Hoanglekhoa:
There are no “standards” or codes for an oily water storage tank. I’ve done projects in many countries, and I never came across such things. You have failed to tell us what the purpose of the tank is and the conditions of storage. Consequently, I can’t come up with any suggestions or recommendations - and I doubt anyone seriously can. We simply don’t know what you intend to do with the fluid(s).
If you have local laws, rules, or ordinances, then follow them. Normally, the area or space you have for such a tank (Nominal 30,000 gallons, total) is what you should guide yourself on. In the simplest scope of work, you would choose an API 650 design - but then you have an even cheaper version: the API 12D. This size of tank is approximately 21.5 ft diameter by 18 feet in height.
What do you mean by a “horizontal” vessel? You don’t mean that you would opt for a cylindrical “bullet” design for this capacity - or do you? I don’t think you could justify that. Please explain yourself better as to the application and use.