Ships began crawling up the Mississippi River at New Orleans in a tightly controlled procession Friday, two days after a massive oil spill shut down a stretch of one of the nation's most critical commercial arteries. The pecking order was based on Coast Guard determination of the economic importance of the ships' cargo, and the pace was slowed by a scrubbing process to remove oil from each hull. A ship carrying refinery-bound oil was the first to get the go-ahead. With more than 200 ships waiting, it was expected to take days to clear the backlog that developed after the tanker Tintomara collided with a barge on Wednesday. About 1.6 million liters of fuel oil spilled from the barge into the Mississippi at New Orleans. The shutdown of a 160 kilometer stretch of river to the Gulf of Mexico halted vessels ranging from oil supertankers to grain barges. Gary LaGrange, executive director of the Port of New Orleans, said a recent economic impact study conducted by the port showed that such a total shutdown could cost the national economy up to $275 million per day.