Question 1

You are loading a general cargo vessel with: 10 tonnes of sodium cyanide (UN 1689, Class 6.1); 5 tonnes of sulphuric acid (UN 1830, Class 8); and 200 tonnes of ammonium nitrate fertiliser (UN 2067, Class 5.1). Plan the stowage.

Accepted Answer

MULTI-CLASS DG STOWAGE PLANNING: STEP 1 — CHECK INDIVIDUAL REQUIREMENTS FOR EACH SUBSTANCE: (1) SODIUM CYANIDE UN 1689, Class 6.1, PG II: requires "away from" acids (separation required — combination with acid produces hydrogen cyanide gas, immediately fatal). Special provision: keep dry, away from acids; (2) SULPHURIC ACID UN 1830, Class 8, PG II: an acid. Must be "separated from" cyanides. IMDG Table 7.2.4 segregation: 6.1 and 8 requires "away from" each other. But the cyanide+acid reaction risk is so severe that specific text in the IMDG entry for sodium cyanide requires "Separated from" acids (at least one hold or deck interval); (3) AMMONIUM NITRATE FERTILISER UN 2067, Class 5.1, PG III: explosive potential if mixed with organic material or in fire conditions. Must be "away from" Class 6.1 substances (minimum one compartment). STEP 2 — SEGREGATION MATRIX (IMDG TABLE 7.2.4): Class 5.1 vs Class 6.1 = "Away from" (one compartment or holds apart, or separated on deck by a minimum distance). Class 6.1 vs Class 8 = "Away from" but cyanide/acid special provision = "Separated from." Class 5.1 vs Class 8 = "Separated from" for sulphuric acid (IMDG Table shows "4" for 5.1 vs 8). STEP 3 — STOWAGE PLAN: (a) Ammonium nitrate: Hold 1 or 3; (b) Sulphuric acid: Hold 5 (opposite end of vessel); (c) Sodium cyanide: must be: SEPARATED from sulphuric acid (different hold, maximum distance); "Away from" ammonium nitrate. Hold 3 may work if Hold 1 = nitrate, Hold 5 = acid, Hold 3 = cyanide with requisite separation distances met. Cyanide + acid interaction: the reaction of sodium cyanide with any acid (including sulphuric acid) produces hydrogen cyanide (HCN) gas — one of the most acutely toxic substances known (IDLH 50 ppm; LC50 for humans approximately 100-300 ppm). This specific reaction hazard is why the IMDG Code imposes more stringent segregation for cyanide/acid combinations than the general Class 6.1/Class 8 matrix might suggest. Masters and chief mates must recognise these specific incompatibilities beyond the general segregation table. Ammonium nitrate as cargo: ammonium nitrate (UN 2067) is the substance responsible for several maritime disasters including the Mont Blanc explosion (1917), Texas City explosion (1947), and more recently the Beirut Port explosion (2020). While UN 2067 is a lower-hazard grade than the military ammonium nitrate, it still requires careful handling and segregation. In fire conditions — ammonium nitrate decomposes to release oxygen (accelerating fires) and can detonate. Multi-cargo DG carriage prohibited (option B): incorrect. The IMDG Code provides a framework for carrying multiple DG classes on the same vessel — this is routine in general cargo operations. The segregation requirements exist precisely to allow multi-class carriage safely. Refusing all three bookings is an over-reaction. Separate packages = segregated (option C): completely wrong. IMDG segregation means physical separation in the cargo spaces — distance or compartmentation. Having incompatible substances in the same hold (even separately packaged) does not meet the segregation requirements. In an emergency (fire, spillage), incompatible materials in the same hold could react catastrophically.

Question 2

As Chief Mate on a chemical tanker, you are planning to load benzene (UN 1114, Class 3, PG I) after carrying methanol. What segregation, tank preparation, and safety measures apply?

Accepted Answer

BENZENE LOADING AFTER METHANOL — CHIEF MATE's PLANNING: CARGO COMPATIBILITY CHECK: (1) Are methanol and benzene compatible for sequence loading? Methanol (Class 3/6.1) and benzene (Class 3) are both flammable — BUT they are not compatible in the sense that residual methanol in a benzene cargo = CONTAMINATION. Check the IBC Code compatibility matrix and the charterer's cargo compatibility requirements; (2) TANK WASHING: tanks previously carrying methanol must be thoroughly washed before benzene loading. Methanol is water-soluble and miscible — water washing followed by recirculation is appropriate. Complete drying is required as water in benzene cargo = haze/contamination; (3) BENZENE PROPERTIES (UN 1114): (a) Class 3, Packing Group I (very high flammability); (b) Boiling point 80°C, flashpoint -11°C (extremely flammable); (c) TOXIC: a human carcinogen (Group 1 IARC). Vapour exposure risk — PPE: SCBA and full chemical protection suit for all operations; (d) Vapour density 2.7 (heavier than air — accumulates in low areas); (4) MARPOL ANNEX II: benzene is Annex II Category X substance (most hazardous). Pre-washing before any discharge — MANDATORY; (5) IBC CODE SPECIAL REQUIREMENTS: benzene requires closed loading, vapour return line (vapour recovery or destruction), enclosed gauging (no open ullaging), and restricted access during loading; (6) SAFETY ZONE: 3-metre exclusion zone around manifold during loading operations. No ignition sources within 30m. IGS may not be required (benzene is non-reactive to inert gas atmospheres) but nitrogen blanket may be required; (7) EMERGENCY: EmS F-E (flammable liquids fire) and S-E (spillage). Benzene as a carcinogen: benzene is classified as a Group 1 carcinogen by IARC (International Agency for Research on Cancer). The IMDG Code and IBC Code impose additional health protection requirements beyond standard Class 3 liquid handling. Exposure limits: the UK COSHH regulations specify a Workplace Exposure Limit (WEL) for benzene of 1 ppm (8-hour TWA). This is extremely low — the vapour pressure of benzene at ambient temperature means significant benzene vapour can be present at open ullage points, manifold areas, and any open sampling. ALL operations involving benzene cargo must use SCBA and appropriate chemical protection. IBC Code Chapter 15 special requirements: Chapter 15 of the IBC Code specifies additional cargo-specific requirements for each chemical. For benzene — these include: (a) vapour emission control; (b) closed operations; (c) emergency shutdown system; (d) health monitoring for crew. MARPOL Annex II Category X: benzene is among the most strictly regulated liquid cargoes under MARPOL Annex II. Discharge into the sea of any benzene-contaminated washwater is absolutely prohibited unless processed through a compliant retention/disposal system. Pre-wash is mandatory before ANY discharge of residues in any sea area. Cross-contamination (option B): loading benzene into a tank with methanol residue creates a contaminated benzene cargo. Methanol is infinitely miscible with benzene — even small traces will reduce benzene purity below specification. Additionally, water from tank washing that is not completely dried creates a benzene/water haze that similarly renders the cargo off-specification.

Question 3

In port, a cargo hold is opened and a strong smell of chlorine is detected. The manifest shows UN 1017 (Chlorine, Class 2.3 Toxic Gas) in the lower hold. Crew members are near the hold. As Chief Mate, what do you do?

Accepted Answer

CHLORINE GAS RELEASE — EMERGENCY RESPONSE: UN 1017, CHLORINE, CLASS 2.3: (a) Extremely toxic gas — IDLH (Immediately Dangerous to Life and Health) 10 ppm; (b) Heavier than air (vapour density 2.5) — accumulates at low levels in holds; (c) Bleach-like odour detectable at 0.5 ppm — if crew can smell it, concentrations may already be dangerous. IMMEDIATE ACTIONS — FIRST 60 SECONDS: (1) SHOUT AND CLEAR: warn all crew in the vicinity to MOVE AWAY UPWIND. Do not send crew into the hold without SCBA; (2) WIND AWARENESS: position crew UPWIND of the hold opening. Chlorine disperses downwind; (3) NO IGNITION SOURCES: chlorine in combination with hydrogen (from bilge gas) could explode — no smoking, no naked flames. However, chlorine itself is not flammable; (4) CALL THE MASTER: immediate notification; (5) SOUND GENERAL ALARM: if the release is significant or risk of spread to accommodation; (6) CLOSE THE HOLD: if it can be done safely from outside without entering — secure the hold to contain the release; (7) EMERGENCY SERVICES: contact port authority and emergency services (shore fire brigade with HazMat team); (8) IMDG EmS: EmS code for UN 1017 is F-C (non-flammable gas fire) and S-U (toxic gas spillage). S-U: keep people away from damaged area. Contact 24-hour emergency advice centre; (9) EMERGENCY CONTACT: the DG manifest must show a 24/7 emergency contact. Call them immediately; (10) DO NOT VENTILATE BY BLOWING AIR INTO HOLD: this could disperse chlorine gas to adjacent areas. Chlorine toxicology: chlorine is one of the most dangerous gases that can be encountered on a cargo vessel. WWI chlorine gas attacks used concentrations above 1,000 ppm — but even at 30 ppm, severe chest pain, vomiting and pulmonary oedema occur. At 430 ppm — death within 30 minutes. Critically: chlorine is HEAVIER than air — it accumulates at the lowest point (bottom of the hold). Crew near the open hatch may not immediately appreciate the concentration at lower levels in the hold. IMDG EmS S-U: the Spillage Schedule for toxic gases (S-U) specifies: "ELIMINATE all ignition sources. STOP leakage only if safe to do so. Move people from danger area upwind. Advise competent authority of potential hazard. Contact chemical experts." "Rag over the face" approach (option C): chlorine at dangerous concentrations is immediately incapacitating. A crew member who enters a hold with chlorine vapour above IDLH using a rag as protection will quickly become incapacitated. This would turn a dangerous goods incident into a rescue situation requiring entry into a toxic atmosphere — putting additional lives at risk. The USCG and MAIB have investigated several cargo incidents where rescue attempts without SCBA resulted in rescuers also becoming casualties. Ventilation with fans (option B): ventilating by blowing air INTO the hold could spread chlorine gas to adjacent spaces, including engine room intakes and accommodation. The correct approach is to CLOSE the hold and control the release, allowing emergency services with HazMat equipment to handle decontamination. Detection threshold vs danger threshold: the detection odour threshold for chlorine is 0.5 ppm. This is BELOW occupational exposure limits (1 ppm ceiling under COSHH). Detection does not mean safe — it means dangerous concentrations may be nearby.