Mercury Southdown Power Station
History
The Mercury Southdown Power Station was commisioned in 1996, as a natural gas-fired combined cycle gas turbine cogeneration power station. Located in a southern suburb of Auckland it was New Zealands nothernmost power station with capacity exceeding 50 MW. Once the site was upgraded to contain 3 LM6000 gas turbines and one steam it produced 170MW of power. In 2015 Mercury announced it was decommissioning the site in line with their commitment to green energy. In May 2022 Ward Demolition began the task of demolishing the structure, sitting on 2.2 hectares of land set to be completely cleared and later sold.
Demolition
Demolition commenced May 2022, kicking off what was set to be a highly technical job with extremely high risk hazards. Stage 1 consisting of deconstructing a wet surface air condenser (cooling tower) nestled in amongst an extremely high voltage, live site. With 11,000 Volt cables suspended on trays only 3 meters from the structure, this job was anything but ordinary. We ran the risk of a news worthy event if we were to merely create excessive dust or vibrations. As we operated in close proximity to a high voltage major switch yard and kiwi rail transformer, that was connected to a large percentage of Aucklands powersupply and transport network.
At this stage the site was still connected by earth grid meaning we were limited to above ground excavations only. Additionally the site contained multiple underground reservoir tanks that we had to be extremely mindful not to track over incase of collapse. This posed a high risk not only to the machine but any team member in or around it. We worked closely with the Murcury team to ensure every team member was thuroughly aware of all hazards. Every morning Mercury attended our prestarts adding their extensive knowledge of hidden hazards to our plans and processes.
This was especially helpful when we had to separate all the pipework that ran underneath the live 11V cables and connected to the structure. We would have regular walk throughs where we discussed what needed to be cut and they would ensure each pipe was completely isolated and explain to us the thickness and material of each so that we could proceed safely and well prepared for the task ahead. Some pipes were up to 80 mm thick (photo provided). Needless to say these were extremely challenging to cut.
Minimizing dust and vibrations was crucial on this job as a major switch yard was located no further than 100 meters from operations and was extremely sensitive to both. Heavy vibrations or dust could result in a fault shutting down the grid connected from Henderson to Otahuhu, a major chunk of the city. Dust can contain iron particles that result in a track that can short the equipment out and shut down the grid, so we heavily relied on our 7500 Dust Fighter to minimise this hazard. Fortunately we’re in our winter months that contain high rain fall that aided our efforts. On the flip side the weather posed further risk to our team as electrical storms posed extremely high risk of lightening strike. We played close attention to weather reports and had to shut down during electrical storms to eliminate the lightening hazard.
Further added precautions on this high risk job included compulsory PPE of 11CAL Fire resistant overalls and rubber gloves for any manual handling, above the standard Steel caps, high viz and helmet. Our health and safety officer was a regular presence onsite constantly ensuring all safety measures were applied.
Method
Our method for demolishing stage 1 included, firstly fully isolating the structure from its suroundings, then seperating structure A and B from each other and the high tensile steel pipe that ran between them. Once completed we were able to proceed with two 50 tonne and two 20 tonne diggers. We demolished structure A first, processing the materials as we went. The large wet sacks were particularly challenging due to their weight and quantity of steel, they required two 50 tonne diggers to move them around site, followed by a full day of cutting with the 50 tonnes shear attachment.
Once structure A was completed this provided room for us to continue processing materials while structure B was being dismantled. As structure B was extremly close to the 11,000 Volt cables we left the outside wall in place to provide a barrier, protecting these cables from the machines. This remained intact until the site was fully isolated for Stage 2. Lastly we broke out the foundations of the large pipe and pushed it off it’s perch so that we could cut it up. Due to the magnitude of the large pipe it worked as an amplifier of the sound and vibrations and set off our monitors when we atempted to cut it with shears. This meant we had to stop and adjust our method before proceeding to cut the entire piece down by hand.
Stage 2
Stage two provided plenty more interesting challenges for our team, lead by Jyoti Jain (project manager) and Bayleigh Ward (Site Supervisor). Including the craning of multiple elements, sorting of exotic metals and operations in close proximity to the live switch yard and powerlines.
The two boiler stacks were among the greatest challenges during this time. Each stack held 3 dense pipe bundles weighing 80+ tonnes each. The top two sections had to be craned off to provide access to these sections for our high reach machines. The top piece weighed 17 tonnes and the bottom was 13 tonnes. It was crucial to get the planning and preparations right to ensure this task was completed safely.
Once the top was off we prep cut the sides and brought in our 70 tonne link belt high reach to assist our PC600 high reach and lower each bundle to the ground. Once on the ground the real challenge began. The bundle was constructed out of dense Inconel pipework, each pipe was encapsulated by stainless steel washers, this combination meant they were extremely tricky to process. Inconel is a nickel chrome-based alloy of high value, so proper processing of the material was vital for the success of the project. Unfortunately, the addition of the stainless steel made it impossible to cut by machine as it “gummed” up and became jammed. We tried gas cutting, thermal lance and enquired about using a wire saw. The only method that worked was using a cut off saw and cutting through the middle by hand for the machine to then pluck the strings out of the bundle one at a time. This was extremely hard tedious work. But we got it done, separated it into its separate grades and loaded it into containers.
100% of the sites Concrete was crushed onsite and recycled into the foundations. All of the office furniture was donated to non profit organisations. Various other items were salvaged for reuse including all of the carpet tiles and two large stainless steel tanks.
