Transpower is a state-owned enterprise with 800 employees, responsible for electrical power transmission throughout New Zealand. Transpower is the owner of New Zealand’s national grid, providing the infrastructure for electric power transmission and the system operator, ensuring real-time operation of the nation's grid and the electricity market.
Their transmission network consists of 170 substations, 25,000 transmission towers, and more than 11,000 kilometers (6,800 miles) of lines.
The existing conductor on these lines had reached the end of its life and was required to be replaced. Under normal circumstances this would be a business-as-usual task however, due to the locality of the line and the significant urban development around and under these lines since their construction in the 1950’s, it was apparent that this would be a very complex task.
The northern section of both lines run through urban Ōtara in Auckland, NZ. The lines cross several high-importance motorways that cannot have full road closures, many smaller roads, schools, live low voltage (LV) crossings as well as many residential and commercial properties.
At the initial stages of the project, the client worried that these risks might be too high and the job impossible.
WHAT WE DID
Groundline’s expertise in stringing, and in particular the use of Catenary Support Systems (CSS), meant we were the obvious experts to be involved in such a complex project.
CSS allowed us to be able to replace the existing conductor whilst providing protection to property, other LV assets and the public. This protective capability was a crucial enabler.
There were three key challenges facing Groundline for this project:
- The use of CSS continuously on every span for long stringing runs
- The complex structural analysis of the existing lattice towers.
- CSS protection required throughout the span
CSS is typically used for very short runs, often limited to 1-2 spans, and typically only protecting 1-2 obstacles within a span. Due to the tight constraints (proximity to property, roads, LV crossing etc) the stringing runs were much longer than considered normal for the use of CSS and therefore required the design to provide full span protection.
This full span protection therefore imposed greater loads on the existing structures.
Although the use of full span protection increased loading, it also allowed a large reduction of conductor tensions. This reduced structure loading, rigging loads and reduced the safety risks associated with pulling conductor under tension.
Groundline developed a specific methodology and loading analysis tool to accommodate the long stringing runs and the resulting structural analysis of the existing lattice towers.
During the development of this analysis tool, Groundline foresaw that any changes would require a lot of iterative analysis. Therefore, the tool was carefully designed to allow changes in the construction order or design to be quickly implemented. This meant that changes could be easily undertaken during the construction phase if unforeseen issues arose, saving significant amounts of time and money. These changes did occur during the construction and this tool allowed our team to quickly and effectively react to the construction contractors’ requests.
The close working relationship between Groundline and Omexom (the construction contractor on this project) also played a key role in the success as it afforded close daily communication between designer and site engineer. This was further enhanced by Groundline’s experienced field engineers being on site during construction to provide real-time changes to be made to the design.
As a result of the innovative design thinking provided by Groundline, and the close working relationship between Groundline and Omexom, the project was delivered well under budget and on a tight timeframe.
The use of Groundline’s expertise in CSS design also meant public safety was upheld and there was minimal disruption to other local utility assets.
Groundline was honored to be recognized for this work in Transpower’s STAR awards; the Safety by Design category recognised the excellent design and delivery of this project.