The circle shown has center $O$, and diameters $\overline{PR}$ and $\overline{QS}$. The length of arc $\overline{PS}$ is $k$ times the length of arc $\overline{PQ}$. If the length of arc $\overline{QR}$ is $36\pi$ and $k=3$, what is the circumference of the circle?