Thursday, July 30, 2009
Criteria for laying up vessels
Thursday, 30 July 2009
IN the current economic conditions, many ship owners are laying up their vessels to avoid unprofitable sailing and to reduce running costs. Some market estimates indicate that as many as 5,000 ships may be withdrawn from service. Given the uncertainties, some owners may prefer a hot lay-up for three months or more, where the machinery on board is still kept in operation and the ship can be promptly re-commissioned, possibly within 24 hours.
However, this is on condition that the ship meets all essential criteria, including whether the fuels kept on board are fit for use.
Marine fuels stored on the ship are vulnerable to quality changes due to storage conditions and handling methods. Polymeric wax, known to cause severe filter clogging, may precipitate out of residual fuel oil, or the fuel could turn into sludge if not stored properly. Either occurrence will mean a huge clean-up bill for the owner during the re-commission.
Off-loading residual fuel oil before lay-up could be the safest option. But as debunkering is an expensive affair and the off-loaded fuel is itself a costly commodity, keeping residual fuel on board during a hot lay-up is the next best option.
Today's residual fuel oils are not comparable in quality to the straight-run fuels of the past. They are the bottom-most products of complex refining processes, blended with distillate cutter stocks to achieve the required viscosity.
Stability is among the most challenging fuel properties to manage. Losing stability means sludge formation, and there is no clear explanation for this occurrence. Although we cannot guarantee that a fuel will not lose its stability during storage, the risk of this happening can be predicted through advanced analysis by experts.
Taking certain practical precautions can also help prevent fuel-related problems. For instance, leaky steam valves in the tanks can cause the stored fuel to be excessively heated. The lighter ends in the fuel in turn vaporise into the atmosphere, increasing both the density and viscosity of the fuel beyond its separation or injection capacity. Constant monitoring of fuel temperature can alert the ship crew to leaking steam valves needing repair.
Attention should also be paid to the potential development of microbes such as bacteria, yeast and fungi in the fuel as these organisms that clog filters, pipes and valves, and also corrode tanks and hull plating.
The presence of water, which may settle at tank bottoms, is an important condition for microbes to thrive. They live in the water phase of the fuel-water interface but draw nutrients from the oil phase.
Unlike fresh water from condensation or leaky steam coils, salt water from the open sea is not likely to promote microbial growth, but it may cause emulsification of oil and water beyond treatment.
Sounding tank level on a daily basis can help the ship crew monitor any water ingress. Draining of free water from tanks where valves are fitted should be done regularly to pre-empt the development of microbes.
Where microbial growth is suspected, a sample should be sent to an accredited testing laboratory for basic quality and microbiological contamination checks.
In addition to water, sediments and abrasive particles settled on tank bottoms may damage engine components if allowed to accumulate over time. Recirculation of fuels at regular intervals is important to prevent such a build-up and to stop the particles from passing over into the engine during the vessel's recommission.
When laying up a ship, the owner should test the quality of the fuel stored in the tanks. The analysis of a tank sample taken prior to the lay-up can, for example, serve as a reference to determine fuel suitability during re-commissioning.
Residual fuel should ideally not be kept on board during a cold or long-term lay-up. However, if the lay-up is in a tropical area, off-loading the fuel may be avoided after careful consideration of the overall fuel quality and pour point, temperature of the sea water and other critical factors.
Finally, before the ship is put back in operation, representative samples of all fuels kept on board (both residual and distillate) should be tested so as to verify that the fuels are fit for use.
Source: Business Times Singapore
