Who built them and where were they used? That should help point you in the right direction. The builder's yard will have plans and maybe photos. Also, if Shell Oil operated them they probably have lots of photos. Shell must have an archive, start with their web site and explain what you want and why.
Lloydd's Register of Shipping will tell you if any are still in service and where. You can then reach out to their new owners or if you can travel then you can do first hand research.
These approaches have worked for me when researching another tug project I am starting.
Just in case someone else has been there done that and made off with the T-shirt, try Taubman's Plans Service. They are also on the web.
Good luck,
Bruce