CMC Vessel-Specific Hazards 2 — Questions and Answers
Question 1: When a marine chemist issues a 'Safe for Workers' certificate for a cargo tank, what condition regarding oxygen must be met?
- Oxygen level must be exactly 21%
- Oxygen level must be between 19.5% and 22% (Correct answer)
- Oxygen level must be above 18%
- Oxygen level must be above 20.9%
Correct answer: Oxygen level must be between 19.5% and 22%
OSHA and NFPA 306 require oxygen levels between 19.5% and 22% for a space to be considered safe for workers without supplied air.
Question 2: Which vessel space presents the greatest risk of hydrogen sulfide accumulation due to organic sludge decomposition?
- Fore peak tank
- Cargo pump room
- Cargo tank after carrying crude oil (Correct answer)
- Engine room bilge
Correct answer: Cargo tank after carrying crude oil
Crude oil cargo tanks accumulate sulfur-containing sludge that decomposes anaerobically to produce dangerous concentrations of H2S.
Question 3: A void space adjacent to a fuel oil tank is being prepared for entry. What is the primary chemical hazard the marine chemist must evaluate beyond flammability?
- Carbon monoxide from combustion
- Hydrocarbon vapor migration through tank boundaries (Correct answer)
- Nitrogen enrichment from inerting
- Chlorine from antifouling coatings
Correct answer: Hydrocarbon vapor migration through tank boundaries
Hydrocarbon vapors can permeate through corroded or poorly sealed tank boundaries from adjacent fuel tanks, creating toxic and flammable atmospheres in void spaces.
Question 4: During a ballast tank inspection on a chemical tanker, the marine chemist detects a slight sweet odor with readings of 1 ppm benzene. What is the appropriate action?
- Proceed with entry using a supplied-air respirator
- Issue a restricted entry certificate requiring air-supplied equipment (Correct answer)
- Declare the space safe since 1 ppm is below OSHA PEL of 1 ppm TWA
- Ventilate for 30 minutes then re-test
Correct answer: Issue a restricted entry certificate requiring air-supplied equipment
Even at 1 ppm benzene (the OSHA PEL-TWA), a restricted certificate requiring supplied-air equipment is appropriate given benzene's carcinogenicity and the need to maintain exposures as low as reasonably achievable.
Question 5: What unique hazard do double-bottom tanks on bulk carriers present that single-bottom vessels do not?
- Higher risk of methane accumulation from grain cargoes
- Difficult ventilation geometry creating dead pockets of inert or toxic gas (Correct answer)
- Greater hydrostatic pressure on personnel
- Increased risk of electrical shock from cathodic protection
Correct answer: Difficult ventilation geometry creating dead pockets of inert or toxic gas
Double-bottom tanks have complex internal framing with longitudinals and transverses that create stagnant pockets where toxic or oxygen-deficient atmospheres can persist despite general ventilation.
Question 6: A marine chemist is called to certify a pump room on a product tanker. Benzene content in the previous cargo was less than 0.1%. Which standard governs the atmospheric testing requirements?
- OSHA 29 CFR 1910.146 confined space standard only
- NFPA 306 in conjunction with applicable OSHA standards (Correct answer)
- IMO MSC/Circ.1401 alone
- SOLAS Chapter II-2 regulations
Correct answer: NFPA 306 in conjunction with applicable OSHA standards
NFPA 306 is the primary standard for controlling fire and explosion hazards on marine vessels in US ports, used in conjunction with applicable OSHA standards.
Question 7: What is the significance of the 'Lower Explosive Limit' (LEL) reading of 10% on a combustible gas indicator when certifying a tank for hot work?
- The space is safe for hot work since it is well below the LEL
- The space must be re-ventilated; hot work requires 0% LEL reading (Correct answer)
- Hot work may proceed with fire watch present
- The reading indicates the meter needs calibration
Correct answer: The space must be re-ventilated; hot work requires 0% LEL reading
NFPA 306 requires 0% LEL (gas-free condition) in any space where hot work is to be performed, and adjacent spaces must also meet this requirement.
CMC Vessel-Specific Hazards 2
When a marine chemist issues a 'Safe for Workers' certificate for a cargo tank, what condition regarding oxygen must be met?