Reduction of the outflow velocity of a closed fallpipe system

Abstract

Subsea rock installation is widely applied in the offshore industry and utilized for a wide range of purposes including but not limited to: pipeline protection, scour protection, insulation of pipelines, upheaval buckling prevention and seabed preparation. Tideway Offshore Solutions is specialized in subsea rock installation and currently operates three state-of-the-art fallpipe vessels. Their vessel 'Flintstone' makes use of an innovative closed fallpipe system to provide high accuracy subsea rock installation.

The presence of rocks in the water column of the fallpipe increase the density of the mixture in the fallpipe. The density difference between the mixture in the fallpipe and the density of the surrounding sea-water results in a water level drop in the fallpipe. To keep this water level drop within acceptable limits extra water is added to the fallpipe system which accelerates the fallpipe flow. The accelerated fallpipe flow can result in high outflow velocities of the rock mixture at the fallpipe exit. High outflow velocities of the rock mixture can eventually result in increased impact velocities of the rock particles on the seabed. Increased impact velocities of the rock particles on the seabed can lead to unsatisfactory rock berm shapes resulting in the need for remedials. To have their fallpipe system perform as efficient as possible Tideway Offshore Solutions was interested in possible measures to reduce the outflow velocity of the fallpipe which resulted in this thesis.

In the first part of this thesis different concepts, that could potentially reduce the outflow velocity of the fallpipe, are generated and conceptually analyzed. The information acquired from this analysis is used as input for a multi criteria analysis that resulted in the selection of the most promising concept, the use of a deflector. The deflector will act as a flow deflector at the fallpipe exit thereby decreasing the impact velocity of rock particles on the seabed. In the second part of this thesis a complete three-dimensional computational fluid dynamics (CFD) analysis is performed on the fallpipe outflow with and without deflector. The CFD program used to simulate these situations is ANSYS Fluent. The simulations for both cases are performed for two different turbulence models, the k – ε and k – ω SST turbulence models, the distance from the fallpipe exit to the seabed is varied as well and a range of deflector angles and dimensions are simulated.

The fluid flow velocities obtained from the CFD analysis are used as input in a MATLAB model to compute the rock particle trajectories in a two-dimensional plane. Combining the rock particle trajectories and their velocity components it is possible to compute the impact velocities of the rock particles on the seabed. The results of the trajectory model set up in MATLAB showed that a substantial decrease in impact velocity of the rock particles on the seabed can be achieved by using a deflector at the fallpipe exit.