Equinor has hailed a major milestone for the North Sea oil and gas industry with first production from its giant Mariner field.

The flagship development east of Shetland has started up after a £6.4billion investment, one of the biggest cash injections for the industry in the last decade.

With up to three billion barrels in place it is one of the UK’s largest oilfields and the project is expected to continue producing through to 2050.

The Norwegian energy giant said Mariner will support more than 700 long-term jobs while contracts worth more than £1billion have been awarded to UK suppliers since work started in 2017.

Around 1,500 people have been involved in the hook up and commissioning of the development around 95 miles east of Shetland, including 800 workers offshore at its peak.

Hedda Felin, Equinor’s senior vice-president UK & Ireland offshore, said: “This start-up of the Mariner field is a huge, significant milestone for us.

“It is Equinor’s first UK operatorship. It is the largest investment in the UK Continental Shelf for the last 10-15 years and it is the second operatorship offshore that Equinor has outside of Norway so there is a lot of focus from all angles.”

Equinor expects to produce more than 300million barrels from the field over the next 30 years and is targeting peak production of up to 70,000 barrels of oil per day.

The heavy oilfield was first discovered in 1981 but it took decades for the industry to work out how to make it viable, with Equinor submitting a field development plan in 2012.

Close to 100 wells are expected to be drilled over the next decade at the field’s two reservoirs.

It is also expected to be the last major infrastructure project in the North Sea until Shell’s Penguins field redevelopment begins in 2022.

Production from Mariner was first expected last year, but a series of delays around “challenging weather conditions” as well as a set of “unofficial” strikes from contractors in December pushed it back.

The project was also expected to cost a total of £5.8billion but that increased by another 10% during construction of the platform in Korea in 2015.

Ms Felin, soon moving on to become Equinor’s top advisor to its CEO, said: “We have been working on the project for a long time. We have had some delays but then we’ve tried to use that time in an optimal way to be robust in order to have a safe start-up.

“It is a big milestone for a lot of people. Many companies have been involved so we’re very grateful to all of them that we are now at this stage of starting-up.”

Equinor describes the project as a “digital frontrunner” with new technologies being used to make the operation safer and more efficient.

Another area the Norwegian firm hopes technology can play a role is on climate change.

Last year, Equinor changed its name from Statoil to reflect its commitment to being a wider energy company with a growing focus on renewables and cleaner forms of output.

However Ms Felin said heavy oil projects such as this will continue to have a crucial in supplying energy.

She added: “We believe the energy mix will consist of oil and gas and renewables but what is important is that we carry out the oil and gas projects as efficiently as we can.

“We think we can have an impact on Mariner, we think we can produce it safely and more efficiently. We’re also using a lot of new technologies and digitalisation so that it can have a natural fit in our portfolio.

“That also continues the systematic work on lowering the carbon footprint of the field. It is quite efficient to be a heavy oilfield, but because it is a heavy oilfield we have to be bold when it comes to taking new investment in technologies that can lower the carbon footprint.”

The three billion barrels of oil in place for the field represents a 50% increase on what was originally estimated, while the amount recoverable has increased 20% to 300million barrels.

With developments in technology, and additional prospects nearby, Equinor will be focussing on the field for years to come.

Ms Felin said: “First now it is safe start-up and continuing to have safe operations and ramping up production levels, getting more wells on stream and working operational excellence, that’s really the focus area on Mariner.

“With a lot of the upside, we’re focussed on getting the maximum out of the field.

“Not only that, but to see if this can be a hub for other areas coming in, that would be interesting for us as there are a lot of opportunities around the Mariner area.”