Procedure for draught survey
Also called draft survey, draught survey refers to the method of determining weights loaded or discharged from one vessel basis the changes in initial and final displacements of the vessel. Draught survey is based on various principles as laws of buoyancy, Archimedes principle and water density. Draught survey has core functions including reading drafts, draft corrections, trim corrections, displacement, and deductibles among others.
A draught survey utilizes different types of equipment such as a strong torch, patent draught mark indicator, calibrated inclinometer or manometer, steel tape measure or stainless steel sounding tape, sea water sampling bucket, calibrated patent hydrometer, calibrated salinity refractometer, ballast water sampling device, and computer or calculator.
Further, draught survey is considered as the basis for which shipping documents are drafted. Documents created through draught survey are accepted to be reasonably accurate. As such, these documents are a convenient means to establish the weights of the cargo shipped on board vessels especially those cargoes that are in bulk.
An ample amount of time should be dedicated for draught survey. The ship master should be advised of the pending survey, and he or she must be requested subject to the safety of the vessel. When the survey is to be conducted by independent surveyors, the cooperation of the ship officers is critical. The role of the chief officer or chief engineer or the appointed respective deputies is crucial to make the draught survey a success.
Before the draught survey should be undertaken, it is highly recommended that the surveyor will make time to examine a general arrangement plan for the purpose of confirming the number and position of the various ballast, fresh water, and oil bunker tanks on the vessel.
With this, the importance of draught survey procedures is apparent. That is, there is a need to obtain a specific reasonable degree of accuracy for a more detailed draught survey. This requires that careful attention must be given when drafting the draught survey which when unusual circumstances arose, these will be handled in a proper and diligent way. Nevertheless, this can be only handled by individuals with due training and experience.
Clear procedures are needed even by the most highly trained and qualified surveyors. There are instances that while conducting a draught survey, the surveyor is under immense pressure from the vessel and also the shore terminal for hastening survey for the purpose of enabling the loading and/or discharging operations. With draught survey, ignoring or missing out on important aspects of the operations will be avoided.
When the surveyor is equipped with all the required perimeters, he or she can proceed with the actual calculations including correction to displacement for trim, correction to displacement for density, nett displacement, constant calculation, and cargo quantity determination, all of which are critical for any draught survey. Needless to say, surveyors should be able to perform calculations to the best of their abilities. This means that, as much as possible, each component of the draught survey must be accurate. Certainly, this can minimize procedural and/or measurement errors.
Indeed, errors are possible, and these will affect the quantity of cargo recorded through the draught survey as being loaded or discharged. There are cumulative errors that should be avoided. Errors may occur when reading and correcting draughts. Errors in the calculation may also be encountered. Errors of the water density in which the vessel is floating may be considered. With the possibility of the occurrence of these errors, a well-conducted draught survey under the reasonable prevailing condition is required in a capable achievement of absolute accuracy of +/- 0.5%.
In sum, because the variance of estimating tonnage within the ship transport chain, accurate assessment of load ensuring transactions are accurate is imperative. This is particularly the case since even a small error can cause large variances in the reported tonnage. This is also called the bill of lading weight. Inaccurate draught surveys are time-consuming, and not to mention, costly. When damages occur, the blame game will start and often point to the surveyor’s capability in carrying out an effective draught survey.
As such, when performing a draught survey, some factors may affect the accuracy of the survey including the accuracy of the hydrostatic tables, varying densities of ballast water or sea water, and even weather and sea state. These factors should be known to the surveyor himself or herself. This leads this report to note that another important factor is the person carrying out the survey itself. The surveyor must be trained and experienced and must have a keen attention to detail which is paramount in fulfilling the draught survey.
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