Subsea rock installation plays an important role in the protection and stabilization of offshore infrastructure, such as pipelines and cables. Fallpipe vessels enable the accurate placement of rocks on the seafloor at large water depths. The fallpipe is used as a large transport system between the vessel and the project depth. The internal dynamics during these operations remain poorly understood. Mixture densities, particle velocities, waterdrop, and air entrainment are important properties of the process that cannot be monitored during operation due to practical limitations. Existing numerical studies largely omit the effects of water inlets and entrained air. This thesis addresses these knowledge gaps through an experimental and numerical investigation of the flow dynamics inside a vertical fallpipe.

A Froude-scaled physical model (scale factor of 11.7) was designed, constructed, and tested at the Boskalis Hydrodynamics lab. A transparent PMMA tube of 4.4 meters in length was used to represent the fallpipe. The translucent material allowed direct visualization of the processes inside the fallpipe. Properties such as density, rock concentration, and production were further recorded using a loadcell and pressure sensors. Full-scale productions ranging from 370 to 1360 tonne/hr were scaled down and tested. Experiments combined production rates between 0.22 and 0.84 kg/s with four water‑inlet configurations (0, 4, 8, and 16 inlets). Each inlet had a diameter of 10 mm and was located at a depth of 0.265 m. The combined measurements from the loadcell and pressure sensors were combined with visual analysis to obtain key parameters, including mixture density, concentration, waterdrop, particle velocity, and air fraction.

Results show that the addition of water inlets substantially lowers the mixture density and rock concentration while increasing the velocity of both the water and rock fraction velocities within the fallpipe. Configurations without inlets experienced full blockage from production above 0.49 kg/s, which in full size would mean at productions above 820 tonne/hr. A configuration with 8 or 16 inlets open showed oscillatory plug forming around the inlets. The cause for this behavior can be attributed to the jet streams entering through the inlets. Rock particles are decelerated once they arrive at the inlets. Air entrainment was observed in all configurations with water inlets and increased with production rate, reducing the mixture density measured between sensors. The waterdrop inside the fallpipe can be reliably estimated from pressure measurements and closely matches visually tracked values.

A one-dimensional drift-flux was created and compared to the test results. The model uses mixture momentum, hindered settling effects, wall effects, and the addition to create simulations of the inside of the fallpipe. Results obtained from model simulation compare well to the results of the conducted experiments. A consistent deviation within the simulation results can be attributed to the absence of a modeled air fraction within the model. The trend that was not captured by the model is the influence of the aforementioned inlet dynamics, as this effect is not included in the model.

This thesis provides the first combined experimental–numerical study of water addition and air entrainment inside a fallpipe. The results provide new physical insights and establish a validated modeling framework that can support and accurately predict efficient subsea rock installation operations. Recommendations are provided, including the incorporation of air fractions and detailed inlet dynamics, for future experimental research.

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