Forced harmonic oscillations of nine configurations consisting of horizontal side-by-side plate elements are performed experimentally and numerically. The configurations are oscillated in vertical direction and represent generalized mudmats of subsea structures. The tests are performed for Keulegan–Carpenter (KC) numbers relevant for force estimation during lifting operations. Hydrodynamic added mass and damping coefficients are presented. The coefficients are found to be amplitude dependent for all tested configurations. The interaction effects between the plates increase with increasing amplitude and decreasing distance between the plates. For small oscillation amplitudes, compared with the gap between the plates, the plates behave approximately like individual plates. A study of the relation between the damping force and the added mass force for the tested structures illustrates the importance of applying representative damping coefficients in numerical analysis of marine operations. Numerical results are obtained using a potential flow solver (BEM) and a viscous flow solver (CFD). Low-KC added mass coefficients predicted with the BEM are in accordance with the experiments. There is acceptable agreement between the CFD and the experiments. Best agreement is obtained for small KC numbers. As the KC numbers increase, the differences are, in general, larger. This is possibly due to the CFD being based on the two-dimensional laminar flow.