Central node supported by a crane and connected to the top section of the mast
We’ve reached a major milestone in the T-pylon project – the first full-size suspension pylon prototype has been put up and is ready for testing.
At the start of January a small team from National Grid (design, construction and maintenance) joined colleagues from the T-pylon’s designers, Bystrup, and DS SM (a Danish steel company – www.ds-sm.dk) to erect the first T-pylon. We, and our Danish partners, were keen to get an idea of where lessons could be learned in putting up T-pylons as we moved, for the first time, away from computer screens, scale models to a real-life installation.
The first observation was that, once up, the pylon looked really elegant. We were pleased with how amazing it looked in real life…sleek, slender and simple, a culmination of 11months of targeted activity. Accelerated development timescales meant that we chose to use uncoated steel for the structure, which just served to give the pylon greater charismatic appeal. The corroded surface appeared similar to weathering steel (e.g. Corten), the same material used for the Angel of the North sculpture. Weathering steel is a surface finish option, alongside painting, that we are considering for use with the final design.
Putting up the first T-pylon on a rainy and cold day in Denmark gave us lots to think about and has allowed us to make some provisional changes to how we are likely to proceed with designing and constructing future pylons. The main point, which greatly encouraged us on the day, was that a multi-skilled team of four people erected the pylon on its pre-installed monopile foundation in a single day.
On our test day, we were hampered by two 200 tonne cranes hired for the heavy lift malfunctioning. In the end we borrowed a crane working on a line for Energinet, the Danish Transmission System Operator we have been working with on this project. We lost a lot of time but still managed to get the construction work done in a day. When all is considered, this still compares favourably, in terms of construction man hours, to today’s lattice pylons.
One of the main lessons we learned was that the cast iron cast connections (by Heger Guss, www.hegerguss.com) between the crossarm to the horn and crossarm to the mast, via the heart at the top of the pylon, are challenging and time-consuming to put together on site. The heart (five tonnes) and horns (one tonne for the pair) are made from cast iron and so are heavy and the hundred or so nuts and bolts which join them are each the length of a forearm and need to be fastened to a specific torque which can only be achieved in a tight space with a torque converter.
This can be a little unwieldy and so we’re actively considering having the crossarm, heart and horns pre-assembled in the factory. They could then be taken to site as two sections, saving a considerable amount of time. Furthermore, we’re seeking to redesign the cast elements with a different iron type and to have threaded holes alleviating the need for nuts to save material and weight.
The assembled crossarm (two crossarm sections, heart and horns) were then connected to the three section mast. The three upright sections (top, middle and bottom) of the mast are slip jointed so they effectively just slot into one another and are jacked together pneumatically creating a strong interference fit to withstand design loads that could cause torsion or bending. The upper two sections (top and middle) were slip jointed horizontally on the ground while the lower joint (middle to bottom) was made vertically after the top sections were lifted into place. Here again, putting up the prototype taught us a valuable lesson. When lifting the assembled crossarm and mast section into place we used a second crane to help lift the lower end of the mast to prevent it being dragged on the ground as it was lifted. It’s important to ensure the steel that forms the slip joint retains its shape as there’s a risk it could be dragged on the ground and either get bent or damage any surface finish applied to the pylon as it’s lifted up in to place.
We’re keen to get the onsite requirements down to one crane, for both cost and sustainability reasons, and so we’re actively considering if the lower end of the mast section could be placed on a hinged platform. This would allow us to lift it with one crane and then take the platform away once it’s above the ground and ready to be fixed into place.
It’s only by having put up a life-size prototype that we can make these discoveries, so the day spent putting up the first suspension T-pylon was a great experience in helping us prepare for the future development of the UK Test Line.
In the coming weeks we’ll be putting the suspension pylon and its diamonds through mechanical tests to verify its design strength. We will use static load tests to simulate climatic (ice and wind) and security loads, dynamic tests (to understand the natural frequency of the structure) and damping tests (to ensure we have a viable solution should we need one in certain installations).
Once we’ve run through these tests we’ll erect a tension pylon alongside to see what the two look like together and then begin the next stage of tests on the tension pylon tubular diamond.
As ever, we’ll be reporting back on progress on this blog, so T-talk readers will be the first to be updated on progress.