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Welsh rare bit

Declining North Sea gas supplies have prompted construction of one of the UK's fi rst Liquefied Natural Gas storage facilities in 13 years. John McKenna reports from Milford Haven.

The government raised fears last month that the UK would face a natural gas shortage this winter leading to an increase in fuel bills. Although this may be news to consumers, the construction industry has been gearing up for a shift in energy supply for over a decade now. The decline of North Sea gas reserves has meant that the UK must resort to importing Liquefied Natural Gas (LNG) to meet the needs of 80M homeowners. One of the first storage facilities for LNG is being built in Milford Haven in Wales.

Between now and January 2006 seven new storage tanks over 30m high will appear on the Milford Haven shoreline to store LNG from the Middle East and Africa. The projects amount to an £874M investment to receive, store and convert imported LNG, the first shipments of which are due in late 2007.

The tanks are being built by two energy companies at locations just outside the town (see map). Two tanks on Petroplus' site in Waterston have just been completed while the first of the remaining five tanks will start construction this week at Exxon Mobil's refinery in South Hook (see box).

Contractor on the Petroplus site is Whessoe Oil & Gas-Volker Stevin joint venture and client is Dragon LNG, a consortium of Petroplus, BG Group and Petronas.

The cylindrical concrete tanks are being slipformed to speed up the construction programme.

Each structure takes just three weeks to build. Construction is labour intensive and a team of 250 site operatives work round the clock to construct the 35m high structures.

'Slipforming meant we were able to shorten the total construction period by six months, ' says Volker Stevin project manager Bauke Lobbezoo.

The tanks have an external diameter of 82.4m and sidewalls 700mm thick. Each container requires about 7,500m 3 of concrete to be built.

To slipform something of this size requires careful planning because, as Whessoe construction manager Peter Francis says, 'once you start, you can't stop'.

Slipforming requires a constant supply of concrete to avoid a cold joint developing, which would lead to a discontinuity in the concrete.

It is a problem Whessoe is addressing by making the concrete on site.

'We have a concrete batching plant here that can produce 90m 3 per hour, which is backed up by a smaller standby plant, ' says Francis. 'If the first breaks down - which has happened a couple of times - the second kicks in, powered by a generator.' Concrete is poured into the formwork by five cranes taking concrete from three skips. However, using cranes, rather than pumps, can cause problems.

The site regularly experiences winds of up to 40km per hour, and anything above this speed means cranes are not safe to use.

But if the cranes are out of use long enough, the concrete could set, a cold joint could form and the whole structure would have to be knocked down and rebuilt. This would set back the project at least six months.

To avoid this, three hours before bad weather is forecast a plasticiser retardant is added to slow down the setting time.

So far, high winds have put cranes on the site out of action for up to 12 hours at a time, so why not just pump the concrete in?

'To do it effectively you would need four pumps which would be much more expensive than cranes. It would also be much more difficult to control their discharge, ' says Francis.

Concrete strength of C23/40 in the tank walls is post-tensioned through a series of vertical and horizontal cables. This ensures they will be robust enough to withstand the pressure of holding 165,000m 3 of LNG which has already been compressed to a six hundredth of its original volume. To remain liquefied, it must also be kept below -162-C.

The LNG is kept at cryogenic levels within a nickel alloy riser tank, covered with a concrete and steel composite roof.

Lobbezoo says the tanks are designed for maximum security: 'The inner tank should do the work, but if there is a leak, the outer concrete tank is designed to withstand the loadings.'

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