Taking in Fuel Oil for a Voyage
A typical modern ship relies on the combustion of fuel oil to power her engines. No matter whether it is a steam ship powered by steam turbine, or a motor ship powered by diesel engines, a tremendous amount of fuel oil is consumed during a voyage. A ship has to store enough fuel oil to last throughout its voyage. It would indeed be a disaster if a ship were caught in the middle of the vast ocean without fuel.
Because of its importance, the Chief Engineer is directly in charge of the fuel. He has to keep track of its usage and its remaining quantity, at any time during a voyage. He has to order sufficient quantity in a timely manner and to use it efficiently. For economical reasons, he must not take in too much excess fuel because it would weigh down the ship.
Bunkering is a term used for receiving heavy fuel oil, diesel oil and lubrication oil directly pumped into the tanks of a ship, for its own engines. For oil tankers, the cargo oil received into the cargo tanks is not referred to as bunkers but as cargo oil.
Bunkering can be done from a floating barge when the ship is at anchorage or from the shore when the ship is alongside the wharf. In both instances, a big hose will be connected from the supplier to the ship.
Some precautions need to be taken before and during bunkering operations:
- No smoking is allowed.
- All scuppers from the deck of the ship are plugged.
- Sawdust or other absorbents are kept ready for use.
- Sounding tapes and rags are kept ready for use.
- Oil dispersing chemicals are kept ready for use.
- All personnel involved have been briefed on their duties.
Because there are many fuel oil tanks on a ship, usually a senior engineer will be at the engine room to operate the filling valves. There will be another group of people on deck to take soundings of the tanks as the oil fills up. The idea is to fill up the tanks in a systematic way so that the oil will not overflow and cause pollution to the environment.
In practice, this can sometimes be quite tricky and all the personnel involved in bunkering need to be extra vigilant.
In winter time, the oil becomes thick, and the level shown on the sounding tapes can be mistakenly read. When bunkering is done at the same time as cargo loading and unloading, the ship tends to heave from one side to another. This will cause error in reading of the oil levels. Miscalculation of tank levels, especially when using the ullage method may lead to wrong readings. Valves at the engine room could be wrongly operated, due to wrong information being relayed from the deck. Sometimes air bubbles trapped in the sounding pipes can indicate a different oil level from the actual. The supplier may be pumping too fast. The list goes on...
How do you check the level of the oil in the tanks during bunkering?
Basically, it's done by measuring tapes. The end of a sounding tape is attached to a metal cylinder with a cone shaped nose. The cylinder with its attached tape is lowered into a sounding pipe on the deck of the ship. The sounding pipe is directly attached to the double bottom tanks of the ship. The person lowering the weight can feel it when it strikes the bottom of the tank. He will then pull the tape up, being careful not to create any kinks in the tape. The oil-wetted part on the tape is the level of the oil from the bottom of the tank.
The above method is messy because the oil needs to be wiped off for the next sounding. If we don't wipe it off, it might smear the sounding pipe and cause errors.
Another method is by using ullage. In this method, instead of dipping the whole sounding tape and weight in oil to find the level, we calculate (or use ship construction references for finding) the height of the deck from the oil level and use this as a basis for calculating the oil level from the tank bottom.
The ullage tape has also got a weighted cylinder at the end. Instead of a cone shaped nose, this one has got a hollowed concave shaped depression at its end. The weighted cylinder will make a popping sound when it strikes the surface of the oil. The person who is doing the sounding needs to be alert to find the point when the cylinder strikes the surface. Normally, he will make a few short pulls each time he lowers a bit of the tape so that he can determine when the tape reaches the surface of the oil.
Although this method is less messy, it could give rise to miscalculations and has to be use with care. Instructions on the correct calculation figures must be very clear.
At the end of the bunkering, normally the supplier will blow through the hose with compressed air to clear it. The people doing the bunkering must be careful even up to this stage. Sometimes oil spills can occur even at this time at the end of bunkering.