Determination of cargo loaded or discharged
Draft surveys are a very common type of survey, they are carried out on many bulk shipments. The draft survey may be requested by shipper, receiver, charterer, shipowner, P&I club, superintending company, or other interested party.
The reasons for making draft surveys are normally to determine cargo loaded, but can also be made when a vessel is having multi port load or discharge.
A draft survey made on a vessel is considered to be a more accurate method of determining cargo, than any other means, although some highly sophisticated weighing systems claim to be accurate. However it should always be noted that any mechanical system is prone to failure, whereas a draft survey carried out on a vessel is static…..there are no moving parts!
C&G have developed a number of methods to increase the accuracy of draft surveys.
Draft damper tubes, used in locations where swell may cause difficulty in making accurate readings.
It is now common practice in bulk loading ports, for terminal operators to require vessels to employ the surveyor to undertake the trimming at the end of loading.
There are sound practical and commercial reasons for this, the surveyor is more accustomed to making the surveys, and it is nearly always a time saving operation. The saving of thirty to sixty minutes may seem trivial, but the ‘knock on’ and cumulative effect can be substantial, when considering the power required to run machinery in idle mode, the effect of delaying vessel sailing, and when there are other vessels waiting, over a period of time the cost saving becomes considerable.
By employing the surveyor, the possibility of vessels overloading is drastically reduced. Not only is the risk factor reduced, there are other positive benefits:
- Cargo lift can be maximized
- The time taken to complete the trimming calculations is usually a lot quicker than relying on the vessel alone, this is not to say that vessels are not capable of doing the trimming calculations, but from a practical perspective the surveyors have local knowledge, both with interaction with the shoreside facility, but also natural phenomenon (the rapid density changes in the Hunter River, Newcastle) is an obvious example.
- The surveyors, it should be added, perform this operation on several hundred vessels a year, whilst a ship’s crew may make half a dozen or less during their contract aboard.
C&G are currently exploring a portable draft gauging system, that utilises laser sighting, and feeds data directly to a tablet app, and while this will not replace physical draft reading, it will allow operators, terminals, and ship’s crew to effectively monitor drafts.
Accuracy of a Draft Survey
This is a question often asked, and the answer, unfortunately can never be fully answered, to explore this question though, there are several considerations, errors can occur for a number of reasons, and by minimising possibility of errors, the overall accuracy of a draft survey can be increased.
- The condition of the sea – high sea or swell, can be compensated by use of draft dampers
- Slack ballast tanks – whilst most vessels have accurate tables to determine actual ballast quantity, soundings may be inaccurate, dues to possible ‘voids’ , or fluid movement in the tank. C&G always urge vessels to ensure tanks are ‘pressed up’ or empty.
- Double defections – the theory of draft surveys, assumes that when a vessel bends, it is a uniform bending, and the formula used to calculate displacement (based on simpson’s rules) allows for this. When a vessel is in heavy ballast however, or is part loaded, the hull bending is not uniform. C&G have an extensive database of vessels, and gathered considerable literature from naval architects etc, to establish how much of an error can occur due to this effect.
- Vessels not ideally suited to draft surveys. Some vessels, usually smaller – non bulk carrier – are not ideal for conducting draft surveys. C&G have an extensive database of vessel types, and issues with particular classes of vessels are flagged.