references
| Year | Issued by | Doc. |
| 2005 | Snam Rete Gas |  |
| 2006 | Plinacro | |
| 2003 | PLE | |
| 1998 | Erdgas | |
| 2000 | Saipem | |
| 1997 | Italgas | |
| 2003 | PLE | |
| 2010 | DESFA | |