This thesis was stimulated by the requirement to decommission offshore steel structures in the North Sea Offshore Oil and Gas Industry, but applies worldwide, where Oil and Gas production has ceased. It initially shows the UK Government assessment of the scale of decommissioning in the UKCS where there are several hundred structures with associated pipelines, risers and wellheads that have to be cut into sub-structures and individual pieces and then removed. Factors other than cessation of production drive the sequence and timing of structural decommissioning, such as the price of oil &gas and what new development might require already installed infrastructure. The nature of decommissioning of underwater steel lattice structure of offshore oil platforms called ‘jackets’ and underwater infrastructure is briefly discussed with the focus on cutting. The comparison of the performance of different underwater cutting tools currently most used in decommissioning is investigated with respect to two new underwater cutting techniques not only in terms of speed of cut but also in terms of deployment time and infrastructure requirements. A number of mainstream cutting tools and deployment systems are briefly introduced to provide a background, showing the range of tools preceding those investigated in more detail in terms of cutting and deployment efficiency. During the initial cutting research it was considered that fibre lasers had developed sufficiently in terms of power density, size and cost to be applied to underwater cutting. They were considered to have potential benefits over other underwater cutting techniques due to the continually increasing power density at the cut and have the advantage of a small envelope due to the small size of the potential cutting head The potential footprint of the laser head was envisaged to be similar to that of the rotating electrode arc tool therefore the deployment issues investigated might apply to both. The later development There is little published information on underwater cutting by laser particularly for the Oil and Gas Industry except for application to cutting rock for drilling. Research has been biased towards the nuclear industry using manufacturing techniques for surface treatment and cooling to reduce stress cracking. Fibre lasers are expensive to buy and logistically difficult to rent therefore it was considered that 1.2 kW CO2 laser at the University of Aberdeen (UoA) could be employed in obtaining an insight into the cutting and deployment issues concerned. Thus, the author designed, built and tested an underwater laser cutting head that would interface to the CO2 laser and would work at different orientations, fully submerged underwater in a tank. The trials were designed to assess the relationship of cutting parameters and performance at three orthogonal attitudes underwater, (beam downward, horizontal and upward pointing) to represent the orthogonal approach to horizontal and vertical primary axis tubular members, that comprise a jacket’s construction. The cutting parameters and their effects were then analysed to determine trends, behaviour and technical issues. Although an underwater laser cutting system could use a fibre laser, the CO2 laser enabled preliminary tests to be carried out and the viability of a cutting head to be explored. In the conclusion the design and performance of the laser cutting head is assessed with respect to the performance of other main-stream underwater cutting tools developed earlier. Offshore deployment of the laser cutting head is also discussed. Recommendations for future research and development work, to enable the realisation of underwater laser cutting, are presented.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:577580 |
| Date | January 2012 |
| Creators | Gledhill, Peter L. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=196133 |
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