PORTLAND — More than a dozen vessels have deployed off the New England coast to test their abilities to respond to a massive oil spill at sea.

Today’s exercise in Portland Harbor is being held 21 years to the day the Exxon Valdez leaked 11 million gallons of crude oil off Alaska.

The Maine Responder and other vessels from Portland, Rye, N.H., and Boston are responding to the simulated collision between an oil tanker and a car carrier 15 miles offshore. The response is being tracked via computer models to measure the performance of more than 50 federal and state agencies and private organizations.

The so-called Spill of National Significance Exercise is held every three years, with the last one taking place on Lake Michigan in 2007.