Arun,

When you input the platform, then its weight is transferred to the vessel and as the platform is offset, the vessel will also see the additional moment. You can see this moment in the Elements and Detail report.

To include the additional weight of the piping you can use "Weight/Piping Dialog". Specify it as an offset weight. This way the natural frequency calculations will consider the effect of the additional mass, weight will be correct and additional mass from the offset that you specify will also be considered.

There may be other ways to handle it too.