Equinor has been forced to delay start-up of its Mariner heavy oil field off the UK to next year, according to Equinor chief executive Eldar Saetre.

There has also been a widening of the scope of hook-up and commissioning work at the £4.5 billion ($5.8 billion) project that has contributed to the delay, supported by challenging weather conditions.

Start-up of field, which was scheduled to come on stream in the fourth quarter of this year, has now been delayed to the first half of 2019, he said at the company’s third-quarter results presentation.

He explained that unusually harsh weather conditions this autumn had led to Prosafe-owned flotel Safe Boreas, which is connected to the field platform, being disconnected several times for safety reasons.

The flotel, which houses 800 workers engaged in hook-up and commissioning work, had been disconnected for about half of the available time, hitting productivity.

However, he said the cost estimate for the project was unchanged and the Norwegian state-controlled operator had also boosted recoverable resources at the field, located south-east of Shetland, by around 20%, or 50 million barrels, to 300 million barrels.

An Equinor spokeswoman attributed this to “greater understanding of the reservoirs, coupled with optimised targeting and drilling of wells, which has improved the drainage strategy”.

The field, expected to produce for 30 years, is being developed through the Mariner A production, drilling and quarters platform that will offload crude to the Mariner B floating storage unit.