The effect of mooring systems on the operability and survivability of a floating offshore wave energy converter : numerical and experimental investigations

Current marine renewable energy practices require the installation of large floating structures offshore. The mooring system is one of the important components for a floating offshore wave energy converter (WEC) such as an oscillating water column (OWC). Moorings can significantly impact the rigid body motions and in turn the hydrodynamic performance of a WEC and are solely responsible for station-keeping during extreme weather events. Accurate assessment of a WEC’s dynamics and mooring responses in operational and survivable conditions is critical when developing a WEC and mooring combination that can consistently produce energy across a wide range of conditions yet mitigate the risk associated with a severe storm. The majority of WEC mooring investigations to date have typically considered a single configuration (often with little rationale for the selection of the mooring configuration) and rarely investigated the effects of the mooring system on the hydrodynamic performance of a floating WEC. The goal of this PhD investigation was to systematically compare the effects of different mooring configurations on the operability and survivability of an OWC WEC. Experimental and numerical studies were performed to investigate the effects of rigid body motion responses and mooring systems on the overall performance of the WEC. The experimental investigation considered a systematic series of scale model tests at the Australian Maritime College which were used to validate the numerical models. The numerical approach considers the development of numerical wave tanks (NWT) using computational fluid dynamics (CFD) techniques due to their compatibility with the essence of extreme waves. The numerical study aims to establish efficient numerical modelling techniques for the dynamic response and structural assessment of the WEC and its moorings during moderate and extreme weather conditions. The geometry of the WEC was developed with a validated CFD method and was based on a column-stabilised semi-submersible platform where an asymmetrical OWC chamber was integrated into the moonpool of the resulting structure. The performance of a 1:36 scale model of the WEC was experimentally tested to evaluate the effect of the external support structure, rigid body motions, and mooring configurations on the hydrodynamic performance of the device. The hydrodynamic response of the model under extreme wave conditions was also investigated. Detailed analysis included wave calibration, physical and numerical decay tests to quantify the natural period of the OWC moonpool, wave- and OWC-structure interactions, turbine damping coefficients, hydrodynamic capture width ratios (CWRs), rigid body motions, mooring/tendon tensions, and wave slamming pressures. The findings of this work are documented in four chapters (Chapter 2 – Chapter 5). Chapter 2 presents the results of an experimental and numerical investigation into the generation of realistic model scale extreme waves for survivability testing of offshore renewable energy systems. The sea state selected for this investigation was based on extreme conditions recorded during a Tropical Cyclone with a waverider buoy. The sea state was experimentally modelled using NewWave theory and was subsequently replicated in a CFD NWT. The NewWave formula was modified by applying an exponential function which intended to explicitly amplify the largest waves in the wavefield while applying minimal amplification to the remainder of the waves. This technique increased the accuracy between the experimental and numerical maximum wave height (yielding 4.245% difference) while maintaining a level of spectral accuracy (R2 = 0.9258). Chapter 3 provides a thorough examination and discussion of a design process developed to improve the hydrodynamic performance of an asymmetrical offshore OWC WEC. The obtained results revealed that the addition of the external support structure improved the OWC’s CWRs from 0.849 at kd~2.53 to 1.541 at kd~2.99 (81.5% increase). It was also observed that the external support structure shifted the peak performance towards higher frequency waves. Chapter 4 presents the results of a systematic experimental investigation into the effects of different mooring configurations on the hydrodynamic performance of the 1:36 scaled OWC WEC model. The WEC was tested in the fixed, free-floating, and moored conditions with three different mooring configurations including a tension leg, a taut mooring with 45° tendons, and a catenary mooring with heavy chains. The rigid-body motions of the device had adversely affected the WEC’s performance with a substantial decrease in the hydrodynamic CWR occurring between the fixed condition and the floating-moored conditions. Of the floating-moored conditions, the 45° taut moored WEC was the best performing, followed by the vertical taut and catenary mooring configuration. Chapter 5 presents the results of an experimental investigation into the survivability characteristics of the floating-moored WEC. The WEC was tested in the floating condition where the WEC was moored into position with a vertical taut mooring, a 45° taut mooring, and a chain catenary. The results and discussion were centred around three extreme sea states that were generated with NewWave theory. The tested sea states included a large singular focused wave and two focused waves that were embedded in an irregular sea state. The singular focused wave condition broke at the immediate site of the WEC and slammed into the forward wall of the OWC, whereas the two embedded focused wave conditions included a large crest and a deep trough, respectively. Each mooring configuration experienced detrimental and adverse responses depending on the shape and type of the focused wave group. Nevertheless, the catenary mooring configuration demonstrated superior survivability characteristics followed by the 45° taut and vertical taut configurations. Following the experimental campaign, results from CFD simulations were compared with various experimental datasets including an OWC moonpool decay test, free floating rigid body decay tests, mooring qualification tests, and a large singular focused wave. The results indicate that CFD is an accurate tool to model the survivability of the floating-moored OWC WEC. The floating-moored configurations investigated in this study had a significant effect on the hydrodynamic performance and survivability characteristics of the OWC WEC. Regardless of the mooring configuration, the rigid-body motions of the device had adversely affected the WEC’s performance with a substantial decrease in the hydrodynamic CWR occurring between the fixed condition and the floating-moored conditions. When considering the floating-moored conditions, it can be concluded that there is an inverse relationship between a WEC’s hydrodynamic performance and survivability characteristics relative to catenary or taut mooring configurations. For example, the 45° taut moored WEC was shown to have the highest CWRs, followed by the vertical taut and catenary mooring configuration. Whereas the catenary mooring configuration demonstrated superior survivability characteristics followed by the 45° taut and vertical taut configurations. Finally, comparison between the CFD and experimental results indicated that these methodologies are appropriate techniques for use when modelling the dynamic and structural response of the WEC and its moorings during extreme weather conditions. This work will provide industry and research organisations with recommendations and advice relevant to mooring selection increased hydrodynamic performance and survivability of floating OWC WECs