THE PROJECT
The project involved the installation of a 2km-long 180mm subsea pipeline from Carness Point to Shapinsay island. The pipeline is believed to be one of the longest in the country used for a water supply. The purpose of the project was to install a pipe under the seabed to bring water to the small island of Shapinsay in Orkney, which means the community there will be supplied from a reservoir on the Orkney mainland. Water quality on the island of Shapinsay had deteriorated to a level where a replacement source was required. HDD was chosen by Scottish Water Solutions as the most cost efficient means of solving the problem; the intervention being to install a replacement water pipeline from Orkney mainland to the island of Shapinsay by drilling 1920 metres through the bedrock.

HDD
Stockton mobilised their 250 tonne HDD rig to Carness Point on mainland Orkney in March 2012. A 317mm diameter pilot hole was drilled through the siltstone and sandstone rock using top of the range HDX tungsten carbide drill bits. The bedrock was shown to have a compressive strength of 100 – 120 MPA proved to be much harder and more compact at the planned drilling depth of 50 metres below entry point elevation. After exiting on the island of Shapinsay it was a simple operation to connect to the prefabricated pipeline and pull it into the drilled borehole.

PIPE PULL OPERATION
The 160mm diameter HPPE pipeline had been prefabricated on the island of Shapinsay and assembled on rollers in one single length. The pipeline had been pressure tested and left full of water at a pressure of 2 bar. Stockton coordinated the pipe pull operation using UHF and VHF radios to ensure good communications between pipeline spotters, the excavator holding the tail of the pipeline and the drilling control cabin. The pipe pulling loads decreased as the pipeline entered the hole and never approached 50% of the yield strength of the pipeline.

DESIGN
The feasibility of this project depended upon the strength of the HPPE pipeline. The design calculations showed that by using an SDR6 pipeline with a wall thickness in excess of 30mm, filled with water to make it more neutrally buoyant, it would be comfortably pulled into the HDD borehole without over stressing the pipeline. Stockton also had to ensure that the drilling fluid in the borehole had a specific gravity no greater than 1100kg per cubic metre. All these parameters were met and the project was successfully completed.