ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER DATABASE OPEN;
SQL>  ALTER DATABASE CHARACTER SET INTERNAL_USE AL32UTF8;
 ALTER DATABASE CHARACTER SET INTERNAL_USE AL32UTF8
*
ERROR at line 1:
ORA-12720: operation requires database is in EXCLUSIVE mode

SQL> show parameter cluster_database

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cluster_database                     boolean     TRUE
cluster_database_instances           integer     2
SQL>

SQL>  alter system set cluster_database=false scope=spfile;
System altered.
SQL> exit

$ srvctl stop database -d orcl -o IMMEDIATE;
$ srvctl start database -d orcl -o mount;
PRCR-1079 : Failed to start resource ora.orcl.db
CRS-5017: The resource action "ora.orcl.db start" encountered the following error:
ORA-01102: cannot mount database in EXCLUSIVE mode
. For details refer to "(:CLSN00107:)" in "/u01/app/12.1.0/grid/log/oora2/agent/crsd/oraagent_oracle/oraagent_oracle.log".
CRS-2674: Start of 'ora.orcl.db' on 'oora2' failed
CRS-2632: There are no more servers to try to place resource 'ora.orcl.db' on that would satisfy its placement policy

$ sqlplus sys/oracle@orcl as sysdba
SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION;
System altered.
SQL> ALTER DATABASE OPEN;
Database altered.
SQL> ALTER DATABASE CHARACTER SET INTERNAL_USE AL32UTF8;
Database altered.
SQL> alter system set cluster_database=true scope=spfile;
System altered.
SQL> exit
$ srvctl stop database -d orcl -o IMMEDIATE;
$ srvctl start database -d orcl -o open;