Cool-down — refrigerated ship
Cooling down is necessary to avoid excessive tank pressures (due to flash evaporation) during bulk loading. Cool-down consists of spraying cargo liquid into a tank at a slow rate. The lower the cargo carriage temperature, the more important the cool down procedure becomes.
Before loading a refrigerated cargo, ship's tanks must be cooled down slowly in order to minimise thermal stresses. The rate at which a cargo tank can be cooled, without creating high thermal stress, depends on the design of the containment system and is typically 10°C per hour. Reference should always be made to the ship's operating manual to determine the allowable cool-down rate.
The normal cool-down procedure takes the following form. Cargo liquid from shore (or from deck storage) is gradually introduced into the tanks either through spray lines, if fitted for this purpose, or via the cargo loading lines. The vapours produced by rapid evaporation may be taken ashore or handled in the ship's reliquefaction plant. Additional liquid is then introduced at a rate depending upon tank pressures and temperatures. If the vapour boil-off is being handled in the ship's reliquefaction plant, difficulties may be experienced with incondensibles, such as nitrogen, remaining from the inert gas. A close watch should be kept on compressor discharge temperatures
Figure 7.4 Cargo tank cool-down using liquid from shore: vapour returned to shore
and the incondensible gases should be vented from the top of the condenser as required (see 7.6).
As the cargo containment system cools down, the thermal contraction of the tank combined with the drop in temperature around it tend to cause a pressure drop in the hold and interbarrier spaces. Normally, pressure control systems supplying air or inert gas will maintain these spaces at suitable pressures but a watch should be kept on appropriate instruments as the cool-down proceeds.
Cool-down should continue until boil-off eases and liquid begins to form in the bottom of the cargo tanks. This can be seen from temperature sensors. At this stage, for fully refrigerated ammonia for example, the pool of liquid formed will be at approximately -34°C while the top of the tank may still be at -14°C. This gives a temperature difference of 20°C. The actual temperature difference depends on the size of the cargo tank and the spray nozzles positions.
Difficulties that may occur during cool-down can result from inadequate gassing-up (too much inert gas remaining) or from inadequate drying. In this latter case, ice or hydrates may form and ice-up valves and pump shafts. In such cases, antifreeze can be added, provided the cargo is not put off specification, or the addition will not damage the electrical insulation of a submerged cargo pump. Throughout the cool-down, deepwell pump shafts should be turned frequently by hand to prevent the pumps from freezing up.
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Once the cargo tanks have been cooled down, cargo pipelines and equipment should be cooled down. Figure 7.4 shows the pipeline arrangement for tank cool-down using liquid supplied from the shore.
Cool-down — semi-pressurised ships
Most semi-pressurised ships have cargo tanks constructed of steels suitable for the minimum temperature of fully refrigerated cargoes. However, care must be taken to avoid subjecting the steel to lower temperatures. It is necessary to maintain a pressure within the cargo tank at least equal to the saturated vapour pressure corresponding to the minimum allowable steel temperature. This can be done by passing the liquid through the cargo vaporiser and introducing vapour into the tank with the cargo compressor. Alternatively, vapour can be provided from the shore.
7.5 LOADING
7.5.1 Loading — preliminary procedures
Before loading operations begin, the pre-operational ship/shore procedures must be thoroughly discussed and carried out. Appropriate information exchange is required and the relevant parts of the Ship/Shore Safety Check List should be completed (see also 5.3.2, 6.4, 6.5 and 10.5). Particular attention should be paid to:
• The setting of cargo tank relief valves and high alarm pressures
• Remotely operated valves
• Reliquefaction equipment
• Gas detection systems
• Alarms and controls, and
• The maximum loading rate
This should all be carried out taking into account restrictions in ship/shore systems.
The terminal should provide the necessary information on the cargo, including inhibitor certificates where inhibited cargoes are loaded (see 2.6). Any other special precautions for specific cargoes should be made known to ship personnel. This may include the lower compressor discharge temperatures required for some chemical gas cargoes (see 7.6). Where fitted, variable setting pressure relief valves, high tank pressure alarms and gas detection sample valves should be correctly set.
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The ballast system for gas carriers is totally independent of the cargo system, as outlined in 3.5. Deballasting can, therefore, take place simultaneously with loading, subject to local regulations. Ship stability and stress are of primary importance during loading. Procedures for these matters are in accordance with normal tanker practice.
The ship's seagoing safety
Trim, stability and stress
The cargo plan should allow for distribution within the ship in order to achieve acceptable structural stress and the required ship trim to meet safe stability conditions when at sea. For these purposes, the weight of the cargo in each tank will need to be known. For ship stability purposes, the weight in question is the true weight-in-air.
As will be seen from the procedures discussed in Chapter Eight, the weight-in-air of liquefied gases, calculated for cargo custody purposes, is not exact in that the cargo vapour in these calculations is assumed to be liquid of the same mass as the vapour.
Thus, the air buoyancy of the cargo vapour spaces has been neglected. However, for practical purposes concerning a ship's stability calculation, this may be ignored.
All gas carriers, as part of the statutory requirements, are provided with stability data, including worked examples showing cargo loaded in a variety of ways. In conjunction with consumables such as fresh water, spare parts and bunkers on board, these conditions provide cargo storage guidelines to ship's officers in order to maintain the ship in a safe and stable condition. Additionally, as part of the requirements to obtain a Certificate of Fitness in compliance with the Gas Codes, the stability conditions must be such that, in specified damaged conditions, the ship will meet certain survival requirements. It is, therefore, essential that all relevant guidance concerning the filling of cargo tanks be observed.
Sloshing
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A further point to be noted in respect of tank filling levels is that, large prismatic cargo tanks, due to their width and shape, may suffer from substantial sloshing of cargo in heavy pitching or rolling conditions. Such tanks, and particularly membrane-type tanks which have no centre line wash bulkheads, may have prohibited filling levels in order to avoid damage to tank structures or internal fittings. Typical controls on such tanks are a prohibition on all filling levels in the 10 to 80 per cent range.
If an unusual cargo distribution is requested and if this involves cargo tanks only being part-filled, then it is usual for the shipmaster to seek further guidance from shipowners. In such cases it is sometimes necessary for the owner to seek confirmation from the ship's classification society before loading can start.
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