Case Study to Investigate the Effects of Modal Damping on Free Spanning Flexible Pipeline, Umbilical and Cable

Abstract Free spans on slender subsea structures laying on the seabed such as flexible pipeline, umbilical and cable might develop due to various reasons, e.g., seabed unevenness, scouring, crossing and end terminations, etc. Due to lack of direct guidance, the industry generally applies a simplified while conservative methodology for these structures, e.g. by applying DNV-RP-F105 Free spanning pipelines, which was developed for rigid pipelines. Although the basic principles of the VIV phenomena are understood to be applicable to any circular cross section, rigid pipelines and these flexible structures have fundamental differences that needs to be acknowledged and addressed in a free span assessment. Different with rigid pipeline, the unbounded flexible structures commonly consist of helix components or unbounded components arranged in a helical geometry which behave with a non-linear structural response. When VIV excitation occurs, a stick/slip behavior of the interior elements of the cross-section initiates which might induce an energy dissipation through friction between different components’ movement which generates an additional amplitude dependent structural damping and tends to reduce the VIV response. This is the so-called modal damping. The effect of modal damping on free spanning flexible pipeline, umbilical and cable is investigated and discussed through a case study.