There is something about the Scottish Highlands that puts mankind's achievements into perspective. A 20 storey tower rising from a four storey plinth the size of two football pitches would dwarf most town centres. Set against the stunning Highland backdrop of Nigg Bay near Inverness, however, Amerada Hess' 180M concrete gravity oil platform looks tiny.
Guided by an unusual combination of naval architecture, design requirements and site constraints, main contractor Taylor Woodrow has come up with some extremely advanced concrete mix designs and innovative construction techniques - even though the sophisticated slipforming process has meant that all 30,000m3 of concrete have ultimately been placed by wheelbarrow
Taylor Woodrow is responsible for designing and constructing the 45M concrete gravity base structure of the oil platform. The base is a hollow concrete box 112m long by 94m wide by 16.5m deep, divided inside by concrete walls into 100 cells that will store oil once the structure is on the sea bed. A 16m diameter hollow concrete tower will rise 63m from its centre.
Later, a tubular steel truss tower will be attached to the side of the base, to carry oil pipes and drilling equipment The production platform will then span the concrete tower and steel truss. Once completed, the whole structure will be floated out to sea and sunk into position in the Danish South Arne Field. A 3m deep steel skirt fixed to the underside of the base will key the structure into the sea bed.
'The starting point is the dock shape,' explains Taylor Woodrow project manager Mike Finlay. 'The dock has a concrete sill with a draught of 11.5m at high tide. We had to design the structural shape so that we could get it out of the dock. Though a bigger base would be more stable at sea, it might not be able to get out of the dock.'
As if floating a 115,000t concrete structure (more than half the weight of the Empire State building) was not difficult enough, the task is complicated by the fact that the steel skirt is currently sunk into the dock floor. Compressed air pumped under the base will lift the structure and free the skirt to allow it to float out.
'Next we had to decide the geometry of the structure,' continues Finlay. 'We looked at various shapes, but the square base was the easiest and simplest to build.'
Site dimensions and structural geometry are constraints on most civil engineering projects but there can be few that have to consider naval architecture. But the structure must be stable at sea. 'The most critical point for stability is during installation,' says Finlay. 'The metacentric height (the key factor in the stability of floating bodies) is crucial. It is governed by weight distribution through the shape of the structure.'
For the structure to be stable once afloat it needs to be much heavier at the bottom than the top. To achieve this the contractor is using three different density concrete mixes.
Concrete in the 1.25m deep base slab has a high target fresh wet density of 2,550kg/m3, achieved by partial replacement of the sand constituent with graded magnetite - fine iron ore that looks like coal dust. 'Total replacement of the sand would give a density of 3,500kg/m3,' says Finlay. 'But that would be too heavy.'
The walls of the base structure use a mix with a fresh wet density of 2,250kg/m3. Part of the coarse aggregate content is replaced with Lytag - 10mm to 12mm lightweight aggregate made from sintered PFA.
To achieve a fresh wet density of 1,850kg/m3 in the 650mm thick base roof and tower, Taylor Woodrow went for total replacement of the coarse aggregate with Liapor - 6mm to 8mm diameter expanded clay balls.
'Liapor gives you lighter and cheaper concrete than Lytag,' says Finlay. 'Originally the base walls and roof were to have the same concrete, but it was getting too heavy so we opted for lightweight.'
All the mixes also have a partial cement replacement with pulverised fuel ash, 'for greater durability and because it reduces the heat of hydration', Finlay explains. To accommodate the lower rate of strength gain that results, characteristic strengths of 45MPa in the base slab, 60MPa in the walls and 50MPa in the roof and tower are measured at 56 days rather
A strict weight control regime is a feature of the project. 'Everything that goes into the structure has a detailed weight report,' says Finlay. 'If we need to add ballast at the end we will pour mass concrete into the cells in the base.'
The ability to change throughout the project has been valuable. 'The concrete mixes have come up through construction,' explains chief engineer Hamish Seaton. 'We developed the mixes in our site labs and at our concrete research facility at Southall in London. The design and build nature of the contract has been crucial to the success of the project. '
Because the structure has to store crude oil on the sea bed, it must be completely watertight. This has led to some complex concrete design issues.
'The whole structure is prestressed for crack control,' says Seaton. 'The E-value is the most important determining factor for the prestressing. The designer needs consistency across the three sections otherwise stresses are locked in. Therefore the closer together the E-values the better.'
Also key is the ability of the concrete to transfer heat. The crude oil stored in the base will have a temperature of around 40degreesC while the water outside will be at 5degreesC. This means there is a temperature gradient of 35degreesC across 500mm of concrete. Concrete behaviour in these conditions is crucial and the designers had to pay attention to the coefficients of thermal expansion and thermal conductivity and Poissons ratio.
The construction method has also been crucial to the concrete design. The need for a watertight structure meant that slipforming was the only viable method of construction. This requires concrete that can remain workable for three to four hours and has meant developing sophisticated admixtures, particularly plasticisers.
Conventional superplasticisers based on napthalene sulphonates were evaluated but none could cope with the combination of high workabilities and unconventional aggregates. In the end a new type of polymer-based superplasticiser from Grace Construction Products was selected. A Grace retarder had to be used, to extend the interval to initial set to eight or even 16 hours.
This has been important in the construction of the tower which relies on a tower crane to deliver the concrete and is therefore susceptible to high wind.
Reinforcement design has also been modified to suit the construction method. The rate of steel fixing dictates the rate of slipforming. The steel fixers can only fix horizontal reinforcement about 300mm in front of the moving forms.
'The reinforcement has to be detailed for slipforming,' says Finlay. 'Fixing for slipforming is continuous therefore the reinforcement design must be kept simple. For example, for manageability the maximum length of any reinforcing bar is 6m.
'Also each face of the structure has its own set of vertical, horizontal and shear reinforcement drawings for simplicity.'
To speed up the steel fixing, Z shaped shear links were used in the slab reinforcement and C links in the walls instead of conventional square links. Tapered threaded couplers have also been used extensively.
The 15.75m high walls of the base structure were split into six slipform pours. The largest was 3,600m3 of concrete for a 596m section of wall which took 12 days to pour. Slipforming has been carried out by Austrian specialist Gleichbau.
Concrete is batched in two Elba EMC60 plants. These are capable of producing 60m3/h but the mixing cycles for the three special mixes reduce rates to 35m3/h for the base slab and walls and 20m3/h for the lightweight mix. 'We slipform the walls at around 35m3/h to 40m3/h,' says Seaton. 'The tower goes at about 5m3/hour.'
The concrete is taken from the batching yard in truckmixers. For the base walls it was pumped up to a number of wet hoppers fixed 2.5m above the slipforming platform at the apex of the cell walls. Four chutes running from each hopper carry concrete to wheelbarrows on the slipforming platform inside each cell.
A static pump line is not possible for the tower because loss of water in the line makes the lightweight concrete difficult to pump. The concrete is lifted in skips and fed into wheelbarrows on the slipforming platform.
'So you see,' says Finlay triumphantly. 'Every drop of concrete on this job is placed by wheelbarrow.'