Oil Tanker Access Hatch Question

I need to know about the access hatches on an Oil Tanker or Barge. I am told there are 18" and 24" access hatches into the holds of Tankers. Are these hatches round bolted flanges like on piping, or oval with latches?

 

Usually oval with a smaller hatch on top for taking soundings. I'll see if I can find a picture.

EDIT:

Try these guys. Most of the ones I've seen are of the raised bolted or raised waterproof type with a sounding port:

http://www.nabrico-marine.com/pages/hatches.html