Scotch and the rocks

Borehole offers effluent solution

The island of Islay, off the West Coast of Scotland, is famous for the production of fine malt Scottish whisky. The distilling industry, which has existed there for more than 200 years, has been threatened by European environmental regulations which place severe restrictions on discharge of low toxicity effluents - principally water and grain residue - into the sea. In order to meet regulations, discharge outfalls have to be placed deep and distant from the shoreline where tidal currents disperse the effluent, rapidly eliminating beach deposits and associated surface effects.

Fraught with difficulties
The Sound of Islay is a fast flowing tidal waterway separating the Islands of Islay and Jura. More than 140-feet deep, the Sound contains tidal currents which frequently exceed 10 knots. Traditionally, discharge outfalls were short - typically less than 30 feet in length - exiting at low-tide water levels. Attempts to place long outfalls on the sea bed have been fraught with difficulties in construction and survival; severe damage or destruction by the sea in less than a year from installation is the rule.

Longbore was contracted by UDV (Distilling Ltd) to install an outfall to exit 500 feet from the shoreline at a depth of 100 feet, by directionally drilling a borehole from an entry point 500 feet inland and 100 feet above sea level. Not easy, particularly when:

  • the rock formation was dalradian quartzite - over 200 million years old - with intensified quartzose, mica and schist;
  • the drill site was very restricted and the borehole entry angle was 15°, to descend steeply below the shoreline;
  • the borehole survey was complex due to topography, surface features and the fact that half of the borehole was drilled under water;
  • the exit point was 100 feet deep in a 10-knot tidal stream.

    The attraction of having a high velocity, deep tidal stream to enhance the effluent dispersion was challenging for completion of the outfall pipeline. The 'slack water' time was less than 20 minutes at high and low tide, so diving operations were extremely restricted. This time restriction and water speed eliminated the ability to pull back the pipeline from the exit so it had to be inserted under compression into the borehole. Longbore technology enabled the welded 1,100-foot-long pipeline to be inserted in just 30 minutes.

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