Cylindrical coordinate for flow simulation

Dear all

I want to solve a flow inside a pipe in 3D and I have (r,theta,z) as my coordinate. how I can define d/dtheta or d/dr in freefem?

BR

If you are in 3d you are using x,y,z but ca can recall as r,theta, z
when of domain is a box [0,R]x[0,2*pi],[Zmin,ZMax]
with periodic boundary condition on theta.