Computations based on CFD for the aerodynamic loading is used to assess the risk for dynamic response due to vortex shedding (VIV) on single tubular members of a flare boom on a North Sea jacket. Computations are performed for bare tubes and for tubes with vortex mitigation devices. Based on the results from the computations, vortex mitigation sleeves are installed on two members of the actual flare boom.
It is very complicated to assess VIV in a consistent manner. Therefore, rather simplified criterions have been used in the past for avoiding VIV due to wind as well as current. These criteria have been based on experience and on model test results. One criterion was connected to the so-called stability parameter, which is a combination of mass and structural damping. More sophisticated criteria have been introduced, and eventually Eurocode (1995) presented a procedure for assessing the risk of VIV and fatigue due to wind. The code is based on empirical data for excitation force and frequency taken from fixed cylinders. Other well-founded criteria may also be used. The flare boom on the Grane jacket is designed by Kværner Oil & Gas (KOGAS). The design is based on similar flare booms in the North Sea. The design principles for avoiding undesired VIV are based on experience and set forth by KOGAS, see Sjursen (1999). The criteria are:
Reynolds number Re > 500,000 when the reduced velocity UR > 4.0
Stability parameter KS > 16, with the structural damping of individual structural members set to 0.0015 of critical. (The stability parameter is the same as the Scruton number, Sc, defined later)