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The answer, my friend. . .

ENVIRONMENTAL GEOTECHNICS

Offshore windfarms are a growing sector of the UK energy industry, and that is good news for geotechnical firms specialising in over-water work. Max Soudain reports.

The UK is the windiest country in Europe. The British Wind Energy Association estimates it has one third of the Continent's potential offshore wind reserves - enough power to meet the country's energy needs three times over.

Offshore windfarms look likely to become common sight on the coastline over the next few years, driven by a combination of public objections to land-based windmills and the Government's drive to ensure that 10% of domestic electricity is generated by renewable resources by 2010.

While the UK only has one working offshore windfarm, 1km off Blyth on the Northumberland coast, the Government has identified 18 potential sites. The most suitable areas are on the east and west coasts. On the east coast, the zone runs from south of the River Humber through Lincolnshire and East Anglia to the Thames Estuary.

On the west coast, the best areas are in the northwest, from Blackpool down to north Wales.

Each facility will have a maximum area of 10km 2and be limited to 30 turbines, producing a minimum of 20MW. The Blyth Windfarm has two 2MW turbines but produces enough electricity to supply 3000 homes.

Sites were chosen on economic and technical considerations - water depth, wind resource and grid connection - and, of course, prevailing weather.

'All of the sites are pretty horrible, ' says Mark Richards, Seacore commercial manager and contract manager for nearshore site investigations in the UK and Ireland. 'They are between 7km and 10km offshore, in under 20m of water with a high tidal range and strong currents.' Offshore drilling contractor Seacore installed the foundations for the Blyth windfarm (GE January 2001) and is now involved in two other projects, at the appropriately named Scarweather Sands off the south coast of Wales, and at Shell Flats, near Fleetwood just north of Blackpool in Lancashire.

The Crown Estate owns the seabed up to 12 nautical miles (22km) from the UK coast and developers have to bid for licences to build and run the windfarms.

After a pre-qualification process, developers, usually private energy firms, assemble a construction team to carry out environmental and geotechnical studies for the bid package.

Before a licence is awarded, bidders must carry out an environmental impact assessment.

Seacore supported developer United Utilities Green Energy's bid for the Scarweather Sands site.

United Utilities says the windfarm, which will cost up to £60M, will have 30 turbines generating enough power for 45,000 homes and will save 3.1Mt of carbon dioxide emissions over its 20 year life.

The first phase of the £0.5M site investigation started at the beginning of May. The site is 7km due south of Porthcawl and 10km southeast of Swansea, next to the Bristol Channel, one of the UK's busiest shipping lanes.

This phase includes installation of a meteorological mast or 'met mast'.

'Five boreholes were put down, one at the met mast and the other four scattered across the site, ' says Richards. The firm is using its Deep Diver jack-up barge for the contract. This powerful rig has 40m long legs, making it suitable for open seas, remote coastal sites and high tidal ranges in water up to 35m deep.

Seacore drilled a 45m deep borehole to investigate the proposed met mast location, using a combination of shell and auger through marine sand and Geobore S triple tube wireline coring system in the underlying siltstone and mudstone.

The firm also carried out static cone penetrometer tests using a Fugro 20t capacity hydraulic penetrometer unit bolted to a sub frame cantilevered from the side of the jack-up.

A stabilising casing was lowered to the seabed using the jackup crane and secured using a hydraulic clamp. This restrained the cone rods laterally during testing. Side friction and cone end resistance were measured using load cells in the cone and pore water pressure was measured using a pressure transducer. Data was sent to an on-board laptop via an umbilical cable through the hollow push rods.

But it has not all been plain sailing. Offshore and nearshore work is usually restricted to the summer months for obvious reasons, but unseasonably bad weather at the end of May and beginning of June epitomises the problems that can beset offshore work.

Downtime of at least 200% is built into any offshore drilling contract, says Richards. 'For example, if a job is scheduled to take 48 hours, we could allow another 96 hours. We usually take this risk, as clients do not like paying for standing.' Storms held up work at Scarweather for nearly two weeks. Even in normal circumstances, with currents up to 4 knots and waves up to 4m high, the environment 'has a number of issues', says Seacore exploration director Marcus Rampley.

'Jack-up stability and the safe transfer of staff on and off the rig are important considerations, ' Rampley says.

Scour is another problem when working with strong currents in relatively shallow water (here it is about 25m).

'Scour around the jack-up legs has to be closely monitored. It tends to affect an area up to three times the diameter of the leg, creating a shallow depression disc, which can make the legs unstable, ' Rampley says.

To combat this, the legs are redriven. 'However, if there is a clay layer below the sand then there is a chance of punch through, with a leg free-falling and causing the platform to tip, ' he warns, though he points out that this rarely happens.

Another major consideration has been unexploded ordnance and shipwrecks. The area is notoriously hazardous, and Rampley says it is 'full of wrecks'.

Once ground conditions are confirmed as suitable, the 90m high met mast will be installed on its 2m diameter, 40m long tubular steel pile. The entire operation takes 9 to 10 days. The met mast is fitted with equipment to measure wind speed and direction. Data will be gathered for at least 12 months before any further geotechnical investigation.

Richards says the second phase will involve obtaining information at each turbine location and investigations for cable laying to land. This is likely to involve drilled boreholes, CPTs, geophysics and vibrocores.

Final design of the turbines will then be carried out. Rampley says the technology is advancing so quickly that it is difficult to know exactly what will be built.

Typically, turbines have 35m to 40m long blades and are mounted on 60m high masts, founded on a steel tube monopile foundation, but gravity platforms have been used. Construction at Scarweather is likely start in early 2004 and Seacore is hoping to be involved in this stage too.

The Shell Flats contract is similar but has a slight twist. Here, Seacore installed two met masts for client Shell.

Richards says the original plan was to carry out site investigation in advance of met mast pile installation, as at Scarweather.

However, because the area is a Ministry of Defence low flying zone there are access issues, including possible conflict with military radar systems.

Instead, the whole operation was fast-tracked, with pile installation following straight after investigations. The first stage was for Seacore to carry out a desk study and to use local British Geological Survey boreholes to build up a general picture of the geology.

Seacore used its Wijslift 6 jackup barge, the largest in the fleet, to carry out the drilling. The barge was fitted with the firm's marine drilling mast, the CRII, with drilling of the 30m borehole carried out using the Geobore S system.

'The system is designed for continuous coring of sediments, uncemented deposits and competent rock, ' Richards explains.

Geology is sand and clay over glacial till. When drilling through the marine sediments, the system was used in its wash boring configuration, with SPTs and a range of insitu sampling carried out.

Coring was then carried out to obtain 102mm diameter cores in the till. Borehole samples were logged on board and a CPT carried out simultaneously, again by Fugro.

Borehole profile data allowed Fugro to make a rapid on-site evaluation (using software modelling) of the suitability of the location for the erection of the met mast. At both locations, the ground was deemed suitable and the 2m diameter up to 54m long piles were installed immediately, embedded about 25m below the mudline, Richards says. The met mast itself is 90m high and was installed by Seacore for Great Yarmouth based contractor SLP.

The poor start to the summer affected this job too. At one stage the crew were stuck on the barge.

'These were exceptional circumstances though, ' says Rampley. The barge is well equipped and is the only one that the crew would be left on, he says.

Despite the hold-ups, investigation and met mast installation took as little as three to four days.

Work was finished last month.

The Wijslift 6 is also fitted with a 280t crane, which Rampley says is ideal for lifting the massive monopiles needed for the wind turbines. With this in mind, Seacore is hoping to win some of the installation work at Shell Flats.

An estimated 1300 2MW turbines are needed to meet the Government's renewable energy targets, so it looks like Seacore's order books will be full for some time yet.

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