From the Telegraph

The document, written by al-Qaeda planner Younis al-Mauretani, suggests planting recruits in jobs which could later be useful in attacks, such as oil or gas transportation, and directing supporters to study chemistry and physics. [...]

 Al-Mauretani identifies the massive petrochemical facility at Abqaiq in Saudi Arabia as a potential target for aerial attack.

The 17-page document includes a suggestion that al-Qaeda should use submarines to explore the layout of underwater gas pipelines, in order to mine them. This idea is given serious consideration in the document which notes that the pipelines have safety valves every 6 miles that need to be taken into account when planting mines.