The bio-fouling sequence
Barnacles and mussels breed throughout the year in tropical waters. In temperate regions, the breeding season is concentrated in the spring and autumn.
The settlement period for larvae lasts up to 8 hours. During this time, they are particularly sensitive to the presence of copper ions.
Without bio-fouling protection, mussels enter the pipework system as larvae and attach themselves to the internal surfaces of pipes where they grow and multiply. This causes a gradual restriction in seawater flow and in more serious cases sections of pipework can be completely blocked and sea valves made inoperable. Blockages can also occur in heat exchangers which seriously reduces their efficiency. As a result, engine cooling systems run at abnormally high temperatures, leading to increased fuel usage which can have a major impact on the running costs of the vessel.
Significant maintenance costs are also involved where complete sections of pipework have to be manually cleaned or replaced, resulting in extended drydocking periods and delays in returning the ship to service.
In addition, the safety of ships can be jeopardised by growth in the pipework systems of fire fighting pumps.
It is estimated that the cost of installing the Cathelco system is recovered in 2-3 years of operation in terms of savings in maintenance costs.
Types of biofouling
Biofouling consists of two separate, but closely related elements ;
- Microfouling comprises bacteria and algae such as those seen on heat exchange surfaces.
- Macrofouling organisms include mussels, barnacles and other soft or hard fouling.
Biofouling causes blockages of pipes and valves, a decrease in the efficiency of heat exchange surfaces, increased fluid frictional resistance resulting in increased back pressure and accelerated corrosion. Fouling also interferes with other processes such as filtration and reverse osmosis.
Biofouling and corrosion
Corrosion can be accelerated in two ways.
The presence of biofouling on a metal surface results in a decreased oxygen concentration below the fouling layer. This area becomes anodic with respect to areas of the metal surface exposed to oxygenated seawater. The result is pitting corrosion under the fouling. An additional problem is the corrosion caused by the action of bacteria. Sulphate reducing and iron bacteria are well known examples of organisms whose biological activity or metabolic by-products cause corrosion. Such bacteria live in areas of low oxygen concentrations, for instance under a layer of aerobic fouling organisms, or in de-aerated water such as that found in oil storage tanks and well flood water. Efficient fouling control results in reduced pitting and crevice corrosion.
Most Cathelco systems have dual action, eliminating bio-fouling and suppressing corrosion. In the case of steel pipework the corrosion suppression anodes are made from aluminium, whereas cupro-nickel pipework is protected against corrosion with ferrous anodes. When these are fitted, corrosion rates are significantly reduced resulting in a longer working for the pipework and any other equipment which is connected to the seawater system.