Copyright 2014, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2014, held between September, 15- 18, 2014, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the final paper submitted by the author(s). The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2014 Proceedings. ______________________________ 1 M.Sc. Senior Engineer – DNVGL, Norway 2 P.Eng. Group Leader, Pipeline Integrity Solutions – DNVGL, Canada 3 Ph.D. Senior Principal Engineer – DNVGL, Norway 4 M.Sc. Geotechnical Engineer – BGC Engineering Inc., Canada 5 P.Eng. Manager, Pipeline Integrity Program, Plains Midstream Canada IBP1941_14 Fatigue of Pipelines Subjected to Vortex-Induced Vibrations at River Crossings Hans Olav Heggen 1 , Richard Fletcher 2 , Olav Fyrileiv 3 , Gerry Ferris 4 , Minh Ho 5 Abstract A model has been developed, based on DNV GL’s Recommended Practice, DNV-RP-F105 “Free Spanning Pipelines” to predict the fatigue lives of onshore pipelines that become exposed at river crossings due to riverbed erosion. In such cases, the flow of water over the exposed pipeline can lead to a phenomenon known as vortex-induced vibration (VIV) which has been accounted for in the design of offshore pipelines for many years. The DNV-RP-F105 methodology for fatigue life calculation has been incorporated into DNV GL’s ‘FatFree’ software, which was used to execute this project. The original FatFree model for river crossings was developed in response to a pipeline failure which resulted in a full-bore rupture of a recently exposed pipeline. The model demonstrated that, when riverbed scour leads to an unsupported pipeline span (known as a free span) that exceeds a certain ‘critical length’ at which the free span’s natural frequency matches the driving frequency of the VIV, fatigue failure can occur quickly at the pipeline girth welds. The situation for onshore pipelines is more severe than for typical offshore pipelines due to the high water current velocities and the flow direction which is usually close to perpendicular to the pipeline in river crossings. The work was then used to create a screening tool for other river crossings where the potential for pipeline exposure exists. A geotechnical study was performed to identify sites where sufficient erosion to expose the pipeline was possible, and to estimate the length of the resulting free span and the current velocity. This was then used to identify crossings with an elevated risk of failure due to VIV and allow mitigative action to be taken. 1. Introduction Vortex-induced vibration (VIV) of pipeline free spans is known to have the potential to lead to cyclic stressing and significant levels of fatigue loading in subsea pipelines. As such, free span assessment is often an important factor in the design of offshore pipeline systems, but has not generally been considered for onshore pipeline systems. However, at river crossings where the potential exists for the pipeline to become exposed, typically as a result of erosion, VIV is also possible and has resulted in pipeline failure in extreme cases. Unlike the case for subsea pipelines where freespans may be an anticipated feature of the pipeline installation and their dimensions and loading can be monitored over time, at river crossings prior to loading the pipeline can be completely supported (buried). The flood event both initiates erosion leading to the development of the free span and then imposes loads on the free span, potentially leading to VIV and, in extreme cases, fatigue failure of the pipeline. There are two possible scenarios when considering the likelihood of VIV at pipeline river crossings; a crossing may exist with a known freespan length for which the impacting water velocities must be determined and the more common situation where the pipeline is not free spanning prior to a large flood event and both the water velocity and freespan length need to be estimated depending on the potential size of the flood. The long-term integrity of the span is