You can go directly from V15.1 to V18.
First, you need to download the V18.0 Base (INC 00) PAX file and install it.
After you have V18.0 Base installed, you need to download and apply all of the V18 Published Solutions. I just checked and there are 211 Published Solutions. After you have applied the maintenance, you will be at V18.0.09 (INC 09) and can being your implementation of V18.0.09.
June