Experiment investigation of scour and self-burial of sagging subsea pipelines in steady current

Fundamental mechanisms of the self-burial process of a subsea pipeline that fully sags into a scour hole and the effects of the timing of pipeline sagging on the self-burial are investigated experimentally. Each self-burial experiment starts with an initial scour hole that is generated by a scour experiment for a particular time, which is defined as the touchdown time T_td. The effects of the touchdown time on the self-burial process are discussed. It was found that, to achieve self-burial, the touchdown time must be delayed to allow a sufficiently deep scour hole to develop. If the touchdown time is too soon, the scour will continue after the pipeline touches down and self-burial will not happen. After touchdown, self-burial initialises at the downstream side of the pipeline and has two stages: pre-self-burial scour and self-burial. The scoured sand behind the pipeline during pre-self-burial scour form a sand dune behind the pipeline and this sand dune is the key mechanisms of self-burial. The growth of this sand dune slows down the flow through the gap and finally stops the scour. Flow visualisation of a sagging pipeline was also conducted using the Particle Image Velocimetry method to understand the flow mechanism at different stages of the self-burial process. If the scour is allowed to fully develop, the pipeline can sag into the scour hole to a vertical distance of 0.7D before self-burial occurs, where D is diameter of pipeline. Based on the visualisation, recirculation of flow downstream to the pipeline plays a significant role in backfilling in the self-burial process.