Updated Design Procedure for Free Spanning Pipelines DNV-RP-F105: Multi-Mode Response

In search for new gas and oil fields, the trend in offshore development points towards deeper waters, harsher environment, increased use of subsea installations and use of pipelines to transport the hydrocarbons to processing facilities onshore or in shallower waters. This also implies installation of pipelines at very uneven seabed causing a high number of spans that can be difficult and very costly to intervene. Conventional free span design according to the DNV Recommended Practice DNV-RP-F105 (2002) allows for vortex induced vibrations (VIV) as long as the integrity of the pipeline is within acceptable limits. However, the 2002 issue of the design code mainly covers short and moderate spans. As the knowledge about very long and/or multiple spans, where several vibration modes may be activated, has been limited, such cases have been treated in an assumed conservative way. This paper discusses the technical advancements in free span design in general and with respect to both long free spans and multi-spanning sections where several vibration modes may be activated simultaneously in particular. These advancements form the basis for the updated DNV-RP-F105 (2006). Changes from the former 2002 version are illustrated by an examples and the technical background is discussed.