During a recent Port State Control (PSC) tanker inspection, a USCG member was exposed to a dangerous concentration of Hydrogen Sulphide (H2S) gas and suffered a serious injury.
The exposure occurred during an examination on board a tanker carrying Grade E sour crude.
A PSC team asked a ship’s crew member to check cargo tank oxygen levels using the ship’s portable gas meter. The crew member accessed the cargo tank via a deck sounding valve.
When the valve was opened, the pressurised cargo tank atmosphere escaped releasing inert gas and H2S vapours.
During the incident, the personal gas meters of two individuals involved - a USCG officer and a crew member – warned of the presence of H2S. The USCG officer, whose alarm sounded was standing 1–2 ft downwind from the sounding valve.
Within days the USCG officer developed severe exposure symptoms consistent with H2S exposure. The other two USCG officers involved were standing upwind. Afterwards, one reported experiencing a minor headache.
The US Occupational Safety and Health Administration (OSHA) notes that H2S is a colourless, flammable gas with a ‘rotten egg’ smell that occurs naturally in crude oil.
Even at low concentrations, this heavierthan- air gas can irritate the eyes, nose, throat and respiratory system with effects delayed for hours, or days. At higher concentrations, nausea, vomiting, headaches, dizziness, unconsciousness, or death, may occur.
While the initial ‘rotten egg’ odour is present, an individual may lose the ability to smell that gas after becoming exposed.
Personal monitoring equipment is, therefore, vital to protect against exposure. An alarm on H2S constitutes an acute exposure and should trigger immediate evacuation and initiation of acute exposure procedures including medical attention.
Prior to using portable gas monitoring equipment, personnel should familiarise themselves with ISGOTT Section 11.8 and safe work practices for conducting, or witnessing these tests.
ISGOTT recommends when sampling tanks, personnel should stand perpendicular to the wind to avoid being downwind, or upwind and creating eddies.
Diligence needed
When monitoring cargo tank atmospheres, all personnel should exercise diligence and great care and in all cases, personnel should completely assess the risks, to include the cargo type, tank pressure, venting arrangements, wind direction/speed and condition of the testing equipment, the USCG advised.
When H2S is suspected to be present, ISGOTT Section 2.3.6.5 recommends that a self-contained breathing apparatus (SCBA) be worn if it is necessary to breach the integrity of the cargo system and if a vapour free atmosphere cannot be guaranteed.
Reduced threshold
The American Conference of Governmental Industrial Hygienists recently reduced the H2S dangerous Threshold Limit Value from 2 parts per million (ppm) to 1 ppm.
As a result the USCG said that it strongly recommends that those involved in cargo tank sampling, or atmosphere testing:
· Be familiar with and adhere to ISGOTT safe work practices.
· Assess personnel risks.
· Don appropriate personal protection equipment, including gas monitoring detectors, respirator, or SCBA.
· If gas monitoring detectors are used ensure:
· Detectors are maintained and properly calibrated.
· Alarm settings values are properly set.
· Be aware and familiar with the hazards of the cargos involved, as well as exposure indicators and emergency response procedures.