Semi-resolved CFD-DEM simulations of the rock behavior inside an inclined fall pipe

Abstract

With the shift to renewable energy becoming more important than ever before and offshore wind farms being one of the sustainable energy resources available, many offshore wind farms are currently being built.
One of the difficulties of these wind farms built with monopiles is that the monopiles and electricity cables must be protected from scouring and falling objects. The protection is usually performed by placing a layer of rocks around and over these objects. For deep water up to 400 meters, vertical fall pipes are used to place the rocks directly under the rock placement vessel. This is impossible when placing rocks around monopiles, as the monopile stands in the way of where the vessel would have to lie during the rock placement procedure.
The inclined fall pipe (IFP) was introduced recently to combat this issue. The IFP, which hangs diagonally next to the vessel, can place rocks a couple of meters to the vessel's side, leaving a safe distance between the vessel and the monopile. Rocks tend to form clusters inside the pipe during placement with an IFP. This means the rocks exit the pipe in bursts of clusters, resulting in uneven rock layers if the rocks are placed during the movement of the vessel. Another result is less accurate placement of the rocks as the rocks at the outer layer of a cluster, during the free fall, are pushed further out, away from the intended placement location.

This thesis investigates the processes of rocks moving inside an inclined fall pipe and simulates this process numerically. This is achieved with a Finite-Volume solver for the fluid coupled with a Discrete Element model for the particles. The results are compared to existing experimental data.

The simulation results show the same rock behavior as in the lab tests for angles up to 60 degrees. The clusters start forming from the small volume concentration differences within the rock layer. The higher densities concentrate and become clusters, while the lower densities become gaps. In the simulations, this process is caused by the differences in the flow velocity of the fluid and the resulting drag differences of the particles. This knowledge is a first step towards finding solutions to lower the formation of clusters and thus, the more accurate placement of the rocks on the sea floor.

Furthermore, multiple points of improvement for the solver have been found. For angles of 75 degrees and steeper, the rock movements of the simulations miss a certain chaoticness, which may be attributed to missing turbulent drag forces in the solver. The velocities predicted by the solver are too high; multiple possible causes are speculated. The simulation results follow the same trends as the lab results, meaning they could be used for qualitative comparisons. Various additions to enhance the solver are proposed.
The influence of a system parameter, the distance factor was noticed to be stronger in shear flows than in homogeneous flows during a system parameter variation. This distance factor relates the fluid-particle interaction radius to the particle diameter. Finding limits for this parameter in shear flow could enhance the solver's prediction capability.

With the shift to renewable energy, more monopiles will be placed. The simulations show which forces cause the cluster formations and can be used to investigate ways of reducing them. This can reduce the spillage of rocks during operation, making the entire rock placement more efficient and will reduce costs.
The solver with the enhancements for better prediction capabilities could be used to further optimize the rock placement accuracy and efficiency. This would reduce wind farm costs and help the shift to renewable energy.