Vortex Induced Vibrations (VIV) are responsible for great part of the fatigue damage caused by the dynamic loeding that occurs In compliant offshore structures such as risers, The cedes, like the API RP 2RD ("Design of Risers for Hosting Production Systems and Tension-Lag Platforms"), typically require the design fatigue life to be larger than 10 times the structure service Hfe. However, because the normal dynamic models available for riser analysis do not have a formulation for the fluid transverse induced force, the fatigue damage is obtained without its consideration. The VIV damage may be assessed by specific models that consider it isolated, but there is no reliable means to calculate the total damage based on these two damage sources. To avoid this Mnd of uncertainty, a strategy is to use VIV supressors, if the risk of having VIV is considered significant, but this solution could be avoided in certain cases ff a better methodology and appropriate model were available. This paper present a summary of a research work that was carried out to improve the design methodology of individual risers, in the sense of integrating the fatigue Hfe assessment by considering all sources of damage, including VIV.
The exploitation of oil in de~pe~ watts has brought a constant change in the design practice of risers. For mstance, the available practical guides do not establish a closed procedure to be applied in the design, although they recommend to account for the influence of vortex induced vibrations in the fatigue calc~ation. During its operation life, a riser it is exposed to different combinations of loads caused by the action of currents, waves and induced platform motion to the top connection. These loa & have a fiu~At~ng parcel that cause cyclic stresses in the riser wall.
