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Performance Standard Under Development for In Water Hull Cleaning

Credit: CleanSubSea
Credit: CleanSubSea

By Dave Smith 2019-09-14 17:25:00

There is an identified need for an internationally accepted performance standard to be developed for the use of in water hull cleaning (IWHC) systems. This will ensure that any cleaning is in accordance with a set of commonly accepted practices and operating conditions. Part of this standard should include relevant test protocols to verify that a particular system has proven efficacy in its capability of safely removing the biofouling mass found on ships hulls.

Applied research scientists at Plymouth Marine Laboratory Applications Ltd are currently collaborating with other U.K. and international bodies in an effort to develop such a standard which will be centered on independent verification using collected scientific data. It is envisaged that the introduction and application of the standard will assist with appropriate local environmental risk assessments leading to the issue of regional licenses or permits for the use of particular IWHC systems at more port locations. 

Propeller Polishing

Propeller polishing in port using divers during a vessels operational program is a long established method of maintaining the full effectiveness of the rotating propeller by smoothing the surface roughness of each blade and thus reducing hydrodynamic cavitation to maintain the designed thrust per revolution.  

This polishing process using divers with hand held  tools to remove biofouling has its own internationally recognized standard of smoothness finish known as the Rupert Scale which can be used as an assessment tool for overall propeller blade roughness before and after polishing by divers. 

Given that the fuel consumption savings as a result of this process are estimated to be in the region of five percent, many shipowners and operators have regular maintenance programs in place which include this de-fouling operation to be carried out during scheduled port visits, in addition to the normal propeller cleaning and repair treatment that takes place during scheduled dry-docking periods.

A comparatively small volume of detritus is released into local waters as a result of this cleaning process due to the fact that the operation is limited to the propeller blades and hub only.

Hull Cleaning

Given the expected gain in fuel efficiency by propeller polishing alone, the potential efficiency gains in keeping a ship’s hull free from significant biofouling during its voyages between mandatory dry-docking periods are considerably greater. 

Anti-fouling paint coating systems (AFS) applied during dry-docking periods are designed to control biofouling development mainly through the controlled release of active biocides or toxicants from the AFS during the periods between the docking schedules. In general terms, some AFS work by the continual removal of microscopic top films of the surface coating of the paint which then expose the under layers containing the active biocides. 

AFS can often require that the ship is regularly moving through the water to achieve this “stripping away” of the paint layers. Such coatings are known as being of a self-polishing type with a predetermined biocide release being achieved throughout the life cycle of the coating until final depletion.  

Whilst AFS are intended to remain as effective as possible between dry- dockings, their continued efficacy requires optimum environmental and dynamic conditions which may not be wholly experienced by a vessel during its actual trading pattern. As a result of this, there will always remain the opportunity for biofilms to initially form on a ship’s hull (microfouling), followed by the consequential growth and accumulation of biological material on the underwater hull and fittings (macrofouling). 

The accretion of biomass and overall percentage level of fouling on the hull can be gauged using what is termed the Floerl Index. This accrued fouling creates resultant hydrodynamic drag on the hull as the vessel moves through the water, with a consequential loss of efficiency in terms of overall speed and fuel consumption.

In order to achieve maximum hull efficiency and hence fuel savings by removing accumulated biofouling growth between  dry-dockings, some shipowners and operators have historically employed methods of IWHC using divers and associated fouling removal equipment at particular port locations.

Cleaning Technologies

Whilst there have been a variety of technologies put forward to achieve this such as those involving the application of hot water surface and cavitation treatment techniques, most IWHC systems employ the use of either high water pressure jets or revolving brushes contained within modules which can attach themselves to the hull by either magnetic force, thrust or applied negative pressure.

“Open circuit” systems allow the detritus to be directly passed into the local receiving waters at the point of removal from the hull. “Capture” systems have a facility to collect the debris laden water from each cleaning unit and send it ashore for filtration and other treatment via an umbilical.

Substantial fuel savings can be gained by the periodic removal of accumulated bio-fouling on both propeller and hull. Quoted values vary greatly and are stated as being in the region of six to 10 percent although higher figures are sometimes claimed. 

Whilst it is acknowledged that there will be tangible fuel savings attained by the application of regular hull cleaning, the algorithms and calculations to quantify these are not definitive due to the varying dynamics that a vessel will experience both pre and post clean. Length of voyage and the effects of wind, tide and sea state all introduce variables which may affect quantifying the overall efficiency of a hull cleaning process with any precision. 

Work to try and more accurately assess the realized hull efficiency values is ongoing and uses a variety of methodologies including applied resistance modeling techniques and empirical fleet performance data.

Dave Smith is a Fellow at Plymouth Marine Laboratory.
 

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.