Understanding of the resistance of tickler chains of beam trawl fishing
N.J. Koorn (TU Delft - Mechanical Engineering)
G. H. Keetels – Mentor (TU Delft - Offshore and Dredging Engineering)
R. L.J. Helmons – Graduation committee member (TU Delft - Offshore and Dredging Engineering)
S.V. Bult – Graduation committee member (TU Delft - Offshore and Dredging Engineering)
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Abstract
The electric pulse fishing technique is no longer an option for fishermen. Therefore, they have to fall back on beam trawl fishing. This method has several disadvantages such as high fuel consumption and much soil disturbance. This system therefore deserves an investigation into improvement.
There is various literature on interaction of offshore structures with the soil. Processes in which a structure is also towed through the seabed are ploughing and cutting. Experiments with tickler chains are done and also numerical models are created.
Experiments were performed to acquire the required information that could not be retrieved from literature. In these experiments a chain element is towed through a sand layer in various configurations in a controlled way. Parameters that are varied are: velocity, chain link diameter, angle of attack, density, grain size.
A decrease of the angle of attack leads to a decrease of the tow force. The force per unit length is defined by dividing the tow force by the projected length. The projected length perpendicular to the towing direction, decreases for a decrease of angle of attack. However, the force per unit length also decreases for a decrease of the angle of attack. That means that the projected length is not the only explanation for the decrease in force.
The experiments indicate that the forces for higher penetration depths and higher chain link diameters are very high. An increase of density leads to a significant increase of the internal friction angle for low confining pressures. The increase of the internal friction angle leads to a higher passive soil pressure coefficient. The increase of this coefficient can certify the high forces.
Two dimensionless parameters are defined: the dimensionless force and the Froude number. An estimate for the increase of the passive pressure coefficient is integrated in the dimensionless force to clarify the high values for dense soil. After plotting the experimental results expressed in these dimensionless parameters, a linear trend line representing the data with reasonable accuracy of R2 = 0.611.