Dawdon Short Sea Outfall Repair Works

The Coal Authority is looking for a specialist contractor to undertake the refurbishment of existing sea outfall structure at Dawdon Mine Water Treatment Scheme, located approximately 1km south of Dawdon, County Durham.

The works will be undertaken in accordance with the NEC4 Engineering and Construction Contract Option B Priced Contract with Bill of Quantities and further details can be found at www.neccontract.com.

For further details and full description of the works, please see VI.3 Additional information of this notice for a more detailed description of the works. Here you will find the Scope and instructions on how to register for the event and gain access to the full tender documentation.

Additional information: The Dawdon short sea outfall consists of two parallel 200mm pipes with a gravity flow approximately 350m to the discharge point 140m metres from the shore. The pipes were installed through directional drilling and emerge from under the sea bed at the discharge point. Each pipe comes to a T piece with duck bill diffuser elements fitted to each end of the T section.

The base of the discharge arrangement was not sufficiently supported and this has allowed the pipework to rotate impacting discharge flows.

This needs to be repaired and returned to the correct position with improvements to the support of the diffusers from a pre cast concrete base. The diffusers also had a protective structure that has been dislodged and this needs to be replaced by a new protective sleeve and cap. The protective sleeve can either be a pre-cast concrete pipe or an HDPE pipe section which will be capped by a pre-cast concrete or GRP lid.

The replacement duckbills will have to be fitted after the sleeve is placed. Alternatively, arch shaped cut-outs could be implemented, to fit over the duckbills. The "duckbill" check valves will protrude slightly through holes/ windows prepared for purpose and will discharge normally.

Construction

Due to operational requirements shut down of the pipelines must be well planned and durations kept to a minimum. Based on our feasibility studies the shutdowns are estimated to be no longer than 3 hours per pipeline. All the rehabilitation operations that require no flow through the pipe must be programmed to take place at the same time so that only one shut-down period is required. The following tasks will need to be completed under shutdown: • Disconnection of the flange adapter and replace. • Align the diffuser to vertical and temporarily secure. • Remove the diffuser Tideflex Check Valves. • Drop the protection sleeve into position. • Replacing the diffuser Tideflex Check Valves

Divers to conduct the following site preparation activities. • Clearing loose rubble around the flange adapter and concrete base footprint. • Rock drilling may be required to level concrete base footprint.

After Pipeline Shutdown • Replace/ rehabilitate and install: • Blank flange o Sacrificial break away spool piece (optional) • Flange Adapter - and right the diffuser to vertical alignment. • Remove the diffuser Tideflex Check Valves. • Place and secure precast concrete base • Drop the protection sleeve into position. • Replace the diffuser Tideflex Check Valves

After Resume Pipeline Flow • Grout fill around flange adapter. • Place and secure protective sleeve cap. • A marker buoy with independent anchorage is also to be installed.

Evidence of quality control measures and that the installation is as per design are required.