Rock behavior in and below an inclined fall pipe

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Abstract

Subsea rock installation is an offshore engineering process where rocks are placed on the seabed as protection of cables and pipelines and as scour protection. The inclined fallpipebis a new piece of equipment specifically designed to install rocks close to submerged structures. This thesis investigates the processes of the rock flow in and below the inclined pipe. This is done with a research question in two parts: What is the rock behavior in the inclined fall-pipe, and what is the behavior below the inclined fall-pipe? The research is questions are supported by sub-questions on the influence of pipe angle, production and rock size.

To answer the research questions model tests have been carried out at a scale of 1:15 in the Dredging Lab at TU Delft. The tests were performed with varying pipe angles, production rates and rock sizes. The tests have been analyzed with special focus on velocity, flow behavior, touch down offset from the pipe, and the spread of the rocks. For analysis, video recordings of the tests have been used and the tests have been analyzed with Particle Image Velocimetry software PIVlab.

The results of the tests reveal that the velocity of the rock flow mostly depends on the pipe angle and production rate, and for a lesser part on the rock size. Steep pipe angles increase rock velocity, increasing production leads to a higher average velocity, and smaller rock sizes increase the velocity.

The spread of the rocks and the offset from the pipe are influenced the strongest by the stand-off (SOD) distance between the pipe and the bed. In the tests the SOD was determined by the pipe angle. To compare the tests, they were also analyzed at the same height below the pipe. The results show that the spread of the rocks is only influenced by the height above the floor. The offset is influenced both by the angle and the production. The influence of the production is only visible at lower angles. The increase in production means an increase in velocity and the rocks falling further away. More horizontal pipe