The effect of Piggyback line on local scour below horizontal pipe lines

Background and Objectives: Waste water or fuel pipelines transmission from the erosion bed of sea and river causes an interaction between water and erosion bed which results some changes in hydrodynamic flow around the tube which caused some scour under the pipeline. As a result, it may leads to instability, bending and even breaking the pipe. To protect the pipelines in counter with potential risks, understanding of amount and scouring pattern around the pipe is necessary.
Materials and Methods: The present study has investigated the scour below the horizontal pipeline in a laboratory flume with three different pipe diameters (2, 3.2, 3.8 cm), located on the erosion bed with two medium-size of sediment (0.3, 1.18 mm) and three different shape of Piggyback line (blade, circle, triangle), under the steady, resistant and a unidirectional in lucid water conditions (V⁄V_C ≈0.9). First of all, using Pi-Bukingham method all the dimensional parameters affecting on local scour under the pipeline analyzed. Then by various experiments some important and dimensionless parameters such as diameter of the pipe, Froude number of flow, depth of flow, average grain size of the sediment of bed and the shape of Piggyback line have been surveyed on maximum depth scour. Results demonstrated that the surface area of barrier opposite of flow effects on final mount of scour and it increases by accretion of pipe diameter, depth of maximum scour and consuming time up to balanced final sour.
Results: According to Experimental results, the surface area of the barrier affect the flow path in the final scour and by increasing diameter of the pipe, a maximum depth of scour and time reaching to the final balance of scour increase.
In this research, by increasing in Froude number in a constant diameter of pipe and soil grading size, maximum depth of scour obtained in Froude number of 3.0. The depth of flow effects on depth of scour at y_n⁄D≤3.5 was almost negligible. In this regard, in small amount of this ratio, depth of scour is independent of depth of flow. On the other hand, average grain size of the sediment doesn’t have a considerable impact on maximum depth of scour. But, because the standard deviation of grain size of the sediment with an average size in 1.18mm, was less than 1.4 (high uniformity), it can be conclude that the standard deviation has a significant effect in amount and location of maximum scour depth and can change the scour profiles significantly.
By using different forms of Piggyback line in equal height, length of hole scour became equal approximately and it just depended on length of obstacle opposite of flow.
The observation showed that the wave length of the leading sediment and scour is affected by Piggyback line. It was in blade Piggyback line form 2.42, triangle Piggyback line form 2.25 and circle Piggyback line form established on main pipe 1.19 times more than single pipe on the detrital bed. Different depth in maximum sour between blade and triangle Piggyback line was less but in accordance with circle ones it was more.
Conclusion: our results demonstrate that any change in stabilized Piggyback line on main pipe can affect scour pattern of under horizontal pipe line. To increase in the maximum depth of scour and transmission of electricity, telephone cables and water-sewer duct along the main pipe, triangle Piggyback line is the better option to replace the spoilers which have been used these days.