** BP upgraded by Citi after falling 5.9 percentyesterday, its biggest fall for four years.
** Faces additional fine of up to $18bln after being foundgrossly negligent in Gulf of Mexico spill.
** Citi estimate less than half that amount at $8.2bln.
** Raises to "buy" from "neutral"; price target to 510p from480p.
** "Although this is now a point of high uncertainty forinvestors, we believe the financial implications of this rulingwill remain significantly below the maximum," Citi said.
** BP shares opened up 0.6 percent in early trading, beforefalling to stand 0.9 percent lower. (RM: alasdair.pal.thomsonreuters.com@reuters.net)