Here is some history, courtesy of "ODI"
Up until September 2000, the Air Traffic Service in the ESB was a full Approach Procedural Service provided by controllers sitting in Aberdeen. The route structure was based on the Aberdeen VOR radials and a track structure from Sumburgh that met the Aberdeen radials at the Basin Boundary.
The tracks from Sumburgh were called track G through track M and the points where these crossed the boundary were called Gates. In addition there were two other Gates, the N Gate and the O Gate, based on a further two radials from ABZ. The theory was that aircraft on different tracks were deemed separated, the Gates were deemed separated, as were the rigs (all 20 of them, except for the Brents and the Ninians due to their proximity to each other), and various reporting points in the Basin were also separated from each other.
Helicopter traffic would enter the Basin at 3000’ and leave at 2000’, but would transit between rigs at, usually, "not above 500’". This is where the fun would be, as the contollers would endeavour to keep separated up to nine helicopters as they shuttled between the rigs before finally departing for home. In good weather it was fairly straightforward as they were allowed to use visual separation - only one pilot needed to be visual and it was not the controller's problem if the pilot lied! The eyesight of pilots is amazing when it is to their advantage!
This all changed for two reasons:
The radar sits on top of one of the Norwegian oil rigs and has recently been approved for full use by the controllers. This means they can now operate a full (SSR only) Radar Advisory Service to the helicopters down to deck level. The track structure was kept in place for a while in case the radar, in fact, wasn’t as good as it should have been, to enable the old system to be resurrected.
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