The UK's Unite trade union is set to go ahead with a 12-hour strike Monday at Total's North Sea oil and gas platforms as a dispute over shift patterns continues, while a separate dispute at the Shetland Gas Plant has been resolved, the union said.

Monday's strike will be the fourth strike day in the dispute, some of them for 12 hours and some 24 hours.

The dispute, suggesting a greater willingness by unions to flex their muscles, comes as major North Sea producers have declared success in bringing down costs and weathering the price downturn.

Total has said the long-term sustainability and efficiency of its North Sea operations was at stake in the dispute.

Total has used some of the downtime for maintenance that was due anyway, but the impact in production terms is prolonged by the time and complexity involved in restarting operations, the company has indicated.

A Unite spokesman told S&P Global Platts further strikes were under consideration for September 3 and 17, and October 1, 15, and 29, adding: "That is if talks fail."

The amount of oil output affected by the strikes at the Alwyn, Dunbar and Elgin-Franklin facilities is thought to be around 70,000 b/d, most being shipments through the Forties pipeline, some of which are exported and some of which feed the Grangemouth refinery.

Gas production at Elgin-Franklin, the main facilities affected, amounts to around 350 MMcf/d.

Separately, Unite said it had resolved a dispute at Total's Shetland Gas Plant, which separates and processes gas and condensate from fields such as Laggan-Tormore West of the Shetland islands.

In exchange for a 15% salary rise and bonuses, Unite had accepted Total's '3/3' rota, entailing three weeks' work followed by three weeks off. The agreement could set a precedent for the dispute at the North Sea platforms, it said.

At the Shetland plant, "while Unite has serious concerns over the 3/3 rota system remaining in place, which we will continue to forensically monitor, we have agreed a generous package available to those that wish to leave and received a commitment by Total not to make adverse changes to the rota system for next three years," the union said.