This is probably part of the dredging operations from Jurong Island to Pulau Ular and bedding layer and wire lay operations from Jurong Island to Pulau Ular that began in Mar 09.
What's at Cyrene Reef?
More on the wildsingapore website with lots of blog posts about Cyrene Reef.
Buoyancy tank removal operations from Jurong Island to Pulau Ular
from Port Marine Notice No. 45 of 2009 dated 16 Apr 09
With effect from 17 Apr 09 to 30 Apr 09. 24 hours daily, including Sundays and Public Holidays. At Selat Pandan, Sinki Fairway and West Keppel Fairway (see attached chartlet):
Buoyancy tank removal entails diving operation and releasing tanks within sectors as indicated in the chartlet. The safety working zones are circular areas of 200-metre radius centred at the diving boat “EM 7”. A safety boat will be deployed in the immediate vicinity of the working area to warn other craft of the project work. Further enquiries relating to the project can be directed to Mr P Bogers, the project manager, (Tel No: 9139 6094 or email email@example.com).