The rig ran aground off Lewis during a storm on Monday while being transported to Malta.
Sightseers have been warned to stay away from the scene of the grounding, and a 300m temporary exclusion zone has been put in place around Dalmore beach.
The Maritime and Coastguard Agency said it was necessary to keep roads clear.
The rig, which is carrying 280 tonnes of diesel oil, broke free from its tug during towing and severe weather prevented the line being reconnected.
A salvage team began an inspection of the structure on Tuesday in preparation for the more detailed assessment.
Hugh Shaw, the Secretary of State's Representative for Maritime Salvage and Intervention, said the initial survey was to check "a number of things including fuel tanks".
Officials said the aim was to carry-out as much work as possible on Wednesday while the weather was favourable, but warned that work to refloat the rig could take some time.
Mark Rodaway, of the Maritime and Coastguard Agency, said it was necessary to keep roads clear for emergency crews and salvage teams.
He said: "We understand that this incident is of interest to people living in the area but we're asking them to stay away to ensure easy access for emergency services and salvage teams.
"Also the last thing we want is for people to be injured or worse trying to get a closer look on remote cliff paths."
Three vessels including the emergency towing vessel Herakles are the scene of the grounding.
Union Bear, a multipurpose offshore vessel, has also arrived at Stornoway Harbour from Aberdeen to assist in the salvage operation.
There were no personnel aboard the 17,000-tonne drilling rig when it became detached from the tug in the early hours of Monday, 8 August.
The semi-submersible platform was being towed from Norway to Malta from where,  it is understood. it was to be taken to Turkey to be scrapped.
The Marine Accident Investigation Branch has begun an inquiry into what happened

Petty Officer Aimee Mejia, 21, was involved in a three-car accident on the island on Saturday, injuring two people, said police.
It is the latest incident involving US troops based in Okinawa.
Many Okinawans have long campaigned against the heavy US military presence.
Police said PO Mejia was driving the wrong way down a street and hit two vehicles. A local woman was left with a cracked breastbone.
Her alcohol levels were about six times the permissible level, Japan's NHK network reported.
Under the US Navy's order, personnel will not be allowed to drink at all, on or off base, and cannot leave the base grounds, except for commuting and necessary errands such as picking up their children or groceries.
The Navy said the "liberty curtailment" would remain in place until all personnel had gone through extra training, while alcohol was banned outright until officials "are comfortable that all personnel understand the impact of responsible behavior".
"These measures are not taken lightly," Rear Adm Matthew Carter, Commander of Naval Forces Japan, said in a statement.
"For decades, we have enjoyed a strong relationship with the people of Japan. It is imperative that each Sailor understand how our actions affect that relationship, and the US-Japan Alliance as a whole."
The US military previously said it was co-operating with the Japanese investigation into the alleged drink-driving case, and expressed sympathies for the injured people
Okinawa is home to more than half of the 50,000 American military personnel based in Japan. Opposition to their presence has rumbled on for years, with anger resurfacing each time a military member is involved in a crime.
US personnel were already under curfew and observing a 30-day mourning period after a civilian military employee was arrested on suspicion of dumping a local woman's body last month.
Prime Minister Shinzo Abe has expressed outrage over the incident, and raised the issue with US President Barack Obama at the G7 summit in Japan.
Salvage teams working at the Transocean Winner oil rig are expected to carry out a detailed inspection later on Wednesday.
[NEXT_CONCEPT]
The US Navy has banned all its personnel in Japan from drinking alcohol and has restricted off-base activity after a sailor was arrested on suspicion of drunk driving.