JUNEAU -- Shell Oil Co. is seeking to change the air permit for one of its drill ships after a spokesman said generator engines tested "slightly above" permit levels for ammonia and nitrous oxide.

Curtis Smith said Shell is working with the U.S. Environmental Protection Agency toward a compliance order that would allow the Noble Discoverer to operate this year. He said this also would set in motion a review process for the permit changes for 2013 and beyond.

EPA spokeswoman Suzanne Skadowski said Thursday that the agency hasn't decided how to proceed yet. She said the regulations for such major permits don't talk about modifications, so the EPA is reviewing whether it's appropriate to make revisions.

Smith said preliminary indications are that "no current engines on the market" can meet the current nitrous oxide and ammonia limits for those specific generators.

"Because the Discoverer's generator engines are not the primary emitters for the project, the Discoverer will still meet ambient air standards," he said in an email.

Shell hopes to begin drilling in the Arctic waters off Alaska in weeks, and Smith said he understands that "compliance orders are fairly common and that this process will not interfere with our 2012 drilling plans."

Shell also is seeking what is considered a minor permit modification for its Kulluk drill rig. Under EPA rules, Shell is able to implement its proposed changes as soon as it files its application with EPA. The agency will have 90 days to decide whether to approve or deny the changes.

The Kulluk is headed to the Beaufort Sea off Alaska's north coast for exploratory drilling during the summer open water season.

The Noble Discoverer is set to drill exploratory wells in the Chukchi Sea off the state's northwest coast.