The present paper originating from the Ormen Lange project, presents a methodology on how to treat the local wall thickness reduction due to internal corrosion with respect to different design criteria for production flowlines and spools. The methodology presented was implemented in pipeline design for the deepwater part of the project where water depth ranged from 250 m to 875 m. The combination of long design life and mildly corrosive well fluid requires a rather high corrosion allowance. With up to 10 mm general corrosion allowance accumulated over the 50 years design life of the pipelines, together with deep water, free spans, global buckling and possibilities for hydrate/ice formation inside the pipe, it would result in a much too conservative design if the full corrosion allowance is used for all aspects of pipeline design. In order to achieve a more realistic estimate of the effect of internal corrosion and a more optimal pipeline design, the internal corrosion was treated in different ways depending on where the corrosion is expected, and what design criteria is considered. By carrying out 2D and 3D finite element analyses of the pipe cross section with the predicted local corrosion distribution, where also the combined loading effects are included, it can be shown that for local buckling and propagating buckling during operation, a significant reduction in the average corrosion depth can be considered in design calculation. Therefore an equivalent corrosion depth, where the corrosion is distributed evenly for the different sections of the route can be estimated. This value can be used directly in the design calculations and allows optimisation of nominal wall thickness, on-bottom stability calculations, free span lengths, global buckling and cost driving factors such as pre- and post-lay seabed intervention and installation methodology.

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