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PART V
APPLICATION TO CARGO TANK OPERATION

The inert gas system should be used during the full cycle of tanker operation as described in this section.

Inerting of Tanks

30. (1) Tanks that have been cleaned and gas freed should be re-inerted according to manufacturers instructions, preferably during the ballast voyage, to allow the inert gas system to be fully tested prior to cargo handling; purge pipes/vents should be opened to atmosphere; when the oxygen concentration of the atmosphere in the tank has fallen below 8 per cent, the purge pipes/vents should be closed and the tank pressurized with inert gas.

(2) During the re-inerting of a tank following a breakdown and repair of the inert gas system, non-gas-free and non-inerted tanks should be re-inerted in accordance with subsection (1); during inerting, no ullaging, dipping, sampling or other equipment should be inserted, unless it has been established that the tank is inert; this should be done by monitoring the efflux gas from the tank being inerted until the oxygen content is less than 8 per cent by volume; this procedure should continue for as long as determined by previous test records when inerting gas-free tanks to ensure that the efflux gas is fully representative of the atmosphere within the tank.

(3) When all tanks have been inerted, they should be kept common with the inert gas main and maintained at a positive pressure in excess of 100 millimetres water gauge during the rest of the cycle of operation.

Discharge of Water Ballast

31. (1) Before discharge of cargo tank ballast is undertaken, the following conditions as given in subsection (2) through (7) should be checked:

(2) All cargo tanks are connected to the inert gas system and all isolating valves in the deck inert gas pipework are locked open.

(3) All other cargo tank openings are shut.

(4) All valves isolating the mast risers from the inert gas system are shut.

(5) The arrangements required by the Fire Protection, Detection and Extinguishing Equipment Regulations, subsection 13(d) and paragraph 13(d)(i), Schedule VII, are used to isolate the cargo main from the inert gas main.

(6) The inert gas plant is producing gas of an acceptable quality.

(7) The deck isolating valve is open.

(8) During the deballasting operation, the oxygen content of the inert gas and its pressure in the inert gas main should be continuously recorded.

Loading

32. When cargo is being loaded, the deck isolating valve should be closed and the inert gas plant may be shut down, unless other cargo tanks are being deballasted simultaneously in which case a close watch should be kept on the inert gas main pressure; all openings to the cargo tanks, except the connections to the mast risers or equivalent venting arrangements, should be kept closed to minimize flammable vapour on deck; before loading commences, the flame screens in the mast risers, or equivalent venting arrangements, should be inspected and any stop valves isolating the cargo tanks from the inert gas main locked in the open position.

Loaded Condition

33. (1) During the loaded passage, a positive pressure of inert gas of at least 100 mm water gauge should be maintained in the cargo tanks and topping up of the pressure may be necessary; when the inert gas pressure is being topped up in the cargo tanks, particular attention should be paid to obtaining an oxygen concentration of 5 per cent, or less, in the inert gas supply before the gas is introduced into the cargo tanks.

(2) On motor tankers, the boiler loading may have to be increased to achieve the low oxygen concentration in the inert gas supply; it may also be necessary to restrict the output of the inert gas blowers to prevent air being drawn down the uptake during the topping up operation; if by these means inert gas of the quality defined in subsection (1) cannot be achieved, then inert gas from an alternate source of supply, such as an inert gas generator, might be used.

Cargo Transfer and Cargo Sampling

34. (1) Ullaging devices of the closed type should be used to avoid the opening of ullage ports.

(2) However, it may be necessary to infrequently relieve the inert gas pressure in the cargo tanks on certain occasions to permit manual tank gauging or cargo sampling before or after cargo is transferred, but during this time, no cargo or ballasting operation is to be undertaken, and a minimum number of small tank openings are to be uncovered for as short a time as necessary to permit these measurements.

(3) Manual gauging or cargo sampling may be performed during the following four periods:

1. at the loading port, prior to cargo loading;
2. at the loading port, after cargo loading;
3. at the discharge port, prior to cargo discharge; and
4. at the discharge port, after cargo discharge.

(4) The tanks should then be re-pressurized immediately after the measurements or cargo samples have been taken.

(5) If the tank is opened prior to cargo transfer, cargo transfer, cargo transfer should not be commenced until all the conditions have been checked and are in order; similarly, if the tank is opened after cargo transfer, normal ship operations should not be commenced until all the conditions have been checked and are in order.

(6) During cargo transfer the oxygen content and pressure of the inert gas main should be continuously recorded.

Crude Oil Washing

35. (1) A crude oil washing system fitted in an oil tanker shall comply with the relevant requirements of the "Oil Pollution Prevention Regulations", Chapter 2 - Oil.

(2) Before each tank is crude oil washed, the oxygen level shall be determined at a point 1 metre below the deck and at the middle region of the ullage space; neither of these determinations shall exceed 8 per cent by volume; where tanks have complete or partial wash bulkhead, the determination should be made at similar levels in each section of the tank; the oxygen content and pressure of the inert gas being delivered during the washing process should be continuously recorded.

(3) If, during the crude oil washing,

1. the oxygen level of the inert gas being delivered exceeds 8 per cent by volume; or
2. the pressure of the atmosphere in the tanks is no longer positive;

then washing must be stopped until satisfactory conditions are restored; operators should also be guided by paragraph 29(5)(b).

Ballasting of Cargo Tanks

36. The conditions for ballasting of cargo tanks are the same as those for loading in section 32; when, however, simultaneous be kept on the inert gas main pressure.

Ballast Condition

37. (1) During a ballast voyage, tanks other than those required to be gas free for necessary tank entry should be kept inerted with the cargo tank atmosphere at a positive pressure of not less than 100 mm water gauge; the oxygen level should not exceed 8 per cent by volume especially during tank cleaning.

(2) Before any inert gas is introduced into cargo tanks to maintain a positive pressure, it should be established that the inert gas contains not more than 5 per cent oxygen by volume.

Tank Cleaning

38. Cargo tanks should be washed in the inert condition and under a positive pressure; the procedures adopted for tank cleaning with water should follow those for crude oil washing in section 35.

Purging prior to Gas-freeing

39. When a tank is to be gas freed after washing, the concentration of hydrocarbon vapour should be reduced by purging the inerted cargo tank with inert gas; purge pipes/vents should be opened to atmosphere and inert gas introduced into the tank until the hydrocarbon vapour concentration measured in the efflux gas has been reduced to 2 per cent by volume, and until enough time, as determined by previous tests on cargo tanks, has elapsed to ensure that readings have stabilized and the efflux gas is representative of the atmosphere within the tank.

Gas-Freeing

40. (1) Gas-freeing of cargo tanks should only be carried out when tank entry is necessary (e.g. for essential repairs); it should not be started until it is established that a flammable atmosphere in the tank will not be created as a result; hydrocarbon gases should be purged from the tank (see section 39).

(2) Gas-freeing may be affected by pneumatically, hydraulically or steam-driven portable blowers, or by fixed equipment; in either case, it is necessary to isolate the appropriate tanks to prevent the leakage of air into inerted tanks, or of inert gas into tanks that are being gas-freed.

(3) Gas-freeing should continue until the entire tank has an oxygen content of 21 per cent by volume, and a reading is obtained on a combustible gas indicator of less than 1 per cent of lower flammable limit.

Tank Entry

41. (1) The entry of personnel to the cargo tank should be carried out only under the close supervision of a responsible ship’s officer, in accordance with national rules and/or with the normal industrial practice laid down in the International Safety Guide for Oil Tankers and Terminals; the particular hazards encountered in tanks previously inerted and then gas-freed are outlined in 9.2.8, 9.3.3 and Chapter 10 of that Guide.

(2) Practical precautions to meet these hazards include:

1. securing the inert gas branch line gas valves and/or blanks in position, or, if gas-freeing is being done with the inert gas blower, isolating the scrubber from the flue gases;
   
2. closing of any drain lines entering the tank from the inert gas main;
   
3. securing relevant cargo line valves or controls in the closed position;
   
4. keeping the inert gas deck pressure in the remainder of the cargo tank system at a low positive pressure, such as 200 mm water gauge; this minimizes the possible leakage of inert or hydrocarbon gas from other tanks through any bulkhead cracks, cargo lines, valves, etc;
   
5. lowering clean sample lines deep into the lower regions of the tank in at least two locations; these locations should be away from both the inlet and outlet openings used for gas-freeing; after it has been ascertained that a true bottom sample is being obtained, the following readings are required
   
   1. 21 per cent on a portable oxygen analyser, and
   2. less than 1 per cent of lower flammable limit on a combustible gas indicator;


6. using breathing apparatus whenever there is any doubt about the tank being gas free, e.g. in tanks where it is not possible to sample remote locations; (this practice should be continued until all areas, checked);
   
7. continuously ventilating and regularly sampling the tank atmosphere whenever personnel are in the tank;
   
8. carefully observing normal regulations for tank entry.

Re-inerting after Tank Entry

42. (1) Once all personnel have left the tank and the equipment has been removed, the inert gas branch line blank, if fitted, should be removed, the hatch lids closed and the gas pressure regulating valve re-opened and locked open to the inert gas main, where appropriate; this will avoid any risk of structural damage when liquids are subsequently handled.

(2) As soon as a gas-free tank is reconnected to the inert gas main, it should be re-inerted (as described in section 30) to prevent transfer of air to other tanks.

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