Scour around submarine pipelines remains a largely complex and not yet fully understood problem. In this study, wave-induced scour around submarine pipelines was investigated. Since various physical processes occur during the development of a scour hole, the effects of each process were considered by employing several nondimensional parameters. To find the effective parameters on equilibrium scour depth, the correlation between independent parameters (e.g. Keulegan-Carpenter number) and dependent parameter (nondimensional scour depth) were determined using different experimental data. Then, an Artificial Neural Network (ANNs) approach was used to develop a more accurate model for prediction of wave-induced scour depth around submarine pipelines. ANN models with different input parameters including gap to diameter ratio, Keulegan-Carpenter number, pipe Reynolds number, Shields number, sediment Reynolds number and boundary layer Reynolds number were trained and evaluated to find the best predictor model. To develop the ANN models, both holdout and tenfold cross-validation methods were used. In addition, an existing empirical method was examined. Results show that the empirical method has a significant error in the prediction of scour depth for the cases with an initial gap between pipe and seabed. It is also indicated that the ANN models outperform the empirical method in terms of prediction capability.

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Submarine pipelines installed on seabed disturb the local flow field and bring imbalance in local sediment transport that leads to scouring. Local scouring can leave the pipeline unsupported in significant distances. The free spans of the pipeline are susceptible to damages arising from environmental forces or human activities. This paper presents results of experimental investigations on the equilibrium profile of the local scouring around the pipe due to unidirectional current. New equations are proposed for the prediction of the scour depth and width before installation of the pipe. It is evident from the experimental results that the equilibrium depth of the current-induced scour is a function of pipe diameter and Shields number. Also scour width at upstream and downstream of the pipe and the total width of the scour are investigated. The results are simplified in the form of diagrams for prediction of the scour depth/width in practice based on flow velocity, bed roughness condition and size of the granular materials. The results can be used either for the design purposes or for the calibration of numerical models. One of the results of this work is that the scour width at downstream of the pipe is about 2.5 times of the width at upstream.

Physical experiments have been conducted to study the scouring around a pipe over an erodible bed under wave motion. The results show that the depth of the wave-induced scour is reduced when the pipe is installed in a primary depth. It is also indicated that when the primary installation depth of the pipe, |e|, exceeds a specified depth, no scouring occurs underneath the pipe. This specific depth is called the equivalent depth of wave-induced scour, Se. The equivalent depth of wave-induced scour is estimated as a function of pipe diameter and the Keulegan–Carpenter number. With respect to the primary installation depth of the pipe, a new equation is suggested to predict the scour depth before installation of the pipe. And the scour width is predicted in two other new equations. The proposed equations are then simplified in the form of diagrams. The experiments cover small KC numbers (KC<6) that represent deep water conditions.

Physical experiments have been conducted to study the scouring around a pipe over an erodible bed under waves motion. The results show that the depth of the wave-induced scour is reduced when the pipe is installed in a primary depth. It is also indicated that when the primary installation depth of the pipe, |e|, exceeds a specified depth, no scouring occurs underneath the pipe. This specific depth is called the Equivalent Depth of Wave-Induced scour, Se. The equivalent depth of wave-induced scour is estimated as a function of pipe diameter and the Keulegan Carpenter number. Also for prediction of the scour depth with respect to the primary installation depth of the pipe, a new equation is proposed. In addition, new equations are proposed for prediction of the scour width before the pipe is installed. The results and proposed equations are next simplified in the form of diagrams to be used in practice. The experiments cover small KC numbers (KC < 6) that represent deep water conditions. The results can be used either for the design purposes or for calibration of the numerical models.