Alaska is seeking bids to conduct a comprehensive risk assessment of the state's oil and gas infrastructure to prevent a repeat of the corrosion and spills suffered in recent years at the Prudhoe Bay oilfield, the largest in the US. The engineering analysis, for which bids are due by April 28, is being conducted on the instructions of Governor Sarah Palin. She ordered the audit - to take two years and cost $5m - after the biggest spill at the BP-operated Prudhoe Bay in 2006 revealed corrosion in the pipelines and forced the closure of half the oilfield.