The purpose of offshore seawater intake is to provide a continuous supply of seawater with proper quantity and quality. The large volume and specified water quality requirements, offshore intake wells are preferred. The adequate and economical supply of seawater from the offshore intake well depends on the performance of the pumping system. In a typical intake well, the pumping equipment is placed below the water surface based on the free surface water oscillation inside the well. The loads exerted by the incoming waves and the free surface water oscillation will affect the pumping performance. This phenomenon should be reduced for smooth and efficient pumping operation. To achieve this, some auxiliary equipment like curtain wall is introduced into the intake well. This type of internal structure can regulate the flow and dissipate the incident wave energy up to some extent. Moreover, they can obstruct the formation of vortices and eddying. The present study focuses on the internal hydrodynamics of an offshore intake well with a porous curtain wall. A 1:20 scale model is tested with regular waves in the shallow wave basin and wave run-up, rundown, free surface water oscillation inside the well and the pressure acting on the curtain wall is measured. Then, the influence of the porosity on the free surface water oscillation is investigated by changing the porosity. © 2021, Springer Nature Singapore Pte Ltd.