Task: Question Answering
Input (English): Passage: Following the purchase of an offshore lease in 2005, Shell initiated its US$4.5 billion Arctic drilling program in 2006, after the corporation purchased the "Kulluk" oil rig and leased the Noble Discoverer drillship. At inception, the project was led by Pete Slaiby, a Shell executive who had previously worked in the North Sea. However, after the purchase of a second offshore lease in 2008, Shell only commenced drilling work in 2012, due to the refurbishment of rigs, permit delays from the relevant authorities and lawsuits. The plans to drill in the Arctic led to protests from environmental groups, particularly Greenpeace; furthermore, analysts in the energy field, as well as related industries, also expressed skepticism due to perceptions that drilling in the region is "too dangerous because of harsh conditions and remote locations". Question: What did Shell initate after its 2005 purchase?
Output (English): its US$4.5 billion Arctic drilling program