Sedimentation-velocity in jet induced flow

Abstract

Embedding of subsea pipelines for protection purposes is usually done in a ‘Trench – Install – Backfill’ operation. The pipeline can also be embedded after it is installed on the seabed. This method is called post-trenching. When using a trailer suction hopper dredger for post-trenching, a large water jet erodes the soil underneath the pipe creating a temporarily trench in which the pipeline can sink. Large steel pipelines are inflexible and need a considerable free span to sink to the desired depth. The length of the trench is limited as the eroded sand that is brought in suspension starts to settle in the trench when the velocity of the suspension decreases. To be able to predict the length of the trench, the sedimentation velocity in jet induced flow is investigated in this research. The research describes the experiment series that is performed to investigate the behavior of a trench that is created by a vertical jet flow, trailing over a sand bed. The focus is on the sedimentation part of the trench. Concentrations and flow velocities are measured to determine the shape of the trench, the distribution profile and velocity profile. The experiments show a clear relation between the depth of the trench and the length that the trench is open. A deeper trench (due to a higher pressure or lower trail velocity) results in all experiments in a longer trench. The near bed concentration and the sedimentation velocity do not change a lot for different settings of the jet. To see if the theory on the sedimentation process can represent the post-trenching process, a calculation model is made that uses the theoretical concentration profile to calculate the sedimentation velocity. A good similarity was found between the model and the experiments. The experimental research on the sedimentation velocity in a trench that is created in a post-trenching process, indicates that sedimentation velocity is largely independent of the jet parameters. To make the trench longer, it has to be deeper to lengthen the path that the bed has to travel. A test with a second run over the bed proved highly effective due to the high porosity of the sand that settles in the first run, the maximum excavation depth is twice the depth of the first run. By implementing a pipeline in the experiment it is investigated if the pipeline has an influence on the flow conditions and sedimentation velocity.