The sanctions imposed on PDVSA only prohibit the company from obtaining either a U.S. export visa or money from the U.S. Import-Export Bank, as well as banning them from attempting to obtain U.S. procurement contracts.
It often moves cargoes under contract with the U.S. government or for projects financed by the U.S. Export-Import Bank, which requires U.S. flagged ships to carry any cargoes it has financed.