Crude oil tankers play an important role in the global oil industry. Tanker ships are the finest means of transport for unrefined crude oil products in mass quantity.
As tanker ships carry different grades of fuel oil in the same cargo tanks, it is important that the tanks are always washed before loading fresh cargo. If you work on a tanker ship or about to join one, it is important that you know everything about the crude oil washing operation on tanker ships.
What is Crude Oil Washing or COW?
Previously, oil tanks on crude oil tanker were cleaned by water, but this method of cleaning increased marine pollution and required bigger slop tanks to store leftover residue and oily water mixture. In order to prevent this problem, a better non pollutant way was introduced where in oil cargo of the tank itself was used to clean the cargo tanks.
When oil cargo is sprayed with pressure on tank walls and surfaces, the sediments sticking to the tank dissolves and converts into useful cargo which can be pumped out to the shore tanks. This system virtually eliminates the requirement of slop tanks on ships and almost all cargo can be transferred to the shore. This process is known as Crude oil Washing or COW.
Procedure for COW
Crude oil washing was made mandatory under MARPOL Annex 1 regulation 13 which states that every crude oil tanker which is 20000 dwt and above must be fitted with COW system for every cargo hold tank.
To perform Crude oil washing in correct and efficient manner every ship must have –
- Efficient and approved COW system and equipment manual
- Skilled Personnel onboard who are properly trained to perform the complete operation
The operation can be divided in to three phases where in following checks must be performed-
Before starting the operation
- Confirm all pre arrival checks are performed
- The complete COW operation to be discussed with ship and shore staff
- Set a communication channel between ship and shore facility for COW operation-Ship shore interface
- Signal and Emergency signs to be discussed to stop the operation between shore and ship staff
- Inert Gas plant to be working and oxygen content must be less than 5 %
- Fixed Oxygen analyser to be checked and calibrated for proper functioning
- Portable oxygen analyser should be made available and checked for proper functioning.
- Oxygen reading in swash bulkhead tanks must be taken from both the sides
- All tanks must be checked for positive inert gas pressure
- Assign duties to all responsible ship staff. One person to be assigned to check the leakage in the pipe line system as soon as the operation starts
- Check all the equipments under COW system for proper functioning
- Check and Set the line and valves for ship to shore under COW system
When the operation is under process
- The inert gas values to be frequently checked- Tank pressure and O2 value
- The crude oil washing must be done in the designated tanks as per the plan including the washing cycle
- A responsible person to be always present on deck
- All deck lines and valves must be frequently checked for any leakages
- Parameters and running condition of all the machineries involved in operation to be frequently checked
- Ullage gauge floats to be raised for the tanks which are being washed
- Trim should be sufficient to assist the bottom washing of tanks
- The level of holding tanks to be continuously monitored to avoid overflow
When the Operation is finished
- Drain tank wash line off crude oil
- Shut all the valves in the line used for the operation
- Stop and drain all the machines involved in the operation
- Drain all the cargo pumps after the operation is finished
Stop the COW operation immediately if you sense any kind of trouble such as failure of IG system or increase of O2 content and drop in the pressure of the cargo tank.