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The job of entering, inspecting and cleaning ship's cargo, ballast, and fuel tanks, void spaces and cofferdams is essentially one of confined space entry. The five basic requirements for safe confined space entry are:

  • The space has adequate oxygen.

  • The space of free of toxic vapours or substances.

  • The tank entry personnel are properly trained.

  • A written procedure is being followed (checklist used).

  • Means of rescue and trained rescue personnel are immediately available.
  • The following procedures refer primarily to cargo tank entry, but the precautions apply to all confined space entry activities on board a tanker. All spaces which are considered 'confined spaces' for manned entry must be prominently labelled as such at each point of access.

    2.15.1 Tank ventilation
    When tank washing is completed, the cargo tank will contain an atmosphere which is not safe for men. If the tank has been ventilated during washing, to maintain a too-lean atmosphere, it may still contain more hydrocarbon vapours than are permitted for 'bare-face' entry. If the tank has been inerted during washing, it will have both insufficient oxygen and excessive hydrocarbon vapours for entry.
    Tanks which have been ventilated during washing are easier to prepare for entry. Ventilation is continued after washing is completed. The ventilation should be arranged so that the an efficient exchange of tank atmosphere is achieved.

    Ventilation by displacement
    The most effective method of ventilating a tank is by extracting the heavier hydrocarbon vapours from the bottom of the tank, while allowing fresh air to enter from the top, displacing the tank atmosphere.
    This method can be used with either fixed or portable equipment as indicated. The advantage of the displacement method is that only a little more than one air exchange is required to ventilate the tank, saving both time and energy. Because it achieves the quickest results, the displacement method is also the safest, since the tank atmosphere will be in the explosive range for the shortest amount of time.

    Ventilation by displacement:
    The heavier tank atmosphere is extracted from the bottom of the tank - allowing fresh air to flow in from the top. A single air exchange usually clears the tank of inert gas or hydrocarbon vapour.

    Ventilation by dilution
    The alternative method of ventilation (and the one most commonly used on smaller tankers), involves blowing air into the top of the tank at one end and forcing the tank atmosphere out a deck opening at the other end of the tank. To be effective, an extension trunk must be used so that the air enters the tank at the bottom. The old (hazardous) atmosphere is removed by dilution. The corners of the tank most distant from the ventilation fan will take a considerable amount of time to reach a safe condition. If the tank is large, with deep bottom framing, the framing bays distant from the fan may retain dangerous pockets of hydrocarbon vapour after the remainder of the tank passes the test for safe tank entry.
    The least efficient method of tank ventilation involves blowing air into the top of the tank without extension trunking. Dilution is very slow with this method and dangerous gas pockets are likely to remain in the distant corners of the tank. This method should be used for small tanks only.

    Ventilation by dilution:
    Turbulent air flow in the tank requires several air changes before the tank atmosphere is diluted to a safe level.

    The atmosphere leaving the tank must exit as an unimpeded vertical flow. If air is being forced out of a tanklid, the tanklid should be fully open. Tank atmospheres should not be exhausted under a catwalk or other structure which would promote accumulations of dangerous vapour at deck level. Generally, a relative wind of more than ten knots will prevent any accumulation of vapours, but care must be taken when ventilating tanks near the superstructure if a relative wind from astern may produce a lee where the ventilation is taking place. Sections 16.4 and 16.5 of the International safety guide for oil tankers and terminals (ISGOTT), provides excellent guidance for tank venting.

    IGS system ventilation
    Inerted tanks can be ventilated in the same manner as non-inerted tanks, but doing so will at some point produce an explosive atmosphere within the tank. This situation can be avoided if, after washing, the tank is first purged with inert gas to remove hydrocarbon vapours. With the hydrocarbon vapour level below 2% (by volume), the tank may then be ventilated with outside air without the tank atmosphere passing through the explosive range. (Note that a conventional Combustible Gas Indicator cannot be used to measure hydrocarbon levels in an inerted tank.)
    It must be remembered that while purging with IGS, an explosive atmosphere can accumulate on the open deck of the ship. In addition, IGS may contain nitrogen dioxide, sulphur dioxide and carbon monoxide, all of which are toxic. It is important that the precautions regarding relative wind speed and direction are applied to the IGS purging.

    If the tank atmosphere measurements show 9% hydrocarbon gas and 6% oxygen (point X) and the inert gas has 3% oxygen and (of course) 0% hydrocarbon gas (point Y), then the tank atmosphere will change along line XY as purging progresses.
    Consequently we can estimate the change of the tank atmosphere while purging and can stop the purging when the atmosphere is below the critical dilution line and then gas freeing can start. In the example if the purging was stopped at point Z (3% hydrocarbon gas and 4% oxygen) then during gas freeing of the tank the atmosphere will change along line ZE to 21% oxygen and 0% hydrocarbon gas and always be outside of the explosive range.

    When ventilating a cargo tank, tank openings of adjacent tanks should be closed to prevent entry by hydrocarbon vapours from the tank being ventilated. All accommodation openings should be closed and windward side only used for access.

    2.15.2 Testing tank atmospheres
    When the tank has been ventilated sufficiently to provide a safe atmosphere for safe bare face entry, it must be tested using a combustible gas indicator (CGI) and oxygen meter. The results of such a test will be valid only if:

  • The test equipment is properly calibrated.

  • The operator is properly trained.

  • Correct test procedures are used.

  • The gas indicator is suitable for testing the atmosphere which is being sampled.
  • The Combustible Gas Indicator (or 'Explosimeter'), should be calibrated before each ballast voyage. To do this efficiently, the ship must have a calibration/test kit suitable for the cargoes it carries and an officer properly trained in the calibration procedure. The test equipment must have the appropriate scales to indicate safe entry levels. Two indicators should be carried, so that a spare is available in the event one is damaged. If a calibration kit is not provided, then the ship will need three indicators, one for immediate use, one as a spare and one ashore being professionally calibrated. If a calibration kit is not provided, the ship must be provided with a gas detector (chemical absorption) tube tank testing set which can be used to verify the results being obtained by the electronic combustible gas indicator. The important concept is that the ship is provided with some means of verifying that the gas indicator in use is functioning correctly.
    The author once served as chief officer of a product tanker on which he found four combustible gas detectors and no calibration kit. Two of the indicators were broken, one was out of calibration giving low readings and one functioned properly but had a contaminated filter! None of the indicators were marked or tagged to indicate their condition! Three indicators were sent ashore for repair and calibration and a calibration kit and spare sampling filters were ordered. The purchasing department responded by asking why the author was requesting services and equipment which no other ship had previously requested!
    For tank testing to be effective, the chief officer must be fully trained in the operation of the test equipment and in the techniques for tank testing. Tank testing equipment should only be purchased if instruction materials and training videos are available in the language of the officer who will be using the equipment. Chief officers should be trained in the same tank testing procedures as that used by chemists certified to issue gas-free certificates.
    Chief officers must be aware that 'catalytic filament' type combustible gas indicators are not suitable for measuring inerted atmospheres because the indicator relies on the presence of oxygen in the sample to accurately measure hydrocarbon levels. Inerted tanks do not have enough oxygen to permit proper operation of filament type CGI's.
    When the chief officer believes that a tank is safe for entry, the complete kit of rescue equipment must be moved to the tank access hatch and set up. No personnel should enter the tank until the rescue equipment and personnel are ready in all respects and the preparation checklist is complete. The tank is first tested using the long sampling hose for the combustible gas indicator. In using the long sampling hose, make sure that it is lowered close to, but not touching, the tank bottom. Coloured tape or paint on the hose should be used to indicate the appropriate length. The chief officer must also know how many times to squeeze the aspirator bulb, or how long to run an indicator with a powered fan, to draw a sample of the tank bottom atmosphere into the testing chamber. Testing must continue until the reading stabilises. If the readings are low, the expanded scale must be used. When the tank has been found to contain safe hydrocarbon vapour levels, it must then be tested for oxygen content. If the last cargo was a crude with high hydrogen sulphide content, the tank must also be tested for H2S gas.
    To determine the safety of a tanks atmosphere, the chief officer must also know the Threshold Limit Value (TLV) of the cargo last carried in the tank. The guidance provided by ISGOTT should be followed in evaluating the tank atmosphere.
    Entry into tanks should not be permitted until testing has demonstrated that the following limitations are met:

  • A gas reading of 1% of the lower explosive limit (LEL), or less is obtained by gas sensor measurement at all areas where gas concentrations are possible.

  • A maximum benzene concentration of 10 ppm as measured by chemical absorption tube indicator (Draeger or equivalent).

  • A maximum hydrogen sulphide concentration of 10 ppm (subject to ongoing regulatory review and reduction) as measured by a chemical adsorption tube indicator (Draeger or equivalent).

  • An oxygen concentration of 21% or more by volume as measured by an oxygen analyser.

  • Maximum hydrocarbon vapour concentration of less than the threshold limit value (TLV), - Time weighted average for the actual mixtures to which personnel will be exposed, as measured by a chemical absorption tube indicator. (Draeger or equivalent).

  • Any other limitation indicated in the material hazard data sheet for the product last carried in the tank.
  • Tanks which were inerted before ventilation may contain traces of toxic gasses. Vigorous ventilation should remove all such gasses, or reduce their concentrations to below their TLV concentrations. If ventilation is questionable, then the tank should be tested for:

  • Maximum carbon monoxide of less than 50 ppm as measured by chemical absorption tube indicator (Draeger or equivalent).

  • Maximum nitrogen dioxide of less than 3 ppm as measured by chemical absorption tube indicator (Draeger or equivalent).

  • Maximum nitric oxide of less than 25 ppm as measured by chemical absorption tube indicator (Draeger or equivalent).

  • Maximum sulphur dioxide of less than 2 ppm as measured by chemical absorption tube indicator (Draeger or equivalent).
  • The tank should be tested for gas every two or three hours. An alternate member of the entry team should conduct the tests to confirm the readings obtained by the first inspector.
    Portable hydrogen sulphide and oxygen alarms, with audible alarm, should be worn by team members in tanks to provide the earliest warning of any change in conditions.

    2.15.3 Tank entry precautions
    When the tank to be entered has been tested and found to have a safe atmosphere at the places tested, the tank entry team is assembled and checked off. Tank entry training should have been conducted for each member of the team and each crew member must be familiar with his duties. Each member of the tank entry team is assigned a particular duty in checking the equipment and in entering the tank. The tank entry and rescue equipment is checked off using a checklist, which is dated and signed by the chief officer and the rescue team leader. The tank entry checklist should include (but may not be limited to):

  • Space(s) to be entered.

  • Names of personnel entering the confined space.

  • Expected duration of in-tank work.

  • Protective clothing and equipment of the in-tank workmen, especially if leaded gasoline tanks are to be cleaned.

  • Condition and operability of the equipment/bucket hoist.

  • Condition of the rescue breathing apparatus (2 sets minimum).

  • Condition of any Surface Air Breathing Apparatus (SABA), unit if used, especially the masks, hoses and hose connectors.

  • Closure/securing of inert gas line valve(s) to the tank(s).

  • Closure/securing/isolation of cargo line valve(s) and controls to the tank(s).

  • Condition/readiness of the resuscitation equipment.

  • Condition of the tank atmosphere testing instruments.

  • Continuous operation of tank ventilation equipment; method of warning in-tank personnel if failure occurs.

  • Closure, labelling and lashing/sealing of all valves to the tank being entered.

  • Tank cleaning buckets, scoops and brooms.

  • Operability of any pumps to be used in the tanks and their associated air and liquid discharge lines.

  • Condition/operability of battery lights or in-tank, air-driven lighting.

  • Correct function of portable radios to be used for communications between the tank entry crew and the deck/bridge watch.

  • Condition of all lines and hardware to be used for lowering/ hoisting equipment into/from the tank.

  • Complete set up and readiness of rescue tripod and equipment.

  • Proper fit of entry team safety harnesses.
  • The tank entry leader (usually the chief officer) then enters the tank with the combustible gas indicator and the oxygen analyser (and H2S detector if required), stopping to test the tank atmosphere at each level as he descends into the tank. The tank entry leader wears a tended lifeline until his initial survey is complete.
    The tank must remain under continuous ventilation during tank entry operations and until all work in the tank is completed and all personnel have withdrawn.
    When the tank atmosphere has been proven safe to the bottom of the tank, each framing bay must be sampled and proven safe before the tank can be declared gas-free. Any framing bay with heavy sludge or scale accumulation may contain a higher concentration of hydrocarbon vapour, so every bay must be tested before work in the tank can begin.

    Enclosed (confined), space entry safety checklist

    After the tank work has begun, the tank atmosphere must be retested, both for hydrocarbon and oxygen concentrations, at least hourly and after every smoke or coffee break. If the tank is not absolutely clear of hydrocarbons, (if the hydrocarbon level is 100 to 300 ppm), change the ill-tank crew and deck support/rescue team crew members after each break. In addition to this precaution, at least two of the in-tank workers should carry belt-mounted hydrocarbon and oxygen deficiency sensors which alarm when unsafe conditions are detected.
    Tank entry is one of the shipboard activities which should be conducted according to a 'Work Permit System' and a shipboard gas-free 'certificate' issued just as if the ship were in port. Indeed, it would seem more urgent to issue a gas free 'certificate' at sea, for the seamen are entitled to the same safety precautions as shore workers and are a lot further from skilled medical help in the event of an accident!

    2.15.4 Work in tanks
    At sea, only 'cold work' should be performed in cargo or bunker tanks. The tank must be gas freed to below the TLV or to 1% LFL, whichever is lower and care must be taken to monitor the gas level in the area of the work.

    Cold work may include:

  • Lifting sludge or scale from tanks,

  • Chipping or hammering,

  • Cutting (with hacksaw or file), or drilling, Use of air driven power tools.
  • Hot work is not permitted at sea and any work involving the following is prohibited:

  • Welding, soldering or flame cutting.

  • Grinding or grit blasting.

  • Fire of any kind.

  • Use of electrical equipment without an explosion proof or intrinsically safe approval.
  • Personnel entering tanks should be equipped with:

  • Hard hat.

  • Safety harness.

  • Hearing protection (if needed).

  • Goggles/eye protection.

  • Gloves.

  • Safety boots with non-slip soles.

  • Full length clothing or coveralls.

  • Flashlight or hand lantern.

  • Activated 12-hour light stick fastened to the shoulder.

  • 30-minute light stick in a pocket (not activated).

  • 10-minute escape air packs.

  • Belt mounted gas-oxygen sensor/alarms (on two men).
  • A full set of breathing apparatus, a fire extinguisher and an intrinsically safe UHF radio should be maintained in the tank with the work party.

    2.15.5 Tank cleaning/de-scaling
    The most common reason for tank entry is to permit a work party to remove loose scale and sediment from the tank bottom. This operation requires sufficient men to perform the functions of:

  • Scooping scale and filling buckets.

  • Passing buckets to the hoisting point.

  • Hooking on/off full/empty buckets.

  • Operating the bucket hoist.

  • Dumping full buckets overboard or into containers and returning them to the hoist.
  • A team of seven or eight men will be required to do this work efficiently, particularly if the tank is large. If the ship has a regular descaling program, the accumulation in any tank should not be significant and the cleaning of a single tank can normally be completed in one day.
    Tanks to be de-scaled must be thoroughly bottom washed with hot water before ventilation and entry. This is done for two reasons. First, the scale tends to hold hydrocarbons, particularly light product and will release these hydrocarbons when disturbed. If any gas is given off by the de-scaling (or 'mucking') operation, the tank may quickly become unsafe for men and the work will have to be suspended until the tank is again washed and ventilated. Secondly, it is illegal for scale or sediment lifted from cargo tanks to be discharged at sea if this material contains oil. Any sediment or scale containing oil must be drummed and retained on board for disposal ashore.
    Scale/sediment should be dumped overboard on the side opposite the engine room intakes to prevent any of the disposed material being drawn into the ship's machinery.

    Efficient safe set-up for tank cleaning to remove sediment and scale. Equipment set up for the de-rusting operation.
    1 Men engaged in lifting scale and sediment. 2 Bucket into which scale is being shovelled. 3 Full bucket being hauled to deck. 4 Single tail block with gantline. 5 Man to handle buckets (sometimes two required). 6 Depositing contents of full buckets overside. 7 Wood chute (metal lined). 8 Pipeline. 9 Suction valve. 10 Suction valve control wheel (lashed or sealed closed). 11 Drums for oily scale.

    The condition of the lift wire, hook and air hoist machinery should be inspected hourly by the chief officer to ensure that it remains free of defects. Each bucket should be inspected before use to verify that the handle and attaching points are in good condition. The man hooking on buckets at the bottom of the tank should not remain under the buckets while they are being hoisted. One of the responsibilities of the bucket-man is to verify the condition of each of his comrades between bucket lifts. This is facilitated by the light-stick fastened to each man's shoulder, which makes it easy to count the men in the darkness of the tank.
    As the work proceeds across the bottom of the tank, the hoist and ventilation must be moved to remain above the work party. Movement of the ventilation should be by first setting up and starting a second blower before the first one is stopped. Two blowers should be in use at all times, one where the men are working and one covering their next work area. The movements of the hoist and blower are coordinated by use of the UHF radios.
    While working in the tank, each member must remain alert for signs of hydrocarbon exposure, either in his behaviour or in the behaviour of others. Excessive hilarity, loss of coordination, slurred speech, or loss of sense of responsibility are signals that the tank cleaning team should evacuate the space. A gas-free tank is only gas free at the time it is tested, the operation of removing accumulated scale can release fresh hydrocarbon vapours which quickly make the tank atmosphere unsafe. Constant ventilation and alertness are essential.
    At the end of the tank cleaning operation the equipment is recovered to the deck using the hoist. The chief officer makes a final inspection with one attendant watching him from the lowest platform of the ladder. When the chief officer is satisfied that the tank is suitably cleaned he returns to the deck and verifies that all the equipment which went into the tank has been accounted for by using the entry checklist.
    Tanks which have carried numerous cargoes of leaded gasoline are a special problem. The tetra-ethyl-lead compound which is added to leaded gasoline is extremely toxic. Men assigned to de-scale such tanks must wear full disposable coveralls, organic vapour respirators and rubber gloves. TEL is readily absorbed through the skin and all contact with scale or sediment from leaded gasoline tanks must be avoided. After tank cleaning the coveralls and gloves must be disposed of.

    2.15.6 Repairs in tanks
    In-tank repairs usually involve the pipelines, valves or valve operators, or the fixed tank gauging equipment. Men working above the tank bottom must work from adequate staging to provide secure footing. If repairs to the tank bulkheads, sounding pipes, or valve actuator lines are required above the tank bottom, adequate ladders must be provided (fibreglass rather than aluminum) and properly secured. Men working on ladders or staging must wear safety harnesses with safety lines properly secured.

    2.15.7 Entry to non-gas free or low oxygen tanks
    In most cases, tanks can be ventilated sufficiently to provide for 'bare face' entry, where a safe atmosphere in the tank permits entry by personnel without breathing apparatus. In some cases, it is not possible to gas free the tank or space for entry. Such cases should be exceptional and men should not be asked to enter unsafe tanks unless no reasonable alternative exists. The work to be completed should be essential to the safety of the ship.
    The requirements for entry to tanks containing a dangerous atmosphere must be strictly observed and only fully trained personnel should be permitted to make the entry.

    The basic requirements for entry are:

  • All entry personnel wear breathing apparatus and a lifeline.

  • Each lifeline is tended by an individual outside the space.

  • For each man in the tank there is a man standing by outside the tank wearing a self contained breathing apparatus.

  • A complete check of all equipment is made prior to tank entry and recorded on a checklist.

  • A meeting is held with all tank entry and support team members to discuss the duties of each man and the exact procedure to be followed. Use a tank diagram.

  • Each tank entry person must wear an activated light stick and carry a flashlight.

  • Escape pack breathing apparatus sets should be used.

  • Signals to be used for tending lines must be understood and rehearsed.

  • The established tank entry permit procedure has been completed and a valid permit issued.

  • Ventilation is provided to the maximum extent possible.
  • The types of tank entry equipment available include fresh air breathing apparatus, self contained breathing apparatus, and surface air breathing apparatus (SABA).
    Fresh air breathing apparatus is the oldest and simplest type, using a hand powered pump to provide air by hose to a full-face face piece. They are easy to use, but require the same care and maintenance as any safety equipment. On one occasion, the author was at the bottom of a gasoline tank, standing over one meter of gasoline, when the inside liner of his air hose began to peel off in flakes and accumulate in the mask. On another occasion, (this time in a heating oil tank), the air hose connection to the face piece snapped off at the bottom of the tank.
    Self contained breathing apparatus provides a completely portable supply of air and is excellent where mobility is relatively unimpeded and minimum set up time is important. A pressure gauge and alarm on the unit indicate the amount of air remaining.
    SABA, or air line breathing equipment permits extended in-tank work and provides better mobility in confined spaces. The air supply is provided by a battery of compressed air bottles ('cascade system'), or by an air compressor with compressed air bottles as backup. Escape packs should be worn with air line units. The maximum length of hose is 90 meters.
    The minimum number of personnel should be sent into the space. If the entry route is complex, it may be necessary to station one or two additional personnel at points inside the tank so that the workers can be kept under continuous observation.

    2.15.8 Tank rescue operations
    The crew attending a tank entry team must be competently trained in confined space rescue procedures and have all the necessary rescue equipment prepared at the tank site before tank entry operations begin. If the tank entry team in a gas-free tank is attended by a single crew member, he must be strictly instructed not to enter the tank, but to pass the word (by radio) and wait for assistance from the rescue team if the in-tank workers encounter difficulties or appear to be incapacitated. No crew member should ever enter a tank to assist another crew member apparently overcome by vapours unless the rescuer is equipped with a tended safety line and breathing apparatus.
    The most dangerous tanks are those with deficient oxygen. Such spaces are never entered bare-faced and the provision of a tended rescue line permits rapid rescue if the man in the tank is injured. This is important because a man without breathing apparatus in an oxygen deficient tank will die in two to three minutes.
    In a tank with an adequate supply of oxygen, personnel incapacitated by hydrocarbon vapour inhalation can remain for some time without harm as long as they are breathing. The following times to death are indications only:

    Time until death in a tank with adequate oxygen and:
    High concentration of petroleum vapour
    Medium concentration of petroleum vapour
    Low concentration of petroleum vapour

    20 minutes
    60 minutes

    If the rescue equipment is ready, there is plenty of time to rescue men overcome in a tank where cleaning operations have generated a low concentration of hydrocarbon vapour. At least three men are needed for a rescue. The rescue team must organise itself, correctly don its equipment and then descend into the tank with the necessary rescue equipment. They must learn from the tank attendant how many men are in the tank and where they were last working. The chemical lights worn by the in-tank workers will aid greatly in locating them.

    If the victims are breathing
    If the vapour concentrations are low, the rescue team may be able to rouse the men in the tank. These men should be fitted with tended lifelines to assist them in climbing out of the tank.
    If the vapour concentrations are medium or high, rescue equipment will be needed and the men will be removed by rescue harness or by stretcher. If the affected personnel have not activated their escape sets (ELSA sets), they should be activated and placed over their heads. Unconscious victims may react violently and will probably vomit when recovering consciousness. A close watch must be maintained as men revive, especially if a hood or face mask has been fitted.

    If the victims are not breathing
    Any men found not breathing should be removed first if resuscitation cannot be given in the tank. Any of the victims who have fallen should be removed next by stretcher. The remaining unconscious personnel can then be removed by stretcher or harness.
    The necessary first aid measures following removal of affected personnel are not within the scope of this text.
    It goes without saying that rescue of personnel from the distant areas of a cargo tank will only be successful if it has been diligently practiced during emergency drills. Without such practice, what might have been a successful rescue could become only an effort ending in futility and tragedy.

    Crew positions and lifting arrangements for a pumproom rescue.
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