It's difficult to speculate.
The two lines were built in the fifties and since that time there haven't been any spills in the Straits of Mackinac. Pipelines have a lot of backup systems, and I would imagine they would engineer it so there would be absolutely no spills in the tunnel. I think the tunnel is an absolute guarantee of no spills into the lake.
My understanding, and from the pictures I've seen, is that the current lines are in good shape so I don't expect any spills until the tunnel is built. The tunnel's going to be 80 feet or 100 feet below lake-bed, and it's going to be somewhere in the range of 15 feet or so. It's going to be well protected.
Again, there are backup systems. Considering all the attention Line 5 is getting, I'm sure it is going to have the best of everything to ensure it will be environmentally safe.
I hope that answers your question, sir.