Metal arc welding under oil (MAW-UO) is a new, revolutionary process for the repair of pipelines without the need of hot tapping and constructing bypass lines. The process uses an automated metal arc welding setup with a continuous wire feed. This process was developed for in-situ internal repairs of in-service cross-country pipelines, tanks and vessels by using a smart-pig with a modified mechanical design that allows for welding. Since some of the cross-country pipeline segments are either buried or under highways making access difficult, the new process will provide great flexibility to the repair of these pipelines. In this paper, the development of the MAW-UO process is described. Process parameters that successfully produced weldments are identified and the consumables tested are reported and discussed. The unique characteristics found in the under oil welds will be discussed in detail and related to the processing parameters and the oil environment. Welding is possible under oil because of the vaporization of the oil to form an oil vapor bubble that surrounds the welding arc plasma and protects the molten steel weld pool. Contrary to general belief, oil does not combust in the presence of a welding arc. The low oxygen content within the oil itself and the low diffusivity of oxygen within the oil even at elevated temperatures are the two main factors that prevent oil combustion during welding. Decomposition of the oil and the decomposed products are expected to significantly affect the weld metal composition and microstructure.
Metal arc welding under oil is a new, revolutionary process for the repairs of pipelines which should minimise the need of hot tapping repairs and the construction of bypass lines. The process uses an automated metal arc welding set-up with a continuous wire feed. This process was developed for the in situ internal repairs of in service cross-country pipelines, tanks and vessels by using a smart pig with a modified mechanical design that allows for joint preparation and welding. The unique characteristics found in the under oil arc are discussed in detail. The large range of temperatures that exists between the arc at the order of 10 000uC (18 000uF) and the surrounding oil (ambient) allows for complex reactions between the oil hydrocarbon chains and the steel weld metal. These reactions may considerably impact the weld microstructure. Details of the dissociation of hydrocarbon chains and ionisation of the resulting species must be known to better control the process. In addition, vaporisation of the oil will form oil vapour bubble to surround the welding arc. The oil vapour bubble will also be high in decomposed products that can significantly affect the weld metal composition. Gas chromatography-mass spectrometry was used to investigate the behaviour of a welding arc under oil and the carbon pick-up. Analyses were conducted on oil samples with different welding exposure times to validate the arc behaviour mechanistic model and the gas chromatography-mass spectrometry analysis results.
Strong demands for energy compels the oil and gas industries to increase productivity. Since transportation of oil and gas products through pipelines is the most efficient way to deliver these products, there is an ever increasing network of pipelines from the production fields to the processing plants. Construction and operating costs, however, limit the proliferation of pipelines. Hot tapping is an economical technology that the oil and gas industries use to minimize oil and gas pipeline segments by connecting a new facility to the nearest operating unit processing the same product. Technologically speaking, hot tapping is challenging especially during field installation. Welding a branch pipe/pipeline to an in-service pipeline has two major concerns: burn through and heat-affected zone (HAZ) cracking. Thin pipe wall thickness and the use of welding electrodes with high hydrogen contents can promote burn through and HAZ cracking. Improper hot-tapping procedures applied on a live pipeline may lead to disasters that can be life threatening to the welding operators. Parameter optimization is essential to ensure successful hot tapping. In this paper, fuzzy logic modeling was used to optimize the hot-tapping procedure, i.e. to minimize burn through and HAZ cracking, for processes that use shielded metal arc (SMA) welding. A total of 324 rules were built to consider multiple welding parameters, pipeline and operating characteristics such as heat input, electrode type, pipe inside surface temperature, wall thickness, hardness, and fluid flow to study the effects of these parameters on the weldment. These parameters were the fuzzy logic model inputs for successful hot-tapping conditions. For each pair of parameters, the output of the model is a processing surface on which successful hot tapping can be conducted. A graphical user inference (GUI) easily retrieves the different sets of parameters and their respective safe hot-tapping surfaces on which burn through and HAZ cracking can be avoided. As conclusion, the present study demonstrates that fuzzy logic modeling can be used to provide guidelines for in-service pipeline hot tapping. Careful modeling can result in safe welding space for hot tapping and allow for the specification of standard welding procedures within the safe welding zone. The guidelines and range of safe welding parameters from fuzzy logic modeling can reduce the size of the experimental matrix required during field weld testing for the determination of the final parameters.
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