Research on the Separation Technology of Kelp and Shellfish Box Based on Shellfish–Kelp Mixed Culture Mode

Abstract

Aiming at the problem of floating shellfish boxes interfering with kelp harvesting when mechanized kelp harvesting is based on shellfish–kelp mixed culture mode, this paper combines the structural characteristics of the shellfish box itself, designs the kelp harvesting unit test bench and develops a shellfish box separator device. The key factors affecting the box separator’s separation effect were derived through the theoretical analysis. The process of separation of a shellfish box by the box separator is simulated and optimized under the derived boundary conditions. The single-factor test for the separating effect of the box separator was conducted with ADAMS kinematics simulation software. The test showed the optimal utility intervals for the key factors under consideration. Further orthogonal tests were conducted for the three key factors, which were ranked in descending order of importance as box separator separation angle θ, box separator taper angle β and box separator placement depth h. The optimal parameter combination is the box separator separation angle of 31°, the box separator taper angle of 30° and the box separator placement depth of 550 mm. Verification experiments have shown that both indicators, the farthest horizontal distance of the shellfish box and the angle of the shellfish box deviating from the box separator, meet the actual production requirements. In summary, the separator can effectively separate the shellfish box from the kelp, and the device is simple in design, quick in operation, and accomplished separation without disturbing shellfish. This study can provide a theoretical basis for the separation technology of kelp and shellfish box under shellfish–kelp mixed culture mode.

1. Introduction

Kelp is one of the important economic algae products in China. It is not only a nutrient-rich marine food but also an important raw material in the fields of medicine and health care, industry and agricultural fertilizer [1,2,3,4,5]. China is the world’s largest kelp breeder, producing 90% of the world’s kelp annually. Kelp harvest is the last and most important part of kelp production. At present, it mainly relies on manual harvesting, which has high labor intensity, low efficiency and significant operational safety risks [6,7,8,9,10]. In recent years, with the transfer of rural labor to the tertiary industry, breaking through the mechanized harvesting of kelp and solving the problem of labor difficulties in kelp harvesting has become the key to the sustainable and stable development of the industry [11].

There are various kelp culture modes, including flat culture, vertical culture, one-stop culture, shallow raft culture, etc. [12,13,14]. Domestically, the flat raft culture mode is mostly adopted. The model consists of two parallel ropes about 75 m long with a spacing of about 5 m to form a floating raft; the two ends are fixed with seabed anchors and floating balls linked to the ropes to make them float on the water surface to form a whole raft. Two 2.5 m long kelp ropes are connected in the middle of the raft, both ends of the kelp rope are connected to the raft with a small hanging rope, and the kelp ropes are arranged in a trapezoidal shape with an interval of 0.75 m. Kelp seedlings are planted 0.15 m apart on the kelp ropes, and small floating balls are added in the middle of the kelp ropes in the middle and late stages of the growth of the kelp [15]. The advantage of this culture model is that the kelp is exposed to uniform light, has a high space utilization rate, large yield, and can meet the demand for kelp in the domestic market [16]. Due to the abundance of sea resources in the kelp farming area, there is an abundance of seafood such as abalone, scallops, and other economic aquatic products. These economic aquatic products mainly feed on microorganisms and algae, and locally farmed kelp is one of the favorite foods of shellfish, thus giving rise to a new aquaculture system—the shellfish–kelp mixed culture model. This model includes the addition of a shellfish box on the base of flat raft culture, and a shellfish box is arranged on the rope at an interval of 1.5 m. It is then connected underwater with a small hanging rope, as shown in Figure 1.

At present, there are relatively few foreign kelp culture producers; the main producers are Japan and South Korea, and they mostly use the vertical culture mode. To harvest kelp, a harvesting ship uses a rope tied to the deck; although the method is simple and efficient, the mode’s low utilization of space creates the problem of low yield [17]. Due to the influence of eating habits and other reasons, the yield of wild kelp in European and American countries can meet the market demand. Therefore, the kelp harvesting machinery in European and American countries is mostly fished after underwater cutting [18,19].

In recent years, some domestic institutions, companies, and universities have conducted research on kelp harvesting machinery in order to solve the problem of mechanized kelp harvesting [20]. Jiang Tao’s team developed an automatic towing and hanging kelp harvesting vessel and developed a matching type of flat raft culture mode [21], but it is not applicable to the general flat raft culture. Liu and Han designed a biomimetic kelp harvesting device based on the main points of manual kelp harvesting [22,23], but it could not be adapted to the shellfish–kelp mixed culture mode, and the phenomenon of entanglement of shellfish box still occurs. There is also the chain-driven kelp harvesting boat, which utilizes the drag chains on both sides of the hull to drag the ropes to drive the raft frame to movement [24,25], and the floating balls on the ropes are often pinched due to the excessive weight of the ropes, and the balls often break, resulting in wastage.

Mechanized kelp harvesting is generally divided into three steps: picking out the kelp, separating the kelp rope from the floating raft and harvesting the kelp. Based on the shellfish–kelp mixed culture mode [26], the process of mechanized kelp harvesting is derived from the shellfish–kelp separation step, i.e., effectively remove the kelp ropes from the water and set aside the shellfish box smoothly to reduce the interference on the life of the scallops and abalones in the harvesting process of the kelp. The step creates the separation of the shellfish–kelp technology—a bottleneck technology that restricts the mechanization of kelp harvesting.

In response to the above problems, this paper designed a shellfish and algae separation device based on the kelp harvesting unit test bench, which realized the reliable separation of shellfish and kelp in the process of kelp mechanized harvesting. This device reduces the interference with the well-being of scallops and abalone, which can be found in the shellfish box, and reduces the shellfish box’s interference with the kelp’s mechanized harvesting. Through theoretical analysis and mathematical calculation, the structure and parameters of the key components of the shellfish box separator are determined. The motion law of the shellfish box on the box separator process is simulated with the help of ADAMS simulation software, the optimal parameter combinations are determined by the method of three-factor orthogonal test, and the operational validity of the optimal parameter combinations is verified.

2. Materials and Methods

2.1. Overall Structure and Working Principle

Based on the demand for mechanized harvesting of kelp and the distribution pattern of shellfish boxes and kelp ropes formed by shellfish–kelp mixed culture mode, a kelp harvesting unit test bench is developed, as shown in Figure 2, which mainly consists of box separator, rope locator, kelp rope picking star wheel, kelp conveyor belt and kelp bracket plate and other components.

When harvesting, the test bench is driven by a generator to simulate the process of the mechanized harvesting of kelp. Firstly, the workers hang the raft rope on both sides on the V-shaped track of the rope limiter so that the whole test bench moves slowly under the guidance of the rope and the kelp rope and the shellfish box are close to the test bench in turn. When the kelp ropes located between the raft move to the box separator position, the kelp located on both sides is gathered inward by the front of the separator, initially widening the horizontal distance between the kelp and the shellfish box. Furthermore, when the kelp encounters the kelp blocking plate, the lower part of the kelp is lifted up by the bracket plate, while the upper part of the kelp follows the kelp rope to move. This assists with the adjustment of the kelp’s placement on the plate and ensures that all of the kelp is lying on the plate. As the test bench continues to move forward, the kelp rope meets the star wheels placed on both sides of the test bench. These star wheels move counterclockwise; the kelp rope is picked up and taken away from the horizontal plane while dragging the kelp in a backward movement. In this process, to avoid damage to the kelp, the star wheel adopts one-way control. Once the kelp rope breaks away from the star wheel, the star wheel will be in a static state. When the shellfish box meets the separator placed on both sides of the test bench, the separator pushes the shellfish box to the outside of the harvesting device. The shellfish box slides backward along the slope of the box separator in a stable and slow way, which minimizes the impact on the life of scallops and abalone in the shellfish box. When the kelp rope moves backward, the hollow design of the conveyor belt and kelp bracket plate minimizes the seawater adsorbed on the kelp, reduces the additional weight when harvesting the kelp, and reduces the mechanical damage to the kelp in the process of dragging and pulling.

In order to minimize the impact of the shellfish box on the kelp harvest and maximize the horizontal distance between the shellfish box and the kelp, the outer side of the box separator is designed as an inward-inclined slope structure. In order to prevent the sudden loosening and tightening of the shellfish box’s hanging rope and the raft rope on the pulley under the influence of the ocean current, an ellipsoidal rope splitter is designed on the outside of the rope limiter. On the one hand, it reduces the friction between the shellfish box and the inclined surface of the separator so the shellfish box glides more smoothly on the separator; on the other hand, it reduces the impact on the well-being of abalones and scallops. When the kelp completely falls on the conveyor belt, the knot between the kelp rope sling and the raft is manually untied to realize the complete separation of the kelp and the shellfish box.

2.2. Determination of Main Parameters of Shellfish Box Separator

2.2.1. Determination of Separator Parameters

(a)

Design of the box separator’s structure

The separator is the key to realizing the separation of the kelp and shellfish box in order to realize the automatic separation of the kelp and shellfish box during the forward process of the kelp harvesting vessel. This is combined with the structure of the kelp and shellfish box, which are distributed in a staggered phase, and the box separator adopts the structure, as shown in Figure 3. In order to reduce the problem, the friction between the shellfish box’s hanging rope and the separator during the separation process may cause the shellfish box to turn over and affect the life of scallops and abalone. The separator has an inward, inclined beveled structure, which realizes the non-contacting backward movement between the shellfish box’s small hanging rope and the separator’s inclined surface and effectively improves the stability of the shellfish box’s attitude during the separation process.

(b)

Determination of separator location

The position of the separator includes a vertical position and a horizontal position. The vertical position (Figure 4a) should ensure both the stability attitude and the stable separation of the shellfish box during the kelp harvesting process as much as possible. Therefore, the separator is generally selected to act on the middle and lower parts of the shellfish box. In the case of ensuring that the horizontal position of the box is far away from the kelp, there should be no problem with the attitude change of the shellfish box being too large to affect the life of the abalone. To this end, the length of the hanging rope from the shellfish box is measured first. (Divided into three groups of 100 roots each, and three groups selected from different farming areas, see

Table 1. Statistics of the length of the shellfish box hanging rope.

Item	Number
Specification (mm)	<750	750–800	800–850	>850
Group 1	11	49	36	4
Group 2	9	42	41	8
Group 3	13	39	46	2

).

From the above table, it can be seen that 84% of the shellfish box’s hanging rope lengths are in the 750–850 mm range. Since the length of the shellfish box’s hanging ropes was artificially controlled by the workers, here we ignore the effect of extreme values and use the average length of the shellfish box hanging ropes, l₁, for subsequent calculations. The average length l₁ of the shellfish box suspension ropes was determined to be 793.5 mm.

According to the field measurements, it is known that the average weight of a one-year abalone shellfish box during the kelp harvesting period is 17.63 Kg, and the average weight of a two-year abalone shellfish box is 21.26 Kg. According to the field measurements, the one-year abalone uses the six-layer shellfish box with a height of 750 mm, and the two-year abalone uses a five-layer shellfish box with a height of 710 mm. Therefore, the length h₂ of the shellfish box is determined to be 710–750 mm. In order to ensure the stability of the shellfish box in the separation process, the position of the box separator acting on the shellfish box needs to be in the center of gravity of the shellfish box and the following position. According to the above measurements, it can be seen that the weight of the shellfish box during the harvest period of the kelp is relatively stable. In addition, abalone growth will not have any impact on the center of gravity of the box position; that is, the center of gravity is in the middle of the shellfish box. Therefore, the vertical position of the box separator is as follows:

$$l_1+1/2h_2\le h_1\le l_1+h_2$$

(1)

Regarding the horizontal position, because the separator is located in the middle of two neighboring rows of kelp, in order to avoid damage to the kelp, the separator needs to be designed with a reasonable working width range B (Figure 4b). Since the separator is in operation, the neighboring rows of kelp are in the unharvested state, and the position of the separator needs to avoid contact with the neighboring rows of kelp. Therefore, the separator separation angle θ is selected to limit the length and width of the box separator. According to the shellfish–kelp mixed culture mode, the shellfish box is placed in the middle of the left and right kelp ropes. Under the wave floating or wind blowing, the left kelp rope, the middle shellfish box and the right kelp rope will be completely closed, so the minimum distance between the left and right kelp ropes is the width of the middle shellfish box. Because the cross-section of the shellfish box is rectangular, the minimum spacing between the two kelp ropes is the length a or the width b of the cross-section of the shellfish box (Figure 5b). The minimum width Bmin is min(a,b), according to the Figure 5a. We can see that the kelp rope is attached to the raft rope with a small hanging rope, while the raft rope is attached to the kelp rope on the other side. The outermost kelp is some distance away from the small hanging rope (Figure 5a). Then, the maximum distance between the first kelp on the left and right sides is the maximum working width of the separator when the ropes on both sides are fully tensioned. The length d₁ of the small hanging rope is 200 mm, and the length d₂ is 150 mm. This determines B for the maximum lateral distance as:

$$B_{\max }=2\left(d_1+d_2\right)$$

(2)

The shellfish box is a multi-layer rectangular box with a cross-section of 430 mm × 340 mm and is placed on the raft rope at intervals of 1500 mm. In order to avoid the mutual influence of the twice-adjacent shellfish box separation process, the previous shellfish box is required to complete the separation-reset action within the range of encountering the next shellfish box. Therefore, the length D of the box separator is 1070~1500 mm, so there is a separator separation angle:

$$\tan \theta ={\displaystyle \frac{B}{D/2}}=0.57\sim 1.31$$

(3)

Solve for: $\theta =$29.5°~52.5°.

(c)

Determination of the taper of the box separator

In order to ensure the shellfish box can slide backward stably, try to make the shellfish box and the guiding surface of the box separator have surface contact to reduce the probability of the shellfish box flipping during the backward sliding process.

This angle β is solved graphically, as shown in Figure 6.

Take the point where the raft rope connects with the shellfish box’s hanging rope as the center and take the free sagging position of the shellfish box as the initial position $O_0{O}_1$. Then, make a parallel line $E_0{E}_1$, with the distance B from the initial position of the shellfish box. In the process of shellfish box separation, the shellfish box can only move around the linking point $O_0$, i.e., the point $O_0$ as the center of the shellfish box A as a rotating body so it is rotated to the position $A^{\prime }$ of contact with the parallel line $E_0{E}_1$, and connected $O_0$ with $O_1^{\prime }$. Then, $\angle O_1^{\prime }{O}_0{O}_1$ is the taper angle β of the box separator, and:

$$\begin{array}{l}\tan \beta ={\displaystyle \frac{1/2\min \left(a,b\right)\cos \beta +B}{O_0{O}_1}}\\ \tan \beta ={\displaystyle \frac{1/2\min \left(a,b\right)\cos \beta +B}{l_1}}\end{array}$$

(4)

Solve for: $\beta =$ 23.2°~34.09°. (Here, for the denominator, we use the approximate value l₁).

2.2.2. Motion Analysis of the Separator Separation Process

For analytical simplicity (ignoring the seawater force on the shellfish box and treating the box as a homogeneous structure), the process of the separator separation is divided into two segments. The first is the equilibrium state of the shellfish box on the separator’s guiding surface under the limit of the shellfish box hanging rope, and the second is the backward sliding movement of the shellfish box along the separator’s guiding surface under the limit of the hanging rope. First, the force on the shellfish box in equilibrium is analyzed in Figure 7.

The shellfish box will be subjected to the force of gravity mg, the pull of the shellfish box hanging rope T and the supporting force $N_1{N}_2$ at the points I and F, which are on the guiding surfaces. Because $N_1$ and $N_2$ are the limit points, both the dispersion force and the single point force are included in the two force ranges; secondly, as long as the force of these two limit points is not 0, it can be ensured that the shellfish box and the separator are in contact with each other, that is, stable sliding without overturning, and the probability of winding with the kelp is reduced. Then, by $\sum M_E$ = 0, there is:

$$mg\cdot GF+N_1\cdot IF=T{\displaystyle \frac{1}{2}}\max \left(a,b\right)$$

(5)

And:

$$\begin{array}{l}GF=HF\sin \left(\angle GHF\right)=\sqrt{E{F}^2+H{E}^2}\sin \left(\angle GHF\right)\\ =\sqrt{{\left(h_1-l_1-{\displaystyle \frac{1}{2}}{h}_2\right)}^2+{\left[{\displaystyle \frac{1}{2}}\max \left(a,b\right)\right]}^2}\sin \left(\angle GHF\right)\end{array}$$

(6)

$$\begin{array}{l}\angle GHF=90°-\angle HFG\\ =90°-\beta -\arctan \left({\displaystyle \frac{EF}{HE}}\right)\\ =90°-\beta -\arctan \left({\displaystyle \frac{h_1-l_1-\frac{1}{2}{h}_2}{\frac{1}{2}\max \left(a,b\right)}}\right)\\ =90°-\beta -\arctan \left({\displaystyle \frac{2h_1-2l_1-h_2}{\max \left(a,b\right)}}\right)\end{array}$$

(7)

$$BF=h_1-l_1$$

(8)

Substituting into Equation (5), we have:

$$mg\sqrt{{\left(2h_1-2l_1-h_2\right)}^2+{\left[\max \left(a,b\right)\right]}^2}\cos \left[\beta +\arctan \left({\displaystyle \frac{2h_1-2l_1-h_2}{\max \left(a,b\right)}}\right)\right]+2N_1\left(h_1-l_1\right)=T\max \left(a,b\right)$$

(9)

Considering the sign of the shellfish box destabilization as $N_1$ = 0, we have:

$$mg\sqrt{{\left(2h_1-2l_1-h_2\right)}^2+{\left[\max \left(a,b\right)\right]}^2}\cos \left[\beta +\arctan \left({\displaystyle \frac{2h_1-2l_1-h_2}{\max \left(a,b\right)}}\right)\right]=T\max \left(a,b\right)$$

(10)

Also, by $\sum y=0$, there is:

$$T=mg\sin \left(90°-\beta \right)=mg\cos \beta$$

(11)

Substituting Equation (11) into Equation (10), we have:

$$\sqrt{{\left(2h_1-2l_1-h_2\right)}^2+{\left[\max \left(a,b\right)\right]}^2}\cos \left[\beta +\arctan \left({\displaystyle \frac{2h_1-2l_1-h_2}{\max \left(a,b\right)}}\right)\right]=\cos \beta \max \left(a,b\right)$$

(12)

It is clear that the upper edge of the separator does not affect the stable slip of the shellfish box, but the lower part (i.e., the separator placement depth) significantly affects the stable slip of the shellfish box. Furthermore, this state of equilibrium is not only related to the length of the shellfish box’s hanging rope but also to the cross-section and length of the shellfish box itself.

The positive pressure of the box on the guiding surface of the separator is:

$$N_1+N_2=mg\sin \beta$$

(13)

Therefore, without considering the effect of seawater on the action of the shellfish box, the friction of the shellfish box sliding along the guiding surface of the separator is:

$$F=\left(N_1+N_2\right)\tan \phi =mg\tan \phi \sin \beta$$

(14)

It can be seen from the above equation that the shellfish box along the box separator guiding surface backward movement distance increases. The shellfish box deviation from the vertical surface in terms of distance becomes farther and farther; that is, the β angle gradually increases, the box on the guiding surface of the positive pressure and friction gradually increases, and when more than a certain value is reached, it may lead to the shellfish box’s flipping phenomenon.

The force analysis of the shellfish box in the guiding surface movement is shown in Figure 8.

That is, when the equilibrium position is not moving, the shellfish box is located at A, and the hanging rope is located in position $O_0{O}_1$. When the shellfish box is to be dragged along the guiding surface, the actual hanging rope position is in the position $O^{\prime }{O^{\prime }}_1$. The upper part of the hanging rope is shifted forward due to the friction force to be overcome, forcing the box to move from its original position A to position $A^{\prime }$. Considering only the forces parallel to the guiding surface, i.e., only the friction force F on the guiding surface of the box and the pulling force T of the hanging rope, we have:

$$T_x=T\sin \delta =F=mg\tan \phi \sin \beta$$

(15)

$\phi$—The angle between the diagonal of the shellfish box and the inclined surface of the box separator is shown in Figure 7.

The simplification is as follows:

$$T={\displaystyle \frac{\sin \beta }{\sin \delta }}mg\tan \phi$$

(16)

$\delta$—The angle of displacement of the shell box hanging rope is shown in Figure 8.

Obviously, during the backward movement of the shellfish box along the guiding surface, the tension of the shellfish box hanging rope is constantly changing due to the changing angle β and δ. Further, the law of motion of the shellfish box for velocity analysis is shown in Figure 9.

The operating speed of the kelp harvesting vessel is shown as V_h (equal to the partial speed of the shellfish box in the forward direction of the harvesting vessel). Due to the backward movement of the rope relative to the harvesting vessel, the shellfish box relative to the shellfish box’s hanging rope and the raft knotting position of the circumferential motion speed is shown as $V_{\tau }$. The actual speed of movement of the shellfish box is $V_m$. The angle between the forward direction of the shellfish box along the harvesting vessel and the direction of the actual speed of the shellfish box’s movement is equal to the separation angle of the box separator $\theta$. The angle between the V_h and $V_m$ is $\delta$. Thus, in the velocity triangle, using the law of sines, we have:

$$\begin{gathered} {\displaystyle \frac{v_h}{\sin \delta }}={\displaystyle \frac{v_{\tau }}{\sin \theta }}={\displaystyle \frac{v_m}{\sin \left(180°-\delta -\theta \right)}} \\ \mathrm{i}.\mathrm{e}.,v_m={\displaystyle \frac{v_h}{\sin \delta }}\sin \left(\delta +\theta \right)={\displaystyle \frac{v_{\tau }}{\sin \beta }}\sin \left(\delta +\theta \right) \end{gathered}$$

(17)

Obviously, the motion speed of the shellfish box is not only affected by the harvesting vessel’s motion speed but also by $\delta$ and $\theta$. Since the angle $\delta$ during the separation process is constantly changing, it leads to the actual motion speed of the shellfish box $V_m$, which agrees with the conclusions of the previous analyses. This determines that the motion of the shellfish box is a process of non-uniform motion. Coupled with the effects of the sea currents, this motion may greatly change the force of the shellfish box’s hanging rope, i.e., its position changes drastically during the kelp–shellfish box separation process.

2.2.3. Separate the Shellfish Box Hanging Rope Outwards

As mentioned previously, the shellfish box’s hanging rope, under the influence of ocean currents, may be completely slack or even entangled in the band pulley, so this kelp–shellfish box separation device is equipped with a rope limiter, as shown in Figure 10.

The rope limiter not only limits the position of the raft rope but also raises the height of the shellfish box. Due to the high installation position of the rope limiter, the raft rope can be lifted up, and the shellfish box can be connected to the raft rope by a small hanging rope, so the shellfish box’s position is increased.

(a)

Design of band pulley structure

As the power is provided by the kelp harvesting ship, the pulley here only plays a guiding role, not as a power wheel, and the band pulley only plays a role in restricting the raft ropes, so standard parts are used. As there is a small hanging rope tied to the raft rope to connect the kelp rope and the shellfish box, the measurement and statistics of the hanging rope knots on the raft rope were recorded (50 hanging rope knots are randomly selected for measurement and statistics). The average diameter of the hanging rope knots is 26.4 mm, which determines that this pulley is suitable for the ordinary V-belt in the E-type structure, with a wedge angle of 40°.

(b)

Determination of the diameter of the ellipsoidal hanging rope-splitter

In order to ensure that the rope does not break away from the groove of the pulley under the action of the shellfish box hanging rope, this study is equipped with an ellipsoidal hanging rope-splitter device on the outer side of the pulley, whose diameter $D_d$ is:

$$D_d=K{D}_J$$

(18)

K = 1.5~2—Hanging rope guide control factor.

D_j—Diameter of band pulley

It should be considered that in the process of separating the shellfish box, the knotting position of the hanging rope and the raft ropes are always ahead of the shellfish box to improve the reliability of the hanging rope winding through the pulley. The convex fan-shaped pattern is uniformly distributed on the circumference of the splitter to increase the friction between the splitter and the hanging rope.

The structure of the splitter adopts the ellipsoidal structure, in which the major axis is equal to the diameter $D_d$ of the splitter device. For the minor axis $D_s$, consider the hanging rope over the surface of the splitter, so in order to reduce the resistance, use the highest point $O_0$ of the device to make a line $O_0{E}^{\prime }$. The angle between $O_0{E}^{\prime }$ and the line $O_0{O}_1$ is less than or equal to the friction angle $\mathsf{\alpha }$ of the hanging rope and the separator with the vertical surface. The intersection with the band pulley rotary axis is at $E^{\prime }$. Then, $O{E}^{\prime }$ is half of the length of the splitter’s minor axis, as shown in Figure 11, where we obtain:

$$O{E}^{\prime }=1/2D_d\tan \alpha$$

(19)

3. Experiment Design

3.1. Selection of Experiment Indicators

3.1.1. The Farthest Horizontal Distance of the Shellfish Box Z

The separation of shellfish and kelp is to prevent interference with the floating shellfish box during the mechanized harvesting of kelp while not affecting the neighboring rows of uncollected kelp. This also requires that the shellfish box be set aside a certain distance, but it cannot be set too far away from the initial position. The farthest horizontal distance of the shellfish box Z is greater than the distance $Z_{min}$ from the outermost end of the section $O_0{O}_1$ on the separator, and less than the distance $Z_{max}$ = 350 mm from the first kelp in the neighboring row, i.e.,

$$Z_{\min }\le Z\le Z_{\max }$$

(20)

3.1.2. The Angle of the Shellfish Box Deviating from the Box Separator γ

The separation of the shellfish box and kelp cannot simply be understood as the separation of the shellfish box. The shellfish box is a tool for aquaculture of sea urchins, abalone, and other aquatic products, so the shellfish box in the process of being separated to maintain the smoothness is also very important. In order to meet the shellfish box under the conditions of the farthest horizontal distance of the shellfish box Z, the angle of the shellfish box deviating from the box separator γ be used to evaluate the smoothness sliding of the shellfish box in the inclined surface of the separator. The γ angle is the angle between when the movement of the shellfish box on the surface of the separator and the inclined plane of the separator. When the angle γ is smaller, the contact surface between the shellfish box and the separator is larger, the support force of the separator is larger, and the movement process of the shellfish box is more stable. The smaller the angle of γ, the more likely the shellfish box will fit the inclined surface of the separator during the separation process, the smoother the sliding process, and the better the separation effect. The evaluation indicators are determined as shown in Figure 12.

3.2. Single-Factor Experiment

According to the above theoretical analysis, it is known that there are three main factors affecting the separator’s separating effect, which are the separator separation angle θ, the separator taper angle β and the separator placement depth h, using ADAMS software to simulate and analyze separation process.

Taking the farthest horizontal distance of the shellfish box Z and the angle of the shellfish box deviating from the box separator γ as the evaluation indexes, we set up the box separator as a steel hard metal material and the shellfish box setting as an acrylic plastic material set up and simulated through the contact parameters in the ADAMS database. The forward speed of the separator was set to 20 m/min, the elastic modulus of the raft rope at 2.8 Gpa, the elastic modulus of the hanging rope at 2.5 Gpa, and the simulation time at 30 s. Then, according to the theoretical analysis in the early stage, the factor levels of the single-factor experiment were chosen, as shown in

Table 2. Factor levels of single factor experiment.

Numbers	Factors	Values	Conditions
1–5	Box Separator separation angle (°)	30, 35, 40, 45, 50	Box Separator taper angle = 28° Box Separator placement depth = 600
6–10	Box Separator taper angle (°)	24, 26, 28, 30, 32	Box Separator angle = 40° Box Separator placement depth = 600
11–15	Box Separator placement depth (mm)	400, 500, 600, 700, 800	Box Separator taper angle = 28° Box Separator angle = 40°

.

The simulation process is shown in Figure 13.

3.3. Orthogonal Experiment Design

The influence of each factor on the two evaluation indexes can be more intuitively seen through the single-factor test, but the interaction effect of the three factors on the rating indexes of Z and γ cannot be seen. Combined with the above simulation results, the separator separation angle θ is taken at three levels: 30°, 35° and 40°; the separator taper angle β is taken at three levels: 26°, 28° and 30°; and the separator placement depth h is taken at three levels: 500, 600 and 700 mm. The orthogonal experiment was conducted using ADAMS simulation software; the coding of the test factors is shown in

Table 3. Coding of factors and levels.

Level	Box Separator Separation Angle θ (°)	Box Separator Taper Angle β (°)	Box Separator Placement Depth h (mm)
1	30	26	700
2	35	28	600
3	40	30	500

.

4. Results

4.1. Analysis of Single-Factor Experiment Results

4.1.1. Box Separator Separation Angle

According to Figure 14a, the greater the separation angle θ, the greater the distance Z. When the separation angle θ is less than 40°, Z increases with time, with a maximum at 22 s and then decreases, indicating that the shellfish box successfully crosses the outermost end of the separator. However, when the separation angle θ is greater than 40°, the outermost end of the separator is too far away from the raft. The shellfish box is unable to cross the outermost end of the separator under the limit of the hanging rope’s limited length, resulting in the shellfish box being stuck on the front side of the outermost end of the separator, and it is unable to be separated. In Figure 14a, the curve of θ = 35° is higher than that of θ = 45° in 13 s. This is because when the simulation is carried out with θ = 35°, the rope of the shellfish box lags behind under the action of the hanging rope-splitter, resulting in the fluctuation of the shellfish box, so the value increases at that time. From Figure 14b, it can be seen that when the shellfish box separation angle θ = 30°, 35° and 40° when the γ angle is smaller, the separation effect is smoother.

4.1.2. Box Separator Taper Angle

From Figure 15a, it can be seen that as the taper angle β increases, the distance Z also increases. The shellfish box crossed the outermost side of the separator at 22 s, and the maximum distance Z appeared. When the taper angle β is 32°, Z is too large, and the separation effect is not ideal. The larger the taper angle, the more the shellfish box is pushed outward so that Z gradually increases. From Figure 15b, when the taper angle is 26°, 28° and 30°, the γ angle is small, and it could be regarded as a smoother separation effect. The two combined evaluation indicators are better separated at a taper angle of 26°, 28° and 30°.

4.1.3. Placement Depth

From Figure 16a, it can be seen that the distance Z decreases with the increase in the separator’s placement depth h. When the depth h increases, the contact point between the lower edge of the box separator and the shellfish box moves downward, the shellfish box position moves upward along the inclined surface of the box separator, the box center of mass is shifted inward along the inclined surface, and the distance Z gradually decreases. From Figure 16b, when the separator is placed at a shallow depth, the lower edge of the separator is located in the upper half part of the shellfish box. This results in the separation process of the shellfish box shaking amplitude to be larger, as well as the γ angle. With the gradual downward movement of the separator, the γ angle tends to 0° for a smoother separation.

4.2. Analysis of Orthogonal Experiment Results

The experimental design plan and test results are shown in

Table 4. Orthogonal experiment test protocol and results.

Test	$\mathit{\theta }$	$\mathit{\beta }$	h	Z/mm	γ/°
1	40	30	600	286	7.9
2	30	28	700	272	6.1
3	35	28	600	317	4.3
4	35	28	600	332	4.5
5	35	26	500	292	8.9
6	35	30	700	286	9.2
7	35	28	600	318	4.7
8	30	28	500	295	5.4
9	40	28	500	305	6.1
10	35	30	500	337	9.1
11	30	30	600	292	6.4
12	35	28	600	321	4.6
13	40	26	600	269	7.3
14	35	26	700	274	8.4
15	40	28	700	265	6.9
16	30	26	600	265	5.7
17	35	28	600	316	4.1

.

The ANOVA analysis of the response surface model of the distance Z is shown in

Table 5. ANOVA analysis of the distance Z.

Source of Variation	Sum of Squares	df	Mean Square	F	p
Model	8746.57	9	971.84	34.09	$<0.0001$
$\theta$-Box Separator angle	0.1250	1	0.1250	0.0044	0.9491
$\beta$-Box Separator taper angle	1275.13	1	1275.13	44.73	0.0003
h-Box Separator placement depth	2178.00	1	2178.00	76.40	$<0.0001$
$\theta \beta$	25.00	1	25.00	0.8770	0.3802
$\theta$h	72.25	1	72.25	2.53	0.1554
$\beta$h	272.25	1	272.25	9.55	0.0176
${\theta }^2$	3277.52	1	3277.52	114.97	$<0.0001$
${\beta }^2$	934.78	1	934.78	32.79	0.0007
h2	315.04	1	315.04	11.05	0.0127
Residual	199.55	7	28.51
Lack of Fit	28.75	3	9.58	0.2244	0.8751
Pure Error	170.80	4	42.70
Cor Total	8946.12	16

.

Establish the regression equation for Z.

$$\begin{array}{l}{Y}_Z=320.80+0.0125\times \theta +12.63\times \beta -16.50\times h-2.5\times \theta \times \beta -4.25\times \theta \times h\\ -8.25\times \beta \times h-27.9\times {\theta }^2-14.9\times {\beta }^2-8.65\times h^2\end{array}$$

(21)

The p-value of the regression model is less than 0.0001, indicating that the regression model is highly significant. The p-value of the model’s lack of fit term is greater than 0.05, indicating that the model’s lack of fit is not significant and the regression model has a high degree of fit. The effects include, in descending order, the box separator placement at depth h, box separator separation at angle θ, and box separator taper at angle β. Based on the analysis results of the regression model, the 3D response surface plots of the interaction effects of the factors are shown in Figure 17.

As shown in Figure 17, when the box separator separation angle increases from 30 ° to 40°, the distance Z shows a tendency to rise and then fall, and the maximum value occurs at 35°. When the box separator taper angle increases, the distance Z also rises; with the increase in the placement depth, the image shows a rising trend. Through the image, we can find that the maximum Z occurs when the θ angle is 31.9°, the β angle is 29.5°, and the h is 500 mm.

The ANOVA analysis of the response surface model of the angle γ is shown in

Table 6. ANOVA analysis of the angle γ.

Source of Variation	Sum of Squares	df	Mean Square	F	p
Model	48.25	9	5.42	37.37	$<0.0001$
$\theta$-Box Separator angle	2.65	1	2.65	18.25	0.0037
$\beta$-Box Separator taper angle	0.6613	1	0.6613	4.56	0.0700
h-Box Separator placement depth	0.1513	1	0.1513	1.04	0.3410
$\theta \beta$	0.0025	1	0.0025	0.0172	0.8992
$\theta$h	0.0025	1	0.0025	0.0172	0.8992
$\beta$h	0.0900	1	0.0900	0.6210	0.4565
${\theta }^2$	0.1601	1	0.1601	1.10	0.3282
${\beta }^2$	28.03	1	28.03	193.38	$<0.0001$
h2	14.88	1	14.88	102.68	$<0.0001$
Residual	1.01	7	0.1449
Lack of Fit	0.7825	3	0.2608	4.50	0.0902
Pure Error	0.2320	4	0.0580
Cor Total	49.76	16

.

Establish the regression equation for γ.

$$\begin{array}{l}{Y}_{\gamma }=4.44+0.5750\times \theta +0.2875\times \beta +0.1375\times h-0.0250\times \theta \times \beta +0.0250\times \theta \times h-\\ \hspace{1em}\hspace{1em}0.15\times \beta \times h-0.1950\times {\theta }^2+2.58\times {\beta }^2+1.88\times h^2\end{array}$$

(22)

The fit of the angle of the shellfish box deviating from the box separator γ is extremely significant (p < 0.0001), and the lack of fit term is not significant. The p-values of A, B, B2 and C2 are less than 0.05, indicating that these terms significantly affect the model. In descending order, the effects are separator separation angle, separator taper angle and separator placement height. Through the ANOVA analysis, we can find that the minimum of γ occurs when the θ angle is 31.9°, the β angle is 27.5° and the h is 581.6 mm.

Based on the above research, parameter optimization and verification experiments were carried out in the test to obtain the best level parameter combination. The regression equations of the distance Z and the angle γ were analyzed, and the constraints were selected according to the theoretical conditions. The objectives and constraint functions are shown as follows:

$$\left\{\begin{array}{c}\max Y_Z\left(\theta ,\beta ,h\right)\\ \min Y_{\gamma }{\left(\theta ,\beta ,h\right)}_{\gamma }\\ s.t.\left\{\begin{array}{l}30°\le \theta \le 40°\\ 26°\le \beta \le 30°\\ 500 \mathrm{mm}\le h\le 700 \mathrm{mm}\end{array}\right.\end{array}\right.$$

(23)

The optimal operating parameters of the box separator are obtained by the Design-Expert optimization solver module as follows: the θ angle is 31.9°, the β angle is 30°, and the h is 550 mm. The simulation separation test is carried out again under the optimal parameters. The farthest horizontal distance Z of the shellfish box is 297.53 mm, and the angle γ of the shellfish box deviating from the box separator is 6.53°.

4.3. Validation Test Results

This work is financially supported by the Key R&D Program Project in Shandong Province, and the overall harvesting unit is supported by a three-body kelp harvesting ship [27]. The best working parameters of the separator separation angle θ is 31.9°, separator taper β is 30°, and the placement depth h is 550 mm for experiments. The team carried out a sea trial in Rongcheng City, Shandong Province, in July 2024, as shown in Figure 18.

We used shellfish boxes on both sides of the raft rope to carry out a sea trial. A total of 50 groups of 100 shellfish boxes were selected for the separation test. The whole separation process was above the water’s surface. We measured the distance Z from the shellfish box to the outermost end of the separator with a tape measure and measured the angle γ between the shellfish box and the separator with a protractor. Since the divider is a triangular pyramid structure, we believe that when the shellfish box moves along the surface of the separator, the maximum distance Z is generated when the shellfish box moves to the outermost end of the separator. Due to the limited length of the hanging rope of the shellfish box, when the shellfish box moves to the outermost end of the separator, the angle between the shellfish box and the separator is the largest. Therefore, we took the γ angle at this time as the standard. If the γ angle at this time is smaller, then the γ angle is smaller during the whole separation process. In addition, the validation test was carried out based on two evaluation indexes: the farthest horizontal distance of the shellfish box Z and the angle of the shellfish box deviating from the box separator γ. The experimental results are shown in Figure 19 and

Table 7. Sea trials results.

Item	50 Groups on the Left	50 Groups on the Right
Average the farthest horizontal distance of the shellfish box Z/mm	279.62	296.42
Average the angle of the shellfish box deviating from the box separator γ/°	11.77	17.60

.

Throughout the experiment, we concluded that the difference between the data obtained from the distance Z test and the simulation experiment data is 3.2%, while the data difference of the angle γ was mainly due to the greater influence of the weather on that day, which led to a larger difference. From the above figure and table, it can be seen that the overall measured data on the right side of the distance Z and the angle γ data are greater than the left side of the data. This is due to the sea current moving from left to right on that day. The influence of the sea current can create a slight bias, but the overall separation effect is better. The data are larger when the ship veers into the raft and out of the raft in the process of harvesting. According to the site’s field observations, the kelp harvesting ship fluctuates when it crosses the rope at both ends of the raft, thus affecting the data. Due to the complexity of the sea operation, the wind direction and wind speed would affect the harvesting effect, but the actual data and the field observations concluded that the separator is of practical significance and had a better effect on solving the problem of shellfish–kelp separation.

5. Conclusions

This article developed a kelp harvesting unit test bench to realize the function of shellfish and kelp separation. We developed a box separator and designed its main parameters, and explored the factors affecting the separation effect through theoretical analysis. This work was based on ADAMS kinematics simulation software to simulate and analyze the separator separation process. The farthest horizontal distance of the shellfish box Z and the angle of the shellfish box deviating from the box separator γ as the evaluation indexes for the single-factor simulation test to derive the optimal utility intervals, and cross-testing of the three factors to derive the optimal parameters of the three factors. The best working parameters of the separator separation angle θ is 31.9°, the separator taper β is 30°, and the placement depth h is 550 mm. We conducted sea trials using a kelp harvesting carrier ship, and the practical results proved the usefulness of the box separator to separate the shellfish and kelp.