DAWDON SHORT SEA OUTFALL, COUNTY DURHAM

Project details

Dawdon Mine Water Treatment Scheme is located approximately 1km south of Dawdon, County Durham. The Coal Authority needed a specialist contractor to undertake the refurbishment of 2no. existing sea outfall structures located at this site. The contract awarded to Southbay included replacement of damaged outfall structures and installation of markers buoys, with works progressed from specialist marine plant.

Specialist divers were used to complete works within the tidal zone.

Due to the tidal location, a Jack Up Barge, complete with 100-tonne crawler crane was used to access the outfall. Initial works included isolating and disconnecting existing pipework. A blanking plate was installed to prevent any debris from entering and blocking the pipework. Using specialist divers, lifting equipment was then attached to the damaged outfall, before the structure was removed and placed onto the deck of the Jack Up Barge for disposal. Divers were then used to commence hydro-demolition to excavate the seabed to the correct level for the instalment of the new outfalls. A prefabricated template was then lowered into position, with measurements taken to determine the position of the new outfall structure. Extra protective measures were put in place when excavating around an existing HDPE Pipe. Once the seabed was prepared, the divers installed grout bags to form a bed for the outfall structures to sit. New precast units were then lowered into position, forming the new outfall structures. The new outfall structures were anchored into the grout bags to prevent potential movement by drilling and grouting dowels bars. All pipework was then subsequently re-attached. Rock bags were then placed around each structure to reduce tidal action on the outfalls. Further works included the installation of marker’s buoys, which were attached to weights positioned on the seabed.

Due to the aggressive tidal environment, the project was delivered in two distinct phases, with the project team demobilising over the winter period. In order to achieve all programme milestones, significant pre-planning was required, which involved closely monitoring daily weather and sea conditions. To accelerate progress, weekend working was employed. The project was delivered to programme, with no reportable incidents.

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