Shell tested its first line of defense against runaway Arctic oil wells for federal inspectors on Monday, marking a major step in the company’s final preparations to launch exploratory drilling in seas north of Alaska.

The tests focused on the capping stack that Shell intends to station between planned drilling operations in the Chukchi and Beaufort seas this summer in case of an emergency.

Marine tracking data showed a Shell ship carrying the capping stack circled an area of Puget Sound today after other Shell support vessels previously docked in Seattle and Tacoma, Wash., moved nearby. A source with knowledge of the deployment confirmed it took place on Monday.

It was unclear whether regulators graded the deployment drill a success or whether they would order additional testing of Shell’s capping stack and containment system.