Supply Chain - Sewerage - Long Sea Outfalls
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Outfall pipe arrival from Norway by sea to the dock in Arklow for preparation and assembly - Courtesy of Van Oord Ireland[/caption]
The proposed outfall pipeline was required to discharge the treated effluent from the wastewater treatment plant to receiving waters. Both marine and river outfall options were initially considered for the proposed development.
However, hydrodynamic and water quality modelling demonstrated that the wastewater discharge standards required for a river outfall would be much more onerous than that of a marine outfall. For that reason, the river outfall was not considered further.
Outfall pipe floated along the quay wall in Arklow prior to preparation and assembly works - Courtesy of Pipelife Norge AS[/caption]
The marine outfall was also designed to be stable in the ‘worst case’ design loads for current and wave loading conditions provided. It was therefore necessary to weight the pipeline with concrete collars and subsequently concrete kennels after installation (float and flood method), to provide a specific gravity of not less than 1.35 that could be achieved without any reliance on backfill material.
This installation weight was provided with 1015kg (290kg/m) (in air) fixed weight concrete collars at 3.5m centres and to satisfy the permanent on bottom stability, 2292kg (654.86kg/m) (in air) concrete kennels were provided at 3.5m centres in between the fixed weight concrete collars. The collars and kennels were provided by Tracey Concrete.
Welding of risers for diffuser section of outfall pipe - Courtesy of Van Oord Ireland[/caption]
Excavation of trench at the onshore connection point using the backhoe dredger ‘Razende Bol’ - Courtesy of Van Oord Ireland[/caption]
(left) Outfall pipe during sinking operation and (right) outfall pipe aligned prior to sinking operation - Courtesy of Van Oord Ireland[/caption]
Outfall pipe concrete collars being fitted in water with special lifting equipment - Courtesy of Van Oord Ireland[/caption]
Once the outfall pipeline had been sunk to the sea bottom and pressure tested, concrete kennels, each weighing 1 tonne were placed in between the concrete collars at 3.5m centre to centre spacings to provide additional permanent weighting. Additionally, six diffusers were installed from the backhoe dredger platform by divers. Each diffuser comprised of a flanged PE pipe, fittings and a riser with a four-port crosshead, each with a flange allowing for Tideflex S35 valves to be fitted directly. Included as part of the diffuser installation, was a removable access flange at the top of the riser to enable access inside the riser.
After completing the diffuser and kennel installation, the pipe was backfilled over its full length to provide 1.5m cover. Rock armour protection was further provided extending approximately 7m around the diffuser section from its centre to protect the area around the diffuser from scour. Also provided was a steel protection frame for the diffuser, fitted with sacrificial anodes to allow a 50yr design life for protection from dropped objects, anchors, trawlers and fishing nets. Finally, on the offshore end a marker buoy and on the onshore end a Saint Andrews Cross were installed to mark the pipeline location.
Due to the turbulent weather at the site location, including hold points in the working method, so that each installation step could be paused at planned intervals, allowed Van Oord to work around the weather. Additionally, working with local contractors who helped to adjust and adapt work methods and techniques within the limited amount of space, allowed the team to prepare and install the outfall system using specialised equipment and marine vessels, with minimal disruption to the environment and public. In particular, a bespoke rigging system was designed and fabricated using a local yard to assist with the efficient installation of the collars whilst working from the water.
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Outfall pipe during sinking operation - Courtesy of Van Oord Ireland[/caption]
(left) Pipe string along coastal path and (right) Pushing pipe lower car park and compound - Courtesy of South West Water[/caption]
(top left) 500t drill rig compound and entry point and (bottom) pipe pushing operation at sunrise - Courtesy of South West Water[/caption]
500 T drilling rig - Courtesy of Galliford Try[/caption]
(left) Drill pipe and pilot drill bit and (right) outfall pipe, roller and guide - Courtesy of Galliford Try[/caption]
900mm diameter pipe welder - Courtesy of Galliford Try[/caption]
Tow and sink operation - Courtesy of NI Water[/caption]
Aligning pipeline over pre-dredged trench - Courtesy of NI Water[/caption]
A range of pipeline diameters and materials were considered to achieve optimum velocities, a balance between pipe diameter and TWL in the proposed header tank. PE pipes were selected for the outfall with varying SR ratings used to offer material cost savings using only higher rated pipes for the marine sections to take account of installation (towing and sinking) stresses on the system.
Specialist hydraulic modelling was then completed by Hydraulic Analysis Limited (HAL) to provide confidence in the performance of the system against all combinations of design flows and tidal levels. The analysis confirmed the pipe diameter selection, siting of air and scour valves and provided specific specifications for pipeline gradients, bends and air valves installation to ensure stable free surface flow conditions were achieved within the mains gravity driven sections.
Barge mobilisation from Bangor Marina - Courtesy of Farrans[/caption]
The final solution was therefore to discharge treated effluent directly from the sample chamber to a gravity outfall which acts in part as a pressurised system. This amendment to the design approach was verified by HAL and removal of the pumping station and header tank was agreed by all prior to finalising the construction details including:
The outfall was installed off the coast at Ballyferris adjacent to a caravan park - Courtesy of NI Water[/caption]
(left) Onshore pipeline assembly - Courtesy of Farrans, and (right) installation of concrete kennels - Courtesy of NI Water[/caption]
Re-excavating the proven trench - Courtesy of Farrans[/caption]
At high tide, the pipe was floated, towed and positioned above the pre-dredged trench. Connection was made to the pre-installed land section of the outfall and the pipe was filled with water allowing a gradual sinking and pipe placement along the trench. Backfilling operations commenced once the pipe was surveyed and verified.
Professionally qualified divers installed the prefabricated diffuser head. Live underwater video surveys were conducted to monitor the installation of all equipment.
The execution of these activities adhered to stringent health and safety protocols, strict environmental considerations, and project timelines, ensuring the successful completion of the project on time and within budget.
NI Water and Farrans held a joint safety initiative for children in caravan parks - Courtesy of NI Water[/caption]