Author(s): Zhimeng Zhang; Chunning Ji; Yee-Meng Chiew
Linked Author(s): Yee Meng Chiew
Keywords: Local scour; Vibrating pipeline; Initial gap ratio; Sediment transport
Abstract: Local scour around a vibrating pipeline can lead to significant structural damage and operational issues. It occurs when the dynamic forces generated by the pipeline's vibrations interact with the surrounding flow or sandy bed directly, resulting in the sediment transport and the gradual formation of scour holes. The size and depth of scour hole are influenced by various factors, including the pipeline's responses, sand properties, as well as the initial gap ratio (Sumer and Fredsøe, 1990; Chiew 1991; Liang et al., 2005; Fuhrman et al., 2014). The initial gap ratio, defined as the ratio of the initial distance between the pipeline and the undisturbed seabed to the diameter of the pipeline, plays a pivotal role in determining the scour evolution and its associated consequences. However, the relationship between initial gap ratio and scour characteristics is complex and non-linear, making it difficult to predict scour behavior accurately. In the present study, the scour characteristics of a forced vibrating pipeline in unidirectional currents are experimentally investigated under clear-water conditions with a water depth of 0.3 m and approach velocity of 0.194 m/s in a glass-sided recirculating flume with dimensions of 11 m × 0.6 m × 0.6 m (length × width × depth) in the Hydraulic Modelling Laboratory at Nanyang Technological University. The pipeline model with a diameter (D) of 3.5 cm was vertically oscillated in a sinusoidal motion with varying amplitudes (A0 = 2–6 cm) and frequencies (f0 = 0.1–0.6 Hz). The initial gap (G0) between the lower pipeline surface and the undisturbed seabed varied from 0D, 0.5D, and 1D. The results indicate that the maximum depth of scour holes increases with the rise in pipeline vibration frequency and amplitude, while it decreases with the increase in the initial gap ratio. By examining the sediment movement under different gap ratios, the impact of the initial gap ratio on the scour patterns has been analyzed, and predictive formulas for the variation of scour depth and length with initial gap ratio, vibration amplitude, and frequency, etc., have been obtained. The dynamic interaction process among flow field changes, pipeline vibration, and sediment movement has also been elucidated.
Year: 2024