Soil scour around a shallowly embedded pile can significantly compromise its lateral response, reducing both stiffness and capacity. Estimation of the lateral pile response must take into account both the scour-hole geometry and the overconsolidation effects on the remaining soil. A series of centrifuge model tests with various scour profiles were conducted at a scale of 1:250 to investigate the effects of both local and general scour on the response of a laterally loaded pile. Measured pile moment distributions and force–displacement data at the pile head were used to derive p–y curves quantifying the lateral pile–soil interaction. The p–y curves derived from various scour profiles were compared for equivalent depths below the new scour base, and below the original soil surface. For the general scour cases, the p–y curves for given depths below the post-scour surface are essentially identical to those at the same depths without scour. In contrast, for the local scour cases, the p–y response at a given depth below the scour-hole base is much stiffer than at the same depth below the original soil surface. As a practical approach to evaluate the effects of scour, the concept of an effective soil depth is introduced to determine the corresponding p–y curves for shallowly embedded piles in sand.
In the natural near-shore environment, waves often coexist with currents, which can induce excess pore pressure in the porous marine sediments and weaken the stability of the seabed. The dynamic interaction of waves, currents, and the seabed has been considered critical in the studies of sediment transport and local scour (e.g.,
Monopile is the most commonly used foundation type for offshore wind turbines. The local scour at a monopile foundation generated by the incoming shear flow has significant influence on both quasi-static lateral responses and dynamic responses of the monopile. This chapter focuses particularly on characterizing the local scour in both spatial and temporal scales and revealing the scour mechanisms associated with the flow field around a monopile. The predicting methods for the equilibrium scour depth and the time scale of scour are detailed under various representative flow conditions in the marine environment. The scale effect while extrapolating the results of model tests to prototype conditions is highlighted. The local scour imposes significant influence not only on the deformation and stiffness of the monopile foundation, but also on the natural frequency and fatigue life of the structure system. Monopiles with diameters up to 10 m have become a feasible option as the industry is currently advancing into deeper waters. More meticulous considerations for monopile design associated with the scour depth prediction and evaluation of scour effects are still in need to efficiently minimize the cost while remaining safety simultaneously.
The spatiotemporal evolution of the turbulent horseshoe vortex (THV) in front of a cylinder vertically mounted on a hydraulically smooth flat-bed was physically modeled in a large water flume. A particle image velocimetry (PIV) system with upward-illumination was, in particular, employed for the junction flow visualization. The examined Reynolds number was varied from 1.28 × 104 to 1.08 × 105, which is above the threshold of turbulent transition for a junction flow. Based on the PIV measurements, the characteristic features were presented for both the time-averaged and the instantaneous flow fields in a sheet flow at the upstream wall-cylinder junction. Statistical analyses on the experimental data are performed to characterize the spatial-temporal evolution of THV strength. The normalized THV strength in the time-averaged flow field increases first and then approaches a constant value with increasing the ratio of cylinder diameter to water depth. Two alternating patterns, i.e., the regular oscillation and the random wandering, are identified for the quasi-periodic oscillating behaviors of the instantaneous THV. It is found that the cumulative distribution curves for the normalized instantaneous THV strength can be described by the Weibul distribution. The present results provide a physical insight and quantitative characterization for the spatiotemporal evolution of THV, which is critical for predicting the associated wall shear stresses.
Pile foundations for offshore wind turbines are subjected to large lateral loads. By mounting wings on the perimeter of regular monopiles, winged monopiles have shown better performance in resisting deformation under horizontal loading. However, the hazardous effect of local scour on the lateral bearing capacity of winged monopiles installed in the sandy seabed has not been systematically evaluated. In this study, a modified Mohr–Coulomb model considering the pre-peak hardening and post-peak softening behavior of dense sand is adopted to simulate laterally loaded winged monopiles in the locally scoured sandy seabed, using three-dimensional finite element analyses. The effect of local scour depth on the lateral capacity of winged monopiles is examined and explained by soil failure mechanisms. The enhancement of lateral capacity with wings attached to the monopile is demonstrated to be more effective than extending pile embedment length. The effects of the relative density of sand and the wing load orientation are also discussed. Finally, the wing efficiency is evaluated to determine the optimal configuration of winged monopiles.
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