Wed, 15 June, 2022
Former National Structural Integrity Research Centre (NSIRC) student Chris Nyamayaro, whose studies were sponsored by Lancaster University and TWI, has recently completed his PhD focused on exploring the theoretical and practical investigation of underwater laser cutting for decommissioning applications.
Chris undertook his PhD with the Joining 4.0 Innovation Centre (J4IC) which carries out research and development (R&D) aimed at digitalising joining and associated manufacturing technologies, and was established as a partnership between Lancaster University and TWI in 2017. Prior to joining NSIRC, Chris obtained his BEng and MSc in Aerospace and Mechanical Engineering at Swansea University and the University of Birmingham, respectively.
The scientific, technical and commercial objectives of Chris’ PhD topic were to: develop an understanding of the underwater laser cutting process and influencing parameters up to a hydrostatic pressure of 20 bar on a 50 mm thick C-Mn steel workpiece; further advance an existing, state-of-the-art, underwater laser cutting technology that can cut the workpiece at depths of up to 200 m; and disseminate the project results, together with the capabilities developed towards post project exploitation.
Using a 10 kW IPG Ytterbium fibre laser system with a wavelength of 1.06 μm, three approaches were taken to investigate underwater laser cutting of C-Mn steel: an experimental study of underwater laser cutting in a 1m3 tank and a high pressure vessel to simulate offshore conditions; development of a power balance theoretical model for underwater laser cutting; and the development and application of a gas jet expansion numerical model for underwater laser cutting.
The outcomes of the experimental aspect of the underwater laser cutting study were: the complete separation of a 50 mm thick C-Mn steel workpiece, which was achieved for the cutting trials in a 1m3 tank using a laser power of 10 kW, a compressed air pressure of 8 bar, a standoff distance of 4 mm and a maximum cutting speed of 125 mm/min; and the successful cutting of a 50 mm thick C-Mn steel workpiece, 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.
A power balance theoretical model for underwater laser cutting that could predict the achieved maximum cut thickness and kerf width was developed as part of the project. The predicted process performance was validated by underwater laser cutting trials carried out in a 1 m3 tank.
A gas jet expansion numerical model for underwater laser cutting was also developed whereby a mixed phase (water, water vapour (steam) and air) medium along the path of the laser beam was observed at the exit of the opening/virtual kerf. A mixed phase region at the exit of the opening can interact with the beam or molten material during underwater laser cutting, affecting both process performance and quality.
During his PhD, Chris also amplified his studies further by presenting at both the NSIRC and Lancaster University annual conferences over a three year period as well as providing a number of project abstracts, and recently producing an associated case study: Underwater laser cutting for decommissioning applications.
Following successful achievement of his PhD Chris, who is now seeking new opportunities as a Postgraduate Researcher, said of his experience "I found that the best way to approach such an in-depth qualification as a PhD was to ‘eat an elephant one bite at a time’! And now I have completed it, my ambition is to develop underwater, fibre-delivered, laser beam cutting technology, primarily for decommissioning applications in the oil and gas, and nuclear sectors.”