On June 28, 2015, about 1000 central daylight time, a Bell 407 helicopter, N311RL, was substantially damaged when strong winds blew it over the side of the helideck during engine start on the South Timbalier 127 oil platform in the Gulf of Mexico. The pilot received serious injuries. The helicopter sustained substantial damage to the fuselage. The aircraft was registered to GM Leasing Company LLC, and operated by Rotorcraft Leasing Company LLC, under the provisions of 14 Code of Federal Regulations Part 135 as a passenger flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan. The flight was originating at the time of the accident for an unconfirmed destination.The pilot reported that he landed on the helideck about one hour prior to the accident. At that time he observed that the helideck was slick due to pressure washing activities that were being performed as a result of a recent fire at the platform. He stated that about 20 minutes prior to the accident that he noticed the weather deteriorating west and northwest of the platform and held a meeting with the passengers. A decision was made to depart the platform. He proceeded to initiate the start sequence for the helicopter, and obtain a flight clearance from company flight operations. A strong gust of wind struck the platform and the helicopter began to slide across the helideck toward the safety skirting. The nose of the helicopter began to rise and then the helicopter fell off of the safety skirting and into the water below. During the mishap, the pilot activated the helicopter floats and the life rafts. He was able to exit the helicopter onto the life raft and was picked up by a nearby boat. A weather report from another oil platform about 28 nautical miles east of the accident site included wind from 250 degrees at 26 knots. The platform where the accident occurred did not have any weather gathering devices installed. Witness reports corroborated the pilot's statement that a strong gust of wind struck the platform resulting in the helicopter sliding off of the helideck. Two of the witnesses reported that the helicopter's main rotors were turning, but were not yet at flight idle speed when the gust of wind struck the platform. At the time of the accident the operator had implemented Enhanced Operational Control Level Two. This is an internal control used when weather conditions dictate more positive control of operations. Since the accident, the operator has amended the procedures for Enhanced Operational Control Level Two to require that pilot's receive permission from Operations before unstrapping the helicopter.