A boat that was moored to an oil rig platform in Cook Inlet sank Thursday, prompting at least 14 people to evacuate from the rig. The Coast Guard said it received a mayday call early Thursday from the captain of the Monarch, an oil supply vessel. The captain reported that the 166- foot vessel was taking on water at the Granite Point platform, which is owned by Chevron Corp. Coast Guard Petty Officer Walter Shinn said seven people on the boat were taken to the platform, which then evacuated nonessential personnel as a precaution. The seven rescued from the boat chose to stay on the rig but 14 people were taken by the platform's helicopter to shore, he said. The Monarch was carrying approximately 35,000 gallons of diesel fuel when it began taking on water. The vessel is partially submerged in the ice-covered waters underneath the oil platform. The Coast Guard said it didn't know why the boat sank, but Shinn said there was "a lot of ice around the platform." It was not immediately known whether any fuel was spilled. A call to Chevron was not immediately returned Thursday. Shinn said the Coast Guard would be speaking with Chevron about plans for raising the vessel. The platform is in the northern section of Cook Inlet about 45 miles southwest of Anchorage.