By Jeffrey Jones

CALGARY, Alberta (Reuters) - Enbridge Inc
aims to file a revised plan for restarting its U.S. oil
pipeline by a Friday deadline imposed by regulators, but the
timing to resume crude flows to thirsty U.S. and Canadian
refineries is still uncertain.

A day after U.S. federal regulators rejected Enbridge's
initial plan on grounds that it would not ensure safe
operations, one refiner hampered by the July 26 rupture of the
line in Michigan warned of fuel shortages as the end of the
summer driving season approaches.

The Pipeline and Hazardous Materials Safety Administration,
an agency of the U.S. Department of Transportation, rejected
Enbridge's plan Tuesday and sent back a host of measures it
wants followed before allowing the restart of the 190,000
barrel a day line.

Those measures include investigating at least four parts of
the pipeline that in-line testing had shown to have "anomalies"
similar to those at the rupture, and performing pressure tests
through some sections.

Enbridge spokeswoman Gina Jordan declined to discuss how
time-consuming or difficult the measures might be.

"We will adhere to all actions, requirements and
regulations that PHMSA imposes in its review and approval of
our plan to safely repair and restart Line 6B," Jordan said.

The company had proposed a series of moves beyond what the
regulator had demanded in a letter late last month spelling out
corrective action, Jordan said. Enbridge had expected more
might be required as the investigation into the rupture and oil
spill progressed, she said.

GAS LINES FEARED

Line 6B is part of Enbridge's massive pipeline system that
carries the bulk of Canadian crude oil exports to the United
States. The segment, which originates in Griffith, Indiana,
carries oil to refineries in the U.S. upper Midwest and the
Sarnia area of southern Ontario, which produce more than
700,000 barrels of fuel a day.

The break near Marshall, Michigan, spilled about 19,500
barrels of Cold Lake blend heavy crude into a tributary of the
Kalamazoo River. Cleanup operations are still under way.

United Refining Co, which which runs a 65,000 barrel a day
plant in Warren, Pennsylvania, warned Wednesday that fuel
shortages are possible if the outage goes on much longer. It
was one of three U.S. refineries that have had to cut output.

"We are entering the final days of summer vacations and the
prospect of gas shortages, empty pumps and gas lines is not a
pretty prospect for northwest Pennsylvania and western New
York," United Chief Executive John Catsimatidis said in a
statement urging a quick restart of the pipeline.

U.S. data on Wednesday showed a drop in crude supplies in
the Midwest over the past week, prompting speculation that the
outage of Line 6B may be a factor.

The Energy Information Administration said crude stocks in
the region fell 2.3 million barrels in the week ended Aug. 6
from a record 97.7 million the previous week.

Shares in Enbridge fell 95 Canadian cents, or 2 percent, to
C$50.45 on the Toronto Stock Exchange amid a broad downturn by
North American equities. Its U.S. affiliate, Enbridge Energy
Partners, fell $1.21, also 2 percent, to $55.39 in New
York.

($1=$1.05 Canadian)
(Additional reporting by Janet McGurty and Joshua Schneyer in
New York; editing by Rob Wilson)