ORA-16000: database open for read-only access
 *Cause:  The database has been opened for read-only access.  Attempts to
          modify the database using inappropriate DML or DDL statements
            generate this error.
 *Action: In order to modify the database, it must first be shut down and
          re-opened for read-write access.

  1* select TRIGGERING_EVENT, count(*) from dba_triggers where owner = 'SYS' group by TRIGGERING_EVENT order by 1
SQL> /

TRIGGERING_EVENT		 COUNT(*)
------------------------------ ----------
ALTER					1
CREATE					2
DROP					2
DROP OR TRUNCATE			1
RENAME					1
SHUTDOWN				1
STARTUP 				1
TRUNCATE				1

8 rows selected.

SQL> select TRIGGERING_EVENT, count(*) from dba_triggers where owner = 'SYS' group by TRIGGERING_EVENT order by 1;

no rows selected

SQL>

SQL> select count(*) from dba_triggers where TRIGGERING_EVENT = 'STARTUP';

  COUNT(*)
----------
         0

SQL> select count(*) from dba_triggers where TRIGGERING_EVENT like '%STARTUP%';

  COUNT(*)
----------
         0

alter database recover managed standby database cancel;

alter system set events '604 trace name errorstack level 10';
alter system set events '16000 trace name errorstack level 10'; 

alter database open read only;

delete from idl_ub1$ where obj#=:1