The presented design criteria, which have been used successfully for nine flare booms installed since 1990, provide a simple method for avoiding vortex shedding induced vibrations in tubular structures exposed to wind. These criteria aim at avoiding vortex shedding-induced vibrations by ensuring a broad band process when vortices are shed from a tubular member, and by providing a built-in resistance against possible lateral forces. Two relationships need to be fulfilled: Reynolds number Re > 500 000 when reduced velocity V r > 4.7, and stability parameter K s > 16. An important part of the described method is to determine the natural frequency of each critical member. For preliminary screening of exposed tubulars, the diameter-to-thickness and the length-to-diameter ratios are controlled. The empirical evidence implies that these criteria are adequate for avoiding vortex shedding-induced vibrations in flare booms. Recent measurements on a flare boom in the UK sector, with independent calculations, support the use of a diameter-to-thickness ratio and a length-to-diameter ratio in design, but also indicate that the presented criteria may be more conservative than necessary.
Vortex shedding induced vibrations (VIV) caused by wind have been widely known ever since the Heimdal flare boom experienced vibrations and cracks in individual members shortly after installation in 1985. Since that time, several cases involving oscillations and cracking have been reported, adding up to a total of about 100 incidents according to the Norwegian Petroleum Directorate (NPD). In addition, several jackets are known to have experienced cracking due to VIV by wind during yard exposure or tow. Some of these cases, where data on vibrating members are available, have been checked against the presented vortex shedding criteria. After the Heimdal flare boom problems, which happened on a relatively slender structure, the pendulum swung to the other extreme. This can be illustrated by the Oseberg "A" flare boom design from 1985/86,
