Lightering (also called lighterage) is the process of transferring cargo between vessels of different sizes, usually between a barge and a bulker or oil tanker. Lightering is undertaken to reduce a vessel's draft in order to enter port facilities which cannot accept very large ocean-going vessels. Lightering can also refer to the use of a lighter barge for any form of short-distance transport, such as to bring railroad cars across a river.
Lightering was practiced for all types of cargo for centuries. The practice became more widespread with the 19th century introduction of steamships too large to enter some of ports they intended to serve, in which case lightering became necessary to reduce the vessels' draft sufficiently to enter the port. Dredging, advances in dock construction, and containerization have reduced the frequency of the practice in dry bulk shipping after the middle of the 20th century. However, the practice remains in common usage in the oil tanking industry ("wet" cargo trade).
Modern process 
Wet bulk 
Lightering for tankers typically occurs in the EEZ, generally between 20 nautical miles (40 km) and 60 nautical miles (110 km) from the shore, and can be performed while the ships are at anchor, drifting, or underway. The product is typically transferred using specialized hoses which offload cargo from the larger vessel to the smaller. Fenders are used to separate the two ships moored to each other and prevent damage while the cargo is being transferred.
Dry bulk 
In many developing nations, such as China and especially India, dry bulk vessels still often lighter in order to meet draft restrictions at ports that either do not have natural deep water access or whose channels have yet to be dredged to sufficient depth to allow some of the larger-size bulk carriers to safely transit.
In dry bulk, lightering can be undertaken one of two ways. If the vessel to be lightered is geared, then it can discharge cargo to smaller, ungeared vessels (typically barges). If the vessel to be lightered is gearless, then floating cranes are often used to transfer cargo to another vessel or barge.
Also, although not very common, vessels will sometimes lighter before (or even between) berthings, shifting to shallower berths in order to discharge more quickly and also free up space for larger vessels.
See also 
- 'Adventures in Energy' animated description of lightering
- SPT Maritime Services information on lightering