The scope of works specifically included relocation of a 150 NB primary gas pipeline for road and bridge works at Albert Avenue, Chatswood.

The existing pipeline ran along Albert Avenue in the roadway. However this section of road was to be lowered by Laing O’Rourke (Main Contractor) in order to allow vehicles over 3.8 metres high to pass under the railway bridge as part of the Chatswood Rail Extension.

Installation

Codmah utilised one small excavation crew on the project to carry out the works. The crew installed the majority of the pipe work by open cut excavation within the existing footpath.

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The first section of works was proposed to be directionally drilled to avoid damaging existing mature trees in the footpath area. After onsite investigation and extensive potholing it was decided by Codmah and Alinta that the risk of damaging existing underground utilities, particularly ten optic fibre cables, was too great to proceed with this method. All was not lost as during the potholing process as it was discovered that an old 12 inch ductile iron gas main was not in use at the location where the HDD was to occur. A risk assessment was carried out by Alinta, WorleyParsons and Codmah to assess the risks of utilising the old main for insertion and from these assessments additional safety requirements were implemented and the old gas main was used for insertion.

There were many challenges on the project; most of these were contributed by the location of the works being in Chatswood CBD:

* Pedestrian and traffic control. * Working space and storage area for the works. * NDT (Radiation) in a CBD area. * Parking requirements for construction vehicles. * Physical space for trench within the footpath area. * Proximity to railway line. * Hydro testing within a CBD area. * Existing underground utilities including electricity, Telstra, Optus, sewer and stormwater. * Deep excavations (over 3 metres deep) to give the new main a minimum of 1,200 mm cover when the road and footpath levels change. * Excavation in limited head room under the existing railway.

Installation of spherical tees was required at each end of the pipeline. This procedure involved welding the tees onto the live gas pipeline. The procedure for welding the tees was developed and a simulation was carried out using a test piece of pipe with cold water running through to give the cooling effect of a live gas main. The procedure required two welders to carry out the process with a trades assistant preheating the pipe and fitting at regular intervals. Each spherical tee was completed in approximately five hours.

Testing and commissioning

The testing procedure for the relocated pipe included pre-cleaning, gauging pigs, filling and stabilisation, pressurisation, testing, de-watering and drying.

The hydro testing of the pipeline presented a challenge to the crew. Not only was the buried pipeline to be tested but additional fabrications were welded on to each end as fabricating back to the tie-ins would not be possible before de-watering and drying. The test section was subjected to a four hour strength test at 8,000 kPa and 24-hour leak tests at 7,600 kPa with the spherical tees were tested using nitrogen for four hours at 3,000 kPa.

During the testing, 24-hour surveillance was carried out at both exposed ends to ensure safety to the general public.

After successful testing was completed and confirmed, dewatering took place where the test water was removed via a vacuum truck and disposed at a licensed liquid disposal facility. Finally the pipeline was dried and the testing was complete, taking a total of four days.

Following testing, the pipe fabrications were cut off at each end and the pipe lined up for the final tie-ins. Commissioning was the carried out by the Alinta crew and again the location of the works challenged the crew for the hot tapping and purging.

The pipeline was completed in December 2007 and took approximately eight weeks to construct, all areas were restored and the footpath opened for the Christmas period.