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Platforms for success

Earthquakes Sakhalin

Detailed soil modelling and focused design is needed for earthquake resistant oil platforms in the Far East, reports Adrian Greeman.

Russia's growing offshore oil industry faces tough conditions in the Far East. Drifting pack ice and storms are hardly the best of working conditions.

Structures, as well as the 'rough necks' working on them, need to stand up to battering by wind, waves and ice in the Okhotsk Sea.

But even bigger problems face companies now trying to open up major new oil and gas fields around Sakhalin Island on the far eastern end of Siberia. For here earthquakes are a major problem too - 'and they're not small ones', says Zygmunt Lubkowski, an associate at Arup Geotechnics and part of a team working on the design of two large platforms for the island.

'These areas are right up at the top in the Russian seismic zoning. A recent event at the town of Neftegorsk in 1995 was 7.2 on the Richter scale.'

The platforms are being built for Sakhalin Energy, a consortium of Shell, Mitsui and Mitsubishi and form part of the £6bn Sakhalin II development.

Platforms generally are massive but the two at Sakhalin, standing in 30m and 48m of water and supporting 32,000t of topside modules each, are especially big.

Designing safe, economically viable structures that are able to resist the extreme loads the environment will throw at them is challenging, Lubkowski says.

In answer to this challenge a 'compliant' approach to earthquakes is being used.

The platforms are being designed to stay operational in a 'rare' earthquake, or in the very worst case, to survive without loss of life in the largest possible earthquake. Their response takes into account the non-linear nature of soils making up the sea bed and the interface between the 10,000m 2, 15m high caisson bases and the ground on which they sit. Energy will be dissipated by allowing the caisson to move up to 500mm in any direction. Further damping is provided by allowing the topside structure to move on four specially designed 2m diameter dished 'friction pendulum' bearings.

Similar design principles - sacrificing a calculated degree of movement for significant savings - are now widely used for bridges and buildings. But the issue is more complicated for a platform. Some 60 pipes used for drilling and as risers for oil and gas connect down through the sea floor. These are potentially vulnerable and are ducted up two of the platforms' 25m diameter legs to protect them from sea action and iceberg strike. But earthquakes pose a different kind of threat.

'A traditional engineering approach would be simply to resist the earthquake with adequate factors of safety, ' says Lubkowski - for example by creating a massive gravity structure. But this would be excessively conservative. Moreover, adding mass to the platform creates its own problems.

'For a gravity platform there is a critical ratio between horizontal and vertical loads, ' explains director of Arup's Advanced Technology Group, Michael Willford. 'As you increase the vertical load, perhaps by ballasting, the horizontal seismic forces also increase.'

Correctly accounting for non-linear behaviour in the soil-structure helps to minimise loads, especially under extreme conditions. 'Simply stated the soil has a damping effect, ' Lubkowski says. But the platform caisson has to resist wind, waves and moving ice.

The designers needed to be very sure about how the structure, its elements and the ground beneath it would interact.

A key tool in this process has been the powerful LS-DYNA software for non-linear finite element analysis, says Lubkowski.

On the Sakhalin II project, computer modelling has allowed complex interactions between the sea bed and the platforms' concrete base structures to be explored. LS-DYNA has also been used to analyse the response of the friction pendulum bearings between the bases and topside modules.

Normally soil is modelled as either fully elastic or elasto-plastic. But this misses some complex intermediate behaviour caused by progressive shearing and failure, says Lubkowski.

One consequence is that there is a tail-off in the magnification caused by many soils in earthquakes. Higher ground motions do not increase as much as might be expected from the effect of low ground motions. As a result, structures move less.

To see what would happen the model is run with a range of soil parameters derived from a comprehensive site investigation carried out by Fugro and detailed laboratory testing by the Norwegian Geotechnical Institute and the University of Western Australia. The ground is modelled as a block of soil 'brick' finite elements to a 100m depth and the platform is represented on top.

Inputs are from real earthquakes recorded in similar locations. 'You need to run at least five different earthquakes, ' says Lubkowski.

Computer modelling also shows graphically how the bearings at deck level help reduce movement not just of the topside structure but also of the legs in a severe earthquake.

Each of four bearings takes some 8,000t vertical load.

Results from the simulations have impacts right through the design, as interactions between all the elements are crucial to understand, says Lubkowski.

Arup has been working closely with designers involved in developing other parts of the platforms, such as the topside modules, and with various Russian Institutes to ensure compliance with Russian regulations.

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