Scour Around A Group Of Piles

Abstract

ABSTRACT Sand scour patterns at three different pile arrays employing two different pile diameters were studied in a two-dimensional wave flume. Important parameters were developed from the dimensional analysis and the experimental data obtained were analyzed to determine the functional relationships between the dimensionless parameters. The primary objective of the study was to determine the parameters that control the "ultimate" scour depth of a group of piles due to oscillatory wave motion; INTRODUCTION In the accelerated exploration and exploitation of gas and oil in deeper waters there is a growing need to place objects on or in the sea bed. Design considerations must include an analysis of their stability, particularly if the sea-bed is composed of fine non-cohesive sediments. In general, a sand bed is in a condition of dynamic stability, as the bottom material is transported by prevailing currents and wave-induced currents. When an object is placed on or in the sea-bed, the equilibrium may be disturbed. Local velocities increase and additional turbulence is generated so that the local flow attains a greater transport capacity. The increased transport capacity causes scouring (erosion) around the structures. The depth of scour is important as the exact position of the mud line must be known for the computation of minimum penetration depth of piles for fixed structures. For objects placed on the bottom, or for "sit-on-bottom" platforms the scour may cause settlement of the supporting members. The aim of this research was to study the effects of scour around the group of piles caused by oscillatory wave motion. No theoretical method for successfully predicting the scour depth has thus far been developed. Hence, this study is primarily directed toward an experimental rather than a theoretical approach. The experiments were conducted in a two-dimensional wind wave channel (2 feet wide × 3 feet deep × 120 feet long). Piles of 1 and 2 inch diameter and three different arrays of pile groups (3, 4 and 6-1egged) were employed in the experiments. The variables included two natural sands and nine characteristic waves. The analysis of data included an attempt to correlate the scour depths obtained with wave characteristics. THEORETICAL CONSIDERATIONS (a) Stokes Third Order Wave Theory Stokes third order wave theory was selected for the analysis of experimental data1,2. Stokes wave theory is an infinite series solution obtained by expanding the velocity potential about the still water line. Stokes theory can be expanded into any order theory but the third, fifth and seventh order theories are the most stable. Stokes third order wave theory can be simplified by tabulating repeating constants in the equations for various values of wave characteristics.5 (b) Dynamics of Scour A schematic view of the general hydrodynamic situation in the vicinity of an obstruction (Fig. 1) reveals the pattern of secondary flows, or turbulence, which accounts for the removal of granular materials through the process of scour. The oncoming flow under one surge pulse is represented by the envelope of flow lines at the left. The scour pit is shown as a clearly defined depression surrounding a cylindrical obstruction.