Question 1

After crude oil discharge, you are required to prepare three tanks for the next cargo (clean petroleum product). Describe the tank cleaning process.

Accepted Answer

TANK CLEANING — CRUDE TO PRODUCT: Cleaning crude oil tanks to accept clean petroleum product requires: MARPOL Annex I controls on all washings; strict IGS discipline throughout. PROCESS: (1) CRUDE OIL WASHING (COW): on crude tankers over 20,000 DWT — COW is mandatory (MARPOL Annex I Reg 33). Using crude oil cargo as the washing medium — crude oil jets dissolve and remove waxy deposits more effectively than water. COW during stripping removes more cargo and reduces slops. COW must only be carried out in port at a terminal equipped to receive slops; (2) BUTTERWORTH WASHING (WATER WASHING): after COW or for vessels without COW — water washing machines (butterworths) are lowered into the tank on hoses and rotate to spray hot or cold seawater. Multiple washes required for changing from crude to clean product; (3) SLOP MANAGEMENT: washings are collected in slop tanks. MARPOL Reg 29 — slops must be retained on board and discharged at a reception facility (or discharged at sea under strict Reg 34 criteria: >50nm from land, below 30 litres/nautical mile, and underway); (4) GAS FREEING: after water washing — ventilate tanks to gas-free status. Monitor with multi-gas detector — must be below 1% LEL before personnel entry; (5) TANK ENTRY (INSPECTION): once gas-free and O2 within safe range — enclosed space entry procedure. Inspect for remaining wax, scale, or residue; (6) MARPOL OIL RECORD BOOK: ALL operations must be recorded in the Oil Record Book Part 1 (machinery spaces) or Part 2 (cargo/ballast) with times and positions. MARPOL Annex I Regulation 33 — COW: crude oil tankers of 20,000 DWT and above are required to be fitted with a crude oil washing system and to carry an Operations and Equipment Manual (OEAM). COW effectively reduces the quantity of slops generated and gives better cargo outturn — but the washings must still be handled under MARPOL rules. MARPOL Annex I discharge criteria for slops (option C): the 12-mile figure is wrong. The MARPOL Annex I Reg 34 special area and general requirements differ, but for crude oil tankers, the standard discharge criteria include: 50 nm from nearest land; below 30 litres per nautical mile; total quantity not more than 1/30,000 of the cargo. Outside special areas. ANY overboard discharge of cargo residues inside special areas (Baltic, North Sea, Antarctic, Arctic, etc.) is PROHIBITED. The Oil Record Book (option D): COW does NOT exempt from ORB entries. Every tank washing operation, slop discharge, oil-water separator use, and ballast pump operation must be entered in the ORB Part II, signed by the responsible officer and the master. False ORB entries are a serious criminal offence — the vessel can be detained and officers prosecuted. 24-hour ventilation (option B): natural ventilation is insufficient and unreliable for gas freeing. Forced ventilation using blowers is required. O2 and hydrocarbon levels must be tested with calibrated equipment before any personnel entry.

Question 2

As Chief Mate on a product tanker, explain the purpose and operation of the inert gas system before cargo discharge.

Accepted Answer

INERT GAS SYSTEM (IGS) — CM KNOWLEDGE: PURPOSE: The IGS prevents flammable hydrocarbon vapours from reaching an explosive concentration in cargo tanks. A hydrocarbon-air mixture is flammable between approximately 1-10% hydrocarbon concentration (LEL to UEL). The IGS keeps O2 concentration below 8% (SOLAS requirement: below 8% O2 by volume) — at this level, the vapour cannot ignite regardless of hydrocarbon concentration. SYSTEM COMPONENTS: (1) SOURCE: combustion gas from the ship's boiler or dedicated inert gas generator (IG generator using gas turbine or diesel exhaust). Typical O2 content of flue gas: 2-5%; (2) SCRUBBER: removes corrosive components (SO2, H2S, soot) from the flue gas before it enters tanks. Water is the scrubbing medium; (3) BLOWER (FANS): propel the scrubbed gas into the distribution main; (4) DECK WATER SEAL: the critical safety device between the blower and cargo tanks. Prevents backflow of hydrocarbon gas to the machinery space if fans fail. Always maintained wet; (5) P/V BREAKER: secondary protection against overpressure/underpressure in tanks; (6) O2 ANALYSER: continuous monitoring of O2 content in the IG supply and individual tanks. PRE-DISCHARGE PROCEDURE: (1) Check O2 content of all cargo tanks — must be below 8% by volume; (2) Ensure IG pressure positive in tanks before beginning discharge (tank pressure > atmospheric); (3) Start IG supply to replace vapour volume as cargo is discharged; (4) Monitor O2 levels throughout discharge; (5) Never open tank in oxygen-enriched atmosphere without gas-free certificate. IGS SOLAS requirement: SOLAS II-2/4.5.5 — inert gas systems are required on crude oil tankers over 20,000 DWT and product carriers over 40,000 DWT. Failure to maintain the IGS is a detainable deficiency under PSC. The flammability triangle: three elements required for fire — fuel (hydrocarbon vapour), oxygen (above 10-11%), and ignition source. The IGS removes oxygen below 8%, eliminating one corner of the triangle. O2 below 8%: at this level, even if there is 100% hydrocarbon vapour in the space, the mixture cannot ignite. This is the safety principle of the IGS. The deck water seal: critical safety device. If the IG fans fail, backflow of hydrocarbon vapour toward the boiler (ignition source) must be prevented. The water seal provides a physical barrier. If the seal runs dry — the IGS is compromised. The CM must ensure the water level is maintained. CO2 confusion (option C): the IG system uses boiler flue gas (N2 + CO2 + water vapour — low O2), NOT pure CO2. The CO2 fixed fire system is separate and is used for firefighting in cargo holds on other vessel types. IGS during discharge (option B): during discharge, as liquid cargo leaves the tank, the vapour space increases. If the tank is not kept under positive IG pressure, air could be drawn in through vapour venting points and create a flammable mixture. IGS must run continuously throughout discharge.

Question 3

Your crude oil tanker is 4 hours from the terminal. As Chief Mate, what pre-arrival preparations do you coordinate?

Accepted Answer

PRE-ARRIVAL CRUDE TANKER — CM PREPARATIONS: (1) SHIP-SHORE COMMUNICATIONS: make contact with the terminal. Confirm: berth availability; cargo order; hose/arm connection details; cargo plan and loading/discharge sequence; free flow/simultaneous operations restrictions; emergency shutdown procedures; (2) CARGO PLAN: prepare/review the loading or discharge plan — cargo sequence, tank allocation, ullages, API gravity, temperatures, transfer rates; (3) STABILITY: calculate stability at all stages of the cargo operation. Ensure GM remains positive throughout. Identify the critical condition (minimum GM stage); (4) TANK CONDITION: check all cargo tank O2 levels — below 8% required. Note temperature and pressure of cargo and tank atmosphere; (5) MANIFOLD PREPARATION: inspect manifolds — blank flanges removed, drip trays clear, manifold valves operational; manifold pressure rating confirmed compatible with terminal pumps; (6) MOORING PLAN: confirm berth orientation; prepare appropriate mooring lines; (7) PUMPROOM INSPECTION: access pumproom — check bilge level; inspect pump seals; ensure cargo pumps ready for operation; (8) CARGO HEATING: if heated cargo — ensure steam coils operational and steam supply available; (9) DECK EQUIPMENT: cargo deck valves operated (any seized valves to chief engineer); hose handling equipment ready; (10) ISGOTT SHIP-SHORE SAFETY CHECKLIST: complete the ISGOTT pre-arrival section of the Ship-Shore Safety Checklist (Part A — ship's responsibilities) before the terminal vessel inspector arrives; (11) HOT WORK SUSPENDED: ensure all hot work is completed and stopped before entering port limits; (12) EMERGENCY SHUTDOWN: brief the duty officer on the ESD procedure and agree a shore contact name. ISGOTT (International Safety Guide for Oil Tankers and Terminals): the industry reference document for tanker terminal operations. The Ship-Shore Safety Checklist in ISGOTT: completed jointly by the vessel and the terminal before cargo operations begin. Part A covers vessel systems; Part B covers terminal systems. Both must be completed and signed. CM role in pumproom (option D): the pumproom is a critical space on a tanker — it contains all cargo pumps, stripping pumps, ballast pumps, valves, and pipework. Pumproom entry has caused numerous casualties (enclosed space, H2S accumulation, flooding). The CM has direct responsibility for cargo operations, which includes ensuring the pumproom equipment is operational. Chief engineer: responsible for maintenance of the cargo pumps as machinery, but the CM is responsible for the cargo operation, which requires those pumps. The Manifold safety: manifold flanges must be compatible between ship and shore hose/arm flanges. The pressure rating of the shore supply must not exceed the ship's manifold rated pressure. MARPOL Annex I: on a crude oil tanker, any oil discharge or spill must be reported. The pre-arrival preparation includes ensuring all drain plugs are in drip trays and that the manifold area is ready to contain any hose drips. Cargo heating: for high-viscosity crudes, the cargo may need to be maintained at 40-60°C or above throughout the voyage. The CM must check that the heating system is functional before arrival at the discharge port.