The closest thing I've found to allow for cutting across a log section is to have a jig with a "V" cut into it. For a log the size you mention, it will need to be fairly wide and deep V. A log will roll, and be pulled into the blade, along with your hands and fingers if you do not find a way to control the downward rotation of the log as it enters into the blade. Early on, I ruined a couple of band blades, and got dangerously close to a serious injury by just trying to hold with my hands.....thankfully, I was not hurt, and learned my lesson!
Is the resaw height of your saw capable of cutting a 15" log? If you have a large bandsaw, it likely will, but at any rate, you need the right blade...preferably 3 tpi, and have the saw tension adjusted to where the blade runs true without much flex. I use both the Woodturners Blade from Highland Woodworking, and Lennox Diemaster 2 blades. The Carter "Green Wood Blade" is 3 tpi, and has the same metal as the Diemaster 2. Expensive though.
I would suggest you find a way to clamp the log to your V jig, so you do not have to rely only on your hand strength. Perhaps a screw or two near the ends where your blade will miss them, or maybe a band clamp of sorts to keep it from rolling.
one can take a 12/4 thick piece of wood, and put their tablesaw blade to 45 degrees and cut each side of the "V" swapping ends for the opposite side from the first cut. A wide V with a depth of 2" and a means of clamping should get you where you want to go with this.
I agree with Richard, that a chainsaw will do it easier and faster.......and probably safer too!