The dilapidated state of the floating storage unit at risk of exploding off Yemen’s coast has been revealed in new photos taken by the salvage team preparing to offload more than 1.1m barrels of oil.

A Boskalis multipurpose support vessel arrived this week to inspect the 406,600-dwt FSO Safer (built 1976) and make preparations to remove the oil without the risk of an explosion that could cause an environmental and economic disaster.

The FSO Safer, one of the largest vessels ever built, has not been maintained since the start of Yemen’s civil war in 2015. Boskalis said the work would include checks of the engine and pump rooms and underwater inspections where required.

The United Nations-backed operation will see the oil transferred to the 307,000-dwt tanker Nautica (built 2008) in the coming weeks. The VLCC was purchased by the UN from Euronav in a $55m deal.

The UN has raised more than $100m for the operation but plans for a permanent mooring for the FSO Safer’s replacement and scrappage plans remain unfunded.

If the FSO Safer spills its cargo, the UN has estimated that any clean-up operation would cost $20bn. Billions more would be lost to global trade owing to the blockage of the Bab al-Mandab strait to the Suez Canal. It said that 17m people would be affected.

In April, the IMO asked member states to send old spill response kits, including oil dispersants and rapidly erected storage tanks, as the oil transfer operation was “complex and inherently risky”.