Free spans constitute a threat to the integrity of offshore pipelines. Ocean currents and wave induced flow near the seabed can cause excitation of vortex induced vibrations (VIV), exposing the pipeline to cyclic loading which may result in fatigue failure. In extreme environmental events, waves and currents may cause significant bending moments due to VIV or direct wave loading, which may cause unacceptably high combined loading potentially resulting in pipeline failure. Safe and reliable free span design is therefore a key aspect of pipeline design in general.
When free spans are located in close proximity to each other, modal response may cause span interaction, effectively coupling potential VIV or direct wave action response of two or more spans. Such spans configurations are called multi-spans. Treatment, and particularly identification of multi-span configurations has long been a challenge in the pipeline industry.
In 2017, a new revision of the DNV GL recommended practice for free span pipeline design DNVGL-RP-F105 was issued. Among other subjects, the new revision provides significantly improved guidance on identification and classification of multi-span free spanning pipelines. The new guidance facilitates the option of automating identification, modal analysis and ultimately fatigue analysis of multi-span configurations.
This paper demonstrates background for and benefits with01 the new revision of DNVGL-RP-F105 in systematic and automated treatment of multi-span identification, fatigue and extreme loading assessments based on recent experience from a selection of pipeline design and operations management case studies.