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GOING STRAIT

A proposed tunnel linking Europe to Africa requires deep coring in strong currents. Damon Sch³nmann investigates.

Some projects are so ambitious that many people write them off as a pipe dream. The scheme for an undersea rail tunnel beneath the Gibraltar Straits to link Europe and Africa might fi into this category, particularly in light of the expense of the Channel Tunnel - about £10bn - and its well documented and long running financial diffi culties. The plan for the mammoth project is for a similar setup to the Channel Tunnel with two main rail tunnels and a third service tunnel with connecting cross passages.

But the Spanish and Moroccan governments are taking it very seriously and are funding a bilateral .27M (£18.5M) feasibility study managed through their respective state-owned companies SECEG and SNED. One of the key things is to investigate the seabed strata between the two countries.

A Norwegian-British joint venture is responsible for the deep coring work with the Scandinavian side Eidesvik Subsea operating the ship Kingfisher, and main contractor Seacore carrying out the drilling work under a £3.8M contract.

The turbulent stretch of water is known for its strong currents with fl w speeds varying at different depths, and the Cornwall-based company plans the work for periods of minimal agitation.

The drill string must descend through up to 300m of water and the team operates inside short drilling windows when currents are slowing, generally between fi ve and nine days. The operation is limited to a maximum safe working current of four knots.

At the time of GE's visit, a force eight gale was blowing, but this was nothing compared with previous operations. Eidesvik Subsea offshore manager Steinar Holte says: 'On the last trip we had 10m waves. The bow was going totally underwater and the waves were hitting the wheelhouse.' Seacore exploration division technical manager Stewart Frazer says: 'We run a riser down to the seabed, drill in a 35t DART (drilling and re-entry template) assembly and tension up the drill string against its weight.'

Two 5.5t lead-filled collars also tension the drill string while the riser is connected and disconnected from the DART, which is left on the seabed between drill sessions.

Tensioning the riser and collars compensates for the vertical heave of the ship, providing a platform from which to use a conventional diamond coring rig.

Seacore contracts manager Mark Richards believes this ability to have 50t tension in the riser while compensating for heave is unique.

'The DART has two functions; it provides the reaction mass to allow tensioning of the drill string, essential in the strong currents that can exceed six or seven knots. It also provides an important re-entry facility to any borehole that was uncompleted in a previous tidal window, ' says Frazer.

The DART has a skirt lined with drill teeth that bores 300mm into the seabed. At the end of a shift Seacore detaches the drill string and at the start of the next drilling trip, the drill string can re-enter via a cone at the top, in a manner similar to air to air refuelling.

Frazer says: 'The original DART only weighed 14t. This is the mark 2 version which is bigger and heavier to enhance stability and has a 2m diameter footprint instead of 1.5m.' Ground conditions are a hard limestone cap overlying a conglomerate and about 100m of weakly cemented sand with breccia in critical areas below.

Critical, because, says Frazer: 'The breccia we are bringing up is a complete mess of broken rock and clay and it's a very difficult material to drill. The whole scope of work has changed because the SpanishMoroccan joint venture can no longer tunnel under the breccia channels as our drilling has shown they go so deep.' Robertson Geologging's geo physical borehole logging uses formation density, neutron porosity and calliper gamma. Senior logging engineer John Kenworthy says: 'This is in your face lithology. [Geophysics] is essential as it speeds the job up immensely - otherwise they'd be rooting through core boxes.

'You could open bore with this [rather than retrieving cores], but it's always good to have the samples.' This is Seacore's second involvement with the project, having been involved in a previous 1998 to 1999 drilling programme.

Work on this phase began in January, and subject to negotiation, may continue until the end of the summer.

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