Such attacks targeting the oil industry in the Niger Delta in recent months have caused Nigeria to lose its position as Africa's largest oil producer.
Oil provides 70 percent of the federal government's revenue. Late last month, Finance Minister Kemi Adeosun said the recent attacks on facilities of U.S.-based Chevron, Dutch-British Shell and Italian Agip cost the government nearly $60 million in May alone.
Militants have said they want a bigger share of Nigeria's oil wealth for residents of the Niger Delta. They also are angry about cuts to an amnesty program that paid militants to guard the installations they had attacked in the past.
The recent attacks have stopped production at two of Nigeria's five oil refineries, disrupted supplies from two export terminals and made buyers cautious of Nigerian oil.
Shell's declaration of force majeure was a move to protect it from contractual obligations because of events beyond the company's control. Exports of Bonny Light crude oil have continued.
The oil company cited a leak in the Nembe pipeline for its declaration in May, but pipeline operator Aiteo Exploration at the time said the trunkline had been damaged in an attack.
Shell says the Nembe pipeline has a capacity of 150,000 barrels per day.
All Nigeria's oil production is in the Niger Delta and offshore of the southern region. Militants and non-violent activists say the industry has massively polluted their lands and destroyed the livelihoods of communities that rely on fishing and agriculture.