A UN-owned ship arrived off Yemen on Sunday for a risky operation to pump more than a million barrels of oil from a decaying tanker and prevent a catastrophic spill.
After years of tense diplomacy between the United Nations, Yemen's Houthis and the internationally recognized government, the Nautica entered Yemeni waters at midday, and was expected to moor soon alongside the FSO Safer, a rusting super-tanker in the Red Sea.
The delicate operation to transfer 1.14 million barrels of Marib light crude to the Nautica, bought by the United Nations for the operation, is expected to begin towards the end of the coming week.
Despite stringent safety checks, concerns remain about a spill or an explosion. The Safer is carrying four times as much oil as was spilled in the 1989 Exxon Valdez disaster off Alaska.
"The risk is high. The risk is very high," said Mohammed Mudawi, the UN Development Programme (UNDP) project manager for the ship Safer.
"But we are hoping with the completion of the project that this will be eliminated."
Maintenance operations on the Safer were suspended in 2015 because of Yemen's war, and the UNDP has for years warned it could "explode at any time".
A major spill could result in ecological disaster, devastate Yemeni fishing communities, and close lifeline ports and desalination plants, according to AFP.
The potential spill -- which could cost more than $20 billion to clean up -- would possibly reach Saudi Arabia, Eritrea, Djibouti and Somalia, the UN has warned.
Scorching summer temperatures, ageing pipes and sea mines lurking in surrounding waters all pose threats to the operation, which has been under preparation since late May by experts from the private company SMIT Salvage.
The team has inspected the vessel, arranged transfer pumps and hoses and pumped inert gas into cargo tanks to lower the risk of an explosion, David Gressly, the UN's resident coordinator for Yemen, told the Security Council on Monday.
Working at the height of summer, when on-deck temperatures soar above 50 degrees Celsius (122 degrees Fahrenheit), is an additional hazard, said Nick Quinn, a senior adviser for the project.
"It becomes really hot, really quickly," Quinn said, noting that this increases the odds of "slips, trips and falls" on deck for workers donning heavy personal protective equipment.
The 47-year-old Safer has been moored off Yemen's coast since the 1980s, when it was converted into a floating storage and offloading unit.
The Nautica sailed from Djibouti on Saturday, arriving in Yemeni waters before midday on Sunday.
Gressly, who was aboard the new ship, told AFP that local officials went out to see it on Sunday.
"We feel pretty confident now that this is going to go forward. We believe through all the statements that we've received and the assurances, both private and public, that the transfer will go through," Gressly said.
Out past mangrove stands and other tankers bearing gas and grains, the Safer's position -- around 50 kilometres from the port of Hodeidah -- is rich in the type of wildlife that would be devastated by a spill.
When AFP visited the Safer on Saturday, dolphins flashed their fins within sight of the ship, and cormorants could be seen on the rudder, which they have made their home for years.
Mudawi said lingering concerns about the Safer's infrastructure require oil-pumping to start during the day, at least 10 hours before sunset, to ensure all connections are secure and that workers can monitor for leaks.
Once underway, UN officials expect the transfer of oil from the Safer to the Nautica to take about three weeks.
