The disruption came on February 16 when a road bridge spanning the Pohangina River was washed out, leaving the 8 in. high pressure gas pipeline that was attached to it suspended in floodwaters. About 120 metres of pipe that had separated from the bridge was still intact, but was deformed into a bow by the force of the current and was being struck by trees and other debris being swept down river.

Pipeline owner/operator, NGC, isolated and depressurised a total section of 16 km of pipe, including the river span, for safety reasons and to preserve linepack downstream of the affected area.

Immediately the damaged section of pipe was isolated and gas network operators and retailers in the Hawke’s Bay region were asked to shed load by ceasing supply to all but essential users and residential customers. Supplies to these customers were maintained from linepack, and it was vital that even they minimised their gas use.

Due to access difficulties and continuous storms sweeping the area, it was to be another four days before supplies were resumed to all customers, after partial recommissioning of the damaged pipeline.

NGC’s initial response to the emergency was hampered by the extensive flooding and multiple road closures in the area. NGC used two helicopters to access the bridge, the main line valves and to carry out surveillance over the pipeline route in the area. Isolation of the damaged section was effected by closing two valves, one about 1.5 km to the east of the bridge, and the other near Palmerston North Airport, about 15 km away.

NGC’s gas control centre in New Plymouth was first alerted to a potential problem in the area at about 7.30 am. A field technician dispatched to investigate found the centre of the Pohangina bridge spanning a gorge on Saddle Road had been washed away. The pipeline was still attached to both ends of it, but about 30 metres of pipe was suspended in the strong current. The pipeline was swaying under the force of the water, and there was a threat of rupture.

There was, however, no immediate danger to people or property because of the remoteness of the bridge and the road closures.

NGC immediately began assessing engineering options to restore gas supplies to Hawke’s Bay, but it was not until February 18 that the river level had subsided sufficiently for heavy machinery to access the river and for NGC to inspect the damaged section of pipe without endangering its people.

Two temporary solutions were considered viable. The first was to recommission the existing pipeline at a reduced pressure if it was safe to do so. The second involved the construction of a temporary river crossing that would replace the damaged section and maintain supplies until a new permanent river crossing could be established.

Options to lay a temporary crossing using barges or pontoons were discounted early due to the width of the river and the strong current, with an aerial crossing determined to be the best temporary solution.

Visual and non destructive testing of the existing line indicated that it could be returned to service at reduced operating pressure. The inspections found that, apart from the deformation of the pipe, the pipe wall had suffered some damage from debris impact and at its former connections points on the bridge. However, none were considered serious enough to prevent the recommissioning of the pipe at partial pressure.

This was achieved on February 20. Following successful pressure testing, and the receipt of regulatory approvals, gas deliveries through the existing pipe were resumed at 30 bar, about half the normal operating pressure but sufficient to maintain flows to meet the normal demand of all customers for that time of the year.

Gas flows and pressures throughout the Hawke’s Bay system were constantly monitored and, based on average consumption, linepack survival time was about 18 days. Trailer-mounted CNG tanks were brought in to supplement gas in some of the local networks.

However, the existing section of line remained at risk of failure, and monitoring and operational constraint conditions attached to its recommissioning included that it be re-isolated in the event of a further flood warning.

Throughout the first two days of the emergency, NGC continued to mobilise equipment and specialist personnel to the site to ensure the supply restoration solutions were completed as soon as possible. Design and engineering planning for the temporary aerial crossing was carried out by New Plymouth-based company P&P Engineering Solutions, while civil and mechanical construction crews began installing new tie-in points either side of the river and welding new pipe sections.

When river conditions allowed, test drilling of the riverbed was conducted to confirm the substrata was appropriate for supporting towers to be installed.

Other activity included liaising with local authorities and safety officials to secure the necessary resource consents to proceed with the work.

The aerial crossing involved 200 metres of new 3 and 4 inch pipe, of which 100 metres are suspended from a cable approximately 6 metres above the river at centre span. After a routine commissioning process, it was completed on March 3 and resumed gas deliveries to Hawke’s Bay at normal operating pressure of 70 bar. The damaged section of pipeline was re-isolated and de-commissioned.

Strong wind and rain associated with frequent new storm fronts crossing the region created very difficult working conditions for crews at the site and delayed progress. However, while subsequent rainfall caused the river level to fluctuate by up to a metre, it did not trigger new flood alerts and continuous gas supply was maintained throughout this period.

Design, engineering and planning work is now well advanced for a new permanent 8 in. pipeline crossing of the Pohangina River. This will be a trenched crossing, about 60 metres downstream of its former location, and completion is scheduled by the end of April.