LONDON French oil major Total's team of crisis engineers carried out a reconnaissance mission at its leaking North Sea Elgin gas platform on Thursday, ahead of a plan to plug the well, the group said.
"The team spent nearly four hours on the Elgin complex to gather preliminary information that will be used to assist in preparation for deploying the necessary equipment to perform a well control operation," the group said in a statement.
The aim of the mission was also to establish zones which can be safely accessed, Total said.
The helicopter took off from Aberdeen at 0930 GMT and returned to the Scottish city at 1550 GMT, the group added, landing personnel on the rig for the first time since the rig was evacuated after the leak began on March 25.
Adverse weather conditions had prevented Total from sending a helicopter to the leaking rig earlier this week, but the weather forecast for Thursday and Friday was more favorable.
Aerial surveillance suggested that a sheen of light hydrocarbons resulting from the leak was evaporating naturally and would have a minimal environmental impact, the UK energy ministry said on Thursday.
Successful relief operations will depend on weather conditions during the next few days.
The UK's MetOffice expected weather around Aberdeen to be "mainly dry and rather cloudy, with still a little light rain mainly towards the Moray Coast", later on Thursday.
"It will be a milder night and winds will stay light," the MetOffice said.
For Friday it forecast conditions to be mainly dry, with light rains and maximum temperatures of 9 degrees Celsius.
Total said earlier this week that the team of engineers would assess conditions on the platform and find out whether a so-called "well kill" was feasible, by pumping mud into the well, and whether any other measures would be necessary.
Another, more expensive option being pursued in parallel is to dig two relief wells to the source of the gas at 4,000 meters depth, far below the sea bed.
Experts have said that option can take up to six months to complete, and Total has said it would push up daily costs to $3 million.
The gas leak was reported on March 25 and is spewing an estimated 200,000 cubic meters of natural gas from the evacuated platform into the air per day, forming a highly explosive gas cloud around the platform.
Total has said the leak is costing it $2.5 million a day so far. Its stock has dropped by almost 8 percent since the leak was reported last week, knocking over 7.5 billion euros ($9.8 billion) off its outstanding share value.
The crew of eight aboard the helicopter is a mix of staff from Total and Houston-based Wild Well Control.
Firefighters and engineers from the Houston-based company are experts at disasters such as oil rig explosions and have been dubbed "Hellfighters" by Hollywood.
The leak began after pressure rose in a well that had earlier been capped.
Two firefighting vessels remain on standby outside a two-mile exclusion zone around the Elgin platform, Total said. ($1 = 0.7623 euros)
(Editing by Jane Baird and Keiron Henderson)