The failure to have had an oil spill does not mean it won't happen.
Oil spills could be like earthquakes - the longer you go without one, the more certain it will happen soon.
If the Trans Mountain pipeline goes through, precautions must include a real time tanker traffic monitoring system that tracks every single tanker, and other cargo ship, in the Salish Sea and Strait of Juan de Fuca. All the ships must be equipped with all possible safety gear. The gear must be transmitting to mainland traffic supervisor sites at all times, including live video feed from the bridge and sensors on steering and engines.
Combine that with shore monitoring facilities that also do real time tracking radar of all cargo ships in these waters.
Combine that with capable tugs on standby along the route.