HOUSTON/WILLISTON, N.D. - Engineers examining why nine giant tendons sank at Chevron Corp’s Big Foot deepwater oil project are considering whether buoys taking on water, combined with a strong current that prevented vessels from repairing them, caused the costly setback, a source familiar with the matter said.

Though it will be months before the investigation is complete, the sinking of the tendons 225 miles south of New Orleans has delayed the project indefinitely, harming Chevron’s plans to produce 75,000 barrels of oil and 25 million cubic feet of natural gas per day by year’s end.

“It’s not appropriate to speculate on cause as we are still assessing damage to the tendons and undertaking an investigation,” Chevron spokesman Cam Van Ast said.

Sixteen tendons, each roughly a mile (1.6 km) long, were designed to float upright with the help of a buoy system until the Big Foot platform could be moved atop them and connected, the source said.

Four tendons were engineered to connect to each of the platform’s corners, and tie the platform to the seabed.

However, the buoyancy system apparently failed to operate as designed and began taking on water in the mile-deep waters of the Gulf.

It is not clear why the buoys failed. But at some point vessels sent to inject them with air and displace the water could not reach them due to the Gulf’s loop current, according to the source.

The loop current is a warm flow of fast-moving water that flows into the Gulf from the Caribbean Sea, past Mexico’s Yucatan peninsula and east toward the Florida coastline before looping in the Gulf Stream and out into the Atlantic.

Its location can change from time to time, making navigation difficult, according to the National Oceanic and Atmospheric Administration.

“You shouldn’t have to add air,” the same source said of the buoy system. “You wouldn’t design something to slowly take on water.”

The source could not be identified because he is not authorized to speak publicly about it. Chevron declined to comment when asked if vessels reached the buoy system or why they filled with water.

It is not clear if the remaining seven buoys have been reinflated.

The delay to the $5.1 billion Big Foot project comes as San Ramon, Calif.-based Chevron deals with setbacks at several mega-projects it needs to lift daily output to 3.1 million barrels of oil equivalent per day (boepd) by 2017.

Statoil ASA and Marubeni Oil & Gas hold minority stakes in the project.