Calibration of Safety Factors for DNV RP-F105 Free Spanning Pipelines

Free spans often become a significant challenge in pipeline design and operation due to uneven seabed or seabed scouring effects. The trend towards deeper waters, harsher environment and installation of pipelines at very uneven seabed often implies a high number of free spans. High costs related to span intervention puts focus on minimizing these costs and still ensure integrity of the pipeline with respect to vortex induced vibrations (VIV) and associated fatigue damage. On the other hand the potential costs related to fatigue failure of a pipeline (recovery costs and economical loss) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits, e.g. as required by DNV-OS-F101 “Submarine Pipeline Systems”. This paper describes the structural reliability analysis performed to obtain the safety factors for free span fatigue design. Accumulation of fatigue damage due to VIV of free spans is associated with various sources of uncertainty. The important stochastic parameters are described, and the basis for the uncertainty modeling given. The calibration scope defined from a set of different pipeline cases, span scenarios, and environmental conditions is presented from which calibration results and sensitivities will be discussed.