A complex procedure to replace a sulphuric acid converter in Runcorn has been solved through smart use of encapsulated scaffolding.
Main contractor Aker contracted scaffolding contractor NSG to work on the £14 million project at Runcorn Rocksavage Works. The subcontractor was brought on board to solve a particular challenge. The site is registered under the Control of Major Accident Hazards Regulations (COMAH) because it produces chlorine and chlorine derivative products. Meanwhile, the 22.6 m high, 8.4 m diameter vessel is made from stainless steel, which can fail when exposed to chloride contamination. Once the converter vessel was completed, it could be coated in aluminium cladding to protect it, but until this happened, it would be exposed to the potential hazard. Therefore until the cladding was installed, a solution was required that would give access to the site and protect the delicate converter vessel.
As a result, the firm developed the concept of a freestanding, fully encapsulated 24 m tall cylindrical scaffold structure. On top of this was a removable roof which needed to be completely watertight, that also needed to withstand storm force winds, lightening strikes and snow. The combination would create a closed-off working area, protected from the elements and gasses, with the roof acting like a lid on top of an encapsulated scaffold ‘jar’. With this in place, material for construction could then be lowered inside with a mobile crane.
Raising the roof
However, given the roof would need to be removed and replaced every time material was lowered into the work area, it was vital that this process was made quick and simple.
NSG UK managing director Mike Carr says: “Because the programme was quite tight, they didn’t want a situation where they had to stand down for a day while the roof was removed and unfastened.” The challenge was solved by using a series of prefabricated aluminium beams that were assembled like a Maltese cross. “We finished that off with scaffolding equipment so we had a roof structure, and a frame that was shrink-wrapped in Enviroclad,” he says. “It’s a shrink on sheeting system that makes the roof weathertight.” The result was a roof that could be lifted on and off within an hour yet still provide the necessary environmental protection.
Building a free-standing scaffold that supports a 1 tonne roof on a chemical site is as challenging as it sounds. Mr Carr says: “Imagine a structure 24 m high near the banks of the Mersey, with a considerable amount of wind. You also can’t tie that structure in to anything, and it has to rest on the floor.”
The firm’s design engineers developed a structure using Contour system scaffolding and a large quantity of kentledge as a counterweight to hold the structure in place. However, the challenges didn’t end there. The site is adjacent to another sulphuric acid converter vessel that remained operational during the construction process. Clearly, should the scaffolding tip over and crash into this vessel, it would be a disaster. As a result, NSG’s design team had to hold numerous briefings with both client Ineos Enterprises and main contractor Aker to prove this wouldn’t happen. “We went through the design brief, the wind loadings, and the imposed loading on the ground with everyone,” says Mr Carr.”The ground was a particular challenge as we weren’t convinced it was solid concrete, but we went through design briefings to overcome this.”
Using system scaffolding was instrumental in making the project a success, according to Mr Carr. “We probably could have installed the roof with tube and fit scaffolding, but there would have been more components used on the project,” he says. “It would have taken a lot longer to build. We managed to cut down on the programme as the majority of the scaffolding we used was system scaffold.” He estimates that using the it trimmed the programme time by 25 per cent.
However, the entire project would ultimately have been near impossible using tube and fit, according to Mr Carr, because of the tight tolerances required. “We had to achieve a perfect circle. The actual tolerance between the inside of the scaffold and the outer face of the vessel was about 30 mm, so we had to be very accurate. Using the system definitely gave us more flexibility,” he says.