A model for riser displacements generated by ocean currents is derived, for the case where the riser is cylindrical with uniform material properties in the axial direction. The current forces are assumed to be proportional to the square of the speed and account for changes due to variable riser diameters (buoyancy modules). Vortex induced vibrations are formulated through time varying drag- and lift coefficients. The model is sufficiently flexible to consider arbitrary currents that vary continuously with depth and time. Solutions of the model equations are developed in the time domain by expanding the riser displacement in terms of axially varying orthogonal functions. Data from a full scale drilling operation is used to validate the model code. It turns out that the oscillation frequency, displacements and mode shapes are fairly well reproduced by the model.
Interactions between fluids and structures occur in many situations related to the offshore oil and gas industry. One of them, the interaction between risers and ocean currents, has received significant attention in recent years due to increasing activity in deeper water. Vortex induced vibration (VIV) is one aspect of the interactions and can take place in steady flows. Experimental studies of VIV have been carried out by several authors, among them Griffin (1980), Huse (1997), King (1977/1986) and Mo and Lie (1997). Field studies of VIV on deepwater drilling risers have also been performed under the "Norwegian Deepwater Programme" (NDP) (see e.g. Fumes et al. 1998).
