From my limited knowledge, it is a mix of pipe tension, angle of pipe lay, water depth, soil friction, bend radius, pipe weight and strength and stiffness of your pipe. One catenery length sounds like a good start point to work from but might be a lot more. Unless you're working for an installation conractor, you probably should get buy in from an installation contractor that this bend can work. If you've got a piggy back be very careful as you will get added torsion and possible twisting, which for the main pipe that's probably Ok, for the piggy back....
Also don't forget that your installation vessel will need to perform a route arc on the surface wider than your final on bottom radius so you might run into what ever you're trying to avoid with your vessel or run out of water depth if you're not careful.....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way