Large-scale transportation of crude oil helps conserve energy and reduce costs over long distances, but many large tankers can't access U.S. ports when fully loaded because their draft - the distance from the water line to the bottom of the boat - is too deep. Lightering - transferring crude oil from supertankers to smaller tankers capable of navigating our waterways - is a simple but effective solution to the problem. More than 25 percent of the 7.5 million barrels of crude oil imported to the United States every day is lightered, mostly in the Gulf of Mexico and along the East Coast.
Lightering can take place either underway at sea or at anchor. Rubber fenders are placed over the ships' sides to prevent damage to their hulls. Crude is offloaded through flexible pipelines from the larger ("mother") ship to the smaller ("daughter") tanker or barge. The daughter ships can then offload their oil cargo at the nearest port or at more distant locations, as markets demand. In some cases, the mother tanker can also proceed to port once sufficient crude has been off-loaded to reduce the ship's draft.
For example, fully loaded Suezmax tankers enter the Delaware Bay with a draft of 55 feet and are lightered to less than 40 feet. The lightered tankers can then proceed to refineries located up the Delaware River.»next