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ABSTRACT Experiments are described whose objective was to investigate the combined effect of current and waves by studying an elastically mounted circular cylinder in a time-varying flow. This is relevant for risers subjected to both current and wave-induced motion. The rigid cylinder section was 75 mm in diameter and 2 meters long and mounted horizontally in a towing tank. The time-varying flow consisted of a constant and a harmonic velocity component. The general conclusion from the experiments is that both the amplitude and acceleration response of a cylinder subjected to constant + harmonic current is smaller than for constant current only. INTRODUCTION Vortex-induced vibrations (VIV) from current alone may represent a severe problem for risers in deep water both by contributing to the fatigue damage accumulation and by increasing the drag force to an extent that causes operational restrictions. Most available VIV prediction tools are developed to account for current only, and the empirical support is relatively extensive for this situation (although critical and supercritical Reynolds number data are scarce). However, the situation with both current and waves is much less known. How, and how much, will the waves affect the transverse response of a riser? The wave direction is often the same as the current direction, and both wave-induced forces and floater motion may very well induce a nearly harmonic relative velocity in addition to the constant current velocity. The main objective of the present work was to investigate the combined effect of current and waves by using an elastically mounted rigid cylinder section in a variable flow. The variable flow consisted of both a constant and a harmonic velocity component. The case where the wave-induced motion totally dominates the velocity (average current velocity equal zero) has been investigated by several researchers.
ABSTRACT This paper presents lift coefficients of an oscillating cylinder based on experiments with a very dense, elastically mounted cylinder, Since the cylinder is dense, it takes many cycles to build up the response to its maximum level. The lift coefficient in phase with the cross-flow velocity of the cylinder is found by requiring that the increase in potential energy is equal to the input energy from the fluid forces minus that dissipated by the structural damping. Estimates of lift coefficients are presented and compared to other empirical data available in the literature. INTRODUCTION The response of risers and cables due to vortex shedding contributes to fatigue damage accumulation and increases the drag forces which may lead to operational limitations. Prediction of vortex-induced vibration (V1V) may be classified in three categories: a) Amplitude response found directly as a function of the mode-shape and a non-dimensional damping parameter (reduced damping). The method may be usable when the current is constant over the whole length, and single low-mode response is likely, (e.g. free-spanning pipelines). Examples of this kind of response model are found in Griffin et. al.(1975), Iwan (1975), Blevins (1977), Sarpkaya (1978), Det Norske Veritas (DNV) (1991), and the Brown & Root lbrmula option in SHEAR7 (1996). b) Solution of the dynamic equation of motion for the system based on empirically determined fluid forces. The force coefficients are functions of different parameters: displacement amplitude and response frequencies at the given cross-section, flow velocity, Reynolds number, roughness, etc. Due to the non-linearity of the interaction between the fluid and the structure an iterative solution of some kind is required. Examples of these semi-empirical prediction programs are SHEAR7 (Vandiver, MIT), VIVA (Triantafyllou, MIT), and VIVANA (Larsen, MAR1NTEK/NTNU).
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