Draghead Analysis

Abstract

In dredging there are a lot of opportunities to improve the production processes and to make the production cycle more efficient, especially the excavation process of the draghead. The aim of this research is to determine the trailing forces and to estimate the production of the draghead. Besides that, the goal is to get more insight into how the draghead behaves, depending on the trailing velocities. The variations in the trailing velocities, soil characteristics, control settings and draghead and suction pipe geometries that can occur do not make it easy to determine the trailing forces on the draghead and suction pipe. Because of this complexity, it is also hard to define how to estimate the production. Therefore, the scope of this research is limited to just one sand type with specific soil characteristics.

An analysis of the draghead and the suction pipe, with a freely suspended visor, showed the physical processes in and around the whole suction pipe system. Because the draghead is fixed to the suction pipe, the influence of suction pipe on the draghead is analysed first. After that, the draghead is divided into two parts, the visor and the visor house. With the use of force and moment balances the trailing forces are determined for every trailing velocity. In addition, the production and its production limits are defined.

The calculations show that the increase of the trailing velocity results in higher trailing forces on the suction pipe and draghead. For a velocity of around 2 m/s the draghead, for a Damen SLK600 used in the case study, will lift of from the bed. It should be notified that, among other variables, the dredging depth has an effect on this ‘floating’ point. Moreover, the results showed that the drag forces at common trailing velocities of 1-2 m/s are relatively low compared to the soil excavation forces and therefore have a small share in the total trailing forces. When the suction pipe system is trailed against the current the dragforce becomes more significant.

The interaction of the draghead with the bed causes several processes to take place. The resulting relevant trailing forces are mapped and determined. The settlement of the draghead causes a hump of sand to be pushed forwards which result in a sled force and a friction force. Besides that, the flow through the pipes will cause impulse forces in the bends and at the end of the jet pipe out of the nozzle. The jets fluidize the sand which results in the largest production contribution. Furthermore, it can be seen that the penetration depth and cavity width of the jets depend on the trailing velocity and determine the amount of sand that is loosened. The cavities can overlap at low trailing velocities, resulting in a jet production limit.

The jets have a significant influence on the behaviour of the visor. The freely suspended visor will drop until a solid bed layer is reached. The cutting force and vacuum force are the dominant forces working on the visor. Application of the equilibrium-moment method showed that the visor is slowly moving upwards when the trailing velocity is increased. However, the cutting layer thickness remains almost constant for an increasing trailing velocity which results in a linear increase of the cutting production. The cutting production contributes, 20-25%, to the total situ production. The total jet and cutting production lead, together with the jet water inserted and ambient water flow, to the total production and mixture density. When the capacity of the dredging pump is insufficient, spillage will occur.

This research shows the best possible estimate of the draghead production and corresponding trailing forces. It should be kept in mind that the calculations are based on a simplification of the suction pipe system and the geometries of the suction pipe system of Damen. Nevertheless, a lot of research can be performed into the processes that occur, to improve the results of the calculations. Suggestions for further research would be to determine the magnitude of the vacuum force, the erosion production which is not considered in this research or the on velocity depending cavity width. Validations of the results is another point of consideration. Thus, to determine trailing forces and production, the new approach used in this study contributes to a better understanding of the draghead.