The announcement of "superior force" was effective from noon yesterday, after Shell reported three new leaks on the pipeline.
One was caused by multiple holes drilled into the pipeline, it said.
The declaration gives Nigeria's biggest oil producer some legal protection against contractual obligations.
Shell said it has repaired 189 crude theft points this year on the Trans-Niger Pipeline and the Nembe Creek Trunkline that carry a daily total of 300,000 barrels of crude at least 15 per cent of the production of Africa's biggest oil producer.
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Experts estimate Nigeria loses about 200,000 barrels a day to theft with most stolen by international criminal gangs that sell it on world markets.
They say some Nigerian politicians and military leaders benefit from the massive thefts in a country where corruption is endemic.
Smaller amounts of oil are stolen by members of communities impoverished by decades of oil spills that have destroyed the livelihoods of farmers and fishermen by polluting farmlands, creeks and mangroves.