# Monitoring-System Development for a Bottom-Set Gillnet through Time-Domain Dynamic Simulations
###### Abstract  
This paper investigates the sensor-based monitoring feasibility of a bottom-set gillnet through time-domain dynamic simulations for various current and wave conditions and failure scenarios. The dimension and design parameters of the bottom-set gillnet were based on an existing model used in Korea, and the measured environmental data were acquired from the southwest coast of Korea and utilized for the dynamic analysis. For efficient numerical modeling of nets, an equivalent net model which uses fewer line elements was considered, and the projected area, wet weight, and axial stiffness were accordingly adjusted. The hydrodynamic forces on the entire gillnet were estimated using a Morison-force model on the instantaneous positions of the net. The designed gillnet provided excellent stretching performance even under low current velocity. The dynamic responses under wave excitations were not significant in operating conditions; however, significant motions were observed in the fishery-prohibition condition. The proposed monitoring system consisted of an accelerometer, tension sensors, and the global positioning system. Numerous line-failure scenarios were simulated, and the proposed monitoring system could effectively detect a specific problem from the combined patterns of sensor signals by a problem-detection algorithm. f [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]  
mooring (SPM) cage system to investigate the fatigue of a mooring line under wave and constant current loads. [PERSON] et al. [6] conducted experiments for a gravity cage and compared hydrodynamic behaviors of the cage and mooring tensions with numerical simulations in waves and currents. Furthermore, the dynamic behaviors of box- and column-shaped net cages under wave excitations and steady currents were compared through experiments and numerical simulations by [PERSON] et al. [7]. Floater-net coupled dynamic analyses in currents and waves have been performed by [PERSON] and [PERSON] [8] and [PERSON] and [PERSON] [9]. [PERSON] and [PERSON] [10] provided a detailed method for developing a cruder, equivalent net model in their time-domain numerical analysis and compared their numerical results, including a wake model, for downstream nets in currents with experiments. Moreover, [PERSON] and [PERSON] [11] have further validated their numerical methods for net dynamics and mooring tensions in currents and waves with experiments. [PERSON] et al. [12] studied the hydrodynamic behaviors of a four-cage-coupled system including a comparison between simulations and experiments. [PERSON] et al. [13] conducted time-domain analyses of a stow net in various current velocities and wave heights and investigated the feasibility of the monitoring system for fishing gears. Authors of [8; 10; 11; 13] have presented many comparisons against various experimental results to consistently validate the developed time-domain simulation tool for various types of nets and cages. In this study, a bottom-set gillnet, which is one of the most widely used types of fishing gear in Korea [16], was selected. A bottom-set gillnet generally consists of a long, slender net (also defined as the net assembly in this study), mooring lines and ropes, and location buoys. Floats and weights are added along the head and ground ropes so that it can stand vertically in the water. The invisible net catches fish while traveling without them being aware of it; thus, stretching performance in the vertical direction is an essential factor in the net design. Also, mooring lines with heavy anchors are located at a specified interval to maintain the position of the net. Location buoys float on the sea surface during operation to visually show the location of the net. The primary purpose of this study was to reliably simulate the dynamic behaviors of the bottom-set gillnet in different current and wave conditions and to develop a cost-effective sensor-based monitoring system that can detect various failures of the bottom-set gillnet.