About 20 high school teams with underwater remotely operated vehicles (ROVs) they”ve built themselves are expected to compete Saturday at Johnson Space Center”s Neutral Buoyancy Laboratory for the right to go to national finals. The all-day competition begins at 9 a.m.

The NBL, near Houston’s Ellington Field, is used primarily to train astronauts for spacewalks. It is 202 feet long, 102 feet wide and 40 feet deep, with a capacity of 6.2 million gallons of water.

Teams, each with four or five members, will compete to retrieve objects from one of three 8- by 6- by 6-foot PVC frames (called Titanics) situated 15 feet below the surface. Each Titanic is divided into staterooms. Staterooms have housings holding targets called C-Probes, “-inch plastic cylinders 6 inches long. The object of the competition is to maneuver the ROVs into the staterooms to retrieve as many of the C-Probes as possible.

Each team has spent months working on its ROV, to be operated from a poolside control box. The ROVs are controlled through 50-foot tethers, which provide operators with pictures from the vehicles. The ROVs were designed and built within stringent safety standards.

The competition is organized by the Marine Advanced Technology Education Center, one of 11 National Science Foundation-funded Advanced Technology Education centers, and the Marine Technology Society”s Remotely Operated Vehicle Committee. Sponsors include about 50 mostly marine-related companies and other organizations, including NASA. Walden Media is a sponsor of the national competition and also provides financial help to the three regionals around the country.

“The top three teams in this competition go on to Massachusetts Institute of Technology in Boston for the national competition,” said Ike Coffman, department chair of the Electronics Technology Program at Alvin Community College. He organized the first competition here a year ago as well as this year”s event.

This regional, which is not open to the public, might produce a contender or two at the nationals. “Some of these designs are fairly sophisticated,” Coffman said.