OTM-7.15	Oil Tanker Manual OTM Tank Washing Atmospheres	Doc No.: OTM 7.15 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

1 TANK WASHING ATMOSPHERES

Non-inert

A non-inert atmosphere is one in which the oxygen content has not been confirmed to be less than 8% by volume. In recognition that tank washing and gas freeing operations in non-inert atmospheres are considered to present a likelihood of increased risk, additional control measures are required to reduce the risk of operations to as low as reasonably practicable. These controls must address two sides of the fire triangle namely – ‘Fuel’ and ‘Sources of Ignition’.

Non-inert cargo tank washing should only be undertaken when both the source of ignition and the flammability of the tank atmosphere are controlled. To achieve this, the following precautions to control ‘sources of ignition’ and ‘Fuel’ MUST be taken for tank washing operations in a non-inert atmosphere condition.

Guidance in this regard can be obtained from ISGOTT sec 11.3.5.2.

A flow chart showing steps to control the “fuel” while tank washing in the non-inert tank atmosphere method is included in the procedure.

Precautions when tank washing in an inert atmosphere

Although the atmosphere in a properly inerted tank is incapable of burning, the following precautions should be observed:

• When portable washing machines are used, all hose connections should be made up before the washing machine is introduced into the tank. Connections should not be broken until after the machine has been removed from the tank. However, to allow draining of a hose, a coupling may be partially opened and then re-tightened before the machine is removed.
• The tank should be kept drained during washing. Washing should be stopped to clear any build-up of wash water.

When there is a need to maintain an inert atmosphere during tank washing, the following points should be observed:

• The purity and pressure of the inert gas being delivered during the washing process should be monitored.
• Before each tank is washed, the oxygen level in the tank should be determined both at a point about 1 metre below the deck and at the middle level of the ullage space. At neither location should the oxygen level exceed 8% by volume.
• If during washing the oxygen level in the inert gas supply exceeds 8% by volume or the pressure of the atmosphere in the tank is no longer positive, washing should be stopped until satisfactory conditions are restored.

Some cargoes carried under an inert blanket are not flammable but the atmosphere is inerted for cargo quality reasons. In this case, gradual decay of the inert atmosphere during cleaning is acceptable. In all other cases, if the inert atmosphere is not maintained then the procedures used and precautions taken should be as for a non-inert atmosphere.

Precautions when tank washing in a non-inert / undefined atmosphere

Sometimes small tankers are not fitted with Inert Gas System. In case tank cleaning operation needs to be conducted in a non-inert or undefined atmosphere after carrying a flammable cargo, the atmosphere in an empty tank should be treated as flammable. The only way to guarantee that an explosion cannot occur during washing in a non-inert atmosphere is to make certain that there can be no source of ignition.

Non-inert cargo tank washing should only be undertaken when both the source of ignition and the flammability of the tank atmosphere are controlled. Reference must be made to ISGOTT Section 11.3.5.2 and/or TSG (C) 7.3.4. The following precautions to control 'sources of ignition' and 'fuel' MUST be taken for tank washing operations in a non-inert atmosphere condition.

Before Washing:

• The tank bottom should be flushed with water, using the main cargo pumps and lines and then stripped. Alternatively, permanent pipe work extending the full depth of the tank should be used. Tank washing machines shall not be used for bottom flushing. The piping system, including cargo pumps, crossovers and discharge lines, should also be flushed with water. The flushing water should be disposed of as required by Annex I or II of MARPOL 73/78 as applicable.
• The tank should be ventilated to reduce the gas concentration of the atmosphere to 10% or less of the Lower Flammable Limit (LFL). Gas tests must be made at various levels and due consideration should be given to the possible existence of pockets of flammable gas. In particular, in the vicinity of potential sources of ignition such as mechanical equipment that might generate hot spots, e.g., moving parts such as found in in-tank (submerged) cargo pump impellers. The gas freeing process must be arranged so as to minimize hazard due to dispersal of flammable and toxic vapours in the atmosphere.
• The venting of toxic or flammable gas during gas freeing should only be performed through the vessel’s approved gas freeing outlets.
• Any portable equipment used shall be intrinsically safe and tested before use.
• Only when the gas level within the tank has fallen to 30% LEL or less and below relevant TLV, can final ventilation be continued at deck level through cargo tank and Butterworth hatches.
• Only tanks being ventilated should have open hatches.
• All deckhouses, stores should be closed and air con must be kept in re-circulation.
• Gas freeing shall not be attempted if there is no wind when vessel is at anchor or has following wind when underway, unless when underway course can be altered to allow wind to pass across the deck.
• Tank washing may only commence once the tank atmosphere reaches 10% or less of the LFL.

During Washing:

• Atmosphere testing should be frequent and taken at various levels inside the tank during washing to monitor the change in LFL percentage. Consideration should be given to the possible effect of water on the efficiency of the gas measuring equipment and therefore to suspension of washing to take readings.
• Mechanical ventilation should, whenever possible, be continued during washing and to provide a free flow of air from one end of the tank to the other. The ability to mechanically ventilate concurrent with tank washing is recommended but, where mechanical ventilation is not possible, the monitoring of the tank atmosphere should be more frequent.
• The tank atmosphere should be maintained at a level not exceeding 35% LFL. Should the gas level reach 35% LFL at any measured location within a tank, tank washing operations in that individual tank MUST immediately cease. Washing may be resumed when continued ventilation has reduced and is able to maintain the gas concentration at 10% or less of the LFL.
• If the tank has a venting system that is common to other tanks, the tank must be isolated to prevent ingress of gas from other tanks.

To Control the 'Sources of Ignition' in The Tank:

No machine shall have a throughput greater than 60 m³/hr, and no nozzle shall have a throughput greater than 17.5 m³/hr. The total water throughput per tank shall be less than 110 m³/hr. Different washing methods give rise to differing risks and the following should be followed for tank washing in non-inert conditions:

• Re-circulated wash water must not be used.
• Heated wash water may be utilized, but use should be discontinued if the gas concentration reaches 35% of the LFL. A hot wash for a low flashpoint product should ONLY take place following a full (i.e. top to bottom) cold wash cycle. If the hot wash water temperature is above 60°C, monitoring of the gas concentration level should be at an increased frequency. Flammable chemical additives should not be used; any additives used should be on the list of IMO approved cleaning additives found in Annex 10 of MEPC.2/ Circ 18. Steam must never be injected into the tank when tank washing and MUST NOT be considered until the tank has been verified as gas free.
• The tank should be kept drained during washing. Washing should be stopped to clear any build-up of wash water.
• If portable washing machines are used, all hose connections should be made up and tested for electrical continuity before the washing machine is introduced into the tank. Portable washing machines should not be introduced into the tank until the LFL level is 10% or less. Connections should not be broken until after the machine has been removed from the tank. To drain the hose, a coupling may be partially opened (but not broken) and then re-tightened before the machine is removed.
• The introduction of sounding rods and other equipment into the tank should be made utilizing a full-depth sounding pipe. If a full-depth sounding pipe is not fitted, it is essential that any metallic components of the sounding rod or other equipment are bonded and securely earthed to the ship before introduction into the tank, and remain earthed until removed. This precaution should be observed during washing and for five hours thereafter to allow sufficient time for any mist carrying a static charge to dissipate. If, however, the tank is continuously mechanically ventilated after washing, this period can be reduced to one hour.

During this period:

• An interface detector of metallic construction may be used if earthed to the ship by means of a clamp or bolted metal connection.
• A metal rod may be used on the end of a metal tape if earthed to the ship by means of a clamp or bolted metal connection.
• A metal sounding rod suspended on a fibre rope should NOT be used, even if the end at deck level is fastened to the ship as a rope cannot be relied upon to provide an earthing path.
• Equipment made entirely of non-metallic materials may, in general, be used, for example a wooden sounding rod may be suspended on natural fibre rope without earthing.
• Ropes made of synthetic polymers should NOT be used for lowering equipment into cargo tanks.
• Measures should be taken to guard against ignition from mechanical defect of machinery, e.g. in-tank (submerged) cargo pumps, tank-washing machines, tank gauging equipment, etc.
• Precautions should be taken to eliminate the risk of mechanical sparks from metallic objects such as hand tools, sounding rods, sample buckets, etc., being dropped into the tank.
• The use of non-intrinsically safe equipment, such as torches, inspection lamps, mobile phones, communication radios, hand-held computers, etc., should NOT be allowed.