Hi Satish,
What version of BusinessObjects are you using?
Although deprecated, you can still use the sWindow=New attribute with openDoc to open target reports in a new tab/window. This only works with BO XI 3.1 SP3 FP 3.5 or higher.
I haven't tested this with BI 4.0. If it doesn't work then the other workaround involves creating a formula in the report that includes the "target=_blank" attribute plus some modification to the web.xml file on the BO Server.
You may refer to SAP Notes 1536937 and 1730817 for more details.
-Abhilash