When I added mod to existing save with oil processing already researched, I could not get drain unlocked.

I changed it's recipe in my mod files from enabled=false, to enabled=true, just so it works without any tech requirement, just for my save.

I'm not sure if that's case, but I believe Factorio had some "event" for mod being ran first time? If so you could check out in this event handler if oil processing is already researched. Worse solution, but still solution, would be to check it on_game_loaded, but I'm not sure if it shouldn't just work like that by default...
I wonder now if Factorio is remembering what recipes are already researched and so that causes problem, or is it only remembering what techs are researched and than unlock appropriate recipes during game-load... If the latter was the case, than maybe I have some different problem, in which case I sorry for false bug-report, but I had no patience to test it thoroughly (my PC launch Factorio in very long time).

I have the same issue, installed the mod after having researched oil processing and a bunch of other things but I can't seem to get the offshore drain item, can't find it and can't research it. How did you fix your problem? What file should I be editing?

Sorry for late response (I'm not often visiting this) the exact way I "fixed" it was :
- I unpacked mod.zip
- Went to prototypes/recipe.lua
- Changed enabled=false to enabled=true under line with name = "offshore-drain",
- Saved,closed,came back to mod folder
- Either create new zip, and name it exactly as orginal one, replacing it, or delete .zip-ped mod (factorio can run mods from folders too)

*disclaimer - my solution is not "real" solution, just changes technologies forthose pumps, so that they're available from start of game. They don't (anymore) need any technology to be researched.

Hey,

So indeed you are all correct, there was an issue with the unlocking of the Offshore Drain when the mod is first loaded into the game, and the fluid handling tech has already been unlocked.

The Mod, will now do a recipe enable, if fluid handling has been researched, and allow you access to the Offshore Drain!

Thanks for the report, nice to know people may actually use the drain :D