How To Convert a Standby database to a Standalone Database After removing it from Data guard environment
Scenario:
You have already removed your standby database from your data guard environment and now you want to converts it to a Standalone database.
Solution:
In your removed standby database:
SQL> select database_role from v$database;
DATABASE_ROLE
------------------------------------------------
PHYSICAL STANDBY
SQL> alter database activate standby database;
Database altered.
SQL> SHUTDOWN IMMEDIATE;
ORA-01109: database not open
Database dismounted.
ORACLE instance shut down.
SQL> select database_role from v$database;
DATABASE_ROLE
------------------------------------------------
PRIMARY
For more information please check Oracle Support Doc ID 2074686.1