Most North Sea helis use Combined Voice & Flight Data Recorders (CVFDR's). These are powered as soon as the bus goes live, when the pilot switches on power. They are stopped by inertia switches and floatation switches. The power required is only about 14 Watts.
Some aircraft use digital recorders which do the same job but are more expensive (£11,000 each). The most common N sea recorder is the Penny & Giles type 900D which uses tape.
IHUMS fitted aircraft take the same data stream that goes to the recorder and monitor it for exceedances in real time. This is ARINC 573 protocol but uses ARINC 717 sync words as well. Exceedances that do occour, allow the system to take a 'snapshot' of the data and save the info to a data card for later use by engineering staff.
IHUMS is fitted to all N Sea aircraft. There are some differences depending on weight of aircraft. Lighter (<5700 Kg) A/C require less params to be recorded (EUROCAE ED 55 spec).
It is possible for Pilots to erase the voice recording after flight, but they cannot erase the data recorded. On tape machines this is 5 hours of data, but digital recorders usually hold a lot more data, normally 10 hours.
Is that it?