Already positioned near the Arctic prospects, he said, is the third line of defense â¿¿ a capping stack. The device looks like a giant spark plug and is designed to provide a metal-to-metal seal on a malfunctioning blowout preventer. The capping stack, he said, could be used to send drilling mud down a hole, simply stopping petroleum flow, or direct oil, gas and water to the surface. It was the most significant change to Shell's safety program after BP's Deepwater Horizon blowout at the Macondo prospect in the Gulf of Mexico, Smith said."The capping stack is modeled after the same one that ultimately stopped the most prolific offshore blowout in the history of North America," he said. "We modified it. We made it Arctic-ready." Shell's is prebuilt, he said. "During Macondo, there was not one available," he said. The fourth method, which remains in Bellingham, is the containment barge carrying the dome. "The containment system is an apparatus that would essentially hover over a compromised well funneling escaping oil, gas and water into this dome," Smith said. He did not have details as to how the dome was damaged. "The cause and the extent of the damage are being investigated right now," he said.