The RigCOSL is a majority owned subsidirary of CNOOC, and I...

The Rig

COSL is a majority owned subsidirary of CNOOC, and I suspect that the majority,if not all, of the work is on behalf of CNOOC.

https://en.wikipedia.org/wiki/China_Oilfield_Services

The rig was built in 2006 - so technology is not that new.

Why were the most recent BG wells so successful, and likely impact on success of 12-8-E (Phase 2 development)

You need to understand how "drilling a well works".

1. A BG Team (CNOOC as they are the operator) plan the well - in very simple terms, the trajectory of the well to get maximum production from the reservoir.  This isn't an exact science as the seismic only gives them a rough idea of the actual shape of the reservoir sand that they plan to drill.
2. They then contract a rig to drill the well.
3. However, the person that controls the day-to-day drilling of the well, is NOT an employee of COSL, but is the member (manager) of the BG team that planned the well.  This is logical as they are familiar with the anticipated shape of the reservoir being drilled.  They are best placed to make slight adjustments to the planned well when the reality deviates from the seismic picture.
4. Irrespective of the rig that we use to drill 12-8-E wells (and it is likely to be 941), we will have the same person that achieved stellar results as the latest 2 infill wells, controlling the drill-bit on he 12-8-E wells.
5. It is likely that this guy (or gal) is really good at their job, and this is the reason that we may get 12-8-E results that significantly exceed expectations.  In other words, it is pretty much independent of the rig used.