The small-gap equations for the stability of Couette flow with respect to non-axisymmetric disturbances are derived. The eigenvalue problem is solved by a direct numerical procedure. It is found that there is a critical value of Ω2/Ω1(Ω1, Ω2 and R1, R2 are the angular velocities and radii of the inner and outer cylinders respectively) of approximately −0·78, above which the critical disturbance is axisymmetric and below which it is non-axisymmetric. In particular for R1/R2 = 0·95, Ω2/Ω1 = −1, the wave-number in the azimuthal direction of the critical disturbance is m = 4. This result is confirmed when the full linear disturbance equations are considered, i.e. the small-gap approximation is not made.

The stability of an inviscid fluid between two concentric rotating cylinders to nonrotationally symmetric disturbances is investigated. The limiting form of the disturbance equations for small gap-to-radius ratios is derived. Computations of the growth rate of unstable disturbances are carried out for the case in which the cylinders rotate in the same direction. It is found that the growth rate of rotationally symmetric disturbances is greater than that of nonrotationally symmetric disturbances.

An extensive model test program was conducted to explore the effect of various parameters on the Vortex Induced Motion (VIM) response of a four column semi-submersible (Semi) floating platform. The paper describes the model test set-up, important parameters that were modeled (including hull geometry, mass, stiffness and damping properties) and considerations of instrumentation and test methodology. The damping contributions from the moorings and risers have traditionally not been included in a VIM model test setup. This is the first time that the damping characteristics of the moorings and risers are systematically evaluated and included in the model test setup. This paper describes the calculation of the full-scale mooring and riser damping characteristics and, the design and construction of an innovative model test damping mechanism. Besides damping, the effect of varying the Semi draft and external hull appurtenances were also evaluated.