VolkerInfra was contracted to lay an underground outfall from a water treatment facility near Torrs Point, south of Kirkcudbright in the far south west corner of Scotland, to an exit point in Kirkcudbright Bay.
The challenge was that a 300mm wide hole had to be drilled through solid rock from high on the cliff top, ‘punching out' within three metres of a designated spot down on the sea bed of Kirkcudbright Bay some 290m away. "An accuracy level of around 1% of the distance is about normal," says VolkerInfra Project Manager Scott Stone, who supervised the drilling process.
Keeping the drill on track was made more difficult than usual because of a jack-up barge which had a magnetic field interfering with that of the earth, which the instruments on the drill rely on to determine their orientation.
Eight articulated lorry-loads of equipment arrived at the site on Monday 8 May 2006, and VolkerInfra's 7-man team immediately began to ready it for drilling. As well as a 150 tonne drilling rig, there was a 14 tonne pump for the special ‘Bentonite' mud used to power and lubricate the drill and carry away its cuttings, and a 22 tonne unit for recycling this fluid, removing the cuttings and other drilling debris. "Typically there will be between 1,000 and 3,000 litres of drilling mud pumped every minute," says Stone.
Drilling commenced on 10 May and after just six days proved successful. "We punched out on the sea bed exactly where we wanted to," says Stone. This accuracy was achieved by keeping close tabs on the position of the business end of the drill, and withdrawing it and re-drilling if it fell outside acceptable limits. "You take a reading every nine metres," says Stone.
The position of the drill was calculated by the usual method of keeping records of the angle of its parts relative to the earth's magnetic and gravitational fields, and the distance drilled. But, to avoid any error brought by interference from the jack-up barge, a system independent of gravity and magnetism was also used, called ‘Tru-Tracker'.
"We didn't have to ream the hole in this case because the diameter needed was narrow enough for us to drill to exactly the right size," according to Stone. Creating holes suitable for larger diameter pipes often means pulling a reaming tool back through.
There was, however, a brief hiatus before the pipe could be fitted. The cause? "We had a number of bad weather days," says Stone. While it would have been no problem for operations on dry land, in this case divers could not safely do the underwater work needed to begin the process of pulling the pipe back into its hole.
Thankfully the weather improved enough on Saturday to take the pipe out to sea, ready for connection and pulling in on Sunday. "We just had to wait for a diving window between the tides at around four on Sunday afternoon," says Stone. "The connection was successfully made at 8.30 p.m. This is when divers, supported by boats, go down and connect the pipe to the end of the drill string so it can be pulled back into the hole."
The pull back itself started at 9.30 p.m. and was successfully completed shortly after midnight.