The chief federal watchdog for worker safety more than halved $99,000 in fines assessed against Williams Olefins over a June 2013 explosion at its plant in Ascension and Iberville parishes that killed two and injured more than 100 people.

The reductions were part of a settlement between the U.S. Occupational Safety and Health Administration and Williams that ended a challenge of six violations and related fines against the Tulsa, Oklahoma, company.

Early this year, Williams contested the violations with the Occupational Safety and Health Review Commission, an independent board in Washington, D.C., that reviews challenged OSHA findings.

Among several changes brought by the settlement, OSHA agreed to reduce the most significant charge from a “willful” violation to a “serious” one and, in the process, appears to have ended the possibility that the agency could refer the case for a future criminal investigation.

The change from willful to serious violation also came with a reduction in Williams’ fine for that violation from $70,000 to $7,000. As a result, Williams’ combined the fines for the deadly explosion fell from $99,000 to $36,000, the settlement says.

OSHA and Williams also have reached a separate settlement on a seventh violation, deemed “serious,” stemming from an incident three months after the deadly blast. The $7,000 fine for the violation is unchanged.

Juan Rodriguez, OSHA spokesman in Dallas, declined comment Monday on the settlements, but under OSHA rules, only a willful citation over an incident that involves the death of a worker brings the risk that OSHA could recommend a criminal investigation to federal prosecutors.

U.S. Attorney Walt Green said Monday that under U.S. Department of Justice policy, he cannot comment whether or not there is an investigation into the explosion.

Tom Droege, Williams spokesman, said the company has cooperated with OSHA in a full and transparent manner.

“These settlement agreements with OSHA bring closure to this phase of our post-incident recovery, and we continue to work diligently to further enhance safety at the Geismar plant and across our entire organization.”

Williams officials added Monday in a news release that the Geismar plant, which has been undergoing repair and an expansion of its capacity, expects ethylene production to begin in January as startup work is nearly finished.

Williams also did not admit fault but, as part of the settlement, the company agreed to a series of upgrades, better training and worker warnings and studies of the plant’s operations.

OSHA isn’t the only agency looking into the explosion.

In late November, the state Department of Environmental Quality also assessed Williams Olefins $194,306 in fines from 32 violations dating back to 2008, including four tied to the June 2013 explosion and fire.

Greg Langley, DEQ spokesman, said Williams has paid those fines.

The U.S. Environmental Protection Agency also has a pending case “being reviewed by our enforcement staff,” but Joe Hubbard, EPA regional spokesman, said he cannot comment on it.

A 200-foot-high fireball ignited June 13, 2013, from a flammable vapor cloud leaking out of ruptured equipment at the plant.

Investigators have pointed to an idled reboiler, a type of heat exchanger where flammable chemicals are mixed and heated under pressure, that failed catastrophically and may have led to the explosion.

A series of lawsuits have been filed against the company in Ascension and Iberville parishes. Plaintiffs have alleged the company ignored internal warnings to upgrade the reboiler and that a key pressure relief valve was improperly closed. The company has said it responded to those warnings appropriately.

OSHA Review Commission documents show both Williams Olefins settlements with OSHA were signed in early September and the fines had to be paid within 45 days. Safiya A. Hamit, review commission spokeswoman, said the settlements did not become final until Nov. 10, some 11 days before DEQ issued the more than $194,000 fines.

OSHA had not disclosed the settlements publicly. As of late afternoon Monday, OSHA’s website still listed both sets of violations against Williams Olefins as being “under contest.”

The settlements came to light this month through a search of online DEQ records.

Rodriguez has said the OSHA website has not been changed because regional officials have not received paperwork indicating the settlements are final.

Follow David J. Mitchell on Twitter @NewsieDave.