I don't want the risk of pregnancy i have had no periods in 18 years and 51 years so from the previous answers does that mean that i am not ovalating?

Many things: If you are 51, it is abnormal for you to have stopped having periods so early. Menopause is considered premature if it happens before age 40. The question is why you stopped so early. Some reasons would mean that you don't have to worry about pregnancy and others wouldn't. Aside form all that, your likelihood od pregnancy is probably small at this point.

No risk: To my knowledge there are 3 documented pregnancies with live birth over the age of 50 in the entire history of the world. That's how rare it is. The pregnancies you see in 50+ year old celebrities are donated oocytes, not natural pregnancies. Most normal women continue to ovulate roughly 10 years past when they can actually conceive.

Menopausal?: You need a test for FSH, a high value would confirm menopause. Yes, you are not ovulating.