Private Sub Workbook_Open()
Dim w As Worksheet, old
Application.ScreenUpdating = False
Set old = ActiveSheet
For Each w In ThisWorkbook.Worksheets
On Error Resume Next
w.Activate
w.Names("DYN").Delete
w.Names.Add Name:="DYN", RefersToR1C1:= _
Replace("=OFFSET(Feuil2!R1C5,0,0,COUNTA(Feuil2!C5),1)", "Feuil2", w.Name)
Next w
End Sub