impdb sys@orcl directory=dumpdir schemas=ov1 content=data_only dumpfile=ov1.dmp logfile=ov1.log 

E:\>impdp help=y

Import: Release 11.2.0.1.0 - Production on Mar. Avr. 22 18:00:23 2014

Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.
...
TABLE_EXISTS_ACTION
Action to take if imported object already exists.
Valid keywords are: APPEND, REPLACE, [SKIP] and TRUNCATE.
...

ORA-39120: Table "OV1"."EVENT" can't be truncated, data will be skipped. Failing error is:
ORA-02266: unique/primary keys in table referenced by enabled foreign keys
ORA-39126: Worker unexpected fatal error in KUPW$WORKER.UPATE_TD_ROW_IMP [59]
TABLE_DATA:"OV1"."EVENT"
ORA-39120: Table  can't be truncated, data will be skipped. Failing error is:

ORA-06512: at "SYS.DBMS_SYS_ERROR", line 105
ORA-06512: at "SYS.KUPW$WORKER", line 8171

----- PL/SQL Call Stack -----
  object      line  object
  handle    number  name
AD4D25DC     18990  package body SYS.KUPW$WORKER
AD4D25DC      8192  package body SYS.KUPW$WORKER
AD4D25DC     18552  package body SYS.KUPW$WORKER
AD4D25DC      4105  package body SYS.KUPW$WORKER
AD4D25DC      8875  package body SYS.KUPW$WORKER
AD4D25DC      1649  package body SYS.KUPW$WORKER
A6098C30         2  anonymous block

Job "SYSTEM"."SYS_IMPORT_SCHEMA_45" stopped due to fatal error at 16:55:30

SELECT sql
  FROM
(
  SELECT 1 rnk, TRIM('ALTER TABLE ' || c_src.owner || '.' || c_src.table_name || ' DROP CONSTRAINT ' || c_list.constraint_name || ';') sql
    FROM dba_constraints  c_list,
         dba_cons_columns c_src,
         dba_cons_columns c_dest
   WHERE c_list.owner             = c_src.owner
     AND c_list.r_owner           = c_dest.owner
     AND c_list.constraint_name   = c_src.constraint_name
     AND c_list.r_constraint_name = c_dest.constraint_name
     AND c_list.constraint_type   = 'R'
     AND c_dest.owner             = UPPER('&&owner')
     AND c_dest.table_name        = UPPER('&&table')
     AND NVL(c_src.position, 1)   = NVL(c_dest.position, 1)
     AND NVL(c_src.position, 1)   = 1
UNION ALL
  SELECT 2 rnk, TRIM(dbms_lob.substr(dbms_metadata.get_ddl('REF_CONSTRAINT', c_list.constraint_name, c_src.owner), 32000, 1) || ';') sql
    FROM dba_constraints  c_list,
         dba_cons_columns c_src,
         dba_cons_columns c_dest
   WHERE c_list.owner             = c_src.owner
     AND c_list.r_owner           = c_dest.owner
     AND c_list.constraint_name   = c_src.constraint_name
     AND c_list.r_constraint_name = c_dest.constraint_name
     AND c_list.constraint_type   = 'R'
     AND c_dest.owner             = UPPER('&&owner')
     AND c_dest.table_name        = UPPER('&&table')
     AND NVL(c_src.position, 1)   = NVL(c_dest.position, 1)
     AND NVL(c_src.position, 1)   = 1
) x
ORDER BY rnk, sql;