Is it safe to pull a continuous 18 amps from a 15 amp outlet?
No it is not safe.

A 15 ampere rated receptacle should not be installed on a circuit protected by a 30 ampere breaker. 

You should not be plugging devices that draw more than 15 amperes, into a 15 ampere rated receptacle. 

The receptacle in your photo is a 20 ampere receptacle, so you shouldn't have a problem plugging in a 20 ampere device.  They make receptacle configurations different, so that you can't plug things into the wrong receptacle.  By making a bootleg extension cord, you're bypassing that safety feature. 

You're going to burn the place down.

Also note that since the device is active for longer than 3 hours, it would be considered a continuous load.  In which case the circuit (overcurrent protection, wiring, and devices) should all be rated for 125% of the load.  That would mean the wiring, overcurrent device, and receptacle should be rated to handle at least 22.5 amperes (18 × 1.25). That means a 30 ampere breaker, 10 AWG copper conductors, and a 30 ampere receptacle.