Vortex-induced oscillations in offshore structures. A case study on helideck dynamics

Abstract

The primary objective of this study is to investigate the dynamic response and structural integrity of offshore helicopter landing platforms subjected to wind-driven vortexinduced oscillations. Marine and offshore structures operate in harsh environmental conditions where strong, continuous air currents pass through their tubular supports, creating alternating shedding vortices. This aerodynamic phenomenon can induce severe structural vibrations that accelerate material fatigue and compromise the overall safety of the platform. To evaluate this critical vulnerability, the tubular supporting framework was assessed by combining an analytical estimation of the natural frequencies of the individual tubular members — based on the closedform beam natural-frequency formulation of DNV-RP-C205, using the tabulated naturalfrequency factors an for the assumed support conditions — with a normative vortex-inducedoscillation check performed in accordance with DNV-RP-C205 (Section 9). The in-line and cross-flow stability criteria were verified through the reduced velocity VR and the stability parameter Ks. The results show that several of the original tubular members fall within the critical reduced-velocity range for cross-flow vortex shedding, the mode that typically governs fatigue, indicating a susceptibility to lock-in resonance in the fatigue-critical base region of the cantilevered frame. The study further shows that a modest increase in member stiffness, achieved by enlarging the wall thickness or the diameter of the affected pipes, shifts their natural frequencies and reduced velocities outside the critical band, thereby removing the resonance risk and improving the long-term integrity of the helideck support without a significant weight penalty.