The Norwegian Petroleum Directorate recommend the structural damping ratio of welded individual members to be taken as 0.15% of critical damping when evaluating vortex induced vibrations. This paper descibres the background for the recommendation which mainly is based on the measurements of the vibrations on the Heimdal flareboom. Severe vortex induced vibrations in Norwegian flarebooms have occured twice. The first occurence was in 1978 on the flareboom at Statfjord A. The second incident was at the Heimdal flareboom in 1984/85. Both incidents gave valuable information about vortex shedding problems.
At Statfjord A the vibration was an overall vibration of the tie, see figure 1. Calculations of the natural frequency of the individual panel sections of the main chord showed that it was close to the recorded frequency and to the vortex shedding frequency for the chord. The calculation of this natural frequency had not been included in the design. The problem was solved by spirally wrapping the main chords of the ties with 25mm diameter ropes. Three ropes was wrapped on each chord at a phase difference of 1200, each rope spiral had a pitch of one metre. The spirals on diagonally opposite chords of the tie had the same rotational direction (clockwise) and the other pair was wound in the opposite direction. Afterwards no more vibrations have been reported (Mobil, 1978). A large measurement program was planed, but was cancelled after the successful use of the ropes. Measurements were later performed to study the overall dynamic effects in the flare boom. It was concluded that the design gave a slight under estimation, but it was compensated for by conservative load calculations (AasJakobsen, 1979). No measurements were performed to actually obtain values for the damping in the vibrating parts of the structure.