Styrene monomer: charterer orders nitrogen inerting on an inhibited cargo
MT Chem Pioneer is an 8,000 dwt stainless-steel chemical tanker preparing to load 1,500 t of styrene monomer at Onne. The cargo certificate states 12 ppm TBC inhibitor and a maximum loading temperature of 28 °C. The ship has nitrogen, tank cooling coils and closed sampling. The charterer instructs the master to inert the cargo tanks to 5% oxygen to reduce fire risk because styrene has a flash point of about 31 °C. The voyage is 14 days through the Gulf of Guinea, with expected sea and deck temperatures of 34 °C. The previous cargo was methanol; the tanks are clean, dry and odour-free. The chief officer has prepared a loading plan showing nitrogen supply connected to No. 4 and No. 5 tanks, and the terminal is ready to start. The charterer's representative says inerting is standard for flammable cargoes and asks for written confirmation before loading.
1. Evaluate the charterer's instruction to inert the styrene cargo to 5% oxygen.
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The instruction should not be accepted as written. Styrene monomer is inhibited with TBC, and TBC depends on dissolved oxygen to work. If the tank is inerted to 5% oxygen, the inhibitor can become ineffective and the cargo may polymerise, even though the intention was to reduce fire risk. Polymerisation is exothermic and can accelerate, leading to tank overpressure, cargo solidification and a serious emergency.
The correct approach is to follow the cargo certificate, manufacturer's guidance and terminal instructions, not a generic flammable-cargo rule. For inhibited styrene, the tank atmosphere should retain sufficient oxygen for the inhibitor, while cargo temperature is controlled below the polymerisation threshold, commonly around 25 °C or lower according to the certificate. The ship should not connect nitrogen for inerting unless the charterer produces a specific written instruction supported by the manufacturer and terminal that addresses inhibitor performance. The master should seek clarification from the technical department and charterer before loading.
2. Describe the operational controls you would require for the voyage.
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The voyage plan should control temperature, inhibitor and contamination. Cargo should be loaded at or below the certificated maximum, and cooling coils should be available and tested before loading. The cargo temperature should be monitored at least daily, with alarm limits set below the certificated threshold, and cooling started early rather than after a rise. The ullage oxygen should be checked and kept adequate for the inhibitor, in line with the certificate; it should not be assumed that 5% oxygen is acceptable.
Inhibitor concentration should be confirmed on the certificate and, where required, by shore analysis at loading. Copper and copper alloys should be avoided because they can catalyse polymerisation. The tanks should be clean and dry, with no residue from the methanol or cleaning chemicals. Sampling should use closed equipment, and the cargo should be protected from overheating, contamination and prolonged storage beyond the inhibitor's effective life. The ship should record temperatures, oxygen readings and any instructions in the cargo log, and the chief officer should brief the watch on the difference between this cargo and a normal flammable cargo.
3. What signs would indicate that polymerisation may be starting, and what immediate actions would you take?
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Warning signs include an abnormal temperature rise that continues despite cooling, increasing tank pressure, a change in cargo appearance or viscosity, and strong odour or vapour evolution. A rising temperature is especially important because polymerisation generates heat and can become self-accelerating. The crew should compare trends, not just single readings, and should check whether cooling is actually working.
Immediately, the watch should inform the master and chief officer, increase monitoring frequency, verify cooling and stop any operation that adds heat or contamination. The ship should not inert the tank as a panic response, and should not open the tank or sample without control. The master should notify the charterer, terminal and technical superintendent, and prepare contingency plans for pressure control and possible cargo transfer if the tank approaches its limits. If pressure rises toward the relief valve setting, the priority is to protect the tank while avoiding ignition sources and avoiding any action that worsens the reaction. The incident should be recorded and investigated as a cargo emergency, not treated as a routine temperature deviation.