Ship hull in-water cleaning and its effects on fouling-control coatings

Figures & data

Figure 1. Dimensions of hull steel panels, with cleaning paths marked in grey, i.e. theoretical area covered by maximum wall shear stress. Locations for dry film thickness measurements are labelled L1 – R3.

Figure 2. Schematic representation (lateral view) of the immersed waterjet.

Table 1. Maximum wall shear stress τ_w,max and stagnation pressure p_s at the wall (Equations 1 and 2, respectively), as a function of flowrate Q through a pipe nozzle with D = 1.6 mm, at standoff distances H = 25 mm and 16 mm.

Q [l min^−1]	u0 [m s^−1]	Re [-]	τw,max [Pa]		ps [Pa]
			H = 25 mm	H = 16 mm	H = 25 mm	H = 16 mm
0.50 (*)	4.14	4.88E + 03	26	63	2.00E + 03	4.89E + 03
1.00	8.29	9.75E + 03	83	202	8.01E + 03	1.95E + 04
1.30 (*)	10.78	1.27E + 04	129	315	1.35E + 04	3.30E + 04
1.50	12.43	1.46E + 04	164	401	1.80E + 04	4.40E + 04
1.80 (*)	14.92	1.76E + 04	223	545	2.59E + 04	6.33E + 04
2.00	16.58	1.95E + 04	266	650	3.20E + 04	7.82E + 04
2.50 (*)	20.72	2.44E + 04	388	947	5.00E + 04	1.22E + 05
3.00	24.87	2.93E + 04	527	1287	7.21E + 04	1.76E + 05

(*) – These flowrate steps were only used on the first and second cleaning events (May and June 2018). Surface forces are given for seawater at 10 °C and salinity 35 ppt (ρ  =  1026.9 kg m^-3; ν  =  1.3602 × 10^-6 m² s^-1).

Table 2. Cleaning schedule, with indication of nozzle standoff distance H used on each date.

Month	t [days]	Panel deployment	Cleaning events on AF and FR panels		H
			Bi-monthly (F1)	Monthly (F2)	[mm]
2018 Apr	0	E, AF, FR
2018 May	29	E		X	25
2018 Jun	61	E	X	X	25
2018 Jul	91	E		X (*)	25
2018 Aug	119	E	X (*)	X (*)	25
2018 Sept	154	E		X (*)	25
2018 Oct	183	E	X (*)	X (*)	25
2018 Nov	215	E		X (*)	25
2018 Dec	245	E	X (*)	X (*)	25
2019 Jan	274	E		X (*)	25
2019 Feb	307	E	X (*)	X (*)	16
2019 Mar	334	E		X (*)	16
2019 Apr	364	–	X (*)	X (*)	16

(*) Intermediate flowrate steps (Table 1: 0.5, 1.3, 1.8 and 2.5 l min^-1) were only used on the first and second cleaning events (May and June 2018). Key: AF – antifouling coating; FR – foul-release coating; E – epoxy anti-corrosive primer; F1-F2 – code for cleaning frequency.

Table 3. US Naval Ships’ Technical Manual fouling rating, fr_NSTM (Naval Sea Systems Command 2006).

frNSTM [-]	Type	Hull fouling condition
0	Undetectable	Foul-free surface.
10	Soft	Incipient slime, visible underlying paint/metal surface.
20	Soft	Advanced slime, obscured underlying paint/metal surface. Juvenile barnacles ≤ 1 mm (this study).
30	Soft	Soft fouling (eg filaments) <76 mm in length and <6.4 mm in height.
40	Hard	Tubeworms <6.4 mm in height. Encrusting bryozoans (this study).
50	Hard	Barnacles <6.4 mm in height.
60	Hard	Combination of tubeworms and barnacles <6.4 mm in height.
70	Hard	Combination of tubeworms and barnacles > 6.4 mm in height.
80	Hard	Tubeworms closely packed and upright from surface, or barnacles on top of each other, < 6.4 mm in height.
90	Hard	Densely packed tubeworms or barnacles, > 6.4 mm in height; presence of mussels or oysters; or slime/grass overlay.
100	Composite	All forms of fouling; soft animal fouling (tunicates) growing on various forms of hard fouling

Figure 3. Areal-mean fouling rating vs time, for different coatings (AF, E, FR) and cleaning frequencies: (a) no cleaning, F0; (b) bi-monthly cleanings, F1; (c) monthly cleanings, F2. For E panels, plotted values correspond to the final stage after each period of one-month exposure, corresponding to 12 different panels deployed and retrieved on a monthly basis. For AF panels, the nozzle translation speed was varied (S1: 0.01 m s⁻¹; S2: 0.02 m s⁻¹; S3: 0.03 m s⁻¹).

Figure 4. Representative examples of fouling rating: (a) E panel (March 2019, t = 11 months): fr_NSTM = 10 (15%), fr_NSTM = 30 (1%); (b) E panel (October 2018, t = 6 months): fr_NSTM = 30 (20-40%), fr_NSTM = 90 (60-80%); (c) non-cleaned AF panel (April 2019, t = 12 months): fr_NSTM = 30 (100%); (d) cleaned AF panel (April 2019, t = 12 months): fr_NSTM = 10 (3-99%), fr_NSTM = 20 (0-1%); (e) non-cleaned FR panel (April 2019, t = 12 months): fr_NSTM = 10 (30-40%), fr_NSTM = 20 (0-30%), fr_NSTM = 30 (0-50%), fr_NSTM = 90 (5-40%); (f) cleaned FR panel (April 2019, t = 12 months): fr_NSTM = 10 (30-60%).

Figure 5. Time-averaged fouling rating, calculated from areal-mean fouling rating along time using the trapezoidal rule (Equation 4(4) $$\overline{\mathit{mean}\left({fr}_{\mathit{NSTM}}\right)}=\mathrm{ }\sum _{i=1}^{n-1}\frac{{\mathit{mean}\left({fr}_{\mathit{NSTM}}\right)}_{i+1}\mathrm{ }+\mathrm{ }{\mathit{mean}\left({fr}_{\mathit{NSTM}}\right)}_i}{2}\times \frac{t_{i+1}-t_i}{t_n-t_1}$$(4) ), where error bars correspond to propagated 95% confidence intervals: (a) AF and FR panels, with either no cleaning or constant translation speed (S0 for no cleaning; S1 for 0.01 m s⁻¹) and varying cleaning frequency (F0 – no cleaning, F1 – bi-monthly, F2 – monthly); (b) AF panels, with varying translation speed (S1: 0.01 m s⁻¹; S2: 0.02 m s⁻¹; S3: 0.03 m s⁻¹).

Figure 6. Adhesion strength, or local cleaning areal forces (in Pa), used in different points in time, depending on cleaning results, for two fouling control coatings (AF and FR), three cleaning frequencies (F0 – no cleaning; F1 – bi-monthly cleanings; F2 – monthly cleanings) and three nozzle translation speeds (S1: 0.01 m s⁻¹; S2: 0.02 m s⁻¹; S3: 0.03 m s⁻¹): (a) maximum wall shear stress τ_w,max; (b) stagnation pressure p_s. Error bars correspond to 95% confidence intervals.

Table 4. Barnacle counting on epoxy anti-corrosive (E) and foul-release (FR) coated panels, individuals dm^-2 (mean ± 95% confidence interval), before/after each cleaning event.

Month	E (1 panel per month)	FR: no cleaning (F0)	FR: bi-monthly (F1)		FR: monthly (F2)
			Before	After	Before	After
2018 May	0	0	0		0	0
2018 Jun	0	0	0	0	0	0
2018 Jul	+	+	0		0	0
2018 Aug	85	0	2 ± 7	0	1 ± 5	0
2018 Sept	+	0	0		0	0
2018 Oct	+	0	0	0	0	0
2018 Nov	0	0	0		0	0
2018 Dec	4	0	0	0	0	0
2019 Jan	567	19 ± 10	10 ± 24		9 ± 20	0
2019 Feb	0	58 ± 28	11 ± 4	0	0	0
2019 Mar	0	52 ± 13	0		0	0
2019 Apr	0	16 ± 17	0	0	0	0

(+) adult barnacles detected but no counting available for that date.

Figure 7. Paint polishing $E_i-E_t$ for each treatment, together with corresponding copper release rate (Kruskal-Wallis test: p = 0.6). Error bars correspond to 95% confidence intervals.

Figure 8. SEM imaging of coating cross-sections (AF), where the surface of the coating is on the left-hand side (approximate leached layer thickness is marked by double arrows), together with element-count profiles along depth, from EDX analysis: (a) newly applied; (b) immersed non-cleaned – AFS0F0; (c) immersed and cleaned bi-monthly – AFS1F1); (d) immersed and cleaned monthly – AFS1F2. Element abundance is expressed as a ratio between EDX counts and average counts on deeper, intact layers of the coating. Scale bars (upper right corner on SEM images) correspond to 50 μm, and error bars correspond to 95% confidence intervals for 30 line scans.

Figure 9. Peak-to-valley roughness height, MHR (Mean Hull Roughness), for anti-fouling paint PPG Sigma Ecofleet® 290 (AF panels) and foul-release PPG Sigmaglide® 1290 (FR panels), before or after immersion and waterjet cleaning (Kruskal-Wallis test: p = 0.0010 and 0.0014, at t = 0 and 12 months, respectively). Error bars correspond to 95% confidence intervals for n cleaned panels under each coating, i.e. n = 18 AF panels (triplicates of six cleaning treatments were pooled) and n = 6 FR panels (triplicates of three cleaning treatments were pooled).

Naval Sea Systems Command. 2006. Chapter 081 – Water-borne underwater hull cleaning of Navy ships. In: Nav Ship’s Tech Man. Washington (DC).

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