Pirates have seized a ship owned by a French company off the Nigerian coast, taking nine crew members hostage. The company - Bourbon - said the captain had assured them that all crew members were unharmed. The hostages are from Nigeria, Ghana, Cameroon and Indonesia. Piracy is common in Nigerian waters, often linked to militants targeting oil companies.Militants attack vessels and strip them of valuables, taking hostages for ransom. The hostages are usually released later.