In this paper, a semi-analytical model based on linear potential flow theory and an eigenfunction expansion method is developed to study wave scattering by an array of structured cylinders in water of finite depth. Each cylinder is formed by closely-spaced array of thin vertical plates, between which fluid can flow, extending through the depth. In order to consider the wave attenuation and energy dissipation in narrow gaps between the thin vertical plates, a viscous boundary layer is introduced at the surface of the fluid occupied by the structured cylinders. In addition to a direct calculation of the energy dissipation, an indirect method based on Kochin functions is derived with the employment of energy identities. The present model is shown to be in excellent agreement with both the published data and those obtained by using different methods. The validated model is then applied to study the effect of a pair of structured cylinders on wave focusing/blocking, scattered far-field amplitude and wave power dissipation. Results show that wave focusing/blocking can be achieved by the appropriate choice of plate alignment. The structured cylinders hold profound potential for wave power extraction.