The first giant 15 MW turbine is up at Germany's largest offshore wind farm

> And it’s being built without any government subsidies.

Important milestone there. It's great to see that green energy projects are largely economically feasible without subsidies these days.

So wind turbines have inertia in the same way coal generators do? Ie rotating mass helps grid stability?

normally no. Modern wind turbines have electric inverters that decouple rotation speed from grid frequency

Pity but I guess it makes sense given variable wind

Depends on model, but some turbines are very much designed to provide frequency stabilisation so long as you can instruct what the phase should be at given time

From Google:

The turbine model V236-15.0 MW from Vestas features a capacity of 15 MW and a 236-meter diameter rotor with hub height of about 145 meter, highest tip of 263 meter.

... 236 meters