Why not to trust GA’s
When looking to charter a marine asset – such as a general cargo ship, heavy lift vessel, barge or tug – having a vessel survey early on is always a worthwhile exercise, especially if it will be the first time you intend to use the asset.
During the chartering process and vessel selection, you may be given access to a vessel’s General Arrangement (GA) and other associated drawings. Although GA’s and drawings can be beneficial and informative, having a surveyor visit the vessel to conduct a suitability and condition survey is of great value.
Design drawings are always subject to change once they hit the shipyard. Ship builds are fluid and often the design drawings do not fully reflect what is finally constructed. As-built drawings may or may not be completed dependent on the regime of the build yard. Items may have been added or removed in the build process, therefore it is not good practice to solely rely on supplied drawings for key engineering decisions.
The issue extends to sister vessels as well. Many vessels on the market are constructed as sister vessels and, as such, should be identical in every way. However, sometimes during the construction process, an owner may request some additional features or changes after the first or second vessel in a series have been built. You should be aware that such, later, modifications are not always captured on a vessel’s drawings.
For convenience, two sister vessels are often constructed side by side and are built as mirror images of one another, meaning where equipment is shown on the port side in a GA, it can actually be located on the starboard side of its sister.
If a GA is based on early stage design then it is entirely possible, you could be planning to roll cargo on over the side of a barge and, when it arrives alongside the berth, you note there are ballast tank vents and other deck furniture where you expected to place the link-span. Bollards and other deck furniture are often added in at the shipyard stage of the construction and can missing completely from the design drawings.
As well as intentional changes in some rare circumstances, due to build quality issues, a vessels transverse frame may be off by a few centimetres forward or aft, and thus not as equally spaced as the ships’ original drawings may indicate. This can cause issues if not checked well in advance, as you may find that where you expect to find a barge transverse frame to connect your grillage to, there is instead unsupported deck.
Finally GA’s might be correct in the detail that they do show but omit important detail that you need to properly engineer the job. For example, you may discover that you designed to weld your stoppers where there are the ship’s fuel tanks, as there was no tank plan shown on the GA.
The above just a short list of issues that incomplete vessel information can cause. As helpful as it is to have a set of vessel drawings, it can never replace the benefit of getting the intimate knowledge of a vessel gained from having a skilled surveyor visit the asset.
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