add_action( 'pre_get_posts', 'sk_portfolio_archive_random' );
/**
 * Set Random order for entries in Portfolio archive page
 *
 * @author Bill Erickson
 * @author Sridhar Katakam
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data
 *
 */
function sk_portfolio_archive_random( $query ) {

	if( $query->is_main_query() && !is_admin() && is_post_type_archive( 'portfolio' ) ) {
		$query->set( 'orderby', 'rand' );
	}

}