Well, first of all, 18 m/s is the object's speed, but not its velocity.
Velocity consists of the object's speed AND direction, and this
particular object's direction is constantly changing, since it's
moving around a circle. So its velocity is constantly changing too.
Centripetal acceleration = (speed)² / (radius)
= (18 m/s)² / (30 m)
= (324 m²/s²) / (30 m)
= 10.8 m/s²
(about 1.1 G)