Higher Order Modal Response of Riser Fairings

Abstract

Riser fairings are designed to rotate freely about the riser axis and to passively align with the direction of incident flow so they will effectively streamline the flow and eliminate VIV (Vortex-Induced Vibrations). This rotational degree of freedom introduces the possibility of a complex dynamic phenomenon involving coupling between the hydrodynamic forces and the fairing / riser motions (e.g. cross-flow translation and rotation). Slocum et al. reported a scaled model test of a long flexible riser model with a freely-rotating riser fairing conducted at MARINTEK at OTC-2004. At low flow speeds, the test showed the fairings to be effective. However, at higher towing velocities they became unstable resulting in high displacements at its first bending mode. This paper presents the work related to a study of effectiveness of fairings and is one of several VIV research activities NDP (Norwegian Deepwater Programme) has conducted at MARINTEK in 2002–2007. The present work is partly a follow up activity to the work reported by Slocum et al. The purpose of the present work was to study possible higher order modal response of faired risers and in particular to find out if such response can occur in higher bending modes than the first. A vertically towed instrumented riser was tested with 2 different fairing designs in uniform current profile with different towing speeds. Fairing II was identical to the one used in Slocum et al (2004) while Fairing I represents an alternative design. The riser model was 9.32m long, had diameter of 20mm and was flexible. Both bare riser configuration and full coverage of the two fairing sets were tested. This study documented first-, second- and third-mode responses at high amplitudes (instability behavior) for Fairing II. Tests with Fairing I showed that the riser was stable, but the riser vibrations were found to be similar with respect to displacement amplitudes and frequencies to the bare riser VIV.