The electrical issue is now being investigated, the company said, and rectification plans are being developed.
First gas is now anticipated by around year-end, it added.
Tolmount is forecast to produce 20,000 to 25,000 barrels oil equivalent per day, net to Harbour, and was expected to contribute around 10,000 boepd to Harbour’s output in 2021.
As a result, the company now cuts production guidance to 185,000 to 195,000 boepd.
Harbour said that its new guidance also includes revisions related to the delayed restart of Elgin Franklin field following summer maintenance.
Operating costs are now forecast at US$15 to US$16 per barrel oil equivalent, unchanged, and it is maintaining the capex budget at US$1.1bn.