HOUSTON (Reuters) - A U.S. National Labor Relations Board administrative law judge has ruled Exxon Mobil's 10-month-long lockout of some 600 union workers at a Texas oil refinery during a contract dispute was legal.

The judge sided with Exxon in his decision on Nov. 21, finding the 2021 and 2022 lockout was to pressure the United Steelworkers union workers toward a deal, not to oust the union from the 369,024 barrel-per-day (bpd) Beaumont, Texas, refinery complex.

The USW had faced a decertification campaign and filed an unfair labor practice complaint during the lockout, alleging an improper effort to break the plant's union.

The union had sought millions of dollars in lost pay and benefits for the workers who were locked out of the plant between May 2021 and March 2022.

"There is little or no evidence that the company locked out the unit employees to unlawfully pressure them to decertify the union," NLRB law judge Jeffrey Wedekind said in an opinion accompanying the decision.

A spokesperson for Exxon did not reply to a request for comment.

Meekie Moseley, president of USW Local 13-243, which represents the workers, said the union is considering its options following the judge's decision. The USW can appeal the decision to the NLRB.

"We believe the decision does not reflect the facts of the case," Moseley said in a statement.

Wedekind's decision came 18 months after hearings began in the case. Those hearings considered whether internal Exxon documents involving negotiating strategy, and a post-contract review could be considered in the case.

But Wedekind excluded the documents, some of which showed Exxon managers early on had weighed a lockout and debated what it would take for union members to vote to decertify, or formally remove, the union.

In documents viewed by Reuters, Exxon managers had early on considered using a lockout and later felt that changes in the makeup of the workforce would benefit the company's strategy in future talks.

A fifth of the workers initially locked out on May 1, 2021, had left the company prior to the contract settlement in March 2022. The replacements "previously worked as contractors for the duration of the lockout - benefits both site performance and long-term labor strategy," wrote a plant manager Jose Diaz, according to the documents that were posted on an internal website and leaked to the USW, according to hearing testimony.

Another document had raised the prospect of getting workers to push out the union during the next round of negotiations in 2027 by dividing the contract between the complex's refinery and the lube oil plant.

"Split the contract in 2027, and the refinery will decertify eventually," former plant manager Jonathan Parsons said, according to the documents.