Session: 04-01-05 Flexible Pipes and Umbilicals V

Submission Number: 157457

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

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. Free spanning these types of subsea structures is a typical concern of ensuring their integrity against static loads generated by seabed roughness, functional loads induced by internal temperature and pressure (if any), and dynamic environmental loads due to Vortex Induced Vibration (VIV) and direct wave action. 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 occurring, a low structural damping initiates a stick/slip behavior of the interior elements of the cross-section. Activation of the stick/slip behavior 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 modal damping has been well investigated for flexible risers which generally have higher modes excited. The higher modes are considered associated with higher structural damping. However, for free spans of flexible structures laying on the seabed, it is still less explored. A possible explanation could be that the number of orders of the vibration mode is limited due to shorter span lengths. Therefore, the effect of modal damping on free spanning flexible pipeline, umbilical and cable is investigated and discussed through a case study.

Presenting Author: Linlin Jiao DNV AS

Presenting Author Biography: Has a Ph.D degree from NTNU. Has a 14 years' working experience on pipeline operation technology, including global buckling/fatigue(VIV)/third party interference study, condition assessment, risk assessment and RBI plan, pipeline integrity management and life extension projects. Has a strong background of hydrodynamics and naval architectures. Has a 7-years research experience in the field of marine technology, especially for hydrodynamics, hydro-elasticity and structural analysis.