Blackmans Bay Outfall, Tasmania

Project: The Kingborough Council is replacing the existing outfall at the Blackmans Bay Waste Water Treatment Plant with a 600 metre long X 500mm diameter pipeline in waters to 18 metres deep.

On this project Dive Works provided video and site assessment to select the underwater route for the outfall, within the constraint that the tie-in structure was to be within 3-12 metres of the existing pipeline, and the alignment due east.

The seabed was rocky and exposed to southerly seas. Dive Works managed the difficult conditions to excavate a three metre wide pathway by manually and mechanically moving the rocks first and then blasting as required. Further offshore the outfall was secured into position on the sandy bottom.
 
                     

The outfall pipeline was constructed in HDPE PN12 pipe and butt welded into 6 X 100 metre long strings along private property in Tinderbox. The location provided a deep launching site with gentle slopes to a gravel beach and sheltered bay. Each string was launched with concrete blocks installed and with additional flotation.

Dive Works undertook towing the string to site by a lead vessel at the front and two additional vessels to the side, to provide control and mobility. Each string was then positioned in its final location and held in place with the aid of tow vessels, anchors and winches and was connected at the surface to the proceeding pipeline. Sinking was initiated when water was introduced into the pipeline and floats were removed, allowing a controlled sinking onto the prepared seabed formation.

Once the laying of the pipeline was completed Dive Works returned to conduct another video survey of the completed outfall. 

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