Oil tankers gas testing procedure

The safe management of operations onboard tankers is often dependent upon the operators ability to determine the composition of the ambient atmosphere or the atmosphere of an enclosed space.

Tanker operators need to measure the oxygen and the hydrocarbon and toxic gas concentrations in the atmosphere. This will enable them to detect the presence of any explosive mixtures, toxic vapours and any oxygen deficiency that may present a risk of explosion or hazard to personnel.

Tankers that are required to be fitted with an inert gas system, have the additional need to measure the oxygen content of inert gas and levels of hydrocarbon and toxic gas concentrations in cargo tanks as part of the safe management of their cargo tank atmospheres.

Summary of gas testing tasks : Atmosphere monitoring

The external atmosphere should be monitored for:
  • Hydrocarbon vapour when undertaking Hot Work. This is achieved by using a flammable gas indicator, capable of measuring gas to the lower flammable limit (LFL) and with the scale graduated as a percentage of this limit.

  • Toxic vapours when loading cargoes containing toxic components and when undertaking gas freeing operations following the carriage of such cargoes. This is achieved by using an instrument capable of measuring concentrations of toxic gases in the human toxicity range, usually calibrated in parts per million.

Enclosed space monitoring

Prior to permitting entry into an enclosed space, measurements must be taken to detect the presence of hydrocarbon gas, to confirm normal oxygen levels and, if applicable, to detect the presence of any toxic vapours.

Measurement to ensure that the atmosphere is free of harmful hydrocarbon vapour is undertaken using a flammable gas indicator capable of measuring gas to the lower flammable limit (LFL) and with the scale graduated as a percentage of this limit (%LFL). An oxygen analyser is used to determine that the normal level of oxygen in air of 21% by volume is present for life support.

Where toxic vapour may be present in the space to be entered, it should also be tested with an instrument capable of measuring concentrations of toxic gases in the human toxicity range, usually calibrated in parts per million.

Inert gas atmosphere management

To ensure compliance with statutory requirements, vessels fitted with an inert gas system must be equipped with an oxygen analyser for determining the quality of the inert gas and for measuring the levels of oxygen in the cargo tanks.

A gas indicator capable of measuring the percentage of hydrocarbon gas by volume (%Vol) in an inerted atmosphere is also required for safe management of operations that include the purging and gas freeing of cargo tanks.

The provision of gas measurement instruments

The SOLAS Convention requires that ships carrying cargoes that are likely to emit a toxic or flammable gas, or to cause oxygen depletion in a cargo space, are provided with an appropriate instrument for measuring the concentration of gas or oxygen in the air, together with detailed instructions for its use.

Implicit in the above provision is the requirement that the operator provides the correct instrument for each gas testing need. It should be noted that the different gas testing functions may be incorporated into a multi-function gas measuring instrument.

The gas measurement instrumentation on board a tanker should form a comprehensive and integrated system that addresses all the necessary applications identified by the operator. The instruments should be fit for the task to which they are applied and users should be made aware of the particular applications and limitations of each instrument.

Users of gas measuring instruments should be trained in the proper use of the equipment, to a level suited to their work duties.

A sufficient number of gas measuring instruments should be available onboard the ship to meet all the identified requirements, whilst allowing for instrument failures, servicing requirements and the capability of the ships staff to undertake repair and certified calibration of the instruments.

Alarm functions on gas measuring instruments

Consideration should be given to the provision of instruments with an alarm facility. Alarms should only be fitted to an instrument that is to be used where an alarm function is necessary, such as a personal gas alarm monitor.

Analytical instruments that are used to provide numerical values for gases and vapours, for dangerous space entry certification, do not need to have an alarm function.

Analytical instruments that are provided with an alarm capability, should be designed so that the alarm inhibit and activate function cannot be changed by the instrument operator. This is to avoid the possibility of inappropriate or accidental inhibition of the alarm function.

The use of different instruments for testing atmospheres for entry certification, and for monitoring atmospheres with a personal monitor during the entry operation, reduces the probability of an accident due to an instrument malfunction. It is therefore not recommended that the testing instrument is also used as the personal alarm instrument during the entry operation.

Sampling lines

Sampling lines should be suitable for the intended service and be impervious to the gases present in the atmospheres being monitored. They should also be resistant to the effects of hot wash water.


The accuracy of measurement equipment should be in accordance with the manufacturers stated standards. Equipment should, on initial supply, have a calibration certificate, traceable, where possible, to internationally recognised standards. Thereafter, equipment should be periodically landed to a recognised testing facility for calibration, either during a vessels refit or when the accuracy of the equipment is considered to be outside the manufacturers stated accuracy. Procedures for management of the calibration certification process should form part of the onboard Safety Management System.

Calibration certificates, showing the instruments serial number, the calibration date and the calibration gas or the method of calibration used, together with reference to applicable standards, should be provided for retention on board.

Instruments are typically calibrated using a calibration gas suitable to the use of the instrument, such as propane or butane. The calibration gas used should be marked on the instrument.

The use of an inappropriate gas for calibration could result in erroneous readings during operation, even though the instrument appears to be operating correctly. Calibration should not be confused with operational testing.

Operational testing

Gas measuring instruments should be checked for accuracy before the commencement of operations requiring their use. Instruments should only be used if the checks indicate that the instrument is giving accurate readings and their alarms, if fitted, are operating at the pre-determined set points.

During extended operations, the operator should determine the frequency at which operational checks should be made.

Instruments not passing these operational checks should be re-calibrated before they are returned to operational use. If this is not possible, they should be removed from service and clearly labelled to denote that they are not to be used. These procedures should be documented in the Safety Management System .

Disposable personal gas monitors

Disposable personal gas monitors should be periodically function tested in accordance with the manufacturers recommendations to confirm that they are operating correctly. Disposable gas detection monitors, which cannot be recalibrated, should be safely disposed of when the calibration for the instrument expires. For this reason, it is important to record when disposable instruments are first commissioned, in order to establish their expiry date.

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