Equilibrium Scour Prediction for Uniform and Non-Uniform Cylindrical Structures Under Clear Water Conditions

Abstract

Offshore Gravity Base Foundations (GBFs) are often designed with non-uniform cylindrical geometries. Such structures interact with the local hydrodynamics which amplify the adverse dynamic pressure gradient, which is responsible for all flow and scour phenomena including the bed shear stress amplification. In this study a method for predicting the effect non-uniform cylindrical structure geometries have on local scour around offshore structures under the forcing of a unidirectional current is presented. The interaction of the flow field with the sediment around these complex structures is described in terms of non-dimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights in the influence a form of the Euler number has on the equilibrium scour around uniform and non-uniform cylindrical structures. Here the Euler number is defined as the depth averaged pressure gradient (calculated using potential flow theory) divided by the product of the square of mean flow velocity and the fluid density. The insights are confirmed through a series of experiments where the equilibrium scour was monitored for different types of structures and flow conditions. The results of this study show that the Euler number is a more appropriate parameter for describing the scour potential of a structure compared to using the equivalent pile diameter. The experimental data show that an increasing Euler number yields an increase in the non-dimensional equilibrium scour. The results of this study also suggest that an increase in the water depth yields a decrease in the equilibrium scour depth for the conical, cylindrical base structures and truncated cylinders and an increase in the equilibrium scour depth for the uniform cylinders which can also be explained in terms of changes in the Euler number. Finally, the Buckingham π theorem in conjunction with the experimental data was used to derive a simple shape correction factor that could be used to determine the scour depth of a non-uniform cylindrical structure based on the equilibrium scour produced for the same flow conditions by a uniform cylinder.

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“…The difference between experimental and numerical results is small enough to be attribured to the slightly different flow profiles that were present in each case. To verify this, the scour prediction correction factor of Tavouktsoglou et al (2016) is applied to the simulation data. Table 2 provides a summary of the data used in this analysis: In the data above < > is the depth-averaged pressure gradient and can be calculated using the method outlined in Tavouktsoglou et al (2016), and is the resulting correction factor.…”

“…To verify this, the scour prediction correction factor of Tavouktsoglou et al (2016) is applied to the simulation data. Table 2 provides a summary of the data used in this analysis: In the data above < > is the depth-averaged pressure gradient and can be calculated using the method outlined in Tavouktsoglou et al (2016), and is the resulting correction factor. From Table 2 it can be seen that the difference of between the simulated and measured cases is 90%.…”