The 45,000 bpd hydrocracker, called the H-Oil Unit, is unique because it converts residual crude under high heat and pressure in the presence of hydrogen and a catalyst into motor fuel, primarily diesel. Most hydrocrackers convert fuel oil into motor fuel.

A Shell spokesman did not reply to a request for comment.

The sources said the timing of the work could shift.

The H-Oil unit had a lengthy recovery from an Aug. 11, 2016 fire that heavily damaged the unit. It was idle for three and a half months and ran at half capacity for another three and a half months before again being idled by a March 18 fire that broke out during an attempt to restart the unit at full capacity.

The H-Oil Unit returned to production at full capacity on May 23.