Hydrodynamic forces on a cylinder under realistic combinations of in-line (IL) and cross-flow (CF) vortex induced vibrations (VIV) have been investigated.
Signals of strain gauges and accelerometers from the Norwegian Deepwater Programme (NDP) tests with a long, slender beam were used to identify cross section orbits. 19 cross sections almost evenly distributed along the pipe were selected, and their motions were applied in controlled motion experiments with a rigid cylinder. Dimensionless parameters like Reynolds number and non-dimensional frequency were identical for the two sets of experiments.
Comparison between hydrodynamic coefficients found from forced motion tests with observed motion time histories and periodic approximations are presented. Force histories are also investigated in detail.
Orbit types for combined IL and CF VIV are categorized based on relative amplitude and phase, and it is shown that IL motions exhibit chaotic character more easily than CF. Amplitude modulation is observed frequently.
Cases where cross section motions are close to periodic have similar hydrodynamic forces as for periodic motion, implying that periodic forced motion tests are relevant to get valid force information. Many cases have amplitude modulated IL motions, while CF motions are quasi-steady. In such cases, IL amplitude modulation can cause abrupt change of IL forces and also 3rd order CF forces, which can accumulate large fatigue damage. When both IL and CF motions are chaotic, the force-motion relationship is impossible to describe by constant coefficients.
