Microsoft office programs are not safe for use in Windows services / IIS. You should use the Microsoft.Office.Interop library only when you have a program with a GUI running in a user session.
Try to find a library which can work with the MS Project files without calling MS Project.
See also: https://support.microsoft.com/en-us/kb/257757
Microsoft does not currently recommend, and does not support, Automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, ASP.NET, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when Office is run in this environment.
By the way, "OutOfMemoryException" is often just an erroneously translated error of a non-managed component.