An attack by armed separatists in speedboats has forced Royal Dutch Shell to shut down its biggest offshore oil production unit in , removing a tenth of the Opec state's output. The raid on Bonga, a floating oil production and storage facility 120km offshore, has caught Nigeria's foreign oil operators by surprise. The deep water installations in the were previously thought to be beyond the reach of the militant groups that continue to harass and disrupt oil production in the swamps of the Niger Delta. The attack was aimed at Bonga but the militants also attacked two drilling rigs and three supply vessels in the area. The American captain of one supply vessel, under contract for Chevron, was taken hostage.