Monitoring-System Development for a Bottom-Set Gillnet Through Time-Domain Dynamic Simulations

applied sciences

Article Monitoring-System Development for a Bottom-Set Gillnet through Time-Domain Dynamic Simulations

Chungkuk Jin 1,* , HanSung Kim 1, Moo-Hyun Kim 1 and Kiseon Kim 2

1 Department of Ocean Engineering, Texas A&M University, Haynes Engineering Building, 727 Ross Street, College Station, TX 77843, USA; [email protected] (H.K.); [email protected] (M.-H.K.) 2 School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Korea; [email protected] * Correspondence: [email protected]; Tel.: +1-979-204-3454

Received: 6 March 2019; Accepted: 19 March 2019; Published: 22 March 2019

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.

Keywords: ﬁshing gear; bottom-set gillnet; computer simulation; dynamic analysis; equivalent net model; numerical sensor; monitoring system; failure scenarios; problem-detection algorithm

1. Introduction In recent decades, marine-environmental pollution from lost fishing gear has been regarded as a serious problem [1]. In particular, one technical report showed that 20% of fishing gear in Korea is lost or abandoned, and within this amount, only roughly 15% is collected [2]. This means that the remaining 85% may be problematic to the marine environment. Therefore, cost-effective methods to prevent the loss of fishing gear is necessary, and developing an economical and functional monitoring system can be a solution. Since a net system is fully submerged and cannot be directly observed, the monitoring system has to use sensors. For this, the underwater dynamic behaviors of a net system must be well understood for a sensor-based monitoring system for fishing gear to be designed. In this regard, the utilization of a reliable numerical-simulation tool is essential for analyzing overall dynamic behaviors of various types of fishing cages/gear in site-specific environmental conditions. Morison-force [3–13] and screen-force [14,15] models are the two popular approaches used to analyze fishing gears and nets. In particular, the Morison-force model has been validated with experiments in current and wave conditions. Lee et al. [4] have suggested a numerical model for a fishing cage with a floating collar and have compared their results with experiments. Huang and Pan [5] investigated a single-point

Appl. Sci. 2019, 9, 1210; doi:10.3390/app9061210 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1210 2 of 20 mooring (SPM) cage system to investigate the fatigue of a mooring line under wave and constant current loads. Zhao 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 Zhao et al. [7]. Floater-net coupled dynamic analyses in currents and waves have been performed by Cifuentes and Kim [8] and Chen and Christensen [9]. Cifuentes and Kim [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, Cifuentes and Kim [11] have further validated their numerical methods for net dynamics and mooring tensions in currents and waves with experiments. Xu et al. [12] studied the hydrodynamic behaviors of a four-cage-coupled system including a comparison between simulations and experiments. Jin 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. The monitoring system may also be applied to protect fishing gear in case of deep-water turbulences. Even though there have been experiments [17,18] and numerical simulations [19,20] investigating the dynamic behaviors of the gillnet, these have only focused on small sections of the gillnet. This means that there has been no publication in the open literature which has attempted to investigate not only the global behaviors of the bottom-set gillnet but also the simulations of transient responses with various scenarios of line failures, which could be applied to the development of a sensor-based monitoring system. A commercial program, OrcaFlex, was used as a base program and the entire gillnet was rigorously modeled through equivalent line elements and three degrees of freedom (DOF) buoys. Previous studies have included authors’ well validated methodologies for the similar applications of fishing cages/gear under waves and currents [3–13]; thus, in this study, we adopted those procedures for dynamic analysis. Since the longitudinal length of a bottom-set gillnet can be up to several kilometers, creating an equivalent net model is essential in modeling, and the equivalent net model was completed by adjusting the projected area, wet weight, and axial stiffness. The relative-velocity-based Morison-force model, which has been suggested by previous research [3–13], was applied at the instantaneous net-position to estimate the hydrodynamic force on the entire system, which is also critical in simulating highly transient phenomena. The global performance of the bottom-set gillnet was investigated in operational and fishery-prohibition conditions. Particularly, in this study, various failure scenarios and the corresponding patterns of numerical sensors were further analyzed for the feasibility of the sensor-based monitoring system. Key monitoring parameters were identified to detect sudden failures of mooring lines and location-buoy-connection ropes. Furthermore, using real-time sensor signals, a simple monitoring algorithm was devised and demonstrated as an example of machine-based problem-detection and decision-making processes. Appl. Sci. 2019, 9, 1210 3 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 20

2. Numerical Model

2.1. Configuration Configuration Figure1 1 shows shows aa schematicschematic viewview ofof thethe entireentire bottom-set-gillnetbottom-set-gillnet systemsystem andand thethe netnet assemblyassembly utilized in this study. The numerical models of the gillnet were based on an actual design used in Korea and the design parameters and material properti propertieses used used in in this this study, study, given in Table 1 1,, werewere obtained from previous research [[21,22].21,22]. As mentioned before, the entire bottom-set gillnet consisted of the net assembly, mooringmooring lines,lines, and location buoys. The net assembly, which was made of nylon monofilament,monofilament, had a length and height of 300 m and 2 m, respectively. The hanging ratio, which is defineddefined as the actual gillnet length divided by the fullyfully stretched length, is considered as an essential factor in catching fish,fish, and a widely used value of 0.5 [[23]23] was adopted in thethe present study. In the net assembly, many floats floats and and weights weights were were fixed fixed to to head head and and ground ground ropes ropes at equal at equal intervals intervals of 0.5 of 0.5m. Wood m. Wood bars bars were were positioned positioned at atfixed fixed intervals intervals of of 50 50 m m along along the the net-assembly net-assembly length length to to maximize the stretching performance of the net assembly. TheThe mooring lines were made of polypropylene and secured by strong strong anchors. anchors. They They were were installed installed at at in intervalstervals of of50 50m mto maintain to maintain the the location location of the of thenet netassembly. assembly. The The original original purpose purpose of ofthe the location location bu buoyoy was was to to check check the the location location of of the underwater portion of the gillnetgillnet visuallyvisually since they floatfloat onon thethe seasea surface.surface. Each locationlocation buoy waswas connectedconnected to the net assembly through a polypropylene rope, and three location buoys were installed at 150 m intervals. The water depth was set as 25 m. The net bottom almost touched the seabed.

(a)

(b)

Figure 1. 2D views of entire bottom-set gillnet (a) and the net assemblyassembly ((b).). Table 1. Material properties and design parameters. Table 1. Material properties and design parameters. Item Value Unit Item Value Unit LengthLength 300300 m m Height 2 m Net Height 2 m Net Material Nylon monoﬁlament - DiameterMaterial Nylon 0.25 monofilament mm - Diameter 0.25 mm Head and ground ropes, mooring lines Material Polypropylene - Appl. Sci. 2019, 9, 1210 4 of 20

Table 1. Cont.

Item Value Unit Material Polypropylene - Head and ground ropes, mooring lines Diameter 10 mm Material Plastic - Floats Volume 12 × 17 × 39 mm3 Material Lead - Weights Size (volume) φ 14 × 30 mm3 Mass 37.5 g Material Styrofoam - Buoys Size 50 L

2.2. Time-Domain Numerical Simulation The bottom-set gillnets were numerically modeled using OrcaFlex, a well-known commercial program [24]. In this study, the gillnet models were entirely created using line and 3-DOF-buoy models, as shown in Figure2. A 3-DOF-buoy model was introduced for modeling other components such as floats, weights, and location buoys, as well as a convenient connection of line models. The Morison-force model for a moving object was employed to estimate the external force of the entire gillnet. In the Morison equation, the added mass coefficient was fixed as 1.0 for the entire line model [25], which corresponded to an inertia coefficient of 2.0. The Morison forces were calculated at each time step at the members’ instantaneous locations and inclinations using the instantaneous normal cross-flow relative velocities and accelerations. The drag coefficient for the net should represent the physical net well, and the formulation suggested by DeCew et al. [26] was adopted in this study. On the other hand, the drag coefficient for mooring lines and ropes was fixed as 1.2 [27]. The equations and detailed descriptions can be found in Jin et al. [13]. Creating an equivalent net model is one of the most important parts of developing a numerical model. If the numerical model is to realize the physical net as it is, significant computational time is unavoidable due to the massive number of lines and 3-DOF-buoys that may represent connecting knots. Thus, to reduce the number of line and buoy elements, an equivalent net model is needed, which is much more efficient in terms of modeling and computational time. In the present study, the wet weight as well as the projected area were matched between the physical and numerical nets. When the projected area was matched to produce equivalent viscous drag forces, the numerical net became heavier and stiffer than the physical net. The heavier numerical net was compensated for by changing the buoyancy in the 3-DOF-buoy model used to connect line models. In addition, the issue related to larger axial stiffness can be solved by introducing a modified Young’s modulus, as discussed by Fredheim [28]. The inertial force term has a much smaller magnitude compared with the drag force term considering the large value of the Keulegan-Carpenter (KC) number and given wave conditions [29]. In this case, the inertial force term in the Morison equation can be neglected. Cifuentes and Kim [10] have thoroughly explained the detailed procedure used to develop an equivalent net model. Except for the net, the other components were modeled as they are. For the convergence-test purpose, two numerical models, which are a cruder model (Model A), as presented in Figure2, and a finer model (Model B), are created. Model A is made up of 568-line and 308 3-DOF-buoy models while Model B has 1831-line and 971 3-DOF-buoy models. Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 20

Diameter 10 mm Material Plastic - Floats Volume 12 × 17 × 39 mm³ Material Lead - Weights Size (volume) ϕ 14 × 30 mm3 Mass 37.5 g Material Styrofoam - Buoys Size 50 L

(a) Side view

(b) Front view

Figure 2. Side (a), front (b), and top (c) views of the designed numerical model of the bottom-set gillnet.

3. Environmental Conditions Table2 summarizes the environmental conditions used in this study. The measured current and wave data, acquired in the southwest coast of Korea for three years, were utilized in this study. The corresponding probabilities of current velocity, significant wave height (Hs), and peak period (Tp) had been analyzed by the authors of [13] in previous research. The operating conditions were selected from the analyzed data. Moreover, the fishery-prohibition condition in Korea, which is a significant wave height of 5 m, was additionally simulated to observe the corresponding global performance of the system [30]. A JONSWAP wave spectrum was utilized to generate irregular-wave time histories. In the operating conditions, the enhancement parameter was fixed as 1.0, which is the same as in the Pierson-Moskowitz wave spectrum. Furthermore, an enhancement parameter of 2.14 for the fishery-prohibition condition was based on the average value of the Korean site [31]. A current profile was produced by the power-law method. The current velocity at the bottom was assumed to be 10% of the free-surface-current velocity. Waves and currents were linearly superposed where their nonlinear interaction effects were not considered. The directions of waves and currents were collinear and were fixed to a positive x-axis (0 degrees). Detailed descriptions for wave and current generations can be found in Jin et al. [13].

Table 2. Wave and current conditions for the parametric study.

Wave and Current Conditions

Signiﬁcant wave height, Hs (m) 1.0 2.0 3.0 5.0 Peak period, Tp (s) 6.0 7.0 8.0 10.0 Enhancement parameter, γ 1.0 1.0 1.0 2.14 Surface current velocity (m/s) 0.2 . . . 0.4 . . . 0.6 . . . 0.8 . . . 1.0 Appl. Sci. 2019, 9, 1210 6 of 20

4. Convergence Test Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 20 Appl.When Sci. 2019 the, 9, x equivalent FOR PEER REVIEW net model was used, it was essential to check its convergence by varying6 of the 20 numbersAppl. Sci. 2019 of lines, 9, x andFOR PEER 3-DOF-buoys. REVIEW In this regard, the results of the less fine Model A and the much-finer6 of 20 3 and 4 show the envelopes of the lateral (X direction) and vertical (Z direction) motions of the net Model3 and B4 show were comparedthe envelopes to show of the that lateral both (X models direction) produce and similarvertical results. (Z direction) Recall motions that Model of the B has net 3assembly and 4 show of Models the envelopes A and B of at the a current lateral velocity(X direction) of 0.6 and m/s. vertical Figure 5(Z shows direction) mooring motions tensions of the at netthe approximatelyassembly of Models three times A and more B at lines a current and 3-DOF-buoys velocity of 0.6 compared m/s. Figure to Model 5 shows A. Figures mooring3 and tensions4 show at the the assemblymiddle location of Models with A currentand B at velocities a current from velocity 0.2 m/of 0.6s to m/s. 1.0 m/s.Figure Waves 5 shows were mooring not considered tensions atin thethe envelopesmiddle location of the lateralwith current (X direction) velocities and verticalfrom 0.2 (Z m/ direction)s to 1.0 m/s. motions Waves of thewere net not assembly considered of Models in the middleconvergence location test. with Model current A produced velocities almost from the0.2 samem/s to motion 1.0 m/s. results, Waves both were in thenot lateralconsidered and vertical in the Aconvergence and B at a current test. Model velocity A produced of 0.6 m/s. almost Figure the5 shows same motion mooring results, tensions both at thein the middle lateral location and vertical with convergencedirections, compared test. Model to AModel produced B. In almostaddition, the ther samee were motion no results,significant both differences in the lateral in theand mooring vertical currentdirections, velocities compared from 0.2to m/sModel to 1.0B. In m/s. addition, Waves ther weree notwere considered no significant in the differences convergence in test. the Modelmooring A directions,tensions between compared the two.to Model The maximum B. In addition, difference there inwere mooring no significant tensions wasdifferences only 3.2 in percent the mooring for the producedtensions almostbetween the the same two. motion The maximum results, both difference in the lateral in mooring and vertical tensions directions, was only compared 3.2 percent to Model for the B. tensionstop mooring between line theat a two. current The velocitymaximum of difference0.4 m/s. Since in mooring the computational tensions was time only of 3.2 Model percent B was for theten Intop addition, mooring there line were at a no current significant velocity differences of 0.4 m/s. in the Since mooring the computational tensions between time the of two. Model The maximumB was ten toptimes mooring more thanline atthat a current of Model velocity A, while of 0.4 both m/s. producedSince the computationalalmost the same time results, of Model Model B was A wasten differencetimes more in mooring than that tensions of Model was onlyA, while 3.2 percent both for produced the top mooring almost linethe atsame a current results, velocity Model of 0.4 A m/s.was timesrepeatedly more usedthan thatto produce of Model the A, ensuing while both results produced from thisalmost point the onsame to results,investigate Model the Aglobal was Sincerepeatedly the computational used to produce time of Modelthe ensuing B was tenresults times morefrom thanthis thatpoint of Modelon to A,investigate while both the produced global repeatedlyperformance used of the to system. produce the ensuing results from this point on to investigate the global almostperformance the same of results, the system. Model A was repeatedly used to produce the ensuing results from this point on to investigateperformance the of global the system. performance of the system.

Figure 3. Envelopes of lateral motions (Z = −23 m) of Model A and Model B at a current velocity of 0.6 Figure 3. Envelopes of lateral motions (Z = −23 m) of Model A and Model B at a current velocity of 0.6 FigureFigurem/s for 3. 3. the Envelopes convergence of lateral test. motions (Z = −23 m) of Model A and Model B at a current velocity of 0.6 m/s for theEnvelopes convergence of lateral test. motions (Z = 23 m) of Model A and Model B at a current velocity of 0.6m/s m/s for forthe the convergence convergence test. test.

Figure 4. Envelopes of vertical motions (net-top motion minus net-bottom motion) of Model A and FigureFigure 4. 4.Envelopes Envelopes ofof vertical vertical motions motions (net-top (net-top motion motion minus minus net-bottom net-bottom motion)motion) ofof Model Model AA and and FigureModel 4.B atEnvelopes a current of velocity vertical of motions 0.6 m/s for(net-top the convergence motion minus test. net-bottom motion) of Model A and ModelModel B B at at a a current current velocity velocity of of 0.6 0.6 m/s m/s forfor thethe convergenceconvergence test.test. Model B at a current velocity of 0.6 m/s for the convergence test.

FigureFigure 5. 5.Convergence Convergence test: test: mooring mooring tensions tensions of of Model Model A A and and Model Model B B at at the the middle middle location location with with Figure 5. Convergence test: mooring tensions of Model A and Model B at the middle location with differentFiguredifferent 5. current currentConvergence velocities. velocities. test: mooring tensions of Model A and Model B at the middle location with different current velocities. different current velocities. 5. Global Performance of the Bottom-Set Gillnet 5. Global Performance of the Bottom-Set Gillnet 5. Global Performance of the Bottom-Set Gillnet 5.1. Effects of Current Velocity 5.1. Effects of Current Velocity 5.1. Effects of Current Velocity Appl. Sci. 2019, 9, 1210 7 of 20

5.Appl. Global Sci. 2019 Performance, 9, x FOR PEER of REVIEW the Bottom-Set Gillnet 7 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 20 5.1. Effects of Current Velocity Figure 6 shows the lateral-response envelopes of the net assembly at various current velocities. WavesFigureFigure were6 6shows showsnot considered the the lateral-response lateral-response in this section envelopes envelopes to ob of ofserve the the net netthe assembly assembly effects atof at various consvarioustant-current current current velocities. velocities. velocity. WavesWavesGenerally, werewere the notnot net considered consideredassembly stre inintched thisthis section sectionwell between toto observeob serverespective thethe effects effectsanchors/mooring ofof constant-currentconstant-current lines for the velocity.velocity. given Generally,Generally,current-velocity the the net net range. assembly assembly Except stretched stre fortched the well currentwell between between veloci respective tyrespective of 0.2 m/s, anchors/mooring anchors/mooring lateral envelopes lineslines were forfor almostthe the given given the current-velocitycurrent-velocitysame because mooring range.range. linesExcept maintained for for the the current current the net veloci velocity positytion of of tightly0.2 0.2 m/s, m/s, without lateral lateral anyenvelopes envelopes more extension. were were almost almost Figure the thesame7 presents same because because the mooring envelopes mooring lines of lines maintainedthe maintainednet heights the net theat posidi netversetion position currenttightly tightly withoutvelocities. without any Current more any extension. more velocity extension. greatlyFigure Figure7influences presents7 presents netthe height, envelopes the envelopes and, ofgenerally, the of thenet net theheights heights higher at atthdi diverseeverse current current current velocity, velocities. velocities. the lower Current the net velocityvelocity height greatly withoutgreatly inﬂuencesinfluenceswood bars net net[17,32,33]. height, height, and, However,and, generally, generally, in the the the present higher higher net the th esystem, current current the velocity, velocity, net he the ightthe lower lower was themaintained the net net height height well without without due to woodwoodthe adoption bars bars [[17,32,33].17 ,of32 ,wood33]. However,However, bars, as shown inin thethe presentin Figure net 7. system, system,As mentioned the the net net hebefore, heightight was wood was maintained maintained bars were well welllocated due due toin tothe the netadoption adoption assembly of of wood at wood 50 mbars, bars,intervals. as as shown shown Because in in Figure Figureof thes 7.7e .As wood As mentioned mentioned bars, the before, net before, heights wood wood around bars bars were werethe woodlocated located bars in inthekept the net nettheir assembly assembly original at at50net 50 m height mintervals. intervals. of 2.0 Because Becausem while of thes of re theseductionse wood wood bars, were bars, the observed thenet netheights heightsbetween around around them. the thewoodThe wood most bars barskeptsubstantial kept their their original reduction original net in net height net height height of of2.0 was 2.0 m m observedwhile while re reductions ductionsat the smallest were were observedcurre nt velocity between of them.0.2them. m/s TheThe since mostmost too substantialsubstantiallow current reduction reduction velocity results in in net net height inheight less wasdrag was observed forceobserved to stretch at at the the smallestthe smallest net assembly current current velocity fully;velocity however, of of 0.2 0.2 m/s m/s its sincereduction since too too lowlowwas current currentnot significant. velocity velocity resultsIn results addition, in in less less net-height drag drag force force reductio to to stretch stretchn at the the high net net current assembly assembly velocities fully; fully; however, however, was largely its its reduction improvedreduction waswasby the not not existence signiﬁcant. significant. of In woodIn addition, addition, bars. net-height Innet-height other words, reduction reductio then atnet at high high assembly current current was velocities velocities well stretched was was largely largely in the improved improved vertical bybydirection the the existence existence at all current of of wood wood velocities, bars. bars. InIn mainly otherother words,words, due to the the netwoodnet assemblyassembly bars. In was wasconclusion, wellwell stretchedstretched the designed inin the the vertical bottom-vertical directiondirectionset gillnet at at allshowed all current current excellent velocities, velocities, stretching mainly mainly due performance due to to the the wood wood in bars.the bars. lateral In conclusion,In conclusion, and vertical the the designed directions designed bottom-set acrossbottom- a gillnetsettypical gillnet showed range showed of excellent current excellent stretchingvelocity. stretching In performance addition, performance as insh theown in lateral in the Figure lateral and 5, vertical mooringand vertical directions tensions directions across increased a across typical with a rangetypicalcurrent of range currentvelocity of velocity. currentbecause velocity. Inof addition,the increased In addition, as shown drag as inforc sh Figureownes on in5 , theFigure mooring gillnet. 5, tensionsmooring Higher increasedtensionsmooring increased withtensions current werewith velocitycurrentobserved becausevelocity in top of mooringbecause the increased oflines the because dragincreased forces current drag on the velocityforc gillnet.es on decreases Higherthe gillnet. mooring with Higher submergence tensions mooring were depth. tensions observed were in topobserved mooring in linestop mooring because lines current because velocity current decreases velocity with decreases submergence with submergence depth. depth.

Figure 6. Envelopes of the lateral responses (Z = −23 m) of the net assembly at different current FigureFigurevelocities. 6.6. EnvelopesEnvelopes of of the the lateral lateral responses responses (Z (Z= −23 = −m)23 of m) the of net the assembly net assembly at different at different current currentvelocities. velocities.

FigureFigure 7. 7.Envelopes Envelopes of of the the vertical vertical responses responses (net-top (net-top motion motion minus minus net-bottom net-bottom motion) motion) of of the the net net assemblyFigureassembly 7. at Envelopesat different different currentof current the vertical velocities. velocities. responses (net-top motion minus net-bottom motion) of the net assembly at different current velocities. 5.2. Effects of Wave Excitations 5.2. Effects of Wave Excitations Figures 8 and 9 present the time histories of the lateral and vertical responses at the middle locationFigures (Y = 80.0 and m, 9Z present= −23.0 m)the in time different histories wave of conditionsthe lateral andand their vertical envelopes responses for theat the significant middle locationwave height (Y = (0.0Hs) m,of 5.0Z = m. −23.0 The m) wave in different and current wave directions conditions were and in theirthe X envelopesdirection, i.e.,for theperpendicular significant waveto the heightnet plane. (Hs) In of this5.0 m.section, The wave the rope and lengthcurrent was directions 100 m and were the in currentthe X direction, velocity i.e.,was perpendicular set as 0.6 m/s. to the net plane. In this section, the rope length was 100 m and the current velocity was set as 0.6 m/s. Appl. Sci. 2019, 9, 1210 8 of 20

5.2. Effects of Wave Excitations Appl.Figures Sci. 20198, 9and, x FOR9 present PEER REVIEW the time histories of the lateral and vertical responses at the middle8 of 20 locationAppl. Sci. (Y2019 =, 0.09, x FOR m, Z PEER = − REVIEW23.0 m) in different wave conditions and their envelopes for the significant8 of 20 Figures 8a and 9a show that the net dynamics were small in typical operating conditions, i.e., Hs was wave height (Hs) of 5.0 m. The wave and current directions were in the X direction, i.e., perpendicular less than 2.0 m. However, in the fishery-prohibition condition (Hs = 5.0 m), significant fluctuations toFigures the net 8a plane. and 9a Inshow this that section, the net the dynamics rope length were wassmall 100 in typical m and operating the current conditions, velocity i.e., was H sets was as were noticed, and their amplitudes increased more than the linear increase. As shown in Figures 8 0.6less m/s. than Figures 2.0 m.8 However,a and9a show in the that fishery-prohibition the net dynamics condition were small (Hs in= 5.0 typical m), significant operating conditions,fluctuations and 9, the lateral motions were usually more significant in the negative direction than in the positive i.e.,wereH noticed,was less and than their 2.0 m.amplitudes However, increased in the fishery-prohibition more than the linear condition increase. ( HAs= shown 5.0 m), in significant Figures 8 directions (direction of wave and current) because mooring tensions could mores effectively restrict the and 9, the lateral motions were usually more significant in the negative direction than in the positive fluctuationspositive-direction were noticed, responses. and theirThe maximum amplitudes lateral increased response more was than observed the linear in increase. the location As shownbetween in direction (direction of wave and current) because mooring tensions could more effectively restrict the Figuresmooring8 and lines9 , because the lateral the motions highly flexible were usually net assembly more significant was free to in move the negative there. The direction vertical than responses in the positive-direction responses. The maximum lateral response was observed in the location between positivewere, on direction the other (direction hand, mostly of wave in the and positive current) direction, because since mooring the bottom tensions of couldthe net more almost effectively touched mooring lines because the highly flexible net assembly was free to move there. The vertical responses restrictthe seabed. the positive-direction They had the largest responses. values The at the maximum positions lateral of mooring response lines was due observed to the effects in the of location wood were, on the other hand, mostly in the positive direction, since the bottom of the net almost touched betweenbars and mooring rope tensions. lines because The mooring the highly lines flexiblehardly re netstricted assembly the vertical was free motions to move because there. the The mooring vertical the seabed. They had the largest values at the positions of mooring lines due to the effects of wood responseslines were were, almost on thelateral, other i.e., hand, perpendicular mostly in theto th positivee net plane. direction, The vertical since theresponses bottom between of the net mooring almost bars and rope tensions. The mooring lines hardly restricted the vertical motions because the mooring touchedlines were the seabed.significantly They coupled had the with largest lateral values concaved at the positions responses of since mooring the net lines was due flexible. to the effects of woodlines barswereand almost rope lateral, tensions. i.e., perpendicular The mooring to lines the hardlynet plane. restricted The vertical the vertical responses motions between because mooring the mooringlines were lines significantly were almost coupled lateral, with i.e., perpendicularlateral concaved to responses the net plane. since The the verticalnet was responses flexible. between mooring lines were significantly coupled with lateral concaved responses since the net was flexible.

(a) Time histories

(b) Envelopes

Figure 8. Time histories of the lateral motions(b) Envelopesof the net at the top-middle location (Y = 0 m, Z = −23 m) under different wave conditions (a) and motion envelopes along the net length at Hs = 5.0 m and FigureFigure 8. 8.Time Time histories histories of of the the lateral lateral motions motions of of the the net net at theat the top-middle top-middle location location (Y =(Y 0 = m, 0 Zm, = Z− =23 −23 m) Tp = 10.0 s (b) (current velocity = 0.6 m/s). underm) under different different wave wave conditions conditions (a) and(a) and motion motion envelopes envelopes along along the the net net length length at atH Hs s == 5.0 m and TpT=p = 10.0 10.0 s s ( b(b)) (current (current velocity velocity = = 0.60.6 m/s).m/s).

(a) Time histories

(aFigure) Time 9. historiesCont. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 20

Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 20 Appl. Sci. 2019, 9, 1210 9 of 20

(b) Envelopes

Figure 9. Time histories of the vertical motions(b) Envelopesof the net at the top-middle location (Y = 0 m, Z = −23 m) in different wave conditions (a) and motion envelopes along the net length at Hs = 5.0 m and Tp = FigureFigure 9. 9.Time Time histories histories of theof the vertical vertical motions motions of theof the net net at the at the top-middle top-middle location location (Y = (Y 0 m,= 0 Z m, = −Z 23= − m)23 10.0 s (b) (current velocity = 0.6 m/s). inm) different in different wave conditionswave conditions (a) and (a motion) and motion envelopes envelopes along thealong net the length net length at Hs = at 5.0 H ms = and5.0 mTp and= 10.0 Tp s= 10.0 s (b) (current velocity = 0.6 m/s). (Figureb) (current 10 velocityshows the = 0.6 time m/s). histories of the mooring tension at the mooring line located in the upper center under different wave conditions and the corresponding distribution of mooring tension FigureFigure 10 10shows shows the the time time histories histories of the of mooringthe moorin tensiong tension at the at mooring the mooring line located line located in the upperin the along the length for an Hs of 5.0 m, considered to be the fishery-prohibition condition. Again, the centerupper under center different under different wave conditions wave conditions and the corresponding and the corresponding distribution distribution of mooring oftension mooring along tension the fluctuations in mooring tensions were small when Hs was less than 2.0 m, whereas mooring tensions lengthalong for the an lengthHs of 5.0for m,an consideredHs of 5.0 m, to considered be the fishery-prohibition to be the fishery-prohibition condition. Again, condition. the fluctuations Again, the in were significantly amplified for Hs = 5.0 m. The high mooring tension under this fishery-prohibition mooringfluctuations tensions in mooring were small tensions when Hweres was small less when than 2.0 Hsm, was whereas less than mooring 2.0 m, tensionswhereas weremooring significantly tensions condition was mainly caused by the sudden increase of tension by snap loading after the mooring amplifiedwere significantly for Hs = 5.0amplified m. The for high Hs = mooring 5.0 m. The tension high mooring under this tension fishery-prohibition under this fishery-prohibition condition was line was slack. In this slack-taut repeating scenario, not only can mooring lines be broken, but also, mainlycondition caused was by mainly the sudden caused increase by the sudden of tension increase by snap of tension loading by after snap the loading mooring after line the was mooring slack. anchors can be dragged out in harsh waves and strong currents. Progressive failures of mooring lines Inline this was slack-taut slack. In repeating this slack-taut scenario, repeating not only scenario, can mooring not only lines can be mooring broken, lines but also, be broken, anchors but can also, be can happen after one mooring line is broken. draggedanchors out can in be harsh dragged waves out andin harsh strong waves currents. and strong Progressive currents. failures Progressive of mooring failures lines of mooring can happen lines aftercan one happen mooring after lineone ismooring broken. line is broken.

(a) Time histories (a) Time histories

(b) Longitudinal distribution (b) Longitudinal distribution FigureFigure 10.10. TimeTime historieshistories ofof thethe mooringmooring tensiontension atat thethe middlemiddle locationlocation (top(top mooringmooring line)line) underunder differentdifferentFigure wave10.wave Time conditions histories (a) (ofa) andthe and themooring the corresponding corresponding tension at spatial th spatiale middle distribution distribution location along (top along themooring thenet netlength line) length atunder H ats = different wave conditions (a) and the corresponding spatial distribution along the net length at Hs = H5.0s = m 5.0 and m andTp = T10.0p = s 10.0 (b)(current s (b) current velocity velocity = 0.6 =m/s). 0.6m/s. 5.0 m and Tp = 10.0 s (b)(current velocity = 0.6 m/s). 6.6. IdentiﬁcationIdentification ofof MonitoringMonitoring ParametersParameters in in Line-Failure Line-Failure Scenarios Scenarios 6. Identification of Monitoring Parameters in Line-Failure Scenarios In this section, the monitoring parameters in different line-failure locations were identiﬁed. Of course, installing more sensors would have provided better accuracy of failure monitoring; however, Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 20

In this section, the monitoring parameters in different line-failure locations were identified. Of course, installing more sensors would have provided better accuracy of failure monitoring; however, considering the cost of the entire bottom-set gillnet, this was unfeasible. Therefore, finding key monitoring parameters was required. In this regard, the limited number of sensors were located in Appl.the Sci.net2019 assembly,, 9, 1210 the location buoy, and mooring lines, as shown in Figure 2. In particular,10 of 20an accelerometer was installed in the center location of the net assembly to measure accelerations in 3 consideringDOFs, a global the positioning cost of the entiresystem bottom-set (GPS) was gillnet,attached this to wasa location unfeasible. buoy in Therefore, the center finding to measure key monitoringdisplacements parameters in 3 DOFs, was and required. tension In sensors this regard, were the po limitedsitioned number at the top of sensorsmooring were lines located at the incenter the netand assembly, end locations the location of the buoy,net assembly. and mooring Since lines, the GP as shownS measures in Figure motions2. In particular,of the buoy, an real-time accelerometer wave wasstatistics, installed e.g., in significant the center locationwave height of the and net peak assembly period, to measurecould be accelerations acquired by insolving 3 DOFs, the a inverse global positioningproblem using system a Kalman (GPS) filter, was attached which has to abeen location done buoyby other in the authors center [34]. to measure Authors displacements have proven that in 3the DOFs, real-time and tension inverse sensors estimation were of positioned ocean waves at the from top heave mooring sensor lines signals at the centerusing an and adaptive end locations Kalman- of thefilter net technique assembly. successfully Since the GPS reproduces measures motions the real-t ofime the buoy,wave real-time elevations wave and statistics, spectra. e.g., The significant proposed wavephysical height system and peak had period,14 mooring could belines, acquired and critical by solving parameters the inverse to problemdetect failure using ascenarios Kalman filter,were whichinvestigated. has been Additionally, done by other a authorsrope connecting [34]. Authors the net have assembly proven thatto a thelocation real-time buoy inverse could estimationbe broken, ofand ocean hence waves the effect from of heave the rope sensor failure signals was usingalso checked. an adaptive Kalman-filter technique successfully reproducesA GPS the cannot real-time deliver wave its elevations signal in the and water; spectra. thus The, it proposed is necessary physical for th systeme buoy had not 14 to mooring be fully lines,submerged and critical during parameters operation. toBesides, detect if failure the length scenarios of the were rope investigated.is too short, it Additionally,can cause significant a rope connectingvertical motions the net of assembly the net by to athe location heave buoy motions could of be the broken, location and buoy. hence These the effect considerations of the rope needed failure wasto be also investigated checked. before key-parameter identifications. Figure 11 shows the time histories of the dry lengthA GPS(or freeboard), cannot deliver which its was signal defined in the as water; the instantaneous thus, it is necessary length of for a buoy the buoy (volume not to= 50 be liters) fully submergedabove the water during surface operation. at different Besides, rope if the lengths. length An of theHs and rope T isp of too 2 short,m and it 7 can s, respectively, cause significant were verticalapplied. motions It was ofseen the that net bythe the rope heave length motions significan of thetly location influenced buoy. the These dry considerationslength of the location needed buoy. to be investigatedWhen the rope before was key-parameter short, the interaction identifications. between Figure the 11 net shows assembly the time and histories the location of the drybuoy length was (orsignificant, freeboard), i.e., which the short was rope defined could as more the instantaneous easily pull down length the oflocation a buoy buoy. (volume On the = 50 other liters) hand, above for a long slack rope, the interaction between the net and buoy became weaker, resulting in smaller the water surface at different rope lengths. An Hs and Tp of 2 m and 7 s, respectively, were applied. Itvariations was seen of that freeboard. the rope length significantly influenced the dry length of the location buoy. When the rope wasFigure short, 12 theshows interaction the time between histories the of netthe assembly vertical motion and the of location the net buoy assembly was significant, in the middle i.e., thelocation. short ropeA case could without more location easily pull buoys down was the also location simulated buoy. to On check the otherthe importance hand, for aof long the slackinteraction rope, thebetween interaction the buoy between and the net asse andmbly. buoy becameDue to the weaker, significant resulting inte inraction smaller between variations the ofnet freeboard. assembly and Figurethe location 12 shows buoy, the time significant histories vertical of the vertical motions motion occurred of the when net assembly the rope in thelength middle was location. 50 m. AHowever, case without the locationinteraction buoys effect was became also simulated weaker to when check the importancerope length of was the interactionincreased. betweenThe vertical the buoymotions and for the the net proposed assembly. design Due to with the significant 100 m rope interaction length were between similar the to netthose assembly without and location the location buoys. buoy,In conclusion, significant the vertical proposed motions location-buoy occurred when and rope the rope designs length with was 100 50 m m. length However, were the able interaction to locate effectthe GPS became sensor weaker with when a view the rope to lengthminimizing was increased. buoy-net The interactions vertical motions and maintaining for the proposed the designbuoy’s withfreeboard. 100 m rope length were similar to those without location buoys. In conclusion, the proposed location-buoy and rope designs with 100 m length were able to locate the GPS sensor with a view to minimizing buoy-net interactions and maintaining the buoy’s freeboard.

Figure 11. Time histories of the dry length of the location buoy in the middle location for Hs = 2.0 m, Tp = 7.0 s, and current velocity = 0.6 m/s (the pink line is a reference level which the dry length should be higher than to prevent the buoy from being fully submerged). Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 20

(a) Rope lengths of 50 m and 75 m

(b) Rope length of 100 m and no buoy or rope

FigureFigure 12. 12.Time Time histories histories of of the the vertical vertical motion motion of of the the net net assembly assembly at at Y =Y 0= m,0 m, Z =Z −= −2323 min min the the middle middle locationlocation for forH Hs =s = 2.0 2.0 m, m,T Tp p= =7.0 7.0 s, s, and and current current velocity velocity = = 0.6 0.6 m/s m/s (the(thebottom-set bottom-set gillnet gillnet without without location location buoysbuoys was was also also simulated simulated for for comparison). comparison).

FiguresFigures 13 13–15–15 show show the the time time histories histories of of mooring mooring tensions, tensions, surge surge motions motions of of the the location location buoy, buoy, andand X-Y X-Y directional directional accelerations accelerations of of the the net net assembly assembly in in the the middle middle location location (height (height center center Z Z = =− −24m)24m) asas mooring mooring lines lines in in the the middle middle location location (Y (Y = = 0 0 m) m) were were broken broken after after 600 600 s. s. The The three three scenarios—top scenarios—top mooringmooring failure, failure, bottom-mooring bottom-mooring failure,failure, andand bothboth failures—were investigated for for HHss == 2.0 2.0 m m and and Tp T=p 7.0= 7.0 s. For s. Forother other wave wave cases, cases, only onlystatistical statistical data have data havebeen presented. been presented. In the case In the of casethe top of mooring the top mooringline being line broken, being the broken, corresponding the corresponding mooring tension mooring became tension zero became since zeroa tension since sensor a tension was sensorlocated wasthere. located When there. the bottom When the mooring bottom line mooring broke, line the broke, mean thevalue mean and value standard and standard deviation deviation of the top- of themooring top-mooring tension tension increased, increased, as shown as in shown Figure in 13. Figure In the 13 case. In of the both case mooring of both lines mooring being lines broken, being the broken,tension-sensor the tension-sensor reading became reading zero, became while zero, the while surge the surgemotion motion of the of location the location buoy buoy increased increased by byapproximately approximately 20.0 20.0 m, m, as as presented presented in in Figures Figures 13 and 1414.. TheThe X-directionalX-directional accelerationsaccelerations abruptly abruptly changedchanged with with failure failure due due to to the the transient transient impulsive impulsive response response at at the the moment moment of of failure. failure. The The signal signal also also showedshowed larger larger variations variations after after failure; failure; however, however, Y-directional Y-directional accelerations accelerations did not change did appreciablynot change afterappreciably failure happens, after failure as shown happens, in Figure as shown 15, since in Figure the symmetry 15, since ofthe the symmetry system in of the the Y system direction in the was Y maintained.direction was Therefore, maintained. for each Therefore, failure for scenario, each failur theree existedscenario, a uniquethere existed combination a unique of combination sensor-signal of changes,sensor-signal which changes, could be which used for could data-based be used problem-detectionfor data-based problem-detection and decision-making and decision-making processes. processes.

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Figure 13. Time histories of the mooring tension in the middle location (Y = 0 m) as mooring lines in

the middle location (Y = 0 m) are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s).

Figure 13. Time histories of the mooring tension in the middle location (Y = 0 m) as mooring lines in the middle location (Y = 0 m) are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s).

FigureFigure 13.13. TimeTime historieshistories ofof thethe mooringmooring tensiontension inin the the middlemiddle locationlocation (Y(Y == 0 m) asas mooringmooring lineslines inin thethe middlemiddle locationlocation (Y(Y == 00 m)m) areare brokenbroken afterafter 600600 s s ( H(Hss == 2.02.0 m,m, TTpp = 7.0 s, current velocity = 0.6 m/s).

Figure 14. Time histories of the surge motion of the location buoy in the middle location (Y = 0 m) as mooring lines in the middle location (Y = 0 m) are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current

velocity = 0.6 m/s). FigureFigure 14. 14.Time Time histories histories of of the the surge surge motion motion of of the the location location buoy buoy in in the the middle middle location location (Y (Y = = 0 0 m) m) as as mooringmooring lines lines in in the the middle middle location location (Y (Y = = 0 0 m) m) are are broken broken after after 600 600 s (sH (H=s = 2.0 2.0 m, m,T T=p = 7.0 7.0 s, s, current current s p velocityvelocity = = 0.6 0.6 m/s). m/s). Figure 14. Time histories of the surge motion of the location buoy in the middle location (Y = 0 m) as mooring lines in the middle location (Y = 0 m) are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s).

Figure 15. Cont.

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Figure 15. Time histories of the X (top) and Y (bottom) directional accelerations of the net assembly in the middle location (Y = 0 m) as mooring lines in the middle location (Y = 0 m) are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s). Figure 15. Time histories of the X (top) and Y (bottom) directional accelerations of the net assembly in Figure 15. Time histories of the X (top) and Y (bottom) directional accelerations of the net assembly the middle location (Y = 0 m) as mooring lines in the middle location (Y = 0 m) are broken after 600 s Figuresin the middle 16 and location 17 show (Y = the 0 m) time as mooring histories lines of mooringin the middle tensions location and (Y X-Y= 0 m) directional are broken accelerations after 600 (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s). of thes net(Hs assembly= 2.0 m, Tp =in 7.0 the s, middlecurrent velocitylocation = (at0.6 m/s).Y = 0 m) as mooring lines at Y = −50 m are broken after 600 s.Figures In this 16 case, and both 17 show the X- the and time Y-directional histories of accelerations mooring tensions show and transient X-Y directional impulsive accelerations peaks at the ofmoment theFigures net of assembly line 16 and failure. in17 theshow The middle thetransient time location histories peak (at in Yof =themooring 0 m)Y direction as tensions mooring occurs and lines X-Y because at directional Y = − the50 mY-directional accelerations are broken aftersymmetryof the 600 net s. assemblyis In suddenly this case, in lost.the both middle After the X-the location and line Y-directional failure, (at Y = bo 0 thm) accelerationsthe as mooringcenter-mooring lines show at transienttensions Y = −50 mand impulsive are Y-directional broken peaks after ataccelerations600 the s. momentIn this case,appreciably of line both failure. the increased X- The and transient Y-directional due to peak the accelerations in in thecreased Y direction dynamics show occurs transient as because a impulsiveresult the of Y-directional thepeaks loss at theof moment of line failure. The transient peak in the Y direction occurs because the Y-directional symmetryneighboring is suddenlylines. When lost. those After two the lineprevious failure, line both-failure the center-mooring events are compared, tensions andthe sensor-signal Y-directional symmetry is suddenly lost. After the line failure, both the center-mooring tensions and Y-directional accelerationspatterns become appreciably quite distinct, increased and due each to failure the increased scenario dynamics can be identified as a result by of observing the loss of the neighboring behavior accelerations appreciably increased due to the increased dynamics as a result of the loss of lines.of the Whensensor those signals. two Similar previous simulations line-failure and events analyses are compared,have been carried the sensor-signal out for other patterns mooring-line- become neighboring lines. When those two previous line-failure events are compared, the sensor-signal quitefailure distinct, scenarios and and each the failure critical scenario parameters can be fo identiﬁedr failure bydetection observing are the summarized behavior of in the Table sensor 3. patterns become quite distinct, and each failure scenario can be identified by observing the behavior signals.Additional Similar simulated simulations statistical and analysesdata for havedifferent been wa carriedve conditions out for other are also mooring-line-failure presented in Tables scenarios 4–6. of the sensor signals. Similar simulations and analyses have been carried out for other mooring-line- and the critical parameters for failure detection are summarized in Table3. Additional simulated failure scenarios and the critical parameters fo r failure detection are summarized in Table 3. statistical data for different wave conditions are also presented in Tables4–6. Additional simulated statistical data for different wave conditions are also presented in Tables 4–6.

FigureFigure 16.16. TimeTime historieshistories ofof thethe mooringmooring tensiontension inin thethe middlemiddle locationlocation (Y(Y == 00 m)m) asas mooringmooring lineslines atat YY == −5050 m m are are broken broken after after 600 600 s s ( (HHss == 2.0 2.0 m, m, TTp p== 7.0 7.0 s,s, current current velocity velocity = =0.6 0.6 m/s). m/s).

Figure 16. Time histories of the mooring tension in the middle location (Y = 0 m) as mooring lines at

Y = −50 m are broken after 600 s (Hs = 2.0 m, Tp = 7.0 s, current velocity = 0.6 m/s).

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FigureFigure 17. 17.Time Time histories histories of of X X ( top(top)) and and Y Y ( bottom(bottom)) directional directional accelerations accelerations of of the the net net assembly assembly in in the the middlemiddle location location (Y (Y = = 0 0 m) m) as as mooring mooring lines lines at at Y Y = =− −5050 mm areare broken broken after after 600 600 s s ( H(Hs s= = 2.0 2.0 m, m,T Tp p= = 7.07.0 s,s, currentcurrent velocity velocity = = 0.6 0.6 m/s). m/s).

Table 3. Key monitoring parameters in different mooring-line-breakage scenarios (TEN = tension of Table 3. Key monitoring parameters in different mooring-line-breakage scenarios (TEN = tension of top mooring line, DX = surge motion of location buoy in the middle location of the net assembly, top mooring line, DX = surge motion of location buoy in the middle location of the net assembly, AX AX = X-directional acceleration in the middle location of the net assembly, AY = Y-directional = X-directional acceleration in the middle location of the net assembly, AY = Y-directional acceleration acceleration in the middle location of the net assembly, TOP = top mooring line, and BOT = bottom in the middle location of the net assembly, TOP = top mooring line, and BOT = bottom mooring line). mooring line). Mooring-Line-Breakage Location Mooring-Line-Breakage Location Key Parameters Y Location Z Location Key Parameters Y Location0 m Z Location TOP TEN (Y = 0 m), AX 00 m m TOPBOT TEN TEN (Y (Y = 0= m),0 m), AX AX 00 m m TOP BOTand BOT TEN TEN (Y (Y = 0 m), AXDX, AX 0 m TOP and BOT TEN (Y = 0 m), DX, AX −−5050 m m TOP TOP AY AY −−5050 m m BOT BOT AY AY −−5050 m m TOP TOP andand BOTBOT AY, AY, TEN TEN (Y =(Y 0 = m) 0 m) −−100100 m m TOP TOP AY AY −100 m BOT AY −100 m BOT AY −100 m TOP and BOT AY, TEN (Y = −150 m) −−100150 m m TOP TOPand BOT AY, AY, TEN TEN (Y =(Y− =150 −150 m) m) −−150150 m m BOT TOP AY, AY, TEN TEN (Y =(Y− =150 −150 m) m) −−150150 m m TOP BOTand BOT AY, AY, TEN TEN (Y =(Y− =150 −150 m) m) −150 m TOP and BOT AY, TEN (Y = −150 m)

Table 4. Statistical data of mooring tension, surge motion of the location buoy, and acceleration of the

net assembly in the X and Y directions in different mooring-line-breakage scenarios at Hs = 1.0 m, Tp = 6.0 s, and current velocity = 0.6 m/s (AVE = average value and STD = standard deviation). Appl. Sci. 2019, 9, 1210 15 of 20

Table 4. Statistical data of mooring tension, surge motion of the location buoy, and acceleration of the net assembly in the X and Y directions in different mooring-line-breakage scenarios at Hs = 1.0 m, Tp = 6.0 s, and current velocity = 0.6 m/s (AVE = average value and STD = standard deviation).

Mooring-Line-Breakage TEN TEN DX AX AY Location (Y = 0 m) (Y = −150 m) (Y = 0 m) (Y = 0 m) (Y = 0 m) Y Z AVE STD AVE STD AVE STD STD STD Location Location (kN) (kN) (kN) (kN) (m) (m) (m/s2) (m/s2) No breakage 0.138 0.023 0.084 0.016 96.330 0.106 0.033 0.000 0 m TOP 0.000 0.002 0.084 0.016 96.579 0.105 0.135 0.000 0 m BOT 0.231 0.025 0.084 0.016 96.341 0.105 0.036 0.000 0 m TOP and BOT 0.000 0.002 0.090 0.016 113.370 1.690 0.232 0.000 −50 m TOP 0.138 0.022 0.084 0.016 96.327 0.105 0.067 0.027 −50 m BOT 0.138 0.022 0.084 0.016 96.327 0.105 0.043 0.010 −50 m TOP and BOT 0.187 0.026 0.094 0.016 96.309 0.105 0.050 0.018 −100 m TOP 0.138 0.022 0.085 0.015 96.326 0.105 0.035 0.020 −100 m BOT 0.138 0.022 0.085 0.015 96.327 0.105 0.034 0.008 −100 m TOP and BOT 0.140 0.023 0.163 0.024 96.325 0.106 0.034 0.024 −150 m TOP 0.138 0.023 0.000 0.001 96.326 0.105 0.032 0.029 −150 m BOT 0.138 0.022 0.138 0.019 96.327 0.105 0.032 0.006 −150 m TOP and BOT 0.133 0.022 0.000 0.001 96.314 0.105 0.035 0.015

Table 5. Statistical data of mooring tension, surge motion of the location buoy, and acceleration of the net assembly in the X and Y directions in different mooring-line-breakage scenarios at Hs = 2.0 m, Tp = 7.0 s, and current velocity = 0.6 m/s.

Mooring-Line-Breakage TEN TEN DX AX AY Location (Y = 0 m) (Y = −150 m) (Y = 0 m) (Y = 0 m) (Y = 0 m) Y Z AVE STD AVE STD AVE STD STD STD Location Location (kN) (kN) (kN) (kN) (m) (m) (m/s2) (m/s2) No breakage 0.152 0.066 0.096 0.046 96.713 0.217 0.070 0.000 0 m TOP 0.000 0.003 0.096 0.045 96.946 0.216 0.264 0.000 0 m BOT 0.247 0.086 0.096 0.045 96.714 0.211 0.102 0.000 0 m TOP and BOT 0.000 0.003 0.101 0.045 114.118 1.680 0.397 0.000 −50 m TOP 0.150 0.061 0.096 0.045 96.700 0.210 0.082 0.042 −50 m BOT 0.151 0.062 0.096 0.045 96.700 0.210 0.078 0.017 −50 m TOP and BOT 0.200 0.074 0.105 0.045 96.671 0.210 0.089 0.042 −100 m TOP 0.151 0.063 0.096 0.043 96.700 0.210 0.074 0.033 −100 m BOT 0.150 0.062 0.096 0.044 96.700 0.210 0.071 0.016 −100 m TOP and BOT 0.153 0.063 0.172 0.064 96.692 0.210 0.071 0.047 −150 m TOP 0.151 0.063 0.000 0.002 96.700 0.210 0.071 0.045 −150 m BOT 0.151 0.063 0.155 0.062 96.700 0.210 0.071 0.015 −150 m TOP and BOT 0.146 0.061 0.000 0.002 96.676 0.209 0.068 0.031

Table 6. Statistical data of mooring tension, surge motion of the location buoy, and acceleration of the net assembly in the X and Y directions in different mooring-line-breakage scenarios at Hs = 3.0 m, Tp = 8.0 s, and current velocity = 0.6 m/s.

Table 6. Cont.

Mooring-Line-Breakage TEN TEN DX AX AY Location (Y = 0 m) (Y = −150 m) (Y = 0 m) (Y = 0 m) (Y = 0 m) −100 m TOP 0.178 0.124 0.116 0.080 96.867 0.266 0.136 0.045 Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 20 −100 m BOT 0.178 0.124 0.116 0.081 96.867 0.266 0.140 0.035 −100 m TOP and BOT 0.180 0.124 0.197 0.129 96.858 0.266 0.144 0.074 −−150150 mm TOP TOP 0.178 0.125 0.125 0.000 0.000 0. 0.002002 96.867 96.867 0.2660.266 0.134 0.134 0.058 0.058 −−150150 mm BOT BOT 0.178 0.125 0.125 0.184 0.184 0. 0.126126 96.867 96.867 0.2660.266 0.139 0.139 0.025 0.025 −−150150 mm TOP TOP and and BOT BOT 0.173 0.121 0.121 0.000 0.000 0.002 0.002 96.833 96.833 0.2670.267 0.141 0.141 0.052 0.052

Finally,Finally, Figure Figure 18 18 shows shows the the time time history history of of the the surge surge motion motion of of the the location location buoy buoy in in the the middle middle locationlocation as as the the connecting connecting rope rope is is broken broken after after 600 600 s. s. AsAs isis clearlyclearly shownshown inin thethe figure,figure, thethe surge surge motionmotion increased increased continuously continuously duedue toto free free drifting drifting on on the the sea sea surface. surface.In In this this case, case, there there should should be be littlelittle changes changes to to other other monitoring monitoring parameters, parameters, simply simply resulting resulting in in the the loss loss of of the the location location buoy, buoy, which which cancan easily easily be be fixed. fixed. When When the the whole whole net net is is drifting drifting with with buoys, buoys, fishermen fishermen can can easily easily check check the the location location ofof the the gillnet gillnet in in real real time time through through GPS GPS and and collect collect it. it.

FigureFigure 18. 18.Time Time histories histories of of the the surge surge motion motion of of the the location location buoy buoy in thein the middle middle location location as theas the rope rope in thein the middle middle location location (Y = (Y 0 m)= 0 ism) broken is broken after after 600 600 s (H ss (=H 2.0s = 2.0 m, Tm,p =Tp 7.0 = 7.0 s, currents, current velocity velocity =0.6 = 0.6 m/s). m/s).

ThroughThrough thisthis kindkind ofof computer-simulationcomputer-simulation tool,tool, aa usefuluseful big-databig-data setset cancan bebegenerated generated andand machine-basedmachine-based problem-detectionproblem-detection oror decision-making decision-making processesprocesses cancan be be established established using using real-time real-time sensorsensor signals. signals.

7.7. Example Example of of Monitoring Monitoring Algorithm Algorithm InIn this this section, section, we we demonstrate demonstrate an an example example of of the the machine-based machine-based problem-detection problem-detection algorithm algorithm basedbased onon sensorsensor signals.signals. Figure 1919 showsshows the the developed developed monitoring monitoring algorithm, algorithm, which which was was based based on onthe the monitoring monitoring parameters parameters identified identified in inthe the previo previousus section. section. For Forsimplicity, simplicity, it was it was assumed assumed that thatthere there are arefour four inputs inputs and andsix outputs. six outputs. The four The fourinpu inputsts were were one tension one tension sensor sensor installed installed at the at top the of topthe ofcenter-upper-mooring the center-upper-mooring line (Y line = 0 m), (Y =X-Y 0 m), accelerometers X-Y accelerometers at the middle at the height middle of height the net of (Y the = 0 net m), (Yand = 0 one m), displacement and one displacement sensor on sensor the middle on the middlelocation location buoy (Y buoy = 0 m). (Y =Since 0 m). accurate Since accurate judgment judgment within withina time a step time is step difficult, is difficult, the thealgorithm algorithm detects detects evidence evidence of ofabrupt abrupt changes changes within within the windowwindow byby consideringconsidering continuous continuous small small windows windows of of the the sensor sensor signals. signals. Mooring Mooring lines lines other other than than the the center center one one at Yat = Y 0 m= 0 are m definedare defined as neighboring as neighboring mooring mooring lines. Tolines. check To the check variation the variation of sensor of signals, sensor thesignals, average the valuesaverage and values standard and deviationsstandard deviations were continuously were continuously obtained inside obtained a small inside window. a small To validatewindow. the To flowchart-based-monitoringvalidate the flowchart-based-monitoring algorithm, various algorith failurem, scenariosvarious werefailure additionally scenarios simulatedwere additionally using a wave condition of Hs = 2.5 m and Tp = 7.5 s. The current was also fixed as 0.6 m/s. simulated using a wave condition of Hs = 2.5 m and Tp = 7.5 s. The current was also fixed as 0.6 m/s.

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Figure 19. Failure detection algorithm.

Figures 20 and 21 show test results for Output 1 = both mooring lines at Y = 0 m are broken and Output 3 = both of the adjacent mooringFigureFigure 19.19. lines Failure at Y detection = –50 m algorithm.are broken. The mooring lines were broken after 600 s. Failures were detected well in both cases and the problems were correctly identified. In FiguresFigures 20 20 and and 21 21 show show testtest resultsresults forfor OutputOutput 1 = both mooring lines lines at at Y Y = = 0 0 m m are are broken broken and and the case of Output 1, there was a small time lag for detection of about 20 s. The slowly varying surge OutputOutput 3 3 = = bothboth ofof thethe adjacentadjacent mooringmooring lineslines atat YY == −–5050 m m are are broken. broken. The The mooring mooring lines lines were were broken broken motion of the location buoy leads to this time delay even if the top mooring tension suddenly became afterafter 600 600 s. s. Failures Failures were were detected detected well well in in both both cases cases and and the the problems problems were were correctly correctly identiﬁed. identified. In theIn zero. However, the time lag for detecting failure was small, which is acceptable for fishing gear failure casethe case of Output of Output 1, there 1, there was was a small a small time time lag lag for fo detectionr detection of of about about 20 20 s.s. The slowly varying varying surge surge monitoring. In the case of Output 3, even though the average value and standard deviation of the motionmotion of of the the locationlocation buoybuoy leadsleads toto thisthis time delay even if if the the top top mooring mooring tension tension suddenly suddenly became became mooring tension varied slowly, X and Y accelerations abruptly changed, which resulted in the fast zero.zero. However, However, the the time time laglag forfor detectingdetecting failure was small, which which is is acceptable acceptable for for fishing ﬁshing gear gear failure failure detection of failure even without the time lag. Although not presented here, we tested a number of monitoring.monitoring. InIn thethe casecase ofof OutputOutput 3, even though the average value value and and standard standard deviation deviation of of the the other failure scenarios, and the developed algorithm was always able to correctly detect and identify mooringmooring tensiontension variedvaried slowly,slowly, XX andand YY accelerationsaccelerations abruptly changed, which which resulted resulted in in the the fast fast the given problem. The overall results demonstrate that the proposed monitoring algorithm can be detectiondetection ofof failurefailure eveneven withoutwithout thethe timetime lag.lag. AlthoughAlthough not not presented presented here, here, we we tested tested a a number number of of used for the machine-based problem-detection system of the bottom-set gillnet. A more extensive otherother failure failure scenarios, scenarios, andand thethe developeddeveloped algorithm was always able able to to correctly correctly detect detect and and identify identify monitoring system can also be developed with more input and/or output variables. thethe given given problem. problem. TheThe overalloverall resultsresults demonstratedemonstrate that the proposed monitoring monitoring algorithm algorithm can can be be usedused forfor thethe machine-basedmachine-based problem-detectionproblem-detection systemsystem of the bottom-set gillnet. gillnet. A A more more extensive extensive monitoringmonitoring system system cancan alsoalso bebe developeddeveloped withwith moremore input and/or output output variables. variables.

FigureFigure 20.20. TimeTime historieshistories ofof failurefailure detectiondetection statusstatus whenwhen bothboth mooringmooring lineslines inin thethe middlemiddle locationlocation (Y(Y == 00 m) are broken after 600 s s ( (Hss == 2.5 2.5 m, m, TTp p= =7.5 7.5 s, s,current current velocity velocity = 0.6 = 0.6 m/s). m/s). Status Status 1 indicates 1 indicates the theOutputFigure Output 20.1 case. 1Time case. histories of failure detection status when both mooring lines in the middle location

(Y = 0 m) are broken after 600 s (Hs = 2.5 m, Tp = 7.5 s, current velocity = 0.6 m/s). Status 1 indicates the Output 1 case. Appl. Sci. 2019, 9, x FOR PEER REVIEW 18 of 20

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FigureFigure 21.21. TimeTime historieshistories ofof failurefailure detectiondetection statusstatuswhen when both both mooring mooring lines lines at at Y Y = =− −50 m are brokenbroken afterafter 600600 ss ((HHss = 2.5 m, Tp == 7.5 7.5 s, s, current current velocity velocity = = 0.6 0.6 m/s). m/s). Status Status 3 3indicates indicates the the Output Output 3 3case. case. 8. Conclusions 8. Conclusions In this work, time-domain numerical simulations were performed to analyze the global dynamic In this work, time-domain numerical simulations were performed to analyze the global dynamic performance of a bottom-set gillnet under various current and wave conditions. Simulation tools were performance of a bottom-set gillnet under various current and wave conditions. Simulation tools developed to design a sensor-based monitoring system and develop the corresponding machine-based were developed to design a sensor-based monitoring system and develop the corresponding problem-detection or decision-making processes. Computer simulations were carried out for a typical machine-based problem-detection or decision-making processes. Computer simulations were carried range of measure environmental data on the southwest coast of Korea. The target net-system was a out for a typical range of measure environmental data on the southwest coast of Korea. The target bottom-set gillnet constructed by employing an actual design used in Korea. A reliable equivalent net net-system was a bottom-set gillnet constructed by employing an actual design used in Korea. A model with Morison forces on instantaneous net/mooring positions was developed. The convergence reliable equivalent net model with Morison forces on instantaneous net/mooring positions was of the equivalent net model was confirmed by increasing the number of elements used for a finer developed. The convergence of the equivalent net model was confirmed by increasing the number of model. From a series of computer simulations, the following conclusions can be drawn: elements used for a finer model. From a series of computer simulations, the following conclusions •can beThe drawn: stretching performance of the gillnet is good under different current loads. • The effects of wave excitations are small in operating conditions at the given submergence depth. • The stretching performance of the gillnet is good under different current loads. Dynamic responses of the net are, however, substantial in the fishery-prohibition condition, which • The effects of wave excitations are small in operating conditions at the given submergence depth. significantly increases the mooring tension through nonlinear snap loading. Dynamic responses of the net are, however, substantial in the fishery-prohibition condition, • Inwhich different significantly line-failure increases scenarios, the mooring the proposed tension monitoring through nonlinear system, insnap combination loading. with three • differentIn different kinds line-failure of sensors, scenarios, can effectively the proposed detect specific monitoring problems system, from in the combination combined patternswith three of sensordifferent signals. kinds of sensors, can effectively detect specific problems from the combined patterns • Theof sensor proposed signals. algorithm, based on the analysis of sensor signals and the corresponding statistical • data,The proposed can be used algorithm, for failure based detection. on the analysis of sensor signals and the corresponding statistical • Andata, example can be used of a machine-basedfor failure detection. problem-detection algorithm using sensor signals has been • developedAn example and of successfully a machine-based demonstrated problem-detection using a number algorithm of different using failuresensor scenarios. signals has been • Indeveloped order to and develop successfully a more demonstrated sophisticated algorithmusing a number for many of different different failure failure scenarios. cases, big-data • analysisIn order combinedto develop with a more machine sophisticated learningmight algori bethm a favorablefor many solution.different failure cases, big-data analysis combined with machine learning might be a favorable solution. Author Contributions: All authors contributed equally contributed to the publication of this article with regard to the design of the target model, validation of the numerical model, parametric study, analysis, and writing. Author Contributions: All authors contributed equally contributed to the publication of this article with regard Funding:to the designThis of research the target is amodel, part of validation a project titled of the ‘Development numerical model, of Automatic parametric Identification study, analysis, Monitoring and writing System. for Fishing Gear’, which is funded by the Ministry of Oceans and Fisheries, Korea. ConflictsFunding: ofThis Interest: researchThe is authorsa part of declare a project no titled conflict ‘Development of interest. of Automatic Identification Monitoring System for Fishing Gear’, which is funded by the Ministry of Oceans and Fisheries, Korea.

ReferencesConflicts of Interest: The authors declare no conflict of interest.

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