By Jeffrey Jones

CALGARY, Alberta, Nov 29 (Reuters) - A deadly explosion on a Enbridge Inc. <ENB.TO> pipeline in Minnesota occurred during routine repairs and was not the result of weakened or corroded pipe, company executives said on Thursday.

Workers were wrapping up repair work on a section of pipe on the system's Line 3 near Clearbrook, Minnesota, when the oil spilled and ignited, Enbridge Vice-President Leon Zupan told reporters during a conference call.

Three weeks earlier, the section had sprung a "pinhole" leak, which led to "a couple barrels" squirting out, but that did not force Enbridge to shut the line down, Zupan said. Instead, crews put a temporary repair sleeve over that section to keep crude moving.

On Wednesday, workers shut the line down to cut out the damaged piece of pipe and replace it with a new, pretested section, using special couplings on both sides, he said.

"It was at the tail end of this repair and replacement process when we restarted Line 3 and we ended up with the release from the line through one of these fittings," Zupan said.

Executives said crews are still investigating the cause of the explosion and fire at the site.

The blast killed two pipeline maintenance workers who lived in the Superior, Wisconsin, area. Company officials said they would not release their identities until the families gave their approval to do so.

The incident was the second involving Line 3 this year. In April, the line ruptured near Glenavon, Saskatchewan, spilling 3,750 barrels. Unlike Wednesday's incident, the break did not occur during maintenance.

Enbridge was forced to run the line at 80 percent of capacity into the summer.

Line 3 is expected to be back in service in the next two or three days. Enbridge's other three adjacent pipelines in the area resumed service on Thursday.

Executives said the Minnesota incident was not the result of inexperienced staff conducting dangerous work.

"All of the employees that were involved in this incident were fully trained and qualified to do their tasks, so it's not an issue of length of service here," Zupan said.

Executives said they will co-operate with U.S. federal and state agencies to determine how the explosion occurred and will commit the necessary resources to make sure its system is safe.

In response to a reporter's question, Chief Executive Pat Daniel said he and his company were not agonizing over whether the tragedy might damage Enbridge's reputation.

"This is an incident that has really shaken us in that it involves the death of two outstanding employees, and our concerns right now are not with the reputation of Enbridge but with the families of the two employees who were involved," Daniel said. (Editing by Rob Wilson)

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