Are chakras and tantiens the same?

Not exactly, although both terms denote major energy centers of the human body. The seven chakras of the yogic system (or nine, if you count the chakras above the head) are primarily the gateways to different levels of human consciousness. They only tangentially influence human physical health, through psychological and psychic interlinks. The three tantiens of the Chinese system are also gateways to the different levels of human consciousness. However, the lower tantien also directly controls all the energy channels, both major and minor, which govern and regulate all aspects of physical health.

Trying to understand the similarities and differences between both systems intellectually is an immensely complex task. From all sorts of functional points of view, key points of both the tantien and chakra systems are sometimes the same, sometimes similar, and sometimes completely different. It is best, at least initially, to study each system entirely on its own terms. After you have experientially studied both systems equally in depth, the connections and interlinks between them will become obvious.