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Call in the marines

SITE INVESTIGATION Drilling in deep water and ferocious weather off Tierra del Fuego gave Seacore's new C100 modular marine drill a chance to earn its stripes.

Rounding Cape Horn could be a fearsome experience for seafarers in the days before the Panama Canal offered an alternative route. Workers for marine drilling contractor Seacore got a taste of what it must have been like, battling the region's extreme weather during an investigation off the Argentinian coast at Tierra del Fuego, close to the notorious Horn.

Deep water added to the challenges of the project, which gave Seacore's new C100 self-contained modular marine drill a chance to prove its worth.

The C100 can drill and take samples in water up to 2000m deep, working from diving support ships, semisubmersibles or jack-up platforms. It fits into ten 12m containers so it can be easily and rapidly shipped overseas. It can be installed and ready to work within 72 hours of arrival on site.

In Argentina, the C100 rig was used from a new multipurpose ship to core the seabed in water depths of up to 85m for new gas platforms on the Aries and Carina gas fields.

Work was carried out for a consortium of Total Austral, Wintershall Energia and PanAmerican Energy. Dutch firm Fugro was subcontracted by Seacore to provide geotechnical engineering services and to undertake cone penetrometer testing.

Coring of the seabed was carried out using 180mm diameter heavy duty casing. A 'piggyback' system was used to prevent the casing buckling and provided a fixed platform referenced to the seabed. This system uses a heave compensator to hold the casing in tension against a seabed template, keeping constant pressure on the drill bit and eliminating the influence of the motion of the vessel on the drill bit.Up to 5m of heave can be compensated by the system.

In the Carina field, high quality 102mm diameter cores were recovered through the 10m thick sediments and into the underlying Tertiary sandstone to depths of up to 74m.

However, an unexpected 32m thick layer of glacio-fluvial gravels and cobbles complicated the investigation at the Aries field. Seacore used a conventional 130mm American Petroleum Institute (API) soils string to penetrate the layer and provide hammer samples.

The piggyback system was then used through the API soils string to drill to 89m below the sediment. An overall penetration rate of 4.5m/h was achieved during one shift with 89.8% core recovery.

Weather encountered during drilling included 111km/h winds, waves up to 7m high, tidal variations of 10.5m and tidal currents up to 7.4km/h. Fibreglass and aluminium fairings were mounted on the drill and the casing to limit drag from the strong currents. The fairings, which swivel around the casing and the drill string, are dynamically shaped to face the current and allow water to pass around the drill to cut drag by 40%.

Despite the adverse weather conditions, fewer than eight days were lost out of 27.Seacore says this low downtime was a result of the new vessel's station-keeping capability, combined with the suitability of the C100 rig and the drilling system employed.

Although primarily a contractor, the firm also designs and manufactures its own equipment.'This means we can adapt to any job and constantly evolve with every contract, ' says Seacore exploration director Marcus Rampley.

The C100 rig is only one of the many pieces of equipment developed at Seacore's headquarters in Cornwall since the company was formed in the mid 1970s. Others include a walking jack-up platform for near shore and overwater drilling work.

Seacore has grown from a marine exploration firm to one of Europe's largest marine geotechnical drilling and civil engineering contractors.

It has two main divisions, exploration and construction, and recently set up Seacore Mining, now working on a diamond mining project off the coast of South Africa.

The firm claims to have drilled some of the largest marine boreholes in the world. On a contract in Hong Kong in 1996, it drilled two 4.1m diameter 70m deep holes in granite beneath 30m of water as part of a contract to build an offshore sewage treatment works. At the time the holes were the largest diameter and deepest holes ever drilled in rock overwater.

Rampley says one of the biggest potential growth areas for large diameter drilling is in offshore windfarm development. Seacore has already installed two wind turbines and their accompanying 3.5m diameter monopile foundations for the UK's first offshore windfarm at Blyth, Northumberland (GE January 2001). Similar projects have been carried out in the Baltic Sea off the coast of Sweden.

With 18 sites for offshore windfarms approved in the UK, the firm is developing its system of installing wind turbines to make the process quicker and more economical, Rampley says. Future windfarms will have up to 30 turbines and promised grants from the government will see a dramatic increase in demand, he adds.

The firm recently installed a 40m tall meteorological monitoring mast for a proposed windfarm 11km off the east coast of Ireland.The mast will determine the meteorological conditions of the site to obtain design parameters for 200 proposed wind turbines, explains Seacore commercial director Richard Reed.

He says the mast and its 1.4m diameter monopile foundation, placed 15m into silts, fine sands and gravels, were installed in a single operation, a method that could be developed for wind turbine installation.

'Although no one in the UK is at this stage of development, we are evolving the procedure to drive down the cost of building wind farms, making the wind energy a viable alternative way producing electricity, ' he says.

'Each turbine costs about £2M and the foundation installation is around 30% of the total. This is not a commercial price and needs to be reduced between now and 2010, when there are plans to have 10% more renewable energy.'

Libyan pipeline

Two more pieces of Seacore equipment developed in-house, the Skate 2C and Skate 3C jack-up platforms, have been used on a shallow water investigation for the Western Libya Gas project.

The £850,000 investigation, due to finish at the end of last month, was carried out in water depths of between 2m and 30m off the coast of Libya, near Mellitah.

Client Agip Gas is proposing to build a jetty with single point mooring facilities and a pipeline to pump oil from Libya to Italy. Fugro was consultant for the investigation.

Work began in January to core a total of 540m of rock in 18 holes up to 60m deep through sands and gravels overlying well-cemented sands and gravels. Testing included 600 SPTs and over 40 CPTs along with pressuremeter testing and piston sampling.

Skate 2C is capable of operating in 1m to 21.5m of water and the Skate 3 in 1m to 30m of water. Their modular design mean that the deck areas can range from between 105m 2to 177m 2for the Skate 2C and 178m 2to 238m 2for the Skate 3. The jack-ups can be fitted with hydraulic thrusters so they can move when the legs are retracted.

Seacore says that in their compact configurations the platforms are ideal for fast accurate positioning in intertidal areas, while in the larger configurations the platforms are capable of working safely in exposed open seas.

All pontoons, components and equipment are designed around a container freight concept; the pontoons double as containers in which the jack-up legs, power units and other ancillary equipment are housed. This allows cost effective international transport by road, rail or container ship.

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