Coastal erosion would continue to threaten gas supplies north through the Maui pipeline and the risks of future pipeline breaks can be minimised but not eliminated, experts say.
A multimillion-dollar project is already under way to upgrade another section of the pipeline under threat from erosion.
The Maui gas pipeline, which runs from the Maui production station at Oaonui to the Huntly power station and feeds gas to much of the North Island, broke near White Cliffs, in North Taranaki, and had to be shut down last October.
The closure cost millions of dollars and crippled commercial, manufacturing and agricultural operations in the north of the North Island.
An investigation by Maui Development Ltd found the leak was due to a sudden overload caused by landslide movement.
"Metallurgical investigations have found that no property, defect, or flaw in the pipe itself contributed to the failure," the report said.
The section of pipeline, at Pukearuhe, has been stabilised through a range of measures, including embedding it in loose granular material to facilitate movement and additional drainage.
Additional monitoring is also being carried out on this section of the pipeline.
MDL chairman Rob Jager said the pipeline traversed a range of challenging topographical and geological areas.
"MDL as the owner and Vector as the technical operator actively manage the risks this creates through a wide range of rigorous maintenance and safety processes, including monthly flyovers, walkovers, full internal inspections of the pipeline, ongoing geotechnical surveys, and scheduled upgrades," Mr Jager said.
"However, the reality is that in a country like New Zealand, the risk natural land movement poses to infrastructure like gas pipelines can be mitigated, but not eliminated entirely."
Before October's break the pipeline, which supplies 75 per cent of New Zealand's natural gas, had operated reliably and without a major incident since being commissioned in 1978. The pipeline has an expected lifespan of 80 years.
"Given its importance to the New Zealand economy, we are focused on learning everything we can from the incident so that best practice in pipeline management continues," Mr Jager said.
The Taranaki Regional Council has issued resource consent to move an almost-two-kilometre section of the pipeline under threat from erosion, just south of the Tongaporutu River between Waikiekie and Ohanga streams.
Mr Jager said $25 million would be spent to upgrade the pipeline.
"That section of the pipeline is close to cliffs susceptible to coastal erosion and adverse weather. It is not under any immediate threat – it would be some years yet before it will be at risk," he said. Engineering design was being done.
The Technical Investigation Report has been submitted to the Ministry of Economic Development as an input to its wider Review of the Maui Pipeline Outage.
- © Fairfax NZ News
Should ratepayers fork out for increased security to keep vandals at bay in Pukekura Park?Related story: Cameras set to catch vandals