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* Leak at Heimdal was "serious" - safety watchdog
* 3.5 tonnes of gas leaked on the platform in 4 minutes
* Statoil says will reply to watchdog, has investigated leak
By Gwladys Fouche
OSLO, Feb 4 (Reuters) - Norway's Statoil must improve safety procedures at one of its North Sea platforms after a gas leak occurred there in May, the Norwegian oil industry safety watchdog said on Monday.
The Heimdal platform had to be evacuated last spring after the testing of two emergency shutdown valves went wrong, leading to some 3.5 tonnes (7,716 pounds) of gas leaking out onto the platform in about four minutes.
Had there been a spark, the leak would have caused an explosion.
"The leak was one of the most serious gas leaks that occurred off Norway for several years," the Petroleum Safety Authority Norway said in a statement. "Gas was detected in a large area of the facility."
In its order, the safety agency instructs Statoil to investigate why improvements ordered after a gas leak at another North Sea platform, Gullfaks B, in 2010 were not implemented at Heimdal.
It also asks the firm for a plan on how it will implement the improvements at Heimdal, and that it should confirm a similar incident cannot take place at any other installations the company operates off Norway.
Statoil has until March 1 to reply to the authority.
At the time of the leak, Statoil described the situation as "serious".
"We take note of the order and we will use the time we have until March 1 to come back to the safety authority with an answer," Statoil spokesman Ola Anders Skauby said on Monday.
"Since the incident, we have worked on measures to avoid this type of event from happening again."
Statoil holds a 39.44-percent stake in Heimdal, which is now mainly a processing centre for other fields, including Huldra, Skirne and Vale. Gas from the Oseberg field is also transported via Heimdal.
Heimdal's own gas production is modest - an estimated 140 million standard cubic metres of gas was expected to be produced in 2012, according to the Norwegian Petroleum Directorate.
Statoil's partners in the field are Centrica (23.80 percent), Total (16.76 percent) and Norwegian state-owned firm Petoro (20 percent).