Figure 7.15 Inerting of cargo tanks
In the past, some grade-changing operations involved the replacement of existing tank vapours with the vapours of the next cargo to be loaded. However, this is now seldom carried out. As shown in Table 2.3(b), this method can only ever be appropriate when switching to compatible grades and when air is not to be introduced into the tank.
Once the cargo system has been satisfactorily freed of liquid and warmed up, inerting operations may start. This involves the replacement of the vapour atmosphere with inert gas or nitrogen. The need of inerting will depend on:—
• A desire to gain tank entry for inspection
• Last cargo
• Next cargo
• Charter party terms
• Requirements of the loading terminal
• Requirements of the receiving terminal, and
• Permissable cargo admixture
Where tanks must be opened for internal inspection, inerting is always necessary. This is to reduce the hydrocarbon content within tank atmospheres to the safe level required before blowing through with fresh air. This safe level will correspond to a point below the critical dilution line (see Figure 2.19) as found on a graph for the product in question. The procedure for inerting after cargo discharge is similar to that described in 7.2.3. During inerting operations, when venting to atmosphere, care must be taken to safeguard personnel and to ensure the absence of any source of ignition.
Figure 7.15 shows how a cargo pipeline system may be set for inerting cargo tanks when using an inert gas generator on board. This diagram shows hydrocarbon gas being returned to the shore but during the operation it is often the case that the gases to be exhausted are directed to the forward vent riser.
7.9.4 Aerating
After the foregoing procedures have been addressed, the cargo tanks can be ventilated with air. The air is supplied using compressors or air blowers and air dryers in the inert gas plant. This should continue until the oxygen content of the whole tank is at 21 per cent and hydrocarbon levels are at the zero percentage of the Lower Flammable Limit. In order to ensure uniformity in the tank atmosphere, various levels and positions in the tank should be monitored prior to tank entry. Figure 7.16 shows a pipeline set up for aerating tanks.
It is important to note that ventilation with air should only take place once the ship's tanks are warmed to ambient conditions. If the tank is still cold when air is allowed inside, any moisture in the air will condense on tank surfaces. This can cause serious problems when preparing the tank for new cargoes. If condensation is allowed to form, its removal can be a protracted and costly operation.
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As covered in 2.5, aeration should continue not only until oxygen levels are satisfactory but also until safe levels of carbon monoxide are established.
Figure 7.16 Aeration of cargo tanks
7.9.5 Ammonia — special procedures
Certain cargoes present particular difficulties when trying to remove all traces of the product. Ammonia is one such case. When a ship is switching from ammonia to LPG, virtually all traces of vapours must be removed from the system. Prior to loading the next cargo, an allowable concentration of ammonia vapour in a tank atmosphere is usually quoted at less than 20 parts per million by volume. This results in a time consuming operation which is covered in more detail below.
The first operation when switching from ammonia is to remove all liquid ammonia from the system. This is important as ammonia, when evaporating to air, is particularly likely to reach super-cooled conditions. Therefore, unless all liquid is removed, dangerously low liquid temperatures can result and tank fractures could ensue. Confirmation that all liquid has been removed can be established, during warming-up, by carefully observing tank temperature read-outs.
Once cargo tank temperatures have been warmed to substantially above the dew point of the air, the ammonia vapours are usually dispersed by blowing warm fresh air through the system. (For ammonia the inert gas plant must not be used due to the formation of ammonia carbamates when ammonia is in contact with carbon dioxide.) The continued use of warm dry air should avoid water vapour condensation, thus limiting the seepage of ammonia into porous tank surfaces. The ventilation of tanks and the cargo system at the highest practical temperature is advantageous as this encourages release of ammonia from rusty surfaces. (Ammonia is released ten times faster at 45°C than at 0°C).
Washing with fresh water to remove ammonia is sometimes carried out. This can be most effective as ammonia is highly water soluble. However, the following points should be noted:
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• The benefit of water washing is limited to certain types of tank. (This technique is not always practical for large fully refrigerated ships with prismatic tanks.)
• When switching from ammonia to LPG, water can hold ammonia in solution and this can be a contaminant for future cargoes. Accordingly, water washing is only recommended for cargo tanks which are completely clean, rust-free and have minimum internal structure, so allowing full and effective drainage.
• All traces of water must be removed at the end of washing to stop the formation of ice or hydrates.
• The high solubility of ammonia in water (300:1) can lead to dangerous vacuum conditions being created within a tank. It is, therefore, essential to ensure adequate air entry into the cargo tank during the water washing process.
After water washing, it is essential that all water residues are removed using either fixed or portable pumps. Subsequently, tanks and pipelines must be thoroughly dried before further preparations for cargo loading are made. In order to maintain maximum dryness, it is important to continue ventilation of the tanks using air with a dew point lower than the tank atmosphere for the reasons discussed above.
7.10 SHIP-TO-SHIP TRANSFER
In recent years the transfer of LPG cargoes from one ship to another has become a common practice in many areas where there is insufficient terminal infrastructure. Detailed recommendations for the safe conduct of such operations are given in the Ship-to-Ship Transfer Guide (Liquefied Gases) (see Reference 2.3). Before any such operations are arranged, it is recommended that this publication be consulted and its procedures be adopted.
Most ship-to-ship transfer operations involve LPG, but there have been a few instances where LNG has been transferred from ship to ship with complete success. However, these instances have been casualty operations occasioned by the disablement of one of the ships. For these transfers of LNG hoses of a composite type were used (see 5.1.1).
Despite a number of studies being carried out, no attempt has yet been made to proceed with projects based on routine LNG ship-to-ship transfer. Due to the special risks inherent to the low temperature of LNG and to the large size of ships involved, such an operation would require extensive detailed preparation. This should include comprehensive studies of the proposed location, the prevailing weather, the ship's movements and the special transfer and mooring equipment to be installed.
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7.11 CONCLUSION
This completes the cycle of gas carrier operations. It is important for every ship to have its own detailed operational procedures clearly listed. What can be done on one ship may not be possible or even desirable on another. However, the basic principles of cargo handling for liquefied gas remain the same for all gas carriers. A safe operation is invariably also an efficient operation and, if in doubt about the safety of any operation, ship's personnel and terminal staff are recommended to seek further advice.
Chapter 8
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