Corrosion to the internal surfaces of cargo tanks is a fact of life on oil tankers, as the steel plates and supports are subjected to corrosive gases, crude oil acids, and seawater along with temperature fluctuations and structural flexing. The effect of the corrosion is to reduce the thickness of the steel plates and supports, leading eventually over a period of time to failure of the structure.
In an effort to prevent this, anticorrosion coatings are applied to the tank internals and regular internal inspections of the tanks carried out to ensure any deterioration is dealt with in an appropriate and timely manner.
The following sections examine the inspection of tanks, the first section looking into the safety precautions that are to be observed by crew members and officers alike before and during tank inspection.
Tank Inspection Safety Precautions
Safety aboard oil tankers is paramount- still top of the list today as it was when I joined my first tanker as a Junior Engineer forty-something years ago, when safety was drummed into us young lads by the Chief and Second Engineers.
It was forcefully brought home as we passed close to the burned out hulk of a Greek tanker on our way up the Persian Gulf, and when the Old Man told me that it was caused by a Junior Engineer smoking in his cabin, of course I believed him!
Anyway I digress; it’s just an age thing and comes to all us old mariners. To continue then
The procedure required for entering an oil tanker cargo tank is controlled by a Permit to Entry system, in the form of a standard checklist. I have listed the fundamental requirements below, which when actioned must be witnessed and signed by the Master or Chief Officer.
A Permit to Enter Cargo Tanks Certificate (PTEC) will be issued once the following checks have been carried out. (Remember, this is just an overview so consult oil tanker rganizations such as the International Marine Organization (IMO), SOLAS or Oil Companies International Marine Forum (OCIMF) for current official procedures for entering cargo tanks.)
- Tanks must be certified gas-free.
Gas-freeing is the responsibility of the Chief Officer who uses the ships inert gas system, be it from a dedicated plant or from the ship's engines or boiler exhaust fumes. (See my article on gas-freeing of Cargo tanks).
Vent fans must be run in extraction mode during tank inspection and regular samples taken to ensure the air within the tanks contains more than 21% oxygen, less than 1% explosive gases with 0% toxic gases present.
- Gas monitoring
Tanks under inspection must be continuously monitored to ensure levels do not exceed recommended values, with this being carried out by officer on watch.
- Cargo tank valves
All oil suction and delivery valves to the tanks under inspection are to be shut, chained, and tagged. (Whilst sailing as a senior engineer on an old ESSO oil tanker, I had to make and insert spades to these valves as the mate thought the valves were passing.)
- Control of entry
A notice must be placed on the tank entry man-way listing the names and ranks of persons inspecting the tank.
- Personnel gas monitors
These must be worn at all times during tank inspection; any alarm must be followed by immediate exit from tank.
- Radio contact
Radio contact with personnel must be maintained at all times during tank inspection.
- Hand torches and extension lights
All hand torches and lights must be certified for use in hazardous places.
- Rescue equipment
All rescue equipment must be readily available in case of emergency evacuation of personal from the tank.
Inspection of Oil Tanker Cargo Tanks
Inspection of tanks will reveal any deterioration to the internals. Cargo tank steel plates are particularly susceptible to corrosion due to the following causes;
- Attack from the acids and fumes inherent to the types of oil they carry.
This is the most common cause of corrosion, but with regular inspections for bubbling or discolouration (tank coatings are usually of a light colour for easy identification of problems) and repair followed by adequate applications of anticorrosion coatings at yearly dry-dock inspection it can be controlled.
- Ship structure flexing
I remember well, on my first trip to sea, watching in amazement the deck rippling from the fo'c'sle head right back to the bridge due to the swells of the North Atlantic as we steamed on ballast towards the Suez Canal and then to our Persian Gulf loading port. I had read about the effect flexing ship structures had on main engine crankshaft and tailshaft alignment, but of course never gave a thought to the stresses the flexing was creating in the cargo tanks.
These stresses were working away, niggling at any small cracks or imperfections not only in the coatings, but in the steel plates and supporting structure.
- Steel used in construction
For a while some Japanese shipyards used high tensile carbon steel plates in the construction of their ships, allowing them to roll the plates with more accuracy, thus keeping the weight down and saving their Yen. However this also meant that the plates ended up being thinner, even before corrosion started eating into it, but thankfully the shipyards are back to using mild steel plates.
- Double hull vessels.
As a result of oil pollution caused by tankers hulls being holed when running aground, double hull design was made mandatory by a consortium of bodies such as SOLAS and IMO. However, it has been found during tank inspections that marine growth (larvae forming worms and crustaceans) normally found on internal side and bottom plates in single hulled vessels had increased dramatically on double hulled vessels.
This escalation in marine growth is thought to be caused by the heat provided by the double hull arrangement. The original single hull plates having been kept cool by proximity with the sea, the double hull acting as insulation with the heat encouraging the extra growth.
This problem is ongoing, however new anti-fouling coatings have proved an effective; but expensive, method of controlling marine growth.
- shipstructure: Strength of pitted plates in tanks
- shipsupplier: Procedure for Inspection of Cargo Tanks
- IMO: Tanker Safety & Pollution Protection