Since 1986 there have been 13 crashes of helicopters servicing oil platforms in the North Sea. 127 passengers and crew have died. The last crash was just last week. Five US built Sikorsky helicopters and 8 Eurocopter machines were lost. All the crashes since 2009 were Eurocopter. European authorities grounded the Eurocopter EC225 in October 2012 and kept it grounded until just a few months ago.
Failure of the main gearbox was responsible for six accidents. Full power of the engines, 5000 to 10000 horsepower flows thru the gearbox which has to gear the 10,000 RPM of the turbines down to 100 RPM or less for the rotor. This is a terrible strain, the slightest weakness, stripping of gear teeth, a crack in the casing, loss of oil pressure, bearing failure, anything, and the gearbox blows apart leaving the helicopter hanging in mid air without power.
Two helicopters were struck by lightening and two other accidents look like pilot error. There was one engine fire, one loss of control (reason not given) and last week's accident where all that is known is the chopper lost power and ditched two miles away from Sumburgh airport.
It's gotten so bad that oil rig workers are reluctant to travel by chopper. Oil companies are chartering ships to transport their workers. This is less than ideal, a three hour flight becomes a couple of rough days at sea. Disembarking from a pitching vessel onto a platform in bad weather is quite dangerous.