Marine Engineering: Inert Gas Protection and Cleaning of Oil Tanker Cargo Tanks

Crude oil and refined hydrocarbons have been shipped in the cargo tanks of oil tankers for many years. The oil tanker design and carrying capacity has altered dramatically in the interim, however one thing that has remained unchanged is the production of toxic fumes produced by the various cargoes

Oil tankers carry various hydrocarbons in their cargo tanks, from crude oil to high octane spirits such as petroleum.

These cargos are loaded by shore-side pumps at different ports of oil producing countries or oil refineries throughout the world. When the destination port is reached the tankers own cargo pumps located at the bottom of the pump-room, discharge the oil.

The pumps discharge the cargo ashore through the chiksan arms; a specialized component connecting the ships discharge pipes to the jetty pipes. (Some modern tankers as the one shown in the Introduction, have these arms incorporated in the tanker deck)

Meanwhile inert gas is supplied to the air space left by the oil and this expels the gases to atmosphere or a specialist gas treatment plant where the gas is treated before admittance to atmosphere.

On completion of cargo discharge the tanker leaves port for its next cargo and in the old days once well clear of land the mates (Deck Officers) always wanted to start gas freeing and clean the tanks. I was an Engineer Officer with Esso Tankers for many years and this was the worst part of any voyage. The whole ship stank of oil fumes and, the mates used our mess-room for meals bringing the reek of oil fumes with them enough to put any ships engineer of his beer - well nearly!

However, I diverge. The tanks are gas-freed using inert gas processed from the main engine, boiler fume exhausts or nowadays from a dedicated inert gas plant. These gases are forced through the cargo tanks by fans and once the oxygen content is at the required level, the cleaning commenced.

Tank cleaning methods have changed over the years the modern one uses pressurized crude oil to wash down the internal surfaces, so gas freeing is sometimes not required. However it is worth examining this process as the tanks still require gas freeing before inspection for corrosion.

This is another article in my series on marine engineering and we shall examine the use of inert gas in preventing explosions and gas-freeing of oil tanker cargo tanks. We will also have a look at the methods to clean cargo tanks when I was at sea, and the current methods.

We begin then with the operation of a typical inert gas system.

Inert Gas System Operation

The inert gas system comprises of the following components;

• A source of inert gas
• Gas Scrubbing
• Inert gas circulation
• A non-return mechanism

• Source of Inert Gas

The gas can be sourced from the main diesel engine exhaust, the main boiler flue gases or a designated inert gas plant.

• Gas Treatment

This comprises of a scrubber which quenches and cools the gas by spraying seawater from strategically placed nozzles onto it as it passes up through the scrubber tower. The gas exits through the demister section at the top of the scrubber.

• Inert Gas Pressurizing Component

This consists of two centrifugal fans or blowers which draw the inert gas through the scrubber and discharge it into the non-return mechanism. A pressure control valve ensures a correct gas pressure and also acts as an automatic shutdown mechanism in the event of a system failure.

• Non-return Mechanism

This consists of a set of non-return valves and a water seal which prevents any hydrocarbon fumes from returning to the oil tankers engine or boilers exhaust system.

From here the inert gas is piped to the cargo tanks via the inert gas distribution system.

The use of Inert Gas on Oil Tankers

Inert gas is used on oil tankers as a means of preventing the risk of explosion and as also to gas-free the tanks if required before cleaning commences. The tanks must also be gas free and purged before any internal inspections are carried out.

The tanks will already have been smothered in inert gas as part of the cargo discharge process, mainly as an explosion deterrent. If gas freeing is required before tank cleaning, the tank vents are opened and inert gas circulated through the cargo tanks.

Sketches of this process are shown below; please click on images to enlarge.

The Mate carries out regular tank gas tests and once the danger from oil fumes has been eradicated he will shut down the inert gas system, topping the tanks up as required.

This completes the gas freeing of the cargo tanks, the procedure being the same before tanks can be internally inspected for corrosion or marine growth. A rigorous and strict tank entry permit system is mandatory and must be applied before any tank or enclosed space is entered.

Cleaning the Cargo Tanks

As I said in the introduction, when I served as engineer on oil tankers many years ago, the tanks were cleaned by means of a Butterworth pump. This device was supplied with saturated steam and lowered into the slowly down the tank, spraying the sides with steam and hot water. I seem to remember an oil emulsifier also being used, to break up the globules of oil.

This all ended up on the bottom of the tanks and was pumped to the sludge tank, the remainder being removed manually.

In the 1970’s, a method using crude oil to wash the tank sides and bottoms was innovated. This system has become mandatory on new builds, having been internationally approved and recognized as the optimum method for tank cleaning.

This method is known as Crude Oil Washing (COW) and uses pressurized crude to clean the tanks, with a final hose down with water.

Crude Oil Washing used the cargo as a cleaner the oil being sprayed onto the surfaces effectively removed remaining oil residues which were clinging to the tank surfaces following cargo discharge. This had twofold advantages in that it reduced pollution and saved the tanker owners money as there were virtually no slops and the oil used in cleaning can be discharged with the next cargo, instead of being pumped ashore as slops.