Currently, all of the city’s wastewater is pumped out to sea via a 2 km ocean outfall. The pump station is on one side of the river and is linked to the ocean outfall on the other side.
However, once the new plant becomes operational, effluent will be transported to the plant for treatment.
It was found that transporting the effluent via the existing 600 m, 1,187 mm diameter concrete pipeline linking the two side of the river would not be possible, as the additional pressure head required to achieve this would have been in excess of the existing under river crossing pressure rating capacity.
As a result a number of basic options for upgrading the existing system were available:
Article continues below…* Construction of a supplementary pump station on the far side of the river so the existing crossing could operate at its existing pressures; * Laying a new pipeline under the river; or, * Upgrading the existing pipeline.
Wanganui District Council opted to upgrade the existing pipeline so that it was able to transport effluent to the treatment plant, as this was the most cost-effective option in the longer term. In order to complete the upgrade, the Council decided to sleeve the existing line by pulling a new liner through the existing pipeline.
The existing concrete pipeline has a diameter of 1,187 mm and is 600 m long. The new line is a 1,000 mm OD SDR 17 HDPE, supplied by Tyco in 15 m lengths.
The pipe was procured directly by the Council and installed by Inframax under a target cost contract. Conhur was the subcontractor responsible for all the pipe welding elements of the work, Smythe Contractors provided the pulling power on the pump station side of the river while Commercial Dive Services cleaned out the existing line and removed an internal repair to facilitate the pipe pulling process.
To execute the upgrade, a directional drilling rig was set up on the pump station side of the river, and a rod pushed through to the far bank. A 15 m test plug was pulled through, then the rods were pushed back, connected to the 600 m pipe string and the whole lot was pulled through in a single operation.
Four excavators and two winch dozers were used to assist the directional drill during the initial phase of the pipe-pulling operation, in order to overcome the frictional forces.
The existing under river crossing was flooded with water and was drilling fluid pumped down the rods, in order to minimise the frictional forces within the existing pipeline, and those generated around the four bends in the pipeline during the pulling process.
