Man Access Hatches on Barge & Tankers

[Randy S]

I could be wrong, but it seems that a clamped cover would be used for cargo hold access and a bolted cover would be used for ballast tanks.

[ScubaCop]

I am told there are 18" and 24" access hatches on Tankers to get into the holds. Does anyone know if these are round with a bolted pipe flange on them or oval with clamps? The ships I am wondering about, would be like the oil tankers used in the BP clean-up in the gulf.