How to set shipping address ?

I have a case where I need to force my user to not change/edit the default shipping address.So I need to first set default address using code.I used session to set shipping_address but it did not work.Could you please help me to figure out which code sets the shipping address?

Thanks,

The selected shipping address is indeed stored in the session. So you're on the right track.
You can see an example of the address being set in the session in the after_shipping function of the file components/com_hikashop/controllers/checkout.php