The operator of the Mariner heavy oil field, one of Britain’s largest offshore developments in years, postponed its startup to summer 2019 on April 10 for safety checks on the platform’s electrical couplings.

Originally due to come on stream in 2017 and with an estimated 300 million barrels of recoverable reserves, the 4.5 billion pound (US$5.9 billion) project has been plagued by delays.

The latest, pushing the expected launch back from the first to the second half of this year, follows a decision taken in November by Norwegian operator Equinor to inspect all of Mariner’s 40,000 couplings.

Initial tests showed the couplings’ failure rate was too high, Equinor’s vice president for major projects, Morten Ruth, told Reuters on April 10.

“They [electrical couplings] can lead to an explosion if gas gets to the platform,” he said. As of this week the company had completed 56% of the work.

“We see the startup getting closer. We see that we will be able to complete all preparations before July, but there are uncertainties,” he said, adding that he now expected the startup between July and the beginning of September.

Ruth declined to say what extra costs the delay would entail.

The North Sea field, discovered east of the Shetland Islands in 1981, was approved for development in 2012.

Mariner is expected to produce about  55,000 barrels per day (bbl/d) at compares with current U.K. production of about 1.2 million bbl/d, according to International Energy Authority data.

Equinor has a 65.11% stake in the project. Its partners are JX Nippon with 20%, Siccar Point with 8.89% and Dyas with 6%.

The platform’s topside was built at a Daewoo Shipbuilding & Marine Engineering (DSME) yard in South Korea.

Mariner has been hit by several delays, and Equinor last October abandoned plans for a late 2018 production start.

Accommodation rig provider Prosafe said in a statement that Equinor had extended the lease of one of its service vessels for work at Mariner by three months until the end of September at a cost of $15 million.