To avoid the deja vu that has affected recent discussions, it might be worth going back and reading the first three pages of this thread.
The posting by Mars in #44 appears to provide some data (first posted by SASless) on why twins are used offshore over a hostile environment.
Just to correct the basis of the statistics: the figure for engine failures (power plant, not core failures) comes out consistently at 1:100,000/flying hour. The probability of failure of an engine in a single is therefore 1 x 10**-5; the probability of an engine failure in a twin (one or the other) is 2 x 10**-5; and the probability of two failures in a twin (from unconnected causes - i.e. one and then the other) is 2 x 10**-5 and (times) 1 x 10**-5 - that is 2 x 10**-10. The consequences of a failure in twin should be negligible providing other elements of the operational code of performance (the performance classes) in Europe are applied as written.
With respect to the requirement for certification in Category A for flights over a hostile environment; it is more to do with the additional protection that is afforded to these helicopters in the design and build processes.
In none of the cases above is the helicopter protected from the vagaries of human factors (as is observed in the offshore statistics - both in the GOM and the North Sea). In this respect, AnFI is quite correct that the complexity of modern helicopters brings with it additional burdens for the operator and the crew members in terms of the requirement to understand complexity, and training to ensure competence it its use.
Jim