|THE ORMEN Lange gasfield is located within a prehistoric slide area with varying water depths from 760-3350ft (250-1100m). Due to the slide area, the seabed is very uneven, including steep slopes and seabed obstacles up to 150ft (50m) high. The major technical challenges with respect to pipeline design in this area are therefore: the extreme seabed topography combined with inhomogeneous soil conditions; uncertainties related to current velocities and distribution; the high number of spans, including some very-long spans; deep water, and therefore difficult and costly seabed preparation and span intervention; and flowlines with a large potential of lateral buckling in combination with free spans.
In order to minimize the cost of span intervention, a major testing campaign and research program was conducted in the Ormen Lange project to come up with a design procedure in compliance with the DNV-RP-F105  design philosophy. The improvements in terms of reduced seabed intervention and rock dumping costs were in the order of several $100,000. The lessons learned and the improved knowledge will also be a great value for other project dealing with similar free-span problems.