WIND VORTEX-INDUCED VIBRATION OF STRUCTURAL MEMBERS IN A STEEL TOWER - A CASE-HISTORY

Gas is flared offshore on the East Brae oil production platform via a 124 m long, inclined flare boom. Each of the tubular elements of the f lare boom could, under certain wind conditions, vibrate due to the for ces induced when vortices are shed from the member. When members on th e flare boom were assessed for wind vortex-induced vibration to Health and Safety Executive (HSE) guidelines,(1) many were predicted to have high levels of vibration leading to low fatigue lives. Selected flare boom members were instrumented. Actual member natural frequencies and damping were measured and accelerometers were fitted to selected memb ers to monitor vibrations. The results showed that narrow-band respons e was not occurring and broad-band response was significantly less tha n that predicted by the HSE guide method. The maximum measured broadba nd responses were typically of the order of 20 to 30% of the predicted maximum response at the critical velocity. Tentative reasons for this variation in response are offered.