The Donegal County Council in Ireland was looking to expand the capacity of an industrial collection network and a municipal collection network. So, they contracted BAM Civil to install the required pipework and a pumping station. The pump station transports the excess water into the Atlantic Ocean through an outfall pipeline with a diffuser system. The official name of this project was ‘Killybegs Main Pumping Station & Outfall DBO’.
BAM Civil instructed VolkerInfra to design and construct the actual outfall pipeline within the strict tolerances of location, with an outflow of 283 litres per second. The VolkerInfra design called for a horizontal directional drill of approximately 640 metres and the installation of a marine pipe section of approximately 150 metres, including a diffuser system with five upright valves. This all had to be executed under the challenging Irish coastal conditions. Rough seas and hard ground, consisting mainly of rock, were to be expected.
After completing the design and survey work, VolkerInfra commenced in March 2011 with the butt fusion welding of the 640-metre HDPE string. Soon after that, the drilling rig was used to start drilling through the hard rock. A pontoon was mobilized to function as a stable work surface for the marine works. Divers closely monitored the exit point of the drill line from the pontoon, and it was kept clear of any washed-in soil or emitted drill cuttings during the punch out and reaming phases with airlifting. The same airlifting method was used to create a trench on the seabed for the marine section of the pipeline to be lowered into.
After the drilling works had been completed, the pre-welded HDPE string was pulled in with an onshore winch. This involved diverting the pulling wire through a submerged sheave block behind the exit point to establish the pulling force in the direction of the sea. Once the HDPE string had been pulled in, it was time to connect the marine section of the outfall system. The marine section had been prepared at a separate work compound in Killybegs Harbour as the drilling work took place.
The marine section, including the diffuser system, was approximately 150-metres long in total and had to be divided into five subsections to be transported to the location. To provide the necessary protection under marine conditions for at least 50 years, each section consisted of the initial 450-millimetre HDPE pipe surrounded by a steel casing with a special coating and attached anodes. Additional weight was required to ensure that the pipe would be stable on the seabed once installed. The annulus between the steel casing and the HDPE outfall pipe had therefore been filled with concrete. And to ensure the required outflow of 283 litres per second, five upright valves were connected to the main pipe.
Once ready, the marine sections were transported to the drill’s exit location and lowered onto the seabed from the pontoon. Divers assembled the sections underwater with specially designed couplings. To provide extra protection, stainless steel protection frames were placed over the upright diffuser valves, and lastly, concrete mattresses were lowered onto the marine section to secure it in place.
VolkerInfra completed this challenging project in August 2011 and is proud of its part in ensuring that the removal of excess water from hundreds of households and businesses will be facilitated with this reliable outfall and diffuser system.
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