Loading and response of a small diameter flexibly mounted cylinder in waves

Abstract

Wave loading on flexibly mounted vertical cylinders is of particular interest to offshore engineers since the dynamic responses of slender tubulars in fixed jacket platforms, marine risers and compliant structures require accurate estimation. This paper describes a series of experiments and an analytical study concerning the behaviour of a spring mounted small diameter cylinder in waves. In-line and transverse forces and cylinder response displacements are examined with particular emphasis on the differences between resonant and non-resonant cases. The influence of vortex shedding on the displacement locus of the cylinder is considered. Force coefficients from the flexibly mounted cylinder are found to be larger than those for the same cylinder but with a rigid mounting. The force coefficients exhibit dependence on the orbital shape parameter Ω for local Keulegan-Carpenter numbers, KC, larger than 14. At lower KC values, the force coefficients become influenced by the ratio of wave frequency to natural frequency. From spectral analyses of the force and displacement data the dominant frequencies are identified in accordance with the theoretical predictions.