The present paper, as originating from the Ormen Lange project, deals with pipeline expansion philosophy on uneven sea bottom. Major design issues are discussed in the form of capacity and serviceability criteria as governing for intervention works. A rational insight into the expansion mechanisms involved on uneven seabed is needed. Analytical as well as numerical simulations are applied to illustrate the different modes of deformation and their governing design parameters, including free span feed-in, shoulder liftoff, as well as vertical and horizontal buckling. The effects on allowable feed-in from soil resistance, pipeline weight and initial geometry are discussed in view of the analyses. As related to pipelines on even seabed the imperfections introduced by topography may play a major role in the selection of expansion locations. A robust design in the form of predictable expansion behaviour may be obtained by combining bottom unevenness, planned routing and seabed intervention. Straight pipeline sections with typical snap-through buckling behaviour, normally imply high unbalanced forces in the system with extra requirement on axial friction between the expansion buckles. The natural imperfections on uneven seabed give the opportunity to obtain a smoother design with less sensitivity to soil parameters. The sensitivity of the allowable axial feed-in and locking distance to bending moment capacity and corrosion is highlighted, together with the need for analyzing cyclic behaviour from repeated shut-downs and start-ups.
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