Can a laser cut underwater? The UK’s TWI, a manufacturing research organisation based in Cambridgeshire, UK, has demonstrated underwater cutting of 12mm thick stainless steel using a 5kW fibre laser.
The system was developed for the nuclear industry for decommissioning underwater vessels and pipelines; decommissioning work on the North Sea oil rig is another possible application. Paul Hilton who led the project at TWI presented results from the work at the Association of Laser Users’ (AILU) Industrial Laser Applications Symposium (ILAS), which took place 17-18 March in Kenilworth, UK.
The system artificially creates a dry zone on the surface of the metal with three gas jets. TWI demonstrated the technology cutting C-Mn steel and Hilton noted that the cutting performance of the fibre laser was similar underwater as in air. The only difference was that the dross created adheres to the part better when cutting underwater, which, contrary to most industrial laser cutting, is actually an advantage in nuclear decommissioning – radioactive dross falling onto the floor has to be swept up, so the more that clings to the part as it is cut the better.
Hilton said that a further advantage with laser cutting underwater for the nuclear industry is that the bubbles and water attenuate the beam rapidly as it passes through the metal. The nuclear industry is generally concerned with what happens to the beam once it has cut through the material.
TWI’s system is not the only underwater laser cutting device; the German research institute, Laser Zentrum Hannover, has also developed similar technology.
TWI began developing laser equipment for nuclear decommissioning in 2010, and, in November 2014, its lasers were used for the first time to cut radioactive material at the Hinkley Point A nuclear power station in Somerset, UK. This is a relatively short timeframe for introducing new technology into the nuclear industry, Hilton commented.
The work at Hinkley Point A involved dismantling radioactive steel waste skips in order to store the material more effectively. The solution proposed by TWI was estimated to save £30 million for processing 300 skips, and there are a total of 2,500 skips stored at Hinkley Point A and Sellafield, another nuclear power plant.
The process starts with a robot-mounted laser cutting each skip into five sections – the four walls and the floor. The laser then performs five-axis milling of the material, removing around 1.3mm over the entire surface of each piece of metal. By removing the radioactive surface material, the process reduces a 450kg skip to 50kg of active, highly compacted waste that can be stored easily, and 400kg of steel that can be recycled. Processing the skips in this manner is estimated to save £0.5 million per skip.
To date, Hilton told delegates at the ILAS event that active skips at Hinkley Point A had been processed, and that the team will return to the site later this year to cut medium-level radioactive skips transported from Sellafield.
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