The WITT (Whatever Input to Torsion Transfer) energy harvesting device converts chaotic motion in six degrees of freedom to a single unidirectional output, which can be coupled to a generator to provide electrical power. The WITT energy device of this size order has an operational sweet spot of approximately 2 Hz, with up to 100 mm amplitude displacements, although it is tunable within the WITT device by varying the pendulum length and mass. To operate in this nature, subsea vortex-induced vibration (VIV) was chosen to drive the system. This is achieved by connecting the WITT energy transmission to cylindrical cross-section pipe sections, creating a modular system that can be varied for sites with different conditions or power requirements. Numerical predictions were performed for a wide range of design variations over a broad range of operating conditions using different VIV calculation techniques. This informed the likely dimensions for different sites and power requirements, so that an off-the-shelf solution could be quickly provided to a client, providing a sealed modular subsea energy solution. Tank testing was performed to compare to the numerical predictions and determine the effect of the WITT energy device on the system dynamics, demonstrating the concept is feasible. A range of possible applications are provided, from powering subsea monitoring equipment through to reduction of riser fatigue by reducing VIV.