The existing systems for monitoring gearbox degradation have demonstrably failed twice with fatal results. The environment in the North Sea is probably harsher than the designers appreciated, with more frequent shock loading of drivetrain components accelerating fatigue crack spreading once a chip or stress point has been formed.
Other areas where helicopters operate in harsh conditions seem to have more stable bad weather. Wind speeds may be higher, but they are more constant. Temperatures also tend to remain more stable over a period of weeks or months. The North Sea can be relatively temperate on day and below freezing the next. Similarly wind speeds are very variable ranging from dead calm to a full storm in a matter of a couple of hours with frequent gusts and squalls.
As usual the planetary gear components need to be hard to prevent wear while being ductile to absorb shock without fracturing. Selective heat treatment and surface hardening are the usual methods used to try and deliver both properties in ine component. A modern day version of damascus steel techniques from the middle ages used to manufacture swords that could cut without snapping when they hit armour. While x-ray, ultrasound and eddy current testing can give an indication that the components have been correctly manufactured, only destructive testing can provide absolute proof. So there is no way of providing an absolute guarantee that every component is flawless.
Chip monitoring and measurement of metallic waste contamination in the gearbox oil filters only works if conducted at frequent intervals and all the chips are actually collected. Even a single hardened metal chip can cause extremely rapid catastrophic destruction of a gearbox if it falls into the mesh between the planetary gears.
As it stands, this gearbox is operating too close to the technical limits of current metallurgy. Either the design needs to be radically changed or the operating limits downgraded, which would severely reduce the operation capabilities of the helicopter.