LONDON — Britain's Prince William has completed his first mission as a Royal Air Force search-and-rescue helicopter co-pilot, plucking a stricken worker from an offshore gas rig, his office said Tuesday.The 28-year-old, who is second-in-line to the British throne, was among the crew of a Sea King helicopter called to a rig in Morecambe Bay, off the coast of northwestern England, on Saturday.William's office said the crew flew through "squally winds" and mild turbulence to collect the rig worker, who had a suspected heart attack, and transport him to a waiting ambulance."Prince William is pleased finally to be able to contribute to the lifesaving work of the search-and-rescue force. He is proud, after two years of intense training, to be able to serve in one of Britain's foremost emergency services," his office said in a statement.