Gholam Hossein Saghi - Managing Director and Chairman of the Board of Directors of Water and Sewerage Company, Bojnourd, North Khorasan, Iran
Majid Vahdani Asadi - Water resources development expert of Water and Sewerage Company, Bojnourd,North Khorasan, Iran.
Mojtaba Dmaroudi - Ph.D student, Department of Civil Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Isfahan, Iran

The pipelines are often laid directly on the seabed and riverbed erosions may change the flow pattern around it, causing local scour around the pipeline and self-burial processes. Therefore, if the pipeline self-burial process accelerates, it can be used as a low-cost option instead of a mechanical trenching. The use of a spoiler is one of the approaches to accelerate the scouring and self-burial processes. In addition, todays, for the transmission of electric and telecommunication cables through the seabed, a pipeline with a small diameter is used on top of the main pipe (known as Piggyback line), which can affect the flow pattern around the pipeline. In this study, the effect of the external geometry of the pipeline (due to the use of spoiler and Piggyback line) on the flow pattern around the pipe under a unidirectional flow were studied. The results showed that the

Local Scour, Pipeline, Spoiler, Piggyback, Self-Burial, Flow Pattern