Experimental study on cavitation water jet cleaning of marine fouling

The accumulation of marine fouling on a ship’s hull can increase its fuel consumption, necessitating removal. The use of a cavitation water jet as an efficient method for cleaning marine fouling has been widely adopted. In the research, experiments are conducted on water jet cleaning of marine fouling to investigate the influence of water jet parameters on cleaning efficiency. The findings reveal that the surface damage to specimens was minimized when maintaining an erosion pressure of 16 MPa, employing a 25 mm standoff distance, and applying a 90° incidence angle. This research can provide theoretical guidance for subsequent optimization of cavitation water jet cleaning parameters.


Introduction
Marine fouling can adhere to the surface of ships, leading to increased surface roughness, higher hydrodynamic resistance, increased operating costs, and carbon emissions [1].Additionally, marine fouling can cause corrosion on ship surfaces, affecting safety [2], and necessitating regular cleaning.Cavitation water jet cleaning technology is a widely adopted method for removing marine fouling from the ship's surface [3].
Cavitation water jet technology introduces cavitation bubbles into the water jet.When these bubbles collapse within a certain distance, they release microjets with extremely high localized stress [4], causing marine fouling to detach.However, while cavitation water jet cleaning effectively removes marine fouling, it can also cause cavitation damage to ship coatings and hull surfaces, resulting in increased surface roughness and hydrodynamic resistance.Changes in internal stresses in the steel plates of ships may affect the material properties, thus impacting the overall structural integrity of the ship [5].The existing experimental research on cavitation damage primarily concentrated on the direct impact of metal specimens, studying their damage mechanisms, with limited attention to cleaning efficiency.Therefore, a water jet cleaning experiment was conducted to analyze the influence of water jet parameters and damage characteristics while ensuring cleaning efficiency.Combining factor analysis with surface microstructure, the damage characteristics of cavitation water jet cleaning on the ship hull are deduced.This research can provide theoretical guidance for subsequent optimization of cavitation water jet cleaning parameters and control of cleaning efficiency.

Specimen preparation
Figure 1 shows the specimen preparation process.The material used for the specimen was marine-grade steel plate Q235, and red and green coatings were applied to simulate ship coatings.The coatings used were specialized chlorinated rubber anti-corrosion coatings for ships.The specimen dimensions were 100×100×2 mm.Seashells were adhered to the specimen surface to simulate marine fouling on ship surfaces.

Experimental device
Figure 2 shows the waterjet cleaning experiment system.The structural parameters of the nozzle used are shown in Figure 3.In this experiment, the incidence angle of the water jet, defined as the angle between the specimen surface and the axis of the water jet, could be adjusted by tilting the specimen placement platform, with a variation range from 70° to 90°.

Experimental scheme
Experimental variables for the study included erosion pressure, standoff distance, and incidence angle, each having three different levels.To ensure accuracy in our experimental results, an L9(3 3 ) orthogonal table is employed.Table 1 shows the factor level of the test, and Table 2 shows the orthogonal test table.In the research, the water jet's cleaning effectiveness is assessed by examining its capability to remove seashells.Furthermore, the water jet's impact width is evaluated by analyzing the specimen erosion area and its impact force is assessed by taking into account the specimen surface roughness and erosion caused to specimens.The duration of each group was 60 seconds.

Experimental results
In the experiment, complete removal of seashells was achieved across each group.To gauge the cleaning performance of the cavitation water jet, erosion area, and surface roughness are assessed as indicators.
Better cleaning performance was indicated by smaller erosion areas and roughness, resulting in less erosion to the hull.After processing the experimental image such as image grayscale conversion, noise reduction, binarization, and feature extraction, the processed Figure 4 was obtained.For detailed experimental data, please refer to Table 3.

Analysis of range (ANOR)
Table 4 displays ANOR for the erosion area.As a preliminary conclusion, it can be inferred that the incidence angle exerted the most significant influence on the erosion area, subsequent to erosion pressure and standoff distance.The optimal parameter for minimizing erosion area was found to be P = 16 MPa, R = 25mm, and θ = 90°.Table 5 reveals that the roughness was most significantly influenced by erosion pressure, subsequent to standoff distance and incidence angle, in that order.The parameter that resulted in the lowest roughness was found to be P = 16 MPa, R = 15 mm, and θ = 90°.It's worth noting that the average surface roughness of the standoff distances of 15 mm and 25 mm differed very little.Since the 25-mm standoff distance also led to the smallest erosion area, it is advisable to employ a 25-mm standoff distance to minimize damage to specimens.Consequently, the configuration that achieved minimal damage while ensuring complete removal of biofouling was P = 16 MPa, R = 25 mm, and θ = 90°.

Regression equation
To obtain the regression parameters, the Quasi-Newton method was utilized to establish the regression equation by using the non-linear curve fit method.Table 6 shows the regression analysis results.The fitting correlation coefficients exceed 93.3% which means the roughness model had high goodness of fit and can well reflect the experimental data.To further verify the accuracy of the regression equation, the experimental value and the predicted value of the experiment were compared.Figure 5 reveals the predicted values are in good agreement with the experimental values.

Microstructure analysis
Figure 6 shows the 200× enlarged microstructure of specimen 5. From Figure 6, it can be observed that the damaged regions can be loosely classified into three areas, ranging from the innermost to the outermost.The jet direct injection area, centered around the jet impact axis, an approximate diameter of 2 mm and exhibited significant peeling and plastic deformation (Figure 6a).The adjacent area to the jet direct injection area was the buffer area, which showed less pronounced plastic deformation compared to the jet direct injection area and lacked extensive peeling, displaying only minor signs of damage (Figure 6b).The outermost area was the turbulent area, characterized by noticeable plastic deformation and fewer erosion pits (Figures 6(c) and 6(d)).
The differences in morphology among these three areas were attributed to the characteristics of the submerged water jet.The water jet ejected from the nozzle gradually diminished as it approached the specimen surface due to the submerged underwater environment, leading to its divergence into a conical shape.Consequently, the center of the jet experienced the highest velocity.The maximum water jet dynamic pressure combined with the explosive force created during bubble collapse impacted the central region of the specimen, resulting in the most severe erosion on the jet direct injection area.The jet within the outermost area caused turbulence and the formation of vortices along with numerous bubbles when it interacted with the surrounding stagnant water.These bubbles continuously collapsed on the outermost area, producing shock waves and micro-jets that caused a little erosion to the specimen, forming the turbulent area.Between the jet direct injection area and the turbulent area, there existed a weaker erosion region where the jet flow rate was significantly slower.Consequently, the pressure from the jet was weaker in the area, and there were fewer bubbles due to its distance from the stagnant water.Therefore, the damage of this area on the specimen was less pronounced compared to the jet direct injection area and turbulent area.

Conclusions
Through the implementation of an orthogonal test that simulated the adhesion of biofouling to the hull surface, the impact of various jet parameters on the ship is assessed, including erosion area, roughness, and microscopic topography, leading to the following conclusions: (1) To achieve minimal damage while ensuring complete removal of biofouling, the optimal configuration involved a 16-MPa erosion pressure, a 25-mm standoff distance, and a 90° incidence angle.
(2) A regression equation for the roughness after erosion was established.The equation has a good fitting effect and can reflect the experimental data.Therefore, within a specific range of parameters, the equation can provide certain guidance for the subsequent cavitation jet parameter optimization.
(3) According to the different levels of damage, the erosion region can be categorized into three areas from the central to peripheral regions: the jet direct injection area, the buffer area, and the turbulent area.

Figure 3 .
Figure 3.The structural parameters of the nozzle.

Figure 4 .
Figure 4.The experimental image after image processing.

Figure 5 .
Figure 5.The comparison between the experimental and predicted values.

Table 1 .
Table of factor level of orthogonal test.

Table 3 .
Orthogonal test result

Table 4 .
ANOR for the erosion area.

Table 6 .
Regression equation and parameter values of specimen surface roughness.