State oil company PDVSA has scheduled stoppages starting later this year at three of its four upgraders, which were nationalised by President Hugo Chavez's government in 2007, a source at the company told Reuters last month.

Petromonagas is a joint venture with BP and is due to be stopped for maintenance between February and March next year. But a union official said that was too long to wait and that the work needed to be done sooner.

"We think the stoppage of Petromonagas has to be brought forward because of the expectation of accidents we found there," Will Rangel, president of the United Federation of Oil Workers, told a news conference.

"We found obsolescent equipment which needs to be taken out of service, and areas that require maintenance."

The union, which is generally supportive of the government, has been inspecting conditions at PDVSA sites since an accident last May when a gas exploration platform operated by the company sank in the Caribbean.

According to the union, there were about a dozen serious accidents last year involving PDVSA workers. Rangel said PDVSA could stop Petromonagas sooner than planned if it wanted to.

"The instructions have already gone out to buy the new equipment and in the first half of this year it will be ready," he said, adding that an inspection would also be carried out in the coming days at the 130,000 bpd Petroanzoategui upgrader.

Petroanzoategui is 100% operated by PDVSA and is scheduled to be stopped for 40 days of maintenance starting in August or September, according to the source at the company.

Another upgrader, the 190,000 bpd Petrocedeno, is due to undergo similar work at the end of next year. It is a joint venture with France's Total and Norway's Statoil .

Venezuela's four upgraders convert heavy tar-like crude into more valuable, exportable oil. According to the Energy Ministry, they produced an average of 390,000 bpd between January and November last year, below 70% of their capacity