Abstract
This research was performed with a focus on two key aspects of energy cost–reduction for offshore OWC devices; improving the power extraction efficiency and reducing the excess margin in structural safety factors by a better understanding of wave–induced loads on these devices. This study utilised information from three different resources. First, 2D and 3D numerical results from fully nonlinear Computational Fluid Dynamics (CFD) simulations performed using the commercial code STAR–CCM+ that was validated in good agreement with physical scale model measurements at each stage of increasing complexity during this research. Second, published experiments in the literature for 2D OWC devices subjected to unidirectional regular waves to validate the 2D CFD models of this study. Third, experiments conducted in the towing tank of the Australian Maritime College (AMC) for 3D offshore stationary and floating–moored OWC devices (at a model–scale of 1:50) subjected to unidirectional regular and irregular waves. These experiments were designed to (1) compare the hydrodynamic performance of both devices, (2) estimate wave–induced loads on the fixed device during operating conditions, (3) investigate the survivability of the floating–moored device with intact and damaged mooring systems and (4) validate the 3D CFD models of this study.
Using the combined CFD and experimental approach, it was found that optimizing the underwater geometry of an offshore stationary OWC device could significantly improve the power extraction efficiency up to 0.97. However, this efficiency could be reduced due to air compressibility effects at full–scale. The surge motion of the floating–moored device improved device efficiency in regular and irregular waves. Furthermore, the effectiveness of deploying offshore OWC devices in deep–water where waves are more energetic was proven by increasing the extracted pneumatic energy by a maximum of about 7.7 times when wave height was doubled (incident wave energy increased four times). The instantaneous position of the floating–moored OWC device and its interactions with a certain wave train was more important than the maximum wave height in an irregular sea state when assessing device survivability. Survivability with a damaged mooring system was the key analysis for mooring design. For this analysis, using an equivalent design regular wave condition along with the current safety factors recommended for offshore oil and gas platforms was found to over–design the mooring system of the floating OWC device. The good agreement between CFD experiments for survivability analysis with intact and damaged mooring systems in regular waves highlighted that CFD is a very promising tool a designer can employ to investigate and assess device survivability under different conditions upon further validations in irregular waves.