gas-free certificates and confined space Practice Questions

1. Under 46 CFR 35.01-1, who is authorized to issue a gas-free certificate for a tank vessel?

• A.A Marine Chemist certified by the National Fire Protection Association, or a USCG-authorized person✓
• B.The vessel's chief officer after testing with a combustible gas indicator
• C.Any licensed tankerman certified by the USCG as Tankerman-PIC
• D.A shore-side safety officer employed by the terminal facility

Why: 46 CFR 35.01-1 specifies that a gas-free certificate must be issued by a Marine Chemist certificated by the NFPA or a person authorized by the USCG, not by ship's crew, before hot work or tank entry for specific operations.

2. A gas-free certificate is required before which of the following operations on a tank vessel?

• A.Welding or hot work on or adjacent to a cargo tank✓
• B.Loading a Class I flammable liquid under inert gas blanket
• C.Transferring ballast water between segregated ballast tanks
• D.Topping off a cargo tank to 98% capacity

Why: A gas-free certificate is required before hot work (welding, burning, grinding) on or near cargo tanks because such work generates ignition sources that could initiate a fire or explosion if residual flammable vapors are present.

3. How long is a gas-free certificate typically valid once issued?

• A.Only as long as conditions remain unchanged — it becomes void if the tank is opened or conditions alter✓
• B.24 hours from the time of testing regardless of any changes
• C.72 hours if the tank remains sealed with no personnel entry
• D.Until the vessel shifts berth or cargo operations are completed

Why: A gas-free certificate is valid only as long as the conditions under which it was issued remain unchanged. Any opening of the tank, change in temperature, or other alteration in conditions invalidates the certificate and requires re-testing.

4. Which type of atmosphere test instrument is used to detect the presence of toxic vapors such as hydrogen sulfide (H2S) in a cargo tank before entry?

• A.A colorimetric detector tube or electrochemical sensor calibrated for H2S in parts per million✓
• B.A combustible gas indicator (CGI) set to the 0–100% LEL scale
• C.An oxygen analyzer measuring percent by volume of O2
• D.A photoionization detector (PID) calibrated for benzene at 100% LEL

Why: Hydrogen sulfide testing requires an instrument that reads in ppm concentration (colorimetric tubes or electrochemical sensors), since the IDLH for H2S is 100 ppm — far below the concentrations a %LEL CGI scale can detect.

5. A Marine Chemist has issued a gas-free certificate for a tank. The next morning, the temperature rises significantly and the vessel begins to take on a slight list. What action should be taken before workers re-enter the tank?

• A.Re-test the atmosphere and obtain a new or updated certificate, since conditions have changed✓
• B.Proceed with entry because the certificate is valid for 24 hours from issuance
• C.Have the vessel master sign a waiver extending the certificate's validity
• D.Ventilate the tank for 30 minutes and proceed without re-testing if the list is minor

Why: A gas-free certificate becomes void when conditions change. A temperature increase can increase vapor generation, and a list may expose previously covered cargo residues. Re-testing is required to ensure the atmosphere remains safe before re-entry.