Um...

   ... can you write out the requirements for it, then hire a mechanical engineer/designer?

   I take it that spilling the 200,000 barrels of oil is bad?

                        JHG

API 650 and API 653 will help you.  Cut out the plate and then weld it back in.  Maintain proper weld spacing.  Add bracing on the tank outside, on both sides of the opening before the door sheet is cut.  The width of the door sheet is not too much of a concern.  The door sheet can go through the horizontal joint or stop short but there are minimum distances, radius of cut, and weld specifications that you need to use.  You will find these all in API 650 and 653.  A reputable tank contractor can do it all for you.

There's really not any design involved, other than following the codes as relates to weld qualifications, weld spacing, etc.

Keep in mind that a large tank can be built without rolling the shell plate, so the piece you cut out may be flat upon removal, which requires a bit more in the way of fitup to reinstall.

Thanks guys youve really been helpful. I think there is a lot of info on API 650 & 653 to help out.

Thanks