The GVS's marked on the chart are usually on the main transmissions lines from offshore. In the event of a mjaor plant upset they can depressurise the plant and for non-routine operations the pipelines.
In terms of frequency this should be less than once a year - the only problem is there would be no warning for the plant upset. The flare (venting is usually ignited) can be up to 100m plus high, the thermal effects considerably higher. If unignited there is some possibility of an engine 'self-fuelling' and destroying itself.
However as Rod1 says the chances of this are pretty small - rather like those rocket NOTAMS and similar stuff.