives a more elastic concrete – and ground granulated blastfurnace slag (GGBS) in all three. GGBS was chosen because it helps control temperature variation during construction and has been shown to help the concrete withstand aggressive environments.

"One of our biggest constraints is the location," explains Finlay. "We're in the middle of nowhere. There is a total of 87,000m3 of concrete in the tanks, so you are looking at bringing in 200,000t of material. The road infrastructure around here is not brilliant, so we tried to use materials that were sourced as locally as possible."

The limestone came from a quarry at Carew, 10 miles away, the sand was dredged from the Severn estuary, cement came from nearby Aberthaw and the GGBS from specialist supplier Civil & Marine in Port Talbot.

Local supply also made it easier to ensure there would be continuity, which is absolutely essential during slipforming, which is a continuous 24-hour process. The contractor could not risk running low on materials, and set up its own batching plant on site, as well as storage for 150t of cement and 150t of GGBS.

Before starting construction, Taylor Woodrow did a lot of tests to get the cement/GGBS proportions right, then moved on to full-scale batcher trials. Once the contractor was happy with the mix, it cast a 12m square trial base and did a 10m x 12m slipform trial. "You don't get a second chance with slipforming," says Finlay. "We were using upwards of 300 labour, so if anything went wrong or we had to stop it would be quite costly."

2. SLIP FORMING
Has cut construction time

3. DAY AND NIGHT CONSTRUCTION
EVEN IN FREEZING TEMPERATURES

The base slab of each 98m-diameter tank ranges from 400mm to 1m in thickness and was formed using 40N/mm2 strength concrete. Each was cast in 13 pours.

A 50N/mm2 strength concrete, in which 50% of the cement content is replaced with GGBS, is being used. It was a fairly fluid mix, with a 175mm slump to enable the concrete to keep its workability during the slipforming process.

Once slipforming was running at full speed, Taylor Woodrow managed to raise the height of the wall by about 5m every 24 hours. "That is fair moving along for a 700mm thick wall," says Finlay. "You're talking about 40-50m3 of concrete per hour." Each tank took between 13 and 15 days of continuous slipforming to construct.

Construction of the domed roof of each tank also required continuous pours, with each one requiring 3,500m3 of concrete poured over six days. This time the 40N/mm2 mix was far less fluid, with a 75mm slump, but it still had to be capable of being pumped through a 60m long line, so contained a range of superplasticisers and retarders.

The roof construction involved CB&I constructing a steel liner within the walls of the tank and using air pressure to lift it up then welding it into position at the top of the wall. The domed roof of the steel liner was strong enough to support the weight of the roof reinforcement and, once this was in place, the tank was closed off and the inside of the steel structure pressurised so that it could carry the load of the concrete as it was cast. Once the roof concrete reached its seven-day strength, the pressure was taken off, but the liner remained in place inside the concrete tank.

Concreting is now finished, and Taylor Woodrow's activities are focused inside the tanks, where insulation and the permanent steel inner wall are being fixed. The entire project is due to be completed later this year, when LNG will start to be shipped in from Qatar to provide 20% of the UK's gas requirement.

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