How to calculate structure accn. inorder to cal. wave force

[pondy08]

Dear Sir,
As per modified morisons equation to calculate forces on members due to wave load,
Fm = row*V*a+row*Cm*Vr*ar --- DNV 6.1.2 (classification note 30.5), if the member or structure vibrates under impact of wave.

ar=relative acceleration (bwt structure and wave)
we can calculate waves acccl. using wave theories available ,

I want to how to calculate the structure acceleartion, please clarify me.

Thanks
P.Ejoumale