Service on the Green Line will be significantly reduced Friday morning because of emergency repairs. There will be single-tracking from U Street to the Georgia Avenue-Petworth station. (Linda Davidson/The Washington Post)

Green Line riders should prepare for disruptions Friday morning because of emergency repairs.

Green Line trains will be single-tracking from the U Street station to the Georgia Avenue-Petworth station during the Friday morning commute, which will result in less-frequent service all along the line, Metro said.

The transit agency is advising riders who usually travel toward downtown D.C. from the northern end of the Green Line to consider using the bus or changing to the Red Line at Fort Totten to complete their journey.

The unplanned maintenance project also will affect service on the Yellow Line. Trains will run from Huntington to Mount Vernon Square, and there will be no Yellow Line service from Mount Vernon Square to Fort Totten.

Metro officials said Wednesday that a recent session of ultrasonic testing — essentially, X-rays of the train tracks — raised concerns about a possible defect outside the Columbia Heights station.

“Temporary measures are already in place to allow trains to continue to operate safely until permanent repairs are completed,” Metro said in a statement Wednesday night.

Maintenance crews are planning to replace a 100-foot section of track, an endeavor that involves cutting electrical power to the tracks, welding together long steel beams, then reconnecting power. The project will begin after the Thursday evening commute, and the work is expected to continue overnight and through midday Friday.

The emergency repair project comes one month after a derailment on the Red Line, which occurred because a 10-feet segment of steel rail cracked and snapped. Metro officials said the piece of rail had been inspected using the ultrasonic testing machine and also had been visually inspected by track walkers, but no defects were found.

On Friday, commuters who usually use the Green Line can opt for Metrobus’s S-line, which runs along 16th Street NW. They can also use the 50 or 59 buses, which travel along 14th Street NW, or the 70 and 79 buses, which run along Georgia Avenue and 7th Street NW.

A D.C. Circulator route runs between Columbia Heights and McPherson Square, while some riders might also choose to board a MARC train at Greenbelt station and ride the commuter rail service into Union Station.

More on Metro:

Two Red Line stations to shut down for 45 days this summer; major work to affect nearly every line in 2018