Pan Pac Forest Products Limited is one of New Zealand’s largest timber pulp mills and they pump tens of thousands of litres of consented, treated waste water from their plant out to sea via a 2.5 km outfall pipeline. The pipeline which was 400mm dia steel encased in a 50mm concrete shell, failed in the surf zone approx. 3m deep in running shingles. Repairs from the outside were attempted but resulted in a sheet pile being punched through the existing steel pipeline filling the pipeline with sea water, running shingles and causing large metal rips / tags to be bent into the pipeline. To add to the complications the pipeline could only be shut down for 8 to 10 hours per day, any longer than this and the factory would have to shut down at a cost of $1,000,000.00 per day. The discharge into the surf zone in front of residential homes had also created national TV media exposure and the threat of abatement notices from councils.
Using divers and a flotilla of boats the pipeline was exposed approx. 300m offshore by airlifting the seabed approx. 10m below sea level. A pup joint section was installed that could be removed and reinstated daily. The pipeline was also exposed on the beach where the same process was undertaken.
We then inserted a drill rig and pushed out to the sea floor. Using a series of different tools and cameras we were able to grind out the steel tag damage in the pipe and bridge the tear. We then flushed and pigged the stones and shingles out of the pipeline. A 300m string of PE was then towed offshore and submerged using buoyancy control and a series of weights to hold it to the correct specific curve to enter the pipeline. We then slip lined the existing pipe and bolted it into position. This was all achieved without shutting the factory down once. This very complex and technically challenging project took months of planning but only 10 days to complete.
