Practical and Theoretical Investigation of Underwater Laser Cutting for Decommissioning Applications

Nyamayaro, Chris and Pinkerton, Andrew and Cheneler, David and de Bono, Paola and Khan, Ali (2022) Practical and Theoretical Investigation of Underwater Laser Cutting for Decommissioning Applications. PhD thesis, Lancaster University.

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Abstract

Energy Act 2008 requires that a large proportion of the North Sea infrastructure will need to be decommissioned in the next 30 years. To perform subsea decommissioning activities, technologies which are flexible (there is large heterogeneity in the structures), fast (deployment costs are significant), reliable (to minimise down-time) and deployable remotely (to avoid the use of divers) need to be developed. Underwater laser cutting provides an innovative cutting technology for decommissioning offshore infrastructures and nuclear waste which is cheaper, safer and relatively faster. In this research, three approaches were used to investigate underwater laser cutting of C-Mn steel using a 10 kW fibre laser. These approaches comprise of an experimental, theoretical, and numerical study of underwater laser cutting. The experimental work consisted of underwater laser cutting trials, conducted in a 1m3 tank and a high pressure vessel. The findings for both cutting trials were analysed for the influence of process parameters on performance and cut quality. The cut quality is assessed in terms of the attached dross height and kerf width because the key requirement in decommissioning applications is that the parts being cut must be separated with minimal secondary waste. A 50 mm thick C-Mn steel workpiece was adequately cut with a maximum cutting speed of 200 mm/min for hydrostatic pressure conditions of up to 20 bar, representing a water depth of ~200 m. The process performance and kerf width results predicted by the theoretical model are validated by underwater laser cutting trials carried out in a 1 m3 tank. A key finding in the numerical study of a 3-dimensional (3D) gas jet expansion model is that a mixed phase (water, water vapour (steam) and air) medium was observed at the exit of the virtual kerf along the path of the laser beam.