The paper investigates the interaction effects due to diffraction by a large volume substructure and an installation barge, during the operation of setting the deck onto the substructure. Linear theory is adopted. The barge motions are constrained by attachments to the fixed substructure, and the effects of the constraints are modeled here by a two-stage hydrodynamic/dynamic analysis. The multi-body diffraction problem is first solved without any constraints between the bodies, and in the second stage the constrained equations of motion are solved. The analysis is validated through comparisons with published results for five interconnected barges. Three configurations of substructure are then investigated for an installation configuration: a non-diffracting steel space framed structure (as a reference case); an array of four large diameter vertical columns mounted on the seabed and a Gravity Based Structure consisting of the same four columns with a subsurface caisson.