(Adds quotes, detail)
By Nerijus Adomaitis and Shadia Nasralla
OSLO/ABERDEEN, Scotland, June 4 (Reuters) - Norway's Equinor and its partners will look to replicate savings from the Arctic Johan Castberg field off Norway at Rosebank, one of the largest undeveloped oil and gas fields on the British continental shelf.
Equinor said it would make a final investment decision by 2022 on Rosebank, which it plans to develop using a floating production, storage and offloading vessel as at Castberg, where it managed to cut development costs to 50 billion Norwegian crowns ($5.75 billion, around half its original estimate.
The breakeven point per barrel came down "from the mid $70s to the mid $30s...it’s a similar type of development. We will take inspiration from Johan Castberg," Torgrim Reitan, Equinor's head of international development and production, said.
Reitan also told Reuters at an industry conference in the eastern Scottish city of Aberdeen on Tuesday that Equinor's Mariner project off Britain was on track to start production in the third quarter.
And regardless of the outcome of Britain's planned exit from the European Union, Equinor was looking to increase its portfolio in the British North Sea, he added.
The Rosebank discovery, which lies some 130 km (80 miles) northwest of the Shetland Islands, could hold more than 300 million barrels of recoverable oil, according to the estimates of its former operator Chevron.
Equinor did not give Rosebank's likely development costs, but energy consultancy Wood Mackenzie last year estimated they would total more than $6 billion.
Equinor and Suncor Energy have each 40% interest in Rosebank, while Siccar Point Energy has the remaining 20%.
The Norwegian firm bought its stake from Chevron last year as the U.S. oil major focuses on developing its shale assets. ($1 = 8.6973 Norwegian crowns) (Reporting by Nerijus Adomaitis, editing by Gwladys Fouche, Louise Heavens and Alexander Smith)