ExxonMobil Corp. violated federal safety regulations while operating the Pegasus pipeline that dumped about 5,000 barrels of oil in a Mayflower, Ark., neighborhood in March and should pay more than $2.6 million in penalties, a federal regulator determined.

The U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA), in a notice issued Nov. 6, claims ExxonMobil, and its ExxonMobil Pipeline Co., did not pay attention to known problems with pipeline integrity in 1991, 2005 and 2006.

“The operator experienced multiple hydrostatic test failures on the Pegasus Pipeline as a result of ERW [electronic-resistance welded] long seam failures in 1991 hydrotesting and subsequent 2005-2006 hydrotesting,” R.M. Seeley, PHMSA southwest region director, wrote in the notice to ExxonMobil Pipeline Co. president Gary Pruessing.

“The pipe manufacturing information, fracture toughness and hydrostatic testing failure history of the Youngstown pre-1970 low frequency ERW pipe in the Patoka [Ill.] to Corsicana [Tex.] segments … provided more than adequate information for the pipe to be considered susceptible to seam failure,” it said.

“Further, the operator did not present an acceptable engineering analysis to PHMSA to demonstrate that the pre-1970 ERW pipe in the Pegasus Pipeline was not susceptible to seam failure.”

Metallurgical tests commissioned by ExxonMobil after the spill show the 22-ft crack in the pipeline was caused by manufacturing defects. The 850-mile pipeline began service in 1948 transporting refined oil north, but flow was reversed in 2006 to carry Canadian heavy crude oil to refineries in Texas.

The 20-in. pipeline split along a faulty seam, sending the heavy crude through the Northwoods subdivision in Mayflower, Ark., a nearby marsh and a cove of Lake Conway. More than 20 homes were evacuated and most are still unoccupied.

ExxonMobil issued a statement saying it was “disappointed” about the notice, but that it was still in review.

It also said “it does appear that PHMSA’s analysis is flawed and the agency has made some fundamental errors.”

PHMSA said ExxonMobil also failed to follow federal regulations involving pipeline integrity monitoring frequency.