Vessel draft survey procedure

5 step for vessel draft survey:

1. Check the vessel condition, ship documents need for draft survey.

2. Reading the draftmark of the ship, which consist of six (6) points of draftmarks, i.e.; Fore, Midship, and After at both sides of the ship,

3. Sampling and testing the sea water or dock water density at the place where the vessel floats,

4. Determining of deductible weights by measuring and sounding of ballast tanks, fuel oil, fresh water that existing onboard at the time of survey,

5. Using Hydrostatic Table provided onboard to calculation.

Step 1: Check the vessel condition, ship documents need for draft survey.

Before go onboard the vessel, we need to quickly assessment  for the vessel condition and document include:

-          Trim and list

-          Trim and list correction sheet for draft and sounding of ballast water, drink water, bunker

-          Hydrostatic table, tank table/ sounding table

Step 2: Reading the Draftmark of the ship

Commonly, all ship are designed with draftmark for working with Draft Survey to determined their actual weight. The draftmark could be find at six (6) points on the below places:

Vessel draft mark

-          Use the small boat to go around the ship and get as near as possible to the draft mark for best viewing. The surveyor should be read all above marks clearly, because reading the draftmark is the first and most essential process. I am not saying that other processses is not essensial, but this process is hard to do and involves many rules of conduct to gain the correctness and accuracy of Draft Survey itself (I will post it later). The draftmark read is recorded on the surveyor notebook, do not try to remember it or write down in your palm hand. Its useless and un-professional.

Step 3: Sampling and testing the sea water or dock water density

After reading the draftmark, directly engage with the sampling of sea water or river water around the ship’s dock.  Why? Because the ship draft will not be the same at different water densities (at the lower density means the ship more sink and at the higher density means the ship more float).  Where as the water density is subject to changes which follow with water tide that carrying different water salinity and temperature on to the ship dock. The sea water density is indeed at density 1.025 and the fresh water at density 1.000. To determine the density of water, we need the instrument named Hydrometer or Density Meter. Inserted the Hygrometer on to the water sample on the Sampling Can or Tube, then we could check the scale pointed on the surface of the sampling water. Records the water dock density as survey data.

Step 4: Determining of deductible weights by measuring and sounding

Deductible Weight could measure by sounding  the tanks which used the Sounding Tape or gauging the tank level by visual inspection. Any deductible weight such as Ballast Water, Fresh Water, Fuel and Diesel Oil, and Bilges is notify to check. Records all in the survey book includes with the density for Ballast and Bilges, and for Oil complete it with density and temperature . The Fresh Water was at density 1.000.

Sounding pipe

Step 5: Using Hydrostatic Table provided onboard to begin calculation

The calculation is uses Displacement Table or usually called Hydrostatic Table. This table is included all data that we need to complete the calculation.

Raw Draft Calculation:

Fore Mean or Fm = (FP+FS)/2, 

Mid Mean or Mm = (MP+MS)/2,

Fore Mean or Am = (AP+AS)/2.

Apparent Trim  or AT = Am – Fm.

The Apparent Trim is the Trim that visually find.

Draftmark posision and correction to perpendicular:

As the ship draftmark is not placed at the perpendicular, the Fore and After draft should be corrected with distance from the draftmark to perpendicular. 

The correction rules is:

if the Trim by Stern, the Fore correction should be minus and After correction plus, and if the Trim by Head (stem), the Fore correction should be plus and After correction minus.

The Midship correction is parallel with the fore correction with the same pattern. Some Hydrostatic table provided with these correction result. But if not the reference pattern is:

Fore Correction or Fc = (Fd x AT) / LBM

After Correction or Ac = (Ad x AT) / LBM.

Where:

Fd = Fore distance to perpendicular,

Ad = After distance to perpendicular,

LBM = Length Between Mark or Length between Fore and After draftmarks  or LBM = LBP – (Fd + Ad).

True Draft Calculation / Draft Corrected:

Fore draft corrected or Fcd = Fm + Fc, 

Mid draft corrected or Mcd = Mm + Mc,

After draft corrected or Acd = Am + Ac.

True Trim or TT:

Actual  Ship Trim after draft corrected or  TT = Acd – Fcd.

Fore and After Mean Draft: FAm = (Fcd + Acd)/2,

Mean of Mean Draft: MM = (FAm + Mcd)/2,

Mean of Mean of Mean Draft : MMM or Quarter Mean = (MM + Mcd)/2.

The above calculation is similar with: MMM = {(Fcd x 1) + (Acd x 1) + (Mcd x 6)}/8.

Coresponding to the MMM or Quarter Mean result, the surveyor could check the value of needed parameters on Hysdrostatic table like; Displacement, TPC, LCF, and MTC. Records them accurately.

Get the Displacement or Disp.

First Trim Correction: FTc = (TT x LCF x TPC x 100) / LBP. Could be plus or minus depend on LCF.

Second Trim Correction: STc = (TT x TT x MTC x 50) / LBP. The result always plus (+).

Displacement corrected by Trim: DispT = D – (FTc + STc).

Density Correction: Denc = DispT x {(Aden – 1.025) / 1.025}. 

Where: the Aden is Actual Density that surveyor has taken sampling and testing previously.

The density correction commonly in minus (-), due to the Actual Density is usually lower than 1.025 (fresh sea water). In case of at some port where the water salinity is high, the density correction could be plus (+).

Displacement corrected by Density: DispDenc = DispT + Denc.

(after corrected by density we will get the actual ship weight as per shown by Draft Survey)

Deductible Calculation.

The same as draft, the deductible also need to corresponding to the table that named Tank Table / Tank Capacity Table. Refer to the sounding records that done before, the surveyor could be calculate the total deductbile existing onboard.

Total Deductible: Deduct =  Ballast Water + Fresh Water + Bilges + Fuel Oil + Diesel Oil,

This total should be minus to the Displacement corected by Density.

Net Displacement: NDisp = DispDenc – Deduct.

The Net Displacement is the actual ship weight after minus with deductible weight.

For Unloading, to estimate the quantity of cargo onboard, the Net displacement should be minus with Light Ship and Constant.

ITVC Global is a leading provider in inspection services in Vietnam expecially in Marine inspection services. With our experience inspector, we are now carrying out the Marine inspection services in Hai phong, Ho Chi Minh, Da Nang, Nghi Son, Vung Tau, Phu My... 

To request for the Marine inspection services - Please contact ITVC Global - you'll get useful advice and the best service.