Evaluation of Free Spanning Pipeline Design in a Risk Based Perspective Available to Purchase

The Ormen Lange subsea pipeline shall be designed to meet a specified risk acceptance criterion, established by consideration of failure probability and consequences of failure. Traditional design for Vortex Induced Vibrations (VIV) of free spans limits the allowable free span length and implies that interventions work may be required. Through a risk based approach the probability of fatigue failure of free spanning pipelines is quantified, and the governing uncertainties identified. A sensitivity analysis of different risk control options is performed. The outcome facilitates to focus in the design process such that a preferred design solution can be identified and implemented via testing campaign in the design stage, prelaid rectification activities and inspection programs. The aim is to obtain a cost efficient design that comply with the given acceptance criterion. Best practices as reflected in DNV-RP-F105 “Free Spanning Pipelines” and updated field specific design guidelines form the basis for the analysis. A probabilistic module is implemented on top of DNV-RP-F105 methodology, which allows application of a dedicated uncertainty modeling for a specific project. Parameters considered include: Pipeline properties, effective axial force, span length and gap, soil properties, ocean current (distribution, depth and directional variation), multiple mode response analysis (VIV response models, natural frequencies, damping, effect of concrete, static deflection), different SN curves, strakes and monitoring. Both the As Laid phase and the Operational phase are considered for different locations along the pipeline route.