A merchant ship is designed and constructed to transport tons of cargo between ports and in addition to cargo, the ship must transport in its own tanks, fuel oil, diesel oil, and various grades of lubricating oil in bulk for the propulsion plant and other auxiliary machinery systems.
Ships also have strategically placed ballast water tanks on both sides to stabilize the vessel when oil or cargo is being loaded or unloaded. Tanks are also supplied for the storage of potable water or feed water for the system.
In order to maintain the ship’s stability, and safety, it is required to monitor the fluid levels at regular intervals. The technique of determining the fluid level in a ship’s various tanks is known as “sounding” of the tanks.
During bunkering operation is is imperious to check the fluid level in tanks at regular intervals, using vessel remote sounding instrumentation or by using a sounding tape. Sounding tape is a measuring tape (in meter or inch) normally made up of brass and steel with a weighted bob attached at the end of the tape using a strap hook.
Despite of vessel in-built sounding method, at the end of bunkering operation the sounding tape value is subsequently utilized in the calculation of the final sounding value, which is determined using the sounding table, taking into account the list and trim of the ship and the temperature at which the fluid (particularly oil) is stored as density of oil is effected by temperature.
Sounding table is a table containing capacity and most importantly the volumetric content of the tank at given depth of sounding or ullage and all vessels have their own specific sounding table documents for each tank containing fluid in bulk. The sounding table is compiled to show the volumetric quantity of fluid at various trims and list for the particular sounding depth in cm.
On board ship, it is essential to maintain an accurate record of the amount of liquids (in all forms) contained in each tank. A ship is equipped with several forms of automatic and hydraulic/ pneumatic/ mechanical sounding measurement systems, allowing the liquid level to be monitored remotely or locally without the need for manually measuring and calculating the amount of liquid within the tank.
However, one cannot rely solely on automation and mechanical devices and manual sounding is always favored in order to reconfirm the fluid level in the tanks, assuring that the tanks will never overflow or run dry.
Malpractices by bunker suppliers during bunkering operations have a significant influence on your Company’s fuel expenses and it is a direct loss that can be prevented with careful observation and attention from the vessel’s crew. It is critical that the vessel’s bunker operating staff takes barge tank measurements carefully, applying the necessary trim/list before and after bunkering, recording the actual temperature of the bunker fuel before/after delivery, and so on. You can read about bunkering operation in here!
In order to learn how to correctly do the tank sounding, regardless of their destination, please watch patiently the below self explanatory video which capture the entire sounding and calculation process.
After you have learned how to correctly perform the bunker tank soundings, the next step is to convert these readings in quantity using, as specified above, the sounding tables applying trim/list and temperature correction.
For exemplification purpose only, let’s consider that vessel receives bunker in only one tank (if vessel receives in multiple tanks the calculation is similar, but quantities must be added up for final quantity). In order to make an accurate calculation of how much fuel has been received, usually below procedure applies:
- take soundings or ullages of all the tanks that have received fuel;
- we first need to know the volume received, vessel trim and heel and the observed temperature of the fuel. We will use the below sounding and correction tables for exemplification only.
- for exemplification purpose, let’s pretend that our ullage reading is 473 cm (so sounding will be 800 cm or 8 m), temperature of delivered fuel is 43 °C. and ship’s condition at the time of sounding is assumed to be as follow:
- Forward draft: 8 m
- Mid draft (P): 8.70 m
- Mid draft (S): 9,30 m
- Aft draft: 10 m
- Heel to starboard: 0.60 m
- from the above data:
- vessel trim: 8 -10 = 2.0 m (by the stern). As per table if vessel trim is by stern the reading is -2.0 m.
- heel angle: Θ = tan-¹ 0.60/30.2 = 1.138 deg. (stb’d)
- therefore the correction value of sounding depth due to heeling is obtained from the “Correction Table due to Heel”
- according to the table, heeling correction value:
0.500 : (1.138 – 1.000) = [-0.019 – (- 0.012)] : [X – (- 0.012)]
X = -0.012 + [0.138 * (- 0.007)]/0.500 = -0.014 m
So, Heeling correction value is: – 0.014 m
Sounding depth corrected for heel is: 8 + (- 0.014) = 7.986 m
- the volume of the tank is calculated from the “Sounding Table” and in our case having a trim of 2 m by the stern and as per attached above sounding table , the actual volume of the tank will be:
2.0 : (798.6 – 798) = (504.99 – 503.66) : (X – 503.66)
X = 503.66 + (0.6 * 1.33)/2.0 = 504.06 m³
So, our measured volume with heel and trim correction is: 504.06 m³, at a temperature of 43 ºC.
- now, after determining the observed volume, you must correct it to the standard of 15 0C and for this you will need to know the density @ 15 ºC of the delivered fuel which can be find into the Bunker Delivery Note (BDN). In our case let’s pretend, for exemplification purpose that the density @ 15 ºC is 996 kg/m³
- once we know the observed volume, temperature and density the next step is to multiply the observed volume with the temperature correction factory (VCF) found in tables such as ASTM 54B or by using a dedicated app or software.
In our case, through interpolation, the VCF will be: 0.9826
So as per above, the gross standard volume is: 504.06 x 0.983 = 495.490 m³ @ 15 ºC.
- as the density is an absolute relationship between mass and volume in a vacuum, it will not be the same as weight to volume in air. So the amount of tones received is:
495.490 x 0.996 = 493.508 Mt in vacuum
- to find the correct density in air, we must multiply the given density with a Weight Correction Factor which can be found in table ASTM 56.
In our case the correct density in air is:
0.996 x 0.99895 = 0.9949 kg/m³
- once this is done, we can multiply the standard volume @ 15 ºC with the corrected density to find the amount of tons of fuel received. So, in our case this will be:
495.490 x 0.9949 = 492.963 Mt in air
It’s critical to have an accurate figure of the density of the fuel, which can later be confirmed by the testing laboratory.
In conclusion, it is of utmost importance that a proper sounding is carried out, applying the necessary trim/list before and after bunkering, recording the actual temperature of the bunker fuel before/after delivery, and so on. Simply measuring temperature correctly can save a lot of money!
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