Free span assessment has more and more become an important part of modern pipeline design. The reason for this is partly that the remaining hydrocarbon reservoirs are located in more challenging places, e.g. with very uneven seabed. Another explanation is that the pipeline design codes a few decades ago did not allow for vibrating free spans, while the modern, state-of-the-art pipeline codes, such as DNV-OS-F101 “Submarine Pipeline Systems” (2013) and its Recommended Practices, opens for long spans that are allowed to vibrate as long as the structural integrity is ensured.
In presence of non-cohesive soils and high on-bottom flow velocities significant free span development may occur over the design life, e.g. due to scouring. Such spans may be associated with a fatigue life capacity less than the design life if the spans are assumed stationary. For non-stationary spans with occasional long span lengths this may not be true since the criticality is strongly linked to the persistence of long spans and the prevailing environmental condition. A realistic fatigue assessment must account for the history of the span (i.e. stress cycles encountered for a critical weld) including predictions into the future development.
High costs related to span intervention puts focus on minimizing these costs while still ensuring 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, economical loss and environmental consequences) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits.
One frequent challenge faced with old pipelines in operations survey reports are that they report several free spans. Old pipelines were not designed to allow any vibration and usually there is scanty information about different parameters such as soil conditions, operational parameters, lay tension, environmental data, etc., thus it’s difficult to determine whether it’s necessary to intervene the span or not.
State-of-the-art free span codes are deterministic in their nature. If the new codes are used to evaluate such old pipeline spans, considering all the before mentioned uncertainties in the input parameters, this would eventually lead to over conservative very low time to failures. The outcome will be that many spans need to be fixed immediately or should have failed already. Such a situation leads to a mistaken conclusion about the conservatism of the codes and not on the way they were applied.
This paper discusses some of the challenges often seen with free spans during the operational phase. The objective of the paper is to demonstrate that for in-service pipelines the lack of reliable information about the free spans is the main source of commonly low life encountered and not the methodology used to evaluate the free span. Some of these challenges are discussed in detail and potential ways forward are outlined.