Artificial barnacles, which simulated biofouling organisms, were constructed from acrylic material using three-dimensional prints of actual barnacle scans. Depending on the species and habitat, barnacles can grow to 3.7–62.9 mm (with an average of 20 mm) (Doell et al., 2017). Based on ASTM D5618-20, a standard test for measuring barnacle adhesion strength, we selected a base diameter of 20 mm and a height of 10 mm ( Figure 1 ).

To ensure that the artificial barnacles had a shell strength similar to that of actual barnacles, we conducted compressive and adhesion strength tests using a 100 kN universal testing machine (UTM; INSTRON 8800, Norwood, MA, USA). The final compressive strength was the average of the compressive values obtained from five artificial barnacles treated at 100 or 150°C for 24 or 48 h (to achieve a compressive strength comparable with that of actual barnacles). The compressive strength test was conducted following the ASTM D695 standard, and the sample was compressed at a deformation rate of 2 mm/min. The compressive strength (MPa) was calculated by dividing the maximum load by the maximum diameter of the artificial barnacle upon fracture.

The adhesion strength test was conducted on artificial barnacles to which various adhesives were uniformly applied following the ASTM D5618-20 standard. Five adhesives commonly used for plastics—acrylic, epoxy, cyanoacrylate, silicone, and polyurethane (A–E, respectively)—were selected. The test panel used for the adhesion was a steel plate coated with epoxy primer and a copper-based antifouling coat. The artificial barnacles were located at least 20 mm away from the edge of the test plate and were attached so there was no interference between the plate and the barnacles. The adhesion strength test was repeated three times for each test panel. After allowing the adhesives on the artificial barnacles to dry at room temperature for over a day, the test plate was immersed in natural seawater for 20 days. The adhesion strength was tested on more than five artificial barnacles for each adhesive. After fixing the test plate (with the artificial barnacles attached) in the UTM, the adhesion strength was tested at 4.5 N/s until the sample was separated from the surface. The adhesion strength (MPa) was calculated by dividing the measured removal strength (N) of the barnacle by its base area (mm²), as follows (Equation 1):

The test plate—a steel plate sheeting (4 m × 1.5 m, with a thickness of 8 mm)—was designed according to the size and load of the cleaning unit. The plate surface was sandblasted with 50 μm alumina particles. The plate used to test biocide release during cleaning was coated with anticorrosive (Jotacote Universal N10, Jotun, Sandefjord, Norway) and a copper antifouling paint (SeaQuantum Pro U paint, Jotun, Sandefjord, Norway), and marked as AC-AF plate. To exclude the influence of the antifouling coating, the test plate used to test the capture of biofouling organisms was coated only with an anticorrosive coating product and marked as AC plate.

The test plate was constructed considering the acceleration and deceleration section of the cleaning unit, and the test area within the test plate was set to be at least 1.5 times the width of the cleaning unit. Artificial barnacles were attached according to the fouling rating using the adhesives selected in the adhesion strength test ( Figure 2 ). According to New Zealand’s guidelines on the conduct of in-water biofouling surveys for domestic vessels (MPI, 2020), organism attachment (and hence biofouling) can be scattered or localized. Artificial barnacles were attached to 15% of the test area based on a fouling rating of 2, the minimum rating that requires capture by in-water cleaning systems in the revised IMO biofouling guidelines (IMO, 2023a). The tests were conducted by assuming the scattered distribution as the worst condition.

The in-water cleaning system used in our tests was a proprietary cleaning unit developed by TAS GLOBAL (Gangseo-gu, Busan, South Korea) and is not yet available. This device removes biopollution and captures the cleaning debris in a transport hose and brush specific to the type of biofouling. In our tests, the cleaning unit was equipped with rotating brushes made of urethane and NC nylon to remove the artificial barnacles from the test plate and a watertight skirt to capture the cleaning debris. The capture equipment comprised a transport pump (100 ton/h capacity) and a 50 m transport hose to direct the captured debris to a treatment system on land.

The biocide concentration change test was conducted at the testing tank facility of the Korea Marine Equipment Research Institute. The size of the test tank was 24 m (L) × 8 m (B) × 8 m (H). The tank was equipped with a 400 m³/h water intake, a drain pump, and a crane for installing the test plate. The AC-AF plate was installed according to the specifications, and the artificial barnacles were then attached to simulate a fouling rating of 2. To examine the quantification of the biocides, sampling was performed at three locations: near the test plate before and after testing (A1 and A2, respectively) and at the cleaning unit during cleaning (A3) ( Figure 3 ). To obtain the sample near the cleaning unit, the rotation direction of the brushes was first determined, and the most likely debris discharge direction considering the design and operation of the equipment was determined; a separate sampling hose was then fixed to this discharge location, and the waste generated during the test period was continuously collected in the capture tank. Samples of approximately 20 L were collected for the analysis of biocides. Replicate samples were obtained at the three sampling locations (A1, A2, and A3) during cleaning, and water quality parameters were measured.

An AC test plate was used in the capture efficacy test. A stainless steel filter (pore size, 50 μm) was installed in the capture tank to collect the captured artificial barnacles. The filtrate was dried at room temperature for more than 24 h, and the dry weight was measured. The capture efficiency (as a percentage) was calculated as the dry weight of the captured barnacles relative to that of the removed barnacles. The weight of the removed barnacles was calculated by subtracting the weight of the remaining barnacles on the test plate after cleaning from that of the attached barnacles before cleaning.

Before and after the cleaning test, the water quality and biocides were analyzed at the three sampling locations in the test tank (A1, A2, and A3). The water temperature, salinity, pH, and turbidity before and after cleaning were measured using a multi-parameter water quality instrument (DS5: Hydrolab, Loveland, CO, USA). To measure the TSS content in natural seawater, 250-ml samples were filtered through a preweighed glass fiber filter (GF/F filter, Whatman) and dried for at least 1 h at 105°C. The particle size distribution of the captured artificial barnacle debris was analyzed according to ISO 13320 using a laser diffraction particle size analyzer (Mastersizer-3000, Malvern Instruments, Great Malvern, UK). The concentrations of copper and zinc, the main biocide components, were analyzed using an inductively coupled plasma atomic emission spectrometer (Agilent 5110, Agilent Technologies, Santa Clara, CA, USA).

Shell strength is a key factor in creating artificial barnacles. To achieve compressive strengths comparable with those of actual barnacles, the artificial barnacles used here were heat-treated. Compressive strength decreased as heat treatment time increased ( Figure 4 ). The compressive strength of artificial barnacles decreased significantly with increasing heat treatment time due to the change in the barnacle’s acrylic structure. The compressive strength was 35.5 ± 1.7 MPa in the unheated control and 8.8 ± 0.8 and 3.8 ± 0.1 MPa after heating at 150°C for 24 and 48 h, respectively.

The compressive strength of barnacles varies significantly among species. Based on the relationship between barnacle volume and compressive strength, the estimated compressive strength of barnacles with a base diameter of 20 mm is 0.5–4.4 MPa (Gubbay, 2009). The compressive strength of the artificial barnacles subjected to heat treatment at 150°C for 48 h (3.8 ± 0.1 MPa) was similar to this estimate. By contrast, the compressive strength of the artificial barnacles treated at 100°C for 24 and 48 h (37.3 ± 3.2 and 35.0 ± 4.3 MPa, respectively) did not differ significantly from that of the control (35.5 ± 1.7 MPa), i.e., it was significantly different from that of natural barnacles. Heating at 100°C, therefore, did not induce structural deformation, even though this temperature was close to the heat deflection temperature of acrylic (95°C).

After the compressive strength test, the fracture pattern was visually determined ( Figure 5 ). The removal of biofouling (such as barnacles and bivalves) using a scraper or cleaning unit sometimes breaks or fragments the shell. During the compressive strength testing, the artificial barnacles treated at 150°C were crushed and broken into small pieces. This fracture pattern closely resembles that observed in real barnacles removed by the cleaning unit.

Adhesion tests were conducted on five types of adhesives widely used for plastic bonding. The adhesive should exhibit high adhesive strength on the surface of the steel plate and at the same time have durability suitable for the marine environment. An adhesive that falls within the natural range was selected. The adhesion strength of the proteinaceous cement secreted by barnacles ranges from 0.1 to over 1 MPa (Yule and Walker, 1987; Watermann et al., 1997). There is an exponential relationship between barnacle diameter and the force required to remove the barnacles, and their adhesion strength depends on the type of substrate to which they are attached: 0.06 MPa for a silicon fouling release surface, 0.6 MPa for a copper base antifouling surface, 1.2 MPa for an epoxy surface, 1.83 MPa for a cathodically protected bronze surface, and 0.7–1.2 MPa for rough and rigid substrates (Swain et al., 1992; Swain and Schultz, 1996; Li et al., 2020; Swain et al., 2022). Based on these values, we set the barnacle adhesion strength at 0.8–1.2 MPa and conducted the adhesion strength test. Polyurethane adhesives (E) exhibited the highest adhesion strength of 1.08 ± 0.32 MPa, followed by silicone adhesives (D) with an adhesion strength of 1.01 ± 0.20 MPa. Acrylic, epoxy, and cyanoacrylate adhesives (A–C) exhibited adhesion strengths of 0.64 ± 0.17, 0.27 ± 0.04, and 0.54 ± 0.11 MPa, respectively, as determined with Equation 1 ( Table 2 ). Polyurethane and silicone were thus the strongest adhesives in this study. Given the presence of chloride ions, these two materials are also the most suitable adhesives for seawater conditions. Polyurethane exhibits strong adhesion in air, humid environments, and water. Solvent-based or water-soluble adhesives can reach an adhesion strength of more than 1 MPa within a short curing time (Lu et al., 2024). Aqueous conditions, characterized by high humidity and seawater, are typically unfavorable for most adhesives, as they may affect their adhesion and durability. When designing a test of the efficacy of biofouling debris capture, it is, therefore, important to select the appropriate adhesive for the specific test design.

In-water cleaning was performed using the AC-AF plate with attached artificial barnacles. The target biocides were copper and zinc, the most commonly used in commercial shipping (Dafforn et al., 2011; Wallström et al., 2011). No dissolved or particulate zinc was observed during the test period. No dissolved copper was detected; however, 0.030 mg/L of particulate copper was detected at A2, suggesting that copper can be released into the water during in-water cleaning ( Table 3 ). In many cases, however, dissolved and particulate biocide can be released into the water during in-water cleaning (Tamburri et al., 2020; Soon et al., 2021). Therefore, regulations that specify the acceptable levels of debris release are required for in-water cleaning in the marine environment.

South Korea still lacks a clear emissions standard for biocides released during in-water cleaning. The country’s marine environmental standards are based on the Marine Environment Management Act; biopollution and biocides generated during biofouling removal may thus correspond to the pollutants specified in Article 22 of this act. According to this act and notices from the Ministry of Oceans and Fisheries, marine environmental standards are divided into those related to seawater quality, submarine sediments, and marine water conditions. These standards specify a maximum copper level of 0.003 mg/L (short-term standard) for marine ecosystem protection and 0.020 mg/L for human health protection. For zinc, these levels are 0.034 mg/L (short-term standard) for marine ecosystem protection and 0.100 mg/L for human health protection. The particulate copper concentrations before and after our test were below the method detection limit (0.020 mg/L), the same as the maximum allowable short-term copper level. For this reason, it was difficult to check whether water quality standards were met. However, the average concentration of particulate copper during cleaning (0.030 mg/L) exceeded the short-term limits for marine ecosystem protection and human health protection. This indicates that copper is not adequately captured, releasing unsafe environmental levels. Therefore, capture efficacy must be improved to reduce copper concentrations to an acceptable level.

The discharge of biofouling is an important source of biological pollution in marine ecosystems. It is critical, therefore, to properly capture, treat, and discharge debris removed from the hull. In the aquatic environment, it is difficult to quantify the capture of the biofouling removed from the hull due to the difficulty in conducting tests controlling for variables such as water flow around the ship and marine conditions. Therefore, this study used a test plate with attached artificial barnacles to examine capture efficacy and environmental impacts in a controlled environment. The capture efficacy test was conducted using the test plate coated only with an anticorrosive (AC plate).

In this experiment, natural seawater was obtained from the coast near the test tank. During the test period, the water temperature in the tank was 15–16°C, salinity was 30–31 psu, and pH was 7.7–7.8. The average TSS in the seawater before the test was 68.33 ± 4.86 mg/L, increasing to 79.17 ± 2.31 mg/L after the test. The elevated TSS before the test may have obscured the apparent impact of uncaptured cleaning debris. This high concentration may have arisen from seasonal factors, such as river discharge and typhoon events. Therefore, consideration should be given to using filtered or artificial seawater in tests to reduce unacceptable variability.

The weight of the artificial barnacles attached to the test plate before the capture efficacy test was 907.02 g, and the 653.59 g of artificial barnacle debris was collected by the filter during the test. After the test, the artificial barnacles were completely removed from most of the test plate, although in some areas, the base plate of the shells remained, apparently affecting capture efficiency ( Figure 6 ). The weight of the barnacles remaining on the test plate was measured and found to be 85.98 g, confirming that 167.45 g was released without being captured. The weight of the removed barnacles, considering the amount attached to the test panel before the test and the amount remaining after the test, was 821.04 g, and the capture efficiency according to Equation 2 was 79.6%. The weight of 167.45 g (approximately 18.5% of the amount attached to the test plate) of uncaptured artificial barnacles may represent complete or fragmented forms, and the release of uncaptured intact viable organisms may have detrimental effects on marine waters (Hopkins and Forrest, 2008; Woods et al., 2012). For this reason, we confirmed the fragmented particle size and distribution of artificial barnacle fragments generated during in-water cleaning. The artificial barnacles were broken into small fragments by the brushes, although some retained their original shape. This incomplete removal is consistent with the partial removal occurring during actual in-water cleaning, when the barnacles’ base plate remains on the hull surface, potentially providing a substrate for further biofouling (Oliveira and Granhag, 2016). The captured particle size ranged from 76 μm to a maximum of 18 mm, with particles >3 mm representing 25.76%. Of the captured particles <3 mm in size, approximately 80% were 500–2,000 μm and 47.5% were 500–1,000 μm ( Table 4 ). In this study, the filter pore size was 50 μm; therefore, microparticles <50 μm were not collected.

Studies have shown that capture methods for biological debris should be used as much as possible, and cleaning technologies should aim to capture debris of at least 50 μm in size (Morrisey et al., 2013; McClay et al., 2015). However, the current processed-water discharge standards for in-water cleaning systems, the BIMCO guidelines (BIMCO, 2023), propose removing >95% of particulate matter >10 μm in diameter, and the Transport Canada (2021) guidelines propose removing particles >15 μm in diameter. Accordingly, the proportion of particles >10 μm in diameter is expected to be an important basic variable for evaluating the performance of water treatments, and a pore size of no more than 10 μm should be used when quantifying barnacle capture efficacy.