CARGO TANK INTERNAL INSPECTION ON CHEMICAL TANKERS- CAPT AJIT VADAKAYIL
Chemical tankers will clean the cargo tanks and associated pipeline systems prior to declaring NOR and arrival at the load berth.
In modern times , it is essential to take plenty of pictures from all angles , with all ports open to allow daylight in. Once you load the pictures it in your computer, you can be surprised that the camera has pinged what your eyes could not --- and you can zoom in too.
A smart tank inspector knows the position of your tankcleaning machines, the blind sectors of the jet and if you gave given machine drops . He knows where to look.
Special attention should be paid to the condition of the tank coating in the case of epoxy coated tanks and, in particular, to the presence of blisters and flaking of the coating.
Unbroken blisters may contain residues from an earlier cargo as will loose or flaking coating.
Depending on the charterer or shipper requirements, the inspector may be asked to estimate the percentage of coating missing or damaged.
Reference to the vessel’s previous cargoes can have a bearing on the further action required from the crew when determining the likely effect the condition of the coating might have on the cargo to be loaded.
Attention should also be paid to the cargo tank, pipes, drop lines, pump casings, suction wells and heating coils. These items can form shadow areas and often receive less than adequate attention during tank washing.
Inspectors cannot, of course, visually inspect the internal condition of pipelines for their full length and the inspection report should clearly state the applicable limitations.
Vent and inert gas lines are usually visible at the tank entry point and could contain prior cargos, polymerized material, or washing residues, that if present, could contribute to contamination.
Deck lines will have drain points and these should be opened in case residues from tank washing operations or prior cargos are present.
Wall wash test by inspectors/ suryeyors :--
Tank surfaces must be dry.
Discoloured or broken coating sections on tank walls should be sampled as follows:
Where such areas are less than 20% of total surface area (excluding deckheads), the washings should be included with those of the rest of the tank.
Non typical areas should be sampled and tested separately. Samples should be tested in a shore based laboratory.
The minimum number of areas to wall wash depends on the tank capacity.
Tank capacity less than 500 cubic metres-- 5 areas
Tank capacity 500 to 1000 cubic metres-- 7 areas
Tank capacity greater that 1000 cubic metres-- 9 areas
Wall wash sampling procedures may cause a previously gas free tank to develop an unsafe atmosphere and personal protective gear should always be worn and breathing apparatus may be required.
There are limitations on the extent to which inspectors can visually inspect all parts of the loading system, vessel and shore, and so they will inquire about the steps taken by the vessel to avoid or reduce the risk of contamination.
Loading/discharge manifolds, vents, and inert gas lines can only be inspected where visually accessible. Extensive areas of the pump and piping systems can only be assessed on the basis of information obtained from the Chief Officer in respect of tank cleaning operations and previous cargoes.
Prior to loading “first foots” in tanks , an initial shore line sample should be drawn and visually examined to ensure the cargo is clean and bright and free of suspended matter. This first foot sample at the manifold must be drawn with the manifold valve SHUT ( you cant do this for screw pumps and reciprocating piston pumps )
Subject to wall wash tests (where applicable) and receiving laboratory clearance, the vessel will then be able to start loading “first foots”.
First Foot samples are required to determine whether any contaminants within the vessels loading systems have survived cleaning regimes that might have a detrimental effect on the cargo to be loaded.
Cargo Tank Coating Suitability
The following is to understanding potential contamination problems that may arise with chemical cargoes.
Zinc silicate, phenolic epoxies and pure epoxies are the most commonly used generic coating systems in cargo tanks for the carriage of chemicals.
They vary in their resistance to cargoes and all have limitations in carriage.
The principal limitation in the use of zinc silicates is that they are only suitable in a narrow pH
range of approximately 5.0 to 9.0.
Phenolic epoxies are generally able to carry a wider range of low molecular weight cargoes and have a higher free fatty acid resistance
The cargo resistance of coatings of the same generic type can vary between manufacturers and, hence, the manufacturers Resistance Lists usually available aboard the vessel should be consulted should there be any doubts as to suitability of any particular cargo.
Some coatings are temperature sensitive. Cargo carriage requirements may require heating, and temperature control should match the coating operability range.
Organic epoxy coatings have the propensity to absorb cargoes during a loaded passage. The contamination potential to a subsequent cargo is therefore considerable as:
Large quantities can be retained
The amount retained after different time periods is not well defined
Variable absorption/desorption characteristics are found between different coating types
Variable absorption/desorption characteristics are found within the same generic types from different manufacturers.
Different rates of absorption/desorption are found between different cargoes
Factors such as coating thickness, temperature and tank cleaning also have an influence on absorption and desorption.
With some chemical cargoes, absorbed material can survive both further intermediate cargoes as well as extensive tank cleaning operations
The shore tank inspector is NOT a bird. He goes down the tank ladder . On the way, he will touch and rub certain areas. Be sure that these areas are good.
At the tank dome itself , before entering , the shore surveyor will smell the tank, and sometimes this smell could come from dirty gasket of the tank dome cover. He will refuse to enter if the previous cargo has any carcinogenic percentage .
Cargo hoses used for looping forms part of the pipelines . Number them and have a list of the last 3 cargoes handles by these hoses.
If the previous cargo was insoluble in water, check the threads of all bolts inside the tank, minutely.
Sometimes it so transpires that after Methanol / potable Ethanol discharge, the tank was vented and the tank fails at the load port for the same cargo, to be loaded again. Just make sure the horizontals and slanted surfaces do NOT have any sediment, by rubbing a wet black/ white cotton lint free cloth.
Nowadays surveyors are hand in glove with shippers ( for a ulterior reasons ) and they fail the tanks for moisture condensation. Don’t be naïve. The principle to apply here is -- warm moist air, always condenses on a cold steel bulkhead. This is the reason why chemical tanker-men cannot be dimwits.
CAPT AJIT VADAKAYIL
29 years in command