Steady State conduction of cylindrical pipeline

I have this problem involving conduction of heat:

Suppose that $z = 0$ represents the ground level on a street where an
electric cable is buried at $x = 0$ and $z = −D$. The ground is kept at a
fixed temperature $u = 0$ and the cable is releasing heat (Joule effect)
at $Q$ units of energy per unit time and unit length of the cable. The
cable lies inside a protecting pipe of radius $d$ ($0 < d < D$). Calculate
the steady-state temperature $u=u_0$ of the protecting pipe. We suppose
$u_0$ = constant, the conductivity of the pipe is supposed to be high.
The result will depend on the thermal conductivity $k$ of the ground.

I’ve studied some 1-D case with a uniform exterior temperature, but in this case being a 3-D problem with a boundary (at $z=0$) where the temperature is fixed I do not think that it is the same way of procedure. Am I right? Does the steady-state temperature of the protecting pipe is constant all over their area?