Field observations of seabed scours around a submarine pipeline on cohesive bed

The seabed scours around a submarine pipeline on cohesive seabed were studied by analyzing the field investigation data of DF1-1 submarine pipeline off the west coast of Dongfang, Hainan Island. The results show that, the tidal current in the study area can cause the seabed scours on both sides of the pipeline (side scours). The maximum scour depths agree well with that calculated according to the formulae proposed by various researches based on experimental studies with sand or silt beds. It is found that the finer sediments were transported mainly in the form of suspended load in the scour process, while the coarser sediments were mainly as bedload. This process caused a noticeable coarsening process of the local sediments. There exists a large number of scour pits under the pipeline (down scours), which are believed to be caused by hurricanes, but the onset of the scour (piping) is a progressive process, and piping occurs only when the critical piping conditions are exceeded.