SQL> 
SQL> -- Update clob column with DBMS_LOB will not fire triggers on the base table.
SQL> 
SQL> drop table t1_a;
drop table t1_a
           *
ERROR at line 1:
ORA-00942: table or view does not exist 


SQL> drop table t1_a_log;
drop table t1_a_log
           *
ERROR at line 1:
ORA-00942: table or view does not exist 


SQL> drop sequence t1_a_seq;
drop sequence t1_a_seq
              *
ERROR at line 1:
ORA-02289: sequence does not exist 


SQL> create sequence t1_a_seq;

Sequence created.

SQL> create table t1_a (id number primary key, c1 clob);

Table created.

SQL> create table t1_a_log (c1 varchar2(100));

Table created.

SQL> insert into t1_a values(1,'xxx');

1 row created.

SQL> insert into t1_a values(2,'yyy');

1 row created.

SQL> commit;

Commit complete.

SQL> 
SQL> create or replace trigger t1_a_trg
  2  before update on t1_a
  3  for each row
  4  declare
  5  pragma autonomous_transaction;
  6  begin
  7  insert into t1_a_log values('updating row, count: '||t1_a_seq.nextval);
  8  commit;
  9  end;
 10  /

Trigger created.

SQL> 
SQL> -- Simple update to the clob column
SQL> update t1_a set c1='xxx + some more text here' where id=1;

1 row updated.

SQL> 
SQL> -- Trgger fired...
SQL> select * from t1_a_log;

C1                                                                              
--------------------------------------------------------------------------------
updating row, count: 1                                                          

SQL> select * from t1_a;

        ID                                                                      
----------                                                                      
C1                                                                              
--------------------------------------------------------------------------------
         1                                                                      
xxx + some more text here                                                       
                                                                                
         2                                                                      
yyy                                                                             
                                                                                

SQL> 
SQL> -- Using BDMS_LOB package to do the update
SQL> declare
  2  x clob;
  3  begin
  4  select c1 into x from t1_a where id=2 for update;
  5  dbms_lob.append(x,' + added text with dbms_lob.append');
  6  end;
  7  /

PL/SQL procedure successfully completed.

SQL> 
SQL> -- Trigger did not fire, trouble here
SQL> select * from t1_a_log;

C1                                                                              
--------------------------------------------------------------------------------
updating row, count: 1                                                          

SQL> select * from t1_a;

        ID                                                                      
----------                                                                      
C1                                                                              
--------------------------------------------------------------------------------
         1                                                                      
xxx + some more text here                                                       
                                                                                
         2                                                                      
yyy + added text with dbms_lob.append                                           
                                                                                

SQL> 
SQL> -- Insert empty_lob then write to it...
SQL> set serveroutput on
SQL> declare
  2  x clob;
  3  icnt number;
  4  begin
  5  insert into t1_a(id, c1) values(3,empty_clob()) return c1 into x;
  6  dbms_lob.write(x,60,1,'Text written to the empty_clob with dbms_lob.write.....................hello!');
  7  end;
  8  /

PL/SQL procedure successfully completed.

SQL> 
SQL> -- Trigger did not fire, trouble here too
SQL> select * from t1_a_log;

C1                                                                              
--------------------------------------------------------------------------------
updating row, count: 1                                                          

SQL> select * from t1_a;

        ID                                                                      
----------                                                                      
C1                                                                              
--------------------------------------------------------------------------------
         1                                                                      
xxx + some more text here                                                       
                                                                                
         2                                                                      
yyy + added text with dbms_lob.append                                           
                                                                                
         3                                                                      
Text written to the empty_clob with dbms_lob.write..........                    
                                                                                

SQL> 
SQL> drop table t1_a_log;

Table dropped.

SQL> drop table t1_a;

Table dropped.

SQL> drop sequence t1_a_seq;

Sequence dropped.

SQL> 
SQL> spool off