A morphological model for the local scour around pipelines is developed. Based on the Navier-Stokes equation and an erodible boundary adjustment technique, the model predicts the equilibrium scour hole without using any sediment transport formula. The calculation results clearly reveal that the lee-wakes behind the pipeline contribute significantly to the local scour hole development, especially to the gentle scour slope observed in experiments. The predicted maximum scour depth and the shape of the scour hole compare favorably with the experimental data published in the literature.
Local scour around offshore pipelines has significant effects on the stability of the pipeline. So far, the prediction of local scour around pipelines has been mainly based on experimental modeling or empirical formulas derived from experiments. A review of the published results relating to local scour in the last 3 decades can be found in Sumer and Fredsfe (1992), and more recently in Whitehouse (1998). There is no doubt that physical modeling is a fundamental and effective method for understanding the mechanisms of the scouring process. However, it is also associated with high costs and relatively long times. In addition, the inadequate knowledge of the scale effect in the model test poses a question mark on the application of experimental results to prototype. In comparison, the numerical model does not suffer from these defects. This can provide design engineers with an alternative way to evaluate and understand the process of local scour around pipelines. It is therefore imperative that more effort is devoted to developing suitable numerical models. Over the last 2 decades, mainly 2 kinds of numerical models for scour prediction have been developed. One is based on the potential flow theory, such as Hansen et al. (1986) and Li and Cheng (1998); the other is based on the k-e model, such as Leeuwenstein and Wind (1984).
