On June 29, 2002, at 1000 central daylight time, a Bell 206L3 helicopter, N255EV, was substantially damaged when the main rotor blades contacted and severed the tailboom while attempting to shutdown while on an off-shore platform in the Gulf of Mexico known as Brazos A-19B. The commercial pilot, sole occupant of the helicopter, was not injured. The helicopter was operated by Evergreen Helicopters International, Inc., an on-demand air taxi operator based at Galveston, Texas. The helicopter was owned by Evergreen Equity, Inc., of Mc. Minnville, Oregon. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 positioning flight for which a company VFR flight plan was filed. The positioning flight originated about 5 minutes prior to the accident from another nearby offshore platform known as Brazos A-19JD. Upon completing his passenger drop-off mission at Brazos A-19 JD uneventfully, the pilot headed back to Brazos A-19B. The pilot reported that he was aware of the heavy rain and convective activity approaching the offshore platform from the southeast. After landing at the helideck at the Brazos A-19B platform, which is located 200 feet off the surface of the ocean, the pilot positioned the helicopter into the prevailing headwind and initiated the engine shutdown procedures. The pilot reported that the wind at this time was from the southeast at 15 to 20 knots, and the rain showers remained about 2 miles southeast of the platform. The pilot further stated that within seconds after shutting down the engine "the wind picked up to 50-60 knots, while the rain was still about 1-2 miles away." The pilot added that he applied the rotor brake after the rotor had slowed to 35-40 percent; however, the rotor brake did not appeared to have any effect in slowing down the rotor system. As the rotor system was buffeted by the strong wind, the rotor system became unbalanced and eventually struck the tailboom approximately 2 feet forward of the tail rotor gear box. The operator reported that the effect of the microburst spun the helicopter about 15 feet to the left, fortunately towards the center of the platform. The pilot reported that he retrieved the helicopter tie-down straps and secured the helicopter to the helideck. The FAA inspector, who traveled to the accident site, confirmed that the rotor brake system was fully operational and reported no anomalies with any aircraft system. .