Parrog Outfall Installation
Kaymac Marine & Civil Engineering were commissioned by Morgan Sindall Plc to assist with Early Contractor Involvement to aid in the design and installation of a new 642m long outfall pipe in the Village of Parrog, Pembrokeshire. The original outfall pipe installed in 1964 had fractured in 2008, which caused treated effluent to enter the sea below the Mean Low Water Springs level, at a point inshore of the consented discharge point.
The quality of the water had been continuously monitored since the fracture and the Environment Agency had stated that unless the test results improved, this popular Newport Sands beach would lose its Eco-Label ‘Blue Flag’ status, which in turn, will result in the small village becoming less popular with tourists who are so vital to the local economy.
The occurrence had resulted in the client determining that the original outfall was in need of replacement in order to meet the requirements of the ‘Blue Flag’ status and to improve the water quality for the local residents and tourist beach users.
Kaymac’s experience in the marine environment allowed the design to be reviewed with regard to its ‘buildability’ and being able to source and transport the required materials to the site. The design was altered so that the original scope of importing thousands of tonnes of rock as backfill material could be replaced with the use of pre-cast concrete weight collars and mattresses that would be fitted to and around the pipeline, with the pipe being laid in a deeper excavation as to what was originally proposed- Thus providing a more cost effective and environmentally friendly solution.
Kaymac planned to undertake the project via both land based and marine based operations. Local residents granted Kaymac permission to use their garden property to store some of the necessary land based plant items required for the works. The land based plant included 2 No. 70T telescopic piling rigs, which were required to install over 530 No. Larssen sheet piles over the course of the project. A temporary stone access ramp was installed from the garden to the beach to allow the plant access to conduct the works. The ramp lay adjacent to an original 130 year old Grade Listed lifeboat slipway- access on the slip was impossible on the slipway as any tracked plant could cause significant damage to the structure.
Access for the land based plant and materials became an issue as the works location was approximately 1 mile away from the nearest usable beach access slipway. Due to a tidal range in the region of 3.8m and with relatively little storage space on the beach, it was decided that the best option would be to import the pipeline, rather than attempt to weld smaller sections of pipe together on the beach. This would also be in the favour of the local residents, as to weld the pipe on site, would mean that the few access roads would not be kept busy with deliveries during the busy Summer months.
With careful planning and internet consultations with a Norwegian supplier, the pipeline was sourced, fabricated to the required design and delivered via a tug boat from Norway to Fishguard Harbour (over 960 nautical miles), to await the necessary preparatory works required by the design. Once delivered, the pipe was split into 3 lengths- 550.0m, 53.0m and 27.0m. Over 200 No. Pre-cast concrete collars each weighing 320 kg, were individually attached to the pipe by Kaymac operatives and steel sinking lids were fixed to the open pipe ends. These sinking lids included fixed valves, which allowed Kaymac to control the influx of water and air into the pipe during the sinking procedure.
The design depth of the beach excavation required sheet piles to be installed by the 2 No. Piling rigs. Over 530 No. Larssen sheet piles ranging from 10-12.0m in length were installed during the twice daily low tidal period. The ground conditions that were encountered required that a suitable support propping system would need to be installed to enable the excavation works to be carried out in a safe manner. A Notice to Mariners was issued to ensure that local boat owners and watercourse users were notified of the sheet piles, as the location is very popular with fishing vessels and local water enthusiasts. As a requirement, the navigatable channel had to be kept open at all times during the project.
The seaward excavation was carried out by a dredging vessel with on-board GPS and 3-Dimensional dig software. Both the beach based and marine based excavations were carried out simultaneously to meet the required depth and also counter the influx of excavated material caused by the rip tides and strong currents.
Once a suitable weather window had been selected, the excavation works was co-ordinated to allow the pipeline to be towed approximately 7 nautical miles (with the 200 No. Pre-cast weight collars attached) to the works location. Due to the underlying water table, the beach excavation was always filled with water at any state of the tide. This allowed the pipeline to be floated into position beneath the support propping system via a shore based manually operated tractor and winch system.
Once the position of the pipeline had been checked by a Kaymac dive team, the pipe was sank by introducing water into the system and allowing the air to escape via the valves fixed to the sinking lids. The sinking procedure had to be undertaken in a controlled manner to prevent damage to the pipe but also, had to be carried out before the rising tide level reached the underside of the support propping system.
Kaymac divers ensured that the pipe was sunk accurately into the trench and that it was angled correctly to allow the later attachment of the diffuser head. Once the position had been confirmed, the pipe sections were connected by divers using marine grade stainless steel fixings. A 50T crane on board a floating work barge was used to lift 53 No. 9T pre-cast concrete mattresses around the pipeline along the majority of its length. Again, Kaymac divers ensured that the mattresses were placed accurately and in the correct locations. The excavation was then backfilled to the original bed level and the marine plant de-mobilized from site.
The sheet piles were removed from the beach and where the design required, concrete was pumped underwater by divers to complete the project.
A dye test which was later carried out, proved that the outfall installation had been a success and that the project had been completed within the budget and to the required deadlines set by the access arrangements.
Contract Value: £1,900,000.00 | Contract Period: June 2011 | Contract Duration: 11 weeks
Parrog Showreel
