Marine Tank Cleaning/Drying   In marine shipping of bulk goods and chemicals, turn-around-time at dockside depends, in part, upon the speed of cleaning and purging of the hold.  Ship schedules for loading, unloading, cleaning and drying are critical.  In many instances, the cleaning and drying process takes place while the vessel is in transit.  Reduction in the time spent on this activity has a direct effect on reducing the down time and lost operation cost of the vessel.  In times of oversupply of vessels the speed of servicing these vessels is even more crutial.   The basic Cleaning Process utilizing washing procedures varies depending upon the cargo last carried.  Methods include cold water washing, hot water washing, solvent application, detergent in water application and various combinations of the above.  These procedures apply to only tanks and holds that carry minimal hazardous cargoes and can be entered by personnel between shipments.   The removal of cleaning fluids from the holds involves pumping to remove gross amounts and then vacuum stripping to remove surface water.  This leaves a damp surface with some water adsorbed in the pores of the surface.  To resolve this problem the typical solution is to blow in ambient air.  This works well as long as the dew point of the ambient air is sufficiently lower than the surface temperature of the steel with the trapped water droplets.   However, when the ambient air is too humid (when the dew point is near or greater than the surface temperature) several methods are employed --- Wait until the dew point drops and then blow in the air, In vessels with double bottoms, fill the double bottom with steam to increase the temperature of the water and steel, In double bottom vessels, spray the interior surface with 82 C (180F) hot water, In vessels with cargo bearing coils, send steam to the holds. All of these methods create a very inhospitable situation if personnel must enter the tanks to inspect for water or other substances.  There is no other way to accomplish a thorough and accurate analysis of water content in a hold when the floor, walls, internal bracing, pipes, pumps, sumps and various nooks and crannies must be visually observed.   The three most common approaches to accomplishing cleaning with respect to schedules are --- Take vessels to a fixed base cleaning facility, Clean at dockside between off-and-on loading, Clean between off-and-on loadings during cruise. There are three phases of drying in a recently washed tank.  The first is the drying of puddles (of any size).  The second is damp surface drying where evaporation from the large surface area into the surrounding air.  The third and final is the removal of the humid air within the tank as it is displaced by dryer (dehumidified) air.   Since drying is best accomplished by contacting dry air with the wet surface, the problem becomes one of directing the dry air to the wet surface.  Typical tank configurations are such that the main mass of dry air passes through the center of the tank, far away from the walls.  The solution requires a mechanism to direct the air towards the surfaces either through a duct or by circulating the air in the tanks.  Condensate waste water from atmospheric air can be drained to a sanitary sewer.  Condensate from recycled air out of the hold may be deemed a hazardous waste and require special treatment.  The waste gases and humid air are vented directly into the atmosphere without treatment.   Energy consumption costs are not significant compared to the cost of delay in drying and cleaning of the hold.In order to speed up the drying process, dehumidification is an effective method of reducing the humidity of the ambient air forced into the cargo tanks and holds.  The rate of drying is increased by the number of air changes per hour, equipment should be sized to deliver no less than 3 air exchanges per hour to the largest hold.