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

Today, ship hull fouling is managed through fouling-control coatings, complemented with inwater cleaning. During cleaning, coating damage and wear must be avoided, for maximum coating lifetime and reduced antifoulant release. When possible, cleaning should target early stages of fouling, using minimal forces. However, such forces, and their effects on coatings, have not yet been fully quantified. In this one-year study, minimal cleaning forces were determined using a newly-designed immersed waterjet. The results show that bi-monthly/monthly cleaning, with maximum wall shear stress up to 1.3 kPa and jet stagnation pressure 0.17 MPa, did not appear to cause damage or wear on either the biocidal antifouling (AF) or the biocide-free foul-release (FR) coatings. The AF coating required bi-monthly cleanings to keep fouling to incipient slime (time-averaged results), while the FR coating had a similar fouling level even without cleaning. The reported forces may be used in matching cleaning parameters to the adhesion strength of the early stages of fouling

Best practise for cleaning of ship hulls

This report summarizes the methods available for cleaning based on a review of commercial equipment and the information is divided into handling and operation, efficacy in removal and capture and impact on paint

Analysis of Pressure Drop Data in Channel Flows Over Foul-Control Coatings

The two-dimensional channel flow is of great interest for experimental as well as numerical studies. From the experimental perspective test in channel equipment is preferred, because it is simple, practical and offers a favorable economic running cost. Especially with the growing interest in marine coating research, it is critical that coating testing equipment delivers realistic flow conditions, is simple to cut the experimental time and effort and yet accurate. In this sense, the flow channel facility offers more advantages than classical cases for experimental investigations. Whereas from the numerical investigations point of view, channel flow exhibits favorable boundary conditions to save computational effort, while providing a deep insight into details of the flow structures. An initiative is taken at Chalmers university to develop the channel flow (or a flowcell) experimental facility to cater for the need of studies on coatings. Therefore, the current paper, in the first step, describes the design and manufacture of the flowcell. Secondly, it presents the thoroughly conducted verification study of the smooth reference test section to demonstrate that experimental facility holds the measurement expectations. Subsequently, skin friction data for selected foul control coatings obtained from the pressure drop measurements in the flowcell are presented

Experimental feeding rates of gelatinous predators Aurelia aurita and Mnemiopsis leidyi at low northern Baltic Sea salinity

The effect of the northern Baltic Sea\u27s low salinity on feeding rates of a native scyphozoan Aurelia aurita and a recent invader to southern Baltic Sea, ctenophore Mnemiopsis was investigated experimentally. Incubations with Acartia spp. prey (4.19-25.16 indiv. l(-1)) were used to estimate clearance rates for both predators. Mnentiopsis leidyi digestion times were measured for several natural prey items. Wet weight (ww):length/diameter relationships as well as clearance rates (0.49 +/- 0.15 1 g(ww)(-1) h(-1) [mean +/- SE] for M. leidyi [mean oral-aboral length +/- SD = 9.6 +/- 1.5 mm]; and 0.18 +/- 0.07 1 g(ww)(-1) h(-1) [mean +/- SE] for A. aurita [mean bell diameter SD = 37.3 +/- 6.9 mm]) and digestion times at salinity 5.7 were within the ranges reported from higher salinities. These preliminary results suggest that the low salinity does not significantly depress the feeding rates or potential predatory impact of these gelatinous predators

Instantaneous Flow Structures and Opportunities for Larval Settlement: Barnacle Larvae Swim to Settle

Water flow affects settlement of marine larvae on several scales. At the smallest scale local flow regime may control the probability of adhesion to the substrate. Our aim was to mechanistically understand the transition from suspended to attached larvae in turbulent flow. Recently it was proposed that opportunities for larval settlement in turbulent boundary layers depend on time windows with suitable instantaneous flow properties. In flume flow we characterized the proportion of suitable time windows in a series of flow velocities with focus on the near-bed flow. The change in the proportion of potential settling windows with increasing free-stream velocities was compared to the proportion of temporary attachment of barnacle cypris larvae at different flow velocities. We found large instantaneous flow variations in the near-bed flow where cyprid attachment took place. The probability of temporary attachment in cyprids declined with local flow speed and this response was compatible with a settling window lasting at least 0.1 s with a maximum local flow speed of 1.9–2.4 cm s-1. Cyprids swam against the near-bed flow (negative rheotaxis) and the swimming speed (1.8 cm s-1) was close to the critical speed that permitted temporary attachment. We conclude that temporary attachment in barnacle cyprids requires upstream swimming to maintain a fixed position relative to the substrate for at least 0.1 s. This behaviour may explain the ability of barnacles to recruit to high-flow environments and give cyprids flexibility in the pre-settlement choice of substrates based on flow regime

Towards an absolute scale for adhesion strength of ship hull microfouling

In-water ship hull cleaning enables significant fuel savings through removal of marine fouling from underwater surfaces. Unfortunately, cleaning may also shorten the lifetime of hull coatings, with subsequent increase in fouling growth rate. Deleterious effects of cleaning would be minimized by matching cleaning forces to the adhesion strength of early stages of fouling, or microfouling. Calibrated waterjets are routinely used for comparing different coatings in terms of adhesion strength of microfouling. However, an absolute scale is lacking for translating such results into cleaning forces, of interest for design and operation of hull cleaning devices. This paper discusses how to determine such forces, namely using Computational Fluid Dynamics. Semi-empirical formulas are derived for forces under immersed waterjets, where the normal and tangential components of wall forces are given as functions of different flow parameters. Nozzle translation speed is identified as a parameter for future research, as this may affect cleaning efficacy

Experimental Quantification of Drag Change of Commercial Coatings Under the Effect of Surface Roughness and Soft Fouling

The performance of ships will be adversely affected by the excessive hull roughness to an extent where financial penalties will be incurred. It is beyond the bounds of possibility to achieve a perfectly smooth hull, as the ship building process and paint application will leave their “fingerprint” on the surface. The roughness of the hull may vary from fine to coarse according to the substrate finish, the coating application methods used and further driven due to exposure to aggressive sea environment. The drag performance data of newly-applied and clean coatings is not sufficient to fully reflect the drag characteristics and efficacy of marine coatings over a typical period between dry-docking. Usually during this period, the increase in surface roughness and development of different fouling stages on marine coatings occur. Therefore, the study focuses on comparison of drag characteristics of hull coatings with relatively smooth, coarse roughness finishes and fouling conditions using time- and cost-efficient approach.\ua0 The study describes experimental tests carried out to quantify the drag change of commercial coatings due to the presence of physical and biological roughness. Firstly, biocidal and non-biocidal coatings with relatively smooth and coarse roughness finishes are tested. Secondly, mentioned coating types and roughness ranges are exposed to fouling growth to explore the extend of algae fouling and its effect on drag characteristics. The results of the study may be useful to estimate the added drag and overall fuel penalty for ships with various coating roughness ranges and soft fouling

Antifouling paints leach copper in excess – study of metal release rates and efficacy along a salinity gradient

Antifouling paints are biocidal products applied to ship and boat hulls in order to prevent the growth and settlement of marine organisms, i.e. fouling. The release of biocides from the surface of the paint film act to repel or poison potential settling organisms. Currently, the most commonly used biocide in antifouling paints is cuprous oxide. In the EU, antifouling products are regulated under the Biocidal Products Regulation (BPR), which states that the recommended dose should be the minimum necessary to achieve the desired effect. For antifouling products, the dose is measured as the release rate of biocide(s) from coating. In this study, the release rates of copper and zinc from eight different coatings for leisure boats were determined through static exposure of coated panels in four different harbors located in Swedish waters along a salinity gradient ranging from 0 to 27 PSU. The results showed the release rate of copper to increase with increasing salinity. Paints with a higher content of cuprous oxide were also found to release larger amounts of copper. The coatings’ ability to prevent biofouling was also evaluated and no significant difference in efficacy between the eight tested products was observed at the brackish and marine sites. Hence, the products with high release rates of copper were equally efficient as those with 4 – 6 times lower releases. These findings suggest that current antifouling paints on the market are leaching copper in excess of the effective dose in brackish and marine waters. Additionally, the results from the freshwater site showed no benefit in applying a copper-containing paint for the purpose of fouling prevention. This indicates that the use of biocidal paints in freshwater bodies potentially results in an unnecessary release of copper. By reducing the release rates of copper from antifouling paints in marine waters and restricting the use of biocidal paints in freshwater, the load of copper to the environment could be substantially reduced