In the past decades, the global demand for energy has increased. The aim to reduces has let to an increase in the production of renewable energy sources, such as wind and solar energy. However, there are more major energy resources available in the oceans, such as wave and tidal power. The estimated potential global energy resource in waves is around 2 Terra Watt. Over the years, many different technologies have been developed to harvest this high-density energy source. Yet, the harsh weather conditions are challenging the survival of the energy converters. A potential device that avoids the risk of high environmental impacts and has a reduced vulnerability is the Neptune: The Neptune is a fully submerged wave energy converter. Inside the structure, a weir and internal air pocket separate two water columns. One column acts as an oscillating water column to pressures of the incident waves. During its oscillations, the inner free surface level exceeds the weir and spills water into the second column. This column acts as a reservoir, and the overflow water is drawn off through an exit pipe, including a turbine. From the net flow through the columns, energy can be extracted. This design has the advantage of being fully submerged and has a single moving part, namely the turbine. The first work on this type of wave energy converter dates back to the mid-1970s. In 2008 a 1/20th scale model was built and tested in a wave basin. However, the work and experiment didn’t include the effects of energy generation. The objective of this thesis is to form a more scientific base concerning this device. A numerical model is made to predict the dynamic behaviour of the system. The equations of motion of the water columns are derived from the equations of conservation of mass and momentum. The excitation forces are obtained from the linear wave theory for regular undisturbed waves. The hydrodynamic coefficients are determined from associated literature. The dynamics of the internal air pressure is derived from the conservation of mass and the pressure density relation for an adiabatic and reversible thermodynamic process...