Oil firm Royal Dutch Shell was told a pipeline had reached the end of its life years before it spilled up to 500,000 barrels of oil, according to court documents seen by the BBC.
Two spills in 2008 affected about 35 sq miles (90 sq km) in southern Nigeria, according to a group suing Shell.
The area included sensitive mangroves.
Shell “dismisses the suggestion that it has knowingly continued to use a pipeline that is not safe to operate,” it told the BBC.
The emails, letters and internal reports submitted to a court in London show that senior Shell employees were concerned before the spill that Shell’s pipelines in the area had reached the end of their lives and needed replacing to avoid danger to lives, the environment and the economy.
The spills took place in Bodo, a town in the Ogoniland region, where people interviewed for an Amnesty International report into the effects of the incident reported headaches and eyesight problems.
Following the spills, the price of fish, a local staple food, rose as much as tenfold, according to Amnesty. People who worked in fishing had to find jobs in other industries which proved more difficult to find.
Oil spills in the Ogoniland region have also contaminated local drinking water sources, seeping into groundwater, according to a report from the United Nations Environment Programme (UNEP). Toxins found by UNEP in the wider Ogoniland area’s drinking water include benzene, which is thought to cause cancer.
The two spills came from the same pipe on the Trans Niger Pipeline, operated by Shell, which takes oil from its fields to the export terminal at Bonny on the coast. It carries about 180,000 barrels of oil per day. The firm disputes the size of the spills and says much of the oil was spilt as a result of thefts and sabotage.
In September 2006 – two years before the spills – a letter from Basil Omiyi, managing director of Shell’s Nigeria business, SPDC, to the governor of Rivers State said that the pipeline was of “immediate and utmost concern”.
Shell had not inspected the pipeline for several years due to difficulties in accessing it, he said.
“There is a risk and likelihood of rupture on this pipeline at any time, which if it happens, could have serious consequences for the safety of life, the environment and the nation’s economy.”
Shell, in documents submitted in its defence, admits its pipeline failed due to a faulty weld, but disputes the volume of spilled oil. It says much of the damage caused to the area comes from other spills, including damage from sabotage and attempts at theft.
Shell will pay “just and reasonable” compensation to those affected, it says.
Shell told the BBC: “SPDC ceased operations in Ogoniland in 1993 following a rise in violence, threats to staff and attacks on facilities. Levels of violence and criminality have remained high over the following 21 years, constraining SPDC’s ability to access the area.
“The condition of the pipeline is regularly assessed.
“Also, SPDC has always made use of the opportunity presented during sabotage [and] crude theft point leak repairs to carry out on-the-spot coating and internal checks to confirm the integrity of the pipeline and coating.”
Shell was unable to use its other leak detection systems such as flow monitoring or acoustic sensors because they were sabotaged or stolen and it was “not reasonable to keep replacing such equipment,” it said.
The claimants say the pipelines were not checked regularly for corrosion or failing welding by robots known in the industry as intelligent pipeline inspection gadgets.. In the UK, pipelines would be inspected in this way every five years or so, according to industry experts.
Shell says it employed contractors to guard and monitor the pipeline and arranged for inspections from the air. The claimants say a lack of boats and too few people to adequately patrol made their job “inadequate”.
The claimants say Shell didn’t want to shut down the pipes that form the Trans Niger Pipeline because of the company’s reliance on them, citing an internal Shell email which says “sustenance/stability via the TNP [pipeline] is key for our survival”