Few years ago, I have received the email from friend that asked regarding Inverse Draft Survey. The Inverse Draft Survey is doing when the ship has to split the cargo for two or more ports. The method is the same as common draft survey calculation, what the ship officer or surveyor needs to do is preventing cargo less or more discharge at the 1st port.
What is the purpose?
– To prevent cargo less or more discharge at one port.
– Surveyor can organize the work effectively.
– To prevent repetition of intermediate draft survey that cost a lot of time.
How is inverse draft survey works?
MV. NORTH SEA has loaded 10,000 MT cargo for China and Malaysia ports.
As Bill of Lading shown :
– 1st Port China with B/L Quantity: 4,000 MT,
– 2nd Port Malaysia with B/L Quantity: 6,000 MT.
At the 1st port the surveyor and ship officer carry out draft survey.
On initial survey report the following:
– Quarter Mean Draft : 7.62 M
– Net displacement : 13,487.20 MT
– Light Ship : 3,300.00 MT
– Constant : 187.00 MT
– Estimated cargo on board : 10,000.20 MT.
– Cargo to be discharged : 4,000.00 MT
– Balance for 2nd port : 6,000.20 MT
After doing initial survey the surveyor and ship officer will be estimated the final quarter mean draft to reach the discharge at 4,000.00 MT of cargo.
To estimate this figure, the ship officer discharging plan data is needed. Such as; how many quantity final of ballast water, fresh water, FO and DO, and expected trim (subject to change).
The calculation will be resulted the final draft survey quarter mean.
Before having the draft survey program, I usually doing the following rough calculation :
1. Determine the Final Net displacement after discharging (FNDAD)
FNDAD = Current Net displacement – Cargo to be discharged at first port
= 13,487.20 – 4,000.00 = 9,487.20 MT,
2. Added with Estimate deductible after discharging
plus (+) Estimate final deductible (ballast, FW, FO, DO),
let’s say deductible 347.30 MT
= 9,487.20 + 347.30 = 9,834.50 MT
3. Times (x) with sea water density and divide (:) with actual density
Where, sea water density: 1.025, actual density: 1.021
= 9,834.50 x 1.025 / 1.021 = 9,873.03 MT
4. Plus (+) 1st and 2nd trim corrections (as per estimate vessel trim)
for example: 1st correction = -16.37 MT, and 2nd correction = 1.25 MT.
= 9,873.03 + (-16.37) + 1.25 = 9,857.91 MT
5. Sum (=) corresponding displacement = 9,857.91 MT
6. Check the quarter mean draft at the table refers to corresponding displacement.
9,857.91 MT on hydrostatic table shown at Draft 6.29 M.
7. Sum (=) estimate quarter mean draft for final survey = 6.29 M.
Here the estimated final survey report after calculating:
– Quarter Mean Draft : 6.29 M
– Net displacement : 9,487.20 MT
– Light Ship : 3,300.00 MT
– Constant : 187.00 MT
– Balance for 2nd port : 6,000.20 MT
How is reconciliation?
As the quarter mean draft noted as rough figure, the spare draft is given to prevent any cargo losses, usually the quarter mean draft plus (+) 5 cm to 10 cm. For the above: 6.29 + 0.05 = 6.34 M. When the mid ship draftmarks is reaches 6.34 M, the discharging will be stopped.
During discharging the surveyor and ship officer will be monitoring the draftmarks (the key is at the mid ship port and starboard side draftmarks).
The intermediate draft survey is carry out to check the actual cargo discharge. For instance, When calculating the discharging cargo shows less about 100.00 MT. The rest is determined the cargo discharge by counting the grabs. Let says the grab capacity is 7.00 M3 with cargo density/stowage factor is 1.50 T/M3.
>> 7.00 x 1.50 = 10.50 MT cargo per lifting grab in full condition. (If the less is more than 200.00 MT, the cargo per truck estimation quantity could be used).
Firstly deciding to discharge at 8 grabs for safer. The last intermediate survey is made after that. Says the cargo less 25.00 MT. The next 2 and 1/2 grabs will be discharged to ensure the cargo in conform for the 1st port.
Then, the actual final draft survey:
– Quarter Mean Draft : 6.302 M (slightly different than estimated 0.012 M)
– Net displacement : 9,487.10 MT
– Light Ship : 3,300.00 MT
– Constant : 187.00 MT
– Balance for 2nd port : 6,000.10 MT.
As per 1st port final draft survey, we are confidence that the 2nd port will receive the intended cargo quantity as per document.
Tips:
– The declared constant is very important to check. Collected the last/previous 10 ships draft survey data to ensure the constant is right (at least last 3 shipments for loading and discharging).
– When the constant goes wrong and more than declared, the 2nd port will be faced cargo losses. (Actual constant at 210.00 MT, but declaring at 187.00 MT. Cargo losses about 23.00 MT).
– Considering the sea condition during survey. In practice, the sea condition was undermined the draft survey (especially in the anchorage area). It needs to give more spare draft for final survey.
The above as my survey experience, and hope that’s clear enough. Please inform me whether you have found another ways/ideas, as I still learning about draft survey as well.