For now, Shell Oil Co.’s first test is excavating a 20-foot wide, 40-foot deep cavern in the seabed that can shelter an emergency device known as a blowout preventer from passing icebergs. Such mud-line cellars are relatively rare — and building the one at Shell’s Burger J well requires the Transocean Polar Pioneer to employ a specialized 20-foot-wide drill bit. It turns at three revolutions per minute, with discs plowing up mud as it burrows down.

Before that work could begin, the rig was anchored over the target and then drilled a pilot hole at the site to check for the presence of gas hazards. “A big part of our well construction is the mud-line cellar,” said Shell drilling superintendent Eric Whatley. The company has scheduled about nine days for the mud-line cellar excavation — but the timeline is unpredictable. During the last sustained round of drilling in U.S. Arctic waters, in the 1980s and 1990s, the mud-line cellars were frequently completed in three days.

By contrast, it took Shell weeks to drill a mud-line cellar at a half-finished Chukchi Sea well during its last attempt in 2012, partly because a floating iceberg forced the company to temporarily flee the site. And as Shell’s newest Arctic rig, the Polar Pioneer has never drilled a mud-line cellar before.

The amount of time Shell spends on the mud-line cellar could be pivotal, affecting the company’s ability to complete a well this year, before the government-imposed Sept. 28 deadline. “If it takes 30 days again we’re definitely in to a two-season well,” Pickard said. “So that to me is the big wild card, that mud-line cellar. That’s such an important piece to get right.”