Experimental Study of Hydrodynamic Damping for Water Intake Risers

Water Intake Riser (WIR), conveying cooling water from the sea, is key to liquefaction of natural gas in the Floating Liquefied Natural Gas (FLNG) facility. Due to the wave-induced vessel motion, WIRs may experience resonant vibrations, which influence its fatigue life. In such situations, the estimate of hydrodynamic damping is critical to the prediction of fatigue life. Due to its small motion amplitudes compared to the diameter of WIR, the Keulegan-Carpenter (KC) for motion-induced flow around WIR is normally small (e.g. KC < 5). For small KC values, the effect of steady current on the hydrodynamic damping is not well understood and the current practice of using the relative velocity Morison model for predicting the hydrodynamic damping with in-line steady current is challenged by guidelines such as DNVGL-RP-C205 and ISO-19902. In this study, the hydrodynamic damping of a smooth WIR oscillating in still water or in steady currents is measured with a series of experiments at KC < 5 and the Reynolds number (Re) in the range of 103 ~ 105. The effect of in-line or cross steady currents on the in-line hydrodynamic damping is investigated and the performance of the relative velocity Morison model for predicting the hydrodynamic damping at low KC is examined. Experiments are also conducted for a WIR with helical strakes in in-line or cross currents. Based on these experimental results, recommendations are made for predicting hydrodynamic damping in the WIR design.