More than four months after a failure was reported on an oil pipeline in Alberta, Canada, regulators said they approved of a conditional restart plan.Operators closed the Rainbow oil pipeline in April after a spill was discovered in northern Alberta. Canada's Energy Resources Conservation Board said about 28,000 barrels of oil spilled from the pipeline, making it one of the largest spills since the 1970s.Plains All American Pipeline, the parent company of Rainbow operator Plains Midstream Canada, was scrutinized in 2006 for a 7,500-barrel leak at a section of the same pipeline in Edmonton. Investigators blamed stress and corrosion for the 2006 leak. The pipeline was built in 1966.The ERCB announced that it gave conditional approval to Plains Midstream to restart the Rainbow Pipeline following a third-party engineering review.The ERCB said Plains must confirm that it would conduct weekly aerial surveillance of the pipeline and submit monthly progress reports on its compliance with regulatory requirements under the restart plan.An assessment of the April incident found stress on an existing crack in one of the welds on the pipeline contributed to its failure. Plains committed to pulling and inspecting all sections of the pipeline containing the type of welds in question, the ERCB added.A restart date for the Rainbow pipeline wasn't confirmed.