Statoil said a natural gas leak occurred at the North Sea platform last weekend during testing of emergency shutdown valves.
"Due to shut down for maintenance on parts of the Heimdal installation the installation was not on stream," the company explained. "However, there were still pressurized systems in areas not included in the turnaround."
The 98 rig workers on the platform were evacuated but returned during the week. There were no injuries reported.
The incident is under investigation in coordination with the Petroleum Safety Authority Norway.
Statoil in March deployed emergency workers down a utility shaft to investigate a leak of the toxic gas hydrogen sulfide at its Statfjord C platform.
Four years ago, around 22,000 barrels of oil spilled from Statfjord while a tanker was loading cargo from a storage buoy. That incident was the second-worst oil spill in Norwegian history.