TAS/CHC have the main Gulf of Thailand work. 2 S92's and 8 S76 in total I believe. I heard a recent rumour that TAS were looking at breaking from CHC but it wasnt from a reputable source so no idea how much truth was in it.
Weststar, along with United Offshore took the work from Si-Chang. Si-Chang moved up to Uttapao.
TAS have a base at the same northern Gulf airfield. The main TAS base is at Nakhon Si Thammarat, but they also have an s76 aircraft on a short contract out of Songhkla utilising TAS/CHC staff.