Concept Design for a Rock Handling System

Abstract

Erosion of the seabed around offshore structures, due to current and waves, is a common occurrence.
Scour protection by rock placement can be done in order to limit erosion and its consequences. Tideway
Offshore Solutions will use the multipurpose vessel ’Living Stone’ to execute such rock placement
operations. Aboard the Living Stone a newly designed inclined fall pipe system will be installed to
transfer the rocks through shallow and possibly high current water towards the seabed. The goal of
this thesis is to design a concept of a rock handling system to feed the inclined fall pipe system aboard
of the Living Stone. The concept requires to be capable of handling armourstone with gradings up to
60-300kg and continuous mass flows up to — tonnes per hour.
With the use of the VDI 2221 design cycle the function, principle and solution structures were created
to realise a morphological overview. Multiple concepts were created and evaluated on criteria
including the maximum allowance of peak production, proven technology, maximum solvability, minimum
required space and minimum costs. Thereafter, a preliminary layout was created for the concept
ranked highest. In order to be sure the concept is capable of generating the demanded mass flows in
terms of capacity and accuracy, discrete element modelling was used. With the use of EDEM-software
the performance of the in Solidworks created model was studied. After calibration of the simulated material,
the performance of the concept was studied by varying the outlet area and the feeders’ velocity.
This resulted in the associated mass flows and their fluctuations. The accuracy of the mass flow was
defined as a percentage of the mass flow in which the flow fluctuated.
The configuration was studied and set for further simulations, using Box-Behnken design of experiments
and Minitab-software. The mass flow, depending on the installed shear height and the velocity
of the feeder, was validated with empirical theory for apron feeders. This work demonstrated that the
ratio between the outlet area, defined as the outlet diameter but realised by the shear height and feeder
width, and the maximum particle diameter is of great influence on the mass flow. The smaller the ratio,
due to an increase of particle size and/or decrease in outlet area, the less accuracy of the simulated
mass flow. The difference between the simulated and theoretical mass flow increases exponential with
the decrease of the ratio between outlet diameter and maximum particle diameter.