CI3, active record contain $offset,$limit

Hello, i already have   model that is this 

PHP Code:

$q = $this->db->query("select * from employee where coloumnA like '%$keyword%' OR columnB like  '%$keyword%' LIMIT $offset,$limit");
return $q; 


I want to change it into query builder sentences,how to do it? I tried several times, but it gave errors.
Thanks