For a variety of reasons, ships rely on the seawater in which they move and one of the primary functions of seawater is to keep the engine and other sections of the ship cool while they are running. The utilized seawater is then discharged back into the sea, and additional seawater is taken in, resulting in a continual flow of salt water exchange between the ship and its surrounding aquatic environment. Despite its benefits, the negative is that seawater contains marine species (both macro and micro), some of which are visible to the naked eye and the majority of which are not. Marine organisms are deposited on the ship’s surface as seawater travels through the different pipelines and sections of the engine. Over time, the deposit accumulates, lowering the effectiveness of the ship’s systems and, in severe situations, choking the entire cooling system of the ship’s leading to the engine’s failure.
Biofouling is the accumulation of aquatic microorganisms, plants and animals on surfaces and structures immersed in or exposed to the aquatic environment. Aquatic organisms may be transferred to new locations and can be harmful and invasive in locations where they do not naturally occur. They can threaten marine environment, human, animal and plant life. Once invasive aquatic species are established in a new location or habitat, they are often impossible to eradicate.
The anti-biofouling system suppresses the growth of micro and macro-fouling on seawater immersed surfaces. The system is designed to safeguards the internal surfaces of enclosed seawater systems, such as pipework, valves, pumps, heat exchangers, and filters.
Marine biofouling and its associated corrosion can occur in two ways:
- Barnacles, mussels, hydroids, and other organisms can cause macro-fouling, which reduces water flow, blocks piping, and increases corrosion.
- Micro-fouling is made up of bacterial slime, micro-algae, and other organisms that impair the heat transfer efficiency of heat exchanger surfaces.
Corrosion can be accelerated in two main ways:
- When biofouling forms on a metal surface, the oxygen content underneath the fouling layer decreases. In areas of metal exposed to oxygenated sea water, this layer becomes anodic. As a result, pitting corrosion occurs beneath the fouling.
- Corrosion produced by bacterial activity. Sulphate reducing and iron bacteria are organisms that induce corrosion by their biological activities or metabolic byproducts. Such bacteria thrive in low-oxygen environments, such as behind a layer of aerobic fouling species or in deaerated water like that found in oil storage tanks and bilge wells.
There are different systems and methods, with different type of electrodes, which are working in pair and the anodes employed are marine growth (MG) type and trap corrosion (TC) type. In general, the system is based on the simultaneous electrolytic generation of copper ions and chlorine, which reduces the levels of existing macro- and microfouling organisms while preventing the growth of new ones. All existing and proposed environmental regulations are compliant with the copper and chlorine concentrations used.
The most used and known system onboard vessels is Cathelco, K.C. Ltd, Petreco etc.
The MG anodes are manufactured from copper and release ions during electrolysis which combine with those released from sea water form an unsuitable environment for entering organisms. The TC anodes are, usually, manufactured from aluminum for use in a system with predominantly steel pipes. The aluminum anodes in reaction with sea water will form aluminum hydroxide who will act as anti-corrosive barrier on the pipework. Lately, due cutting cost policy all over the places, the TC anodes are mixed metal oxide (MMO) type.
The copper and MMO electrodes are the anodes in the electrical circuit, which is set up between the strainer body and basket and these act as the cathode to the anodes. The current supply to the electrodes is controlled by thyristor current control modules, which are located in the control panel. The control panel also houses the supply transformer and surge suppression circuits, together with all instruments and controls.
A pulse dosing technique is used for ensuring the correct conditioning of the sea water. This is achieved by switching the power supply to the electrodes on and off using timers; typically the power supply is on for 3 minutes and then off for 3 minutes. The current supplied to each electrode is adjustable at the current setting potentiometer. The controller has facilities for operating the port or starboard electrodes or both sets of electrodes as necessary. Under normal circumstances the system is set for automatic operation and the current at each electrode is controlled by the controller.
The effectiveness of the system can only be evaluated through examination, and the manufacturers recommend that if after 6 months of operation, the opportunity to examine a strainer, length of pipe, or heat exchanger comes, that this to be done.
If there are symptoms of infestation, increase the current to each anode in the relevant marine suction filter by a maximum of 0.2 amps. If there is no evidence of an infestation, the current can be lowered by up to 0.2 amps. This routine should be repeated at regular intervals, with the current adjusted accordingly.
The anodes have a life expectancy, usually, of 2.5 years. Routine inspection of the anodes and sea strainers will show when replacement is required.
The procedure for operating the system is very simple and can resumed as follow:
- Check that the sea water system is operating and that sea water is flowing through the strainer, either the high or low strainer.
- Ensure that the selector switch is set to the port or starboard sea suction strainer as required, but it may also be set to the DUAL position if both sea suctions are open.
- Check if the main breaker switch is set to the ON position and that the POWER ON lamp is illuminated.
- Set the selector switch to the AUTO position.
- The protection system timer will start, usually, with the OFF period. When the timer enters the ON period, the indicated currents for the chlorine (MMO) electrodes and the copper electrode should be checked and adjusted if necessary, by means of their individual potentiometers. The timer may be set for off and on periods to suit conditions.
- If you want to switch off the system, after stopped ensure that the sea water flows through the strainer for a few minutes to ensure any copper, chlorine or hydrogen gas is removed from the vessel.
As mentioned the system is quite simple to operate and doesn’t require extensive maintenance if is operated correctly. There are some precautions that need to consider when operated by the responsible engineer and few of these, but not limited to, are mentioned below:
- Gases are released at the electrodes; chlorine at the anode and hydrogen at the cathode. It is essential that sea water is flowing over the electrodes whenever the electrodes are supplied with current, in order to flush these gases away. If the sea water pumps are switched off the power supply to the electrodes must also be switched off. If this is not the case the gases can build up resulting in an explosion.
- The copper electrode and the chlorine (MMO) electrode currents must be set to suit water flow which is influenced by the sea water temperature as a higher sea temperature requires a higher water flow to give the correct cooling capacity.
- The current settings should be set according to the sea water flow for optimum performance and efficiency. The setting pro rata is given on a table supplied by a manufacturer or shipyard.
- Care must be taken to ensure that the current at the electrodes is set correctly for the water flow rate. Excessive current will result in rapid wear of the electrodes. Too high a current at the copper electrodes will cause heavy deposits on the sea suction filter element resulting in flow restriction and even greater deposits. The higher the current the shorter the anode life.
- If the vessel is in fresh or brackish water, the system should be switched off as missing sea water will make it inoperable and the control system may be unable to reach the recommended current setting causing the alarm to activate.
- During maintenance, before opening the sea chest strainer, make sure that the system is switched off and that the strainer casing is pressure less and water is drained. It happens very often that the sea chest valves are not properly holding, thus making nearly impossible to open the filter and posing a very high risk of flooding if is not handled correctly.
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