concrete society 2008 awards winner civil engineering catagory

NOVEMBER 2008 CONCRETE 32

South Hook LNG Tanks,Milford Haven, SW Wales

Civil Engineering

Category

WINNER

Figure 1 below: LNG tanks at South Hook

with (Inset) jetty.

Figure 2 bottom: Preparation for a tank

base.

THE CONCRETE SOCIETY Awards EXCELLENCE IN CONCRETETHE CONCRETE SOCIETY Awards EXCELLENCE IN CONCRETE

Innovative techniques have enabled slipforming to meet the particular requirements of liquefied natural gas (LNG) tanks and hence saved six months construction time. Concrete mixes have been designed using local materials to suit construction needs and a high-quality product has been achieved.

Imported LNG provides the UK with a replacement for dwindling North Sea gas reserves, and increases diver-

sity of the country’s energy sources. Natural gas is cooled to -160°C, at which point it liquefies, occupying 600 times less volume than in gaseous form and hence economical to transport by ocean-going tanker, in this case from Qatar.

Project detailsThe South Hook terminal, west of Milford Haven, will be one of the world’s largest LNG terminals, supplying one-fifth of the UK’s natural gas. Five tanks, the size of the Albert Hall, are situated in a cutting to minimise impact on the adjacent National Park. LNG tanks are essentially giant thermos flasks with an outer concrete shell providing pro-tection and support for insulated inner steel tanks storing liquid natural gas.

The South Hook LNG Company is the client and CB&I UK is principal contractor, with Taylor Woodrow as sub contractor responsible for the prestressed concrete outer containment tanks.

Design and planningReinforced concrete is an obvious choice for LNG contain-ment structures but previously these were constructed by jumpforming. At South Hook, however, Taylor Woodrow saved six months from the client’s original programme by slipforming.

A crucial requirement for the 700mm-thick walls was to cast continuous vertical steel embedment plates into the inner face, flush at 1.8m centres. A steel liner plate subse-quently welded to these embedments creates a vapour bar-rier. Although slipforming is significantly quicker than jumpforming, and avoids construction joints, an inherent disadvantage is that embedments become recessed into the concrete because the shutters are inclined inwards. Had this occurred, the benefits of slipforming would have been lost.

To overcome this difficulty, Taylor Woodrow, and its long-term slipforming partner Gleitbau, separated the inner shutter and integrated the embedments into the moving surface. Factory and full-scale site trials were undertaken to prove the process.

Following discussions between Taylor Woodrow, pre-stressing subcontractor SSL and CB&I’s designers, the design was amended to suit slipforming. For example, the walls that originally tapered in from base up were changed to a parallel-sided design. Reinforcement was detailed by Taylor Woodrow to suit slipforming and the prestressing system was simplified to achieve the specified force enve-lope with fewer tendons than originally designed.

An early key decision was how best to obtain concrete. Consistent plastic properties and a reliable supply available around the clock were vital. Consequently, Taylor Wood-row hired a dedicated batching plant and designed con-cretes in-house. Local sources of suitable limestone aggre-gate and sea-dredged sand were identified and cement and GGBS were obtained from the closest sources. Trials deter-mined relationships between setting times and retarder dosages, pumping properties, measured heat of hydration and proved the characteristic strength of the concrete.

One of the more difficult requirements for the con-crete was to maintain a high level of workability through the placing process. This process required concrete to be

Figure 3: Slipforming work at night on Tank 2.

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CONCRETE Nov 08 17-32.indd 32CONCRETE Nov 08 17-32.indd 32 23/10/2008 09:36:3523/10/2008 09:36:35

pumped 30m vertically into a holding hopper, then flow by gravity into a 0.25m3-capacity barrow, for transport to the wall. Investigation of many additives eventually led to a plasticiser normally used for floor-slab construction. This, combined with a nominal amount of retarder to hold initial set and pumping aid, produced the required concrete.

Environmental initiatives included recycling waste cement slurry through the batcher, crushing surplus con-crete for reuse as hardstandings and partial replacement of cement by GGBS, which also limited heat of hydration.

Rigorous quality-assurance processes were imple-mented, with Taylor Woodrow establishing separate verifi-cation and production engineering teams, as well as an independent UKAS-accredited on-site concretelaboratory.

ConstructionThe programme required three slipforms completed by Christmas 2005, with the remaining two by June 2006, after a site start in May 2005. A 10m-long half-height slipform trial provided hands-on experience to all con-struction personnel, highlighted improvements to detailing and construction methods and gave assurance to the client. It also determined how retarder dosages should be varied

to slow the rate of rise of the slipform, this being necessary to install a large continuous horizontal steel embedment.

Although working conditions for the labour and engi-neering teams were arduous – particularly for the first four slipforms that were affected by rain, winds and snow – the teams performed superbly, completing four slipforms in 2005, and the final one in March 2006. Tolerances were tightly controlled on the walls ensuring that CB&I’s auto-matic welding equipment could be deployed on the inter-nal steel tanks. Concrete finishing was undertaken from a platform hanging beneath the slipform, with minimal remedial work necessary to meet the specified standard of finish.

Following completion of walls, CB&I fabricated the

CONCRETE NOVEMBER 2008 33


Figure 4 far left: Reinforcing bars form the capping beam with both vertical and horizontal ducts for the post-tensioning visible. Structural Systems used a cryogenic BBR VT Cona Multi-Strand bonded post-tensioning system.

Figure 5 centre: Slipforming of the first tanks gets underway. The tanks are 98m wide and 30m tall.

Figure 6 above: Trial panel of wall for one of the tanks. The holes in the main wall are the post-tensioning ducts, which are stressed from the other side of the buttress. The buttress shows the termination live end points for the tendons coming from the opposite direction.

Figure 7 left: The horizontal post-tensioning strands being stressed from the live end anchorages using a 500-tonne stressing jack. Each tendon ran around the half-perimeter and was stressed from opposing buttresses. Each live end consists of nineteen 15.7mm strands.


steel roof liner at base level in the tanks, and fixed a 2m-deep steel ‘tension ring’ around the top of the walls. The liner was then raised to the top of the walls by CB&I using air pressure from below and welded to the tension ring. Taylor Woodrow then constructed a 2.3m-deep ring beam around the roof, after which the walls wereprestressed.

A roof trial also led to improvements. Originally, the roof concrete was specified to be placed in two layers, separated by a construction joint. The trial showed that the concrete could be placed in one layer without slumping down the dome’s steep gradient and led to the horizontal construction joint being omitted, improving quality and reducing construction duration.

The roofs were reinforced then concreted, with the steel liners acting as permanent shutters. The first roof was con-creted in a continuous operation around the clock but con-crete finish was improved on later roofs by concreting the flatter outer perimeter during day shifts, with just the dome concreted in a continuous four-day, three-night operation.

The fifth roof was completed in November 2007 and the experience has been taken by CB&I and Taylor Woodrow to their next joint project, the LNG terminal at the Isle of Grain in Kent. ■

Figure 11: Completed LNG tanks with pipeline installation.

NOVEMBER 2008 CONCRETE 34

Judges’ commentsThere can be few if any projects in the UK at present

that compare with the South Hook LNG Terminal

development in terms of pure scale of the physical

infrastructure. This project is undoubtedly impressive

in its size, volumes and vital statistics. However, it is

very much more than just a large project using a lot

of concrete.

The project team has clearly invested time

and money up front to develop the right solution.

Technological boundaries have been pushed and

the construction team has worked closely with the

(very receptive) client. In addition, the team has

demonstrated extraordinary attention to detail and

meticulous planning with military precision. The result

is slipforming on the largest-ever scale in the UK.

There has been a clear awareness of the

local community’s needs and concerns. Also

an awareness of sensitivity of work within the

Pembrokeshire Coast National Park, hence the low

visual impact of project within the wider landscape.

The highest accolade that any contract team

could hope for has been received by Taylor Woodrow

from its client choosing to take this revised design

and construction method and making it standard for

all its future terminals.

This project is of the highest standard and reflects

all that is best in innovative use of concrete in civil

engineering.

Figure 8 right: Tank 5 – internal fabrication.

Figure 9: Upward view of Tank 4.

Figure 10: The tanks nearing completion,

with the lids ready to receive the

reinforcement prior to being concreted.

• The five tanks are each 95m diameter and 42m high to the top of the domed roof.

• Each slipform required 6300m3 concrete to be placed and 670 tonnes of reinforcement bar and ducts to be placed during a 15-day continuous pour.

• Each tank required 13,000m3 of concrete and 2500 tonnes of reinforcement.

• Approximately 1,000,000 man-hours were expended in building the five concrete containment tanks. Labour force levels peaked at 500 during slipforming.


South Hook LNG Tanks, Milford Haven, SW WalesOwner: South Hook LNG CompanyConsulting Engineer: CB&I UKContractor: Taylor Woodrow ConstructionLabour Supplier: Bowercross ConstructionSlipform Shutter Supplier: GleitbauPrestressing Subcontractor: Structural Systems (UK)


concrete society 2008 awards winner civil engineering catagory

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