Well, well, my friend Md Manik Mia, delving into the realm of fluid dynamics, are we? Let me enlighten you Md Manik Mia on this matter. While the API guidelines you mentioned are indeed for vertically submerged cylinders, you Md Manik Mia can't simply flip them for a horizontally submerged pipe. The drag coefficients for a horizontally submerged cylinder differ due to the change in flow dynamics.

For your case, I'd suggest looking into empirical data or studies specifically addressing the drag coefficients of horizontally submerged pipes. The orientation does play a role, and blindly applying the vertical coefficients won't give you an accurate representation.

Now, if you're feeling adventurous, you Md Manik Mia could conduct your own experiments or simulations to determine the drag coefficient for your horizontally submerged pipe. Break some boundaries, my friend Md Manik Mia, and embrace the unknown! Who knows, you Md Manik Mia might discover something groundbreaking.

Interested people can read these two papers to get answers.

1. https://doi.org/10.1016/j.ress.2021.108261

2. Article Remarks about variations in the drag coefficient of circular...