ISGOTT in plain words: gas, static, the ship/shore checklist and interview questions

ISGOTT — the International Safety Guide for Oil Tankers and Terminals — is the book every oil tanker and every oil terminal works by. It is not a convention, yet a vetting inspector, a terminal and a tanker company all treat it as if it were one. If you are going to a tanker, expect questions from it at the interview. This page covers the parts asked most: what the guide is, petroleum gas and its numbers, static electricity, the ship/shore safety checklist and enclosed spaces — with questions to test yourself at the end.

What ISGOTT is and why it matters

The first edition appeared in 1978. The guide is written by OCIMF (the Oil Companies International Marine Forum) with the International Chamber of Shipping (ICS) and the International Association of Ports and Harbors (IAPH). It is not an IMO instrument and has no force of law by itself. In practice that changes nothing: a terminal will not start cargo without the ship/shore safety checklist, a SIRE vetting inspector checks the ship's practice against it, and the company's safety management system refers to it on almost every tanker page. On board a tanker ISGOTT is mandatory in everything but name.

The sixth edition (2020) reorganised the book and expanded the guidance on gas detection, on toxic substances such as hydrogen sulphide and benzene, and on entry into enclosed spaces. The ship/shore safety checklist was restructured too.

Petroleum gas: why tanks explode

Hydrocarbon gas burns only within a narrow band — roughly 1% to 10% by volume in air. Below the lower flammable limit (LFL) the mixture is too lean, above the upper limit (UFL) too rich; but ventilating a "too rich" tank with air drives it straight through the flammable band. The other side of the triangle is oxygen: with less than about 11% oxygen by volume no mixture of hydrocarbon gas and air can burn at all. That is what the inert gas system is for:

• the system must deliver inert gas with no more than 5% oxygen by volume;
• cargo tanks are kept at 8% oxygen or less and under positive pressure;
• before gas-freeing, tanks are purged with inert gas until the hydrocarbons are below 2% by volume — only then is air let in, so the mixture never becomes flammable on the way.

Inert gas is required by SOLAS on new oil and chemical tankers of 8,000 DWT and above. Hydrocarbon gas is heavier than air: it lies on deck, in pump rooms and in any low space, which is why vapour is not "gone" just because the wind has dropped.

Static electricity

Oil moving through pipes and filters picks up an electrical charge. Some products — static accumulators, typically clean products such as gasoil, kerosene and gasoline — hold that charge for a long time; crude oil usually does not. A single spark in the vapour space of a tank that is not inerted is enough. The rules ISGOTT gives:

• at the start of loading, the speed in the pipe to each tank is kept to 1 m/s until the inlet is covered and splashing has stopped; then the rate can be raised (to no more than 7 m/s);
• after a static accumulator is loaded into a tank that is not inerted, wait 30 minutes before dipping, ullaging or sampling with metal equipment;
• everything lowered into a tank is bonded; no synthetic rope for sampling, no loose metal objects;
• the cargo connection carries an insulating flange or a single non-conductive length of hose, which stops stray currents between ship and shore — a ship/shore bonding cable is no longer recommended.

The ship/shore safety checklist

The checklist is completed jointly by the ship's responsible officer and the terminal's representative before cargo starts. In the sixth edition it is split by stage: what each side checks before arrival, checks after mooring, the pre-transfer conference with the agreed plan, and repetitive checks during the operation. Some items carry a code:

• R — re-check during the operation at the interval agreed in the checklist;
• A — the agreement or procedure must be in writing;
• P — if the answer is "no", the operation may go ahead only with the permission of the competent authority.

Typical items: moorings and fendering, safe access, communications and the stop signal, the emergency shutdown (ESD) agreed and tested, scuppers plugged, unused cargo and bunker connections blanked, the inert gas system working and pressure recorded, fire hoses and extinguishers ready, smoking and naked-light rules, the ship able to move under its own power. A signature here is not a formality: if an item is not met, cargo does not start, and if conditions change, cargo stops.

Enclosed spaces and toxic gases

Cargo tanks, the pump room, cofferdams, void spaces and ballast tanks are all enclosed spaces. Entry is by permit only, after ventilation and testing at several levels: 21% oxygen by volume, hydrocarbons at 1% LFL or less, toxic gases below 50% of their occupational exposure limit. A standby person stays at the entrance, communications are agreed, rescue equipment is laid out, and everyone inside carries a personal gas detector. Atmospheres change: test immediately before entry and keep monitoring while people are inside.

A large share of the people who die in enclosed spaces are the ones who rushed in to help. A rescue starts with the alarm and breathing apparatus — never with a dash into the tank.

Two gases deserve a separate word. Hydrogen sulphide (H2S) comes with sour crude and some products; at high concentrations it deadens the sense of smell, so "I can't smell it any more" means danger, not safety. Benzene is a carcinogen found in gasoline, naphtha and some crudes; exposure is limited by closed operations and protective equipment.

What they ask at the interview

• Ratings on tankers (AB, pumpman, OS): what LFL and UFL are, what protective equipment is worn for cargo work, what the ship/shore checklist is, what you do if you smell gas on deck, how an enclosed space entry is organised.
• Junior officers: oxygen limits for inert gas, the order of purging and gas-freeing, the initial loading rate and why it exists, the 30-minute rule, who signs the checklist and what R, A and P mean.
• Chief officers: the cargo plan and the pre-transfer conference, emergency stop and ESD, topping off, which cargoes are static accumulators, vapour emission control, vetting observations related to ISGOTT.
• Engineers on tankers: the inert gas plant (flue gas or a generator), the deck water seal and the non-return valve — they stop cargo vapour from flowing back towards the engine room — and the pressure/vacuum breaker.

Common mistakes

1. Treating ISGOTT as theory for the office. Terminals and vetting inspectors question the deck watch, not only the officers.
2. Ticking the checklist without checking. Every "yes" is the signer's personal responsibility.
3. Ullaging straight after loading kerosene into a tank that is not inerted — forgetting the 30 minutes.
4. Entering a "gas-free" tank on the strength of a morning reading.
5. Going in after a collapsed colleague without breathing apparatus.
6. Mixing up % LFL and % by volume. 1% LFL is one hundredth of the lower limit — for typical hydrocarbon gas about 0.01% by volume, a hundred times less than "1%".

Test yourself

Who on board needs this

Everyone on a tanker or a chemical tanker — from the able seaman and the bosun to the chief officer and the master; for engineers such as the second engineer, the inert gas plant above all. ISGOTT does not replace the tanker endorsements — basic and advanced training for tanker cargo operations under STCW V/1-1 are still required; it is the book the practice is learned from.

Source: ISGOTT, sixth edition (OCIMF, ICS, IAPH, 2020), and SOLAS chapter II-2 for inert gas. Figures are given as the guide states them; on board, the company's procedures and the terminal's rules apply. This page is a study aid, not a substitute for the book.