Year

1998

Degree Name

Master of Engineering (Hons.)

Department

Department of Mechanical Engineering

Abstract

The location of oil and gas reserves in Australia and the distribution of centres of population have created a need for an extensive network of land based transmission pipelines. There is a continuing need to expand this network but this must be achieved within increasingly stringent cost targets and in accordance with changing environmental constraints. The pipeline industry in Australia has traditionally employed smaller diameter and thinner wall pipe materials than are common in many other parts of the world. In order to improve cost effectiveness there is also a trend to increase the yield strength of the material and to further reduce wall thickness. Girth welding of pipelines in the field represents a significant proportion of the installation cost and is currently performed by highly skilled manual welders using the shielded metal arc process (SMAW), cellulosic electrodes and a 'stovepipe' technique. The use of lower wall thickness has the potential to reduce the number of weld runs and the overall cost. The higher yield strength low alloy pipe materials have been shown to have excellent weldability but as yield strength increases there may be an increased risk of weld metal cracking, particularly if overmatching welds are required. The current Australian field welding practice has been analysed and the local constraints identified. Based on this review a list of desirable attributes for suitable processes was drawn up. The international literature on process options for mechanised girth welding was reviewed and techniques, which fitted the Australian requirements were identified. The most attractive mechanised girth welding options for further investigation were identified as: • GMAW (New technology controlled transfer) • Pulsed plasma keyhole welding • High current (buried arc/keyhole) GTAW <• MIAB welding The rationale behind these suggestions is discussed and the implications for the pipeline industry in adopting mechanised girth welding are explored. Suggestions regarding the design of suitable systems are put forward. A practical assessment of the first of these short listed options was conducted and found to offer considerable promise. This process is tolerant to gap variations, inaccuracy in groove preparation and pipe misalignment. High welding speeds (735mm/min) are also achieved, with high quality welds deposited without varying welding parameters as the arc progresses along the circumference. Monitoring the weld quality in the real time was also achieved. The preliminary calculations of root weld strength indicates that clamp could be released at 23% of the root weld completed. Such early clamp release would significantly decrease construction time and project cost. An assessment of comparative costs was made based on observed field practice, reports of previous trials and discussions with industry experts.

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