Dear Zahid,
Indeed, there is a bug in HydroDyn whereby the external and total buoyancy written to the HydroDyn summary file is incorrect for the version of HydroDyn included in FAST v8.15 and v8.16. This bug has been discussed in the following forum topic and will be fixed in the next release of HydroDyn: Water ballasting & buoyancy loads - #4 by Greg.Hayman. The time series is not effected.
Best regards,