The Strait of Gibraltar site investigation was successfully completed in June 2005. The Anglo-Norwegian joint venture combined Seacore's R100 drilling rig with Eidesvik Subsea's multi-purpose vessel Kingfisher. Extreme conditions were encountered in the deepest part of the strait in the middle of the busy shipping lanes.
The samples taken will be used to establish and characterise the seabed geology and determine the depth, alignment and design of a proposed rail tunnel crossing between Europe and Africa.
The investigation project is part of a comprehensive and detailed €27 million feasibility study by the Moroccan and Spanish state owned companies, Sociètè Nationale d'Etudes du Détroit (SNED) and Sociedad Española de Estudios para Ia Comunicacióni Fija del Estrecho de Gibraltar, (SECEG).The project to build a rail tunnel linking Europe and Africa was first discussed in the 1980s. It is expected the tunnel will be about 40 km long and 300m below the sea bed at its deepest point and a service tunnel will be the first to be built.
Seacore, one of Europe's leading specialist marine exploration drilling contractors, installed its R100 drill rig on the powerful, multi-purpose offshore vessel Kingfisher to provide the drilling and sampling services. The R100 drill was originally designed and built by Seacore for drilling and core sampling near the North Pole along the Lomonosov Ridge in the Arctic Ocean in the summer of 2004.
For the Strait project 14 boreholes were drilled to determine the extremity of two pockets of clayey flysch quaternary breccia: with the exception of one, the boreholes were concentrated in a 3km central section of the narrow strait. The drillers encountered what is believed to be a smectite clay, which swells by about 20% in volume when disturbed and is not particularly suitable for tunnelling. The expanding clay posed a major challenge when coring and to prevent blockages of the core barrels Seacore used a unique mix of drilling mud. Drilling to depths of 300m below the seabed has never been done before in the Strait and even Kingfisher the most powerful drilling ship available, was limited to drilling in tidal currents of around 4 knots. Currents in the Strait reach 6 knots, just over 3m/s, and the Seacore/Eidesvik team operated in low tidal cycles of between 5-9 days, followed by periods back in port when currents exceeded 4 knots.
Key to the project's success was Kingfisher's ability to stay accurately on station: the vessel's GPS is linked by computer to an array of powerful thrusters, enabling the vessel to stay within one metre of its target. The vessel was equipped with Seacore's heave compensated R100 drill rig and a novel, purpose-built borehole drilling and re-entry template, known as the DART. This has been adopted in place of a normal seabed frame and allows Seacore to disconnect the hollow drill string and internal sampling core barrel in 10 minutes. The DART, designed and built by Seacore, looks like a giant hole cutter with a central short hollow pilot drill protruding below a 2m diameter can with serrated cutters round its base. On top of the DART is a conical docking guide with a special low torque thread, for rapidly disconnecting the drill string in the event of an emergency, leaving the DART, which weighs 30t, implanted in the seabed.
At each borehole location Seacore's drilling crew, working round the clock in two 12 hour shifts, first lowered the DART through the R100's drilling platform and the open moon pool, gradually adding sections of hollow riser and special aerofoil type fairings. The fairings freely rotate round the riser in the direction of the prevailing current, eliminating vortex induced vibration and minimising drag. This process continues until the DART is suspended just above the seabed. At the same time the crew control a remotely operated vehicle with on board camera hovering just above the seabed. The ROV is used to find a suitable flat location on the variable and complex seabed prior to planting and rotating the DART to sink it vertically to a depth of about 500mm into the sea floor. On previous campaigns seabed templates had been positioned using bathymetry information and a camera deployed within the riser. The visualisation provided by the ROV has proved to be far more effective in selecting sites to secure implantation of the DART. Seacore's ability to put up to 50t tension in the riser considerably helps to minimise riser deflection. The R100's heave compensation caters for the vessel’s rise and fall in 4m swells in the strait - well within the rig's 6m capability.
With the DART implanted vertically in the seabed Seacore's drill crew ran a 61mm diameter HQ Series 3 core barrel system down the inside of the hollow riser to take core samples to a maximum depth of 325m below the seabed. If the coring was not completed to full depth within a working window, the core barrel was pulled out and the riser undone at the DART using the low torque threaded coupling. Back on station the DART was captured by lowering the riser into the conical re-entry docking guide and rotating the riser to remake the fixed connection ready for coring to continue. When coring was completed the barrel was removed and the riser and its attached DART rotated slowly to break the seabed seal prior to full recovery by the R100 ready to move to the next location. The recovered cores were sent for on-shore logging and testing. Seacore completed its challenging seabed investigation in 148 days.