Vortex-induced vibration (VIV) in oscillatory flow is experimentally investigated in the ocean basin. The test flexible cylinder was forced to harmonically oscillate in various combinations of amplitude and period with Keulegan-Carpenter (KC) number between 26 and 178 in three different maximum reduced velocities, URmax=4, URmax=6.5, and URmax=7.9 separately. VIV responses at cross-flow (CF) direction are investigated using modal decomposition and wavelet transformation. The results show that VIV in oscillatory flow is quite different from that in steady flow; features, such as intermittent VIV, hysteresis, amplitude modulation, and mode transition (time sharing) are observed. Moreover, a VIV developing process including “building-up,” “lock-in,” and “dying-out” in oscillatory flow, is further proposed and analyzed.

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Model tests with a Gravity-Based Structure (GBS) platform in extreme waves are presented. First, the background for the experiments is given, based on the analysis of a possible late life production scenario in the Statfjord field. The statistical approach applied in the specification of the wave conditions, and the laboratory reproduction is then described. This is focused on the generation of a statistical ensemble of 50 extreme crests sampled from random wave realisations. The model test set-up and instrumentation is described. Finally, results on deck impact loads and nonlinear wave amplification effects are presented and discussed.