Sammendrag
Pipeline plays a very important role for transporting oil and gas from offshore fields to on-shore facilities. The recent trend of exploring oil and gas in deeper water exposes extreme challenges for pipeline laying as well as repair. For the sake of human health, the maximum depth of diver assisted operation is limited to 180 meters. Fully remote-controlled system should be developed for deeper water operation. Furthermore, extension of existing pipeline network by using hot-tapping technology also requires diverless techniques. Both deep water repair and hot-tapping methods rely on the reliability and robustness of welding methods. Numerical modeling and simulations have a useful role in development of welding methods and for assessment of welding joint properties. However, systematic modeling of hyperbaric welding has not been performed. In practice, hyperbaric welding residual stress and cold cracking problems are the main concerns from the structural integrity point of view.
In this study, single-pass hyperbaric welding of X70 pipe has been studied by using WeldsimS. The water depth is 1000 to 2500 msw (Meters of Sea Water), which corresponds to a pressure range from 100 to 250 bar, respectively. A 2D axisymmetric model has been considered in this study to simulate circumferential welding of a pipe. The welding parameters used in the simulation are taken from real welding experiments. Phase transformations and transformation plasticity during the welding procedure have been taken into account. The main aim of the study is to predict the hyperbaric welding residual stresses and investigate their effect on structural integrity. The temperature evolution and the microstructure were also studied. Results show that residual stresses induced by hyperbaric welding are significant within the pressure range investigated, which should be well assessed for the sake of structural integrity.
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