A band of saboteurs that calls itself the Niger Delta Avengers has been prowling the swamps of Nigeria’s petroleum-rich south for four months, bombing pipelines and diving underwater to destroy equipment. The damage has helped tip Africa’s biggest economy toward recession, and has cost Nigeria its position as the continent’s top oil producer—a distinction inherited by Angola. The Avengers struck again before dawn on Friday. A group of militants sneaked through marshland to bomb two pipelines, one owned by Royal Dutch Shell RDS.A 1.25 % PLC and the other owned by Italy’s Eni E 0.56 % SpA, according to Nigeria’s navy. Shell confirmed signs of a spill from one of its pipelines and said it is still evaluating potential damage. Eni confirmed the attack but said it didn’t contribute to any new supply disruption. […]