Ocean Renewable Power Company, LLC (ORPC) has successfully completed the OCGen® Module Mooring Project (Project).The Project made a significant step in the development of designs, methodologies and practices related to floating and mooring of marine hydrokinetic (MHK) devices. Importantly for ORPC, the Project provided a sound basis for advancing a technically and commercially viable OCGen® Power System. The OCGen® Power System is unique in the MHK industry and, in itself, offers distinct advantages over MHK devices that are secured to the seabed using fixed structural frames. Foremost among these advantages are capital and operating cost reductions and increased power extraction by allowing the device to be placed at a more energetic level of the water column.
This Project required an extensive research, design, development, testing, and data collection and analysis effort conducted with respect to a positively buoyant, submerged MHK device secured to the seabed using a tensioned mooring system. Although the Project was based on ORPC’s OCGen® Power System, it has wide applicability to other MHK systems and to other ocean-based technologies, including offshore oil and gas.
Different analytic tools were evaluated for their utility in the design of submerged systems and their moorings. Three analytical approaches were evaluated: basic computational fluid dynamics (CFD), an ORPC developed lumped-parameter modeling effort, and use of commercial mooring analysis codes. These analytical efforts were supported by a limited scale model test effort. Commercial mooring codes offered significant advantages over the other approaches, in that a well developed and validated code offered higher confidence in the analysis results. Scale model testing was shown to have been qualitatively valuable by rapid testing of selected mooring configurations.
Deployment and testing of a prototype OCGen® system provided significant data related to mooring line loads and system attitude and station keeping. Mooring line loads were measured in situ and reported against flow speeds. The data set generated was one of the few data sets available for such mooring systems. The data will prove useful for additional validation work of newer design approaches.
The most important overall measure of the technical and economic effectiveness of the methods and techniques used in the Project was the full demonstration of the stability of the OCGen® device in reversing tidal currents and the efficacy of the tensioned mooring system. There was no doubt after completion of this Project, that the basic design assumptions of ORPC’s OCGen® Power System have been technically and operationally proven and that both capital and installation costs associated with the tensioned mooring system are significantly less than those incurred in ORPC’s fixed frame power system, the TidGen® Power System.
Costs of concrete and welded steel anchors were lower than the construction for the tubular space frame structures used for the TidGen® device. While overall masses may be equivalent between the anchors for OCGen® and the bottom support frame of the TidGen® device, the mass for the OCGen® anchors was mostly concrete as opposed to high quality steel. Costs of mooring lines were small compared to other project costs.
Installation costs for the buoyancy OCGen® system were much lower than for a piled foundation used for the TidGen® Power System. Installation for the TidGen® Power System would not be expected to be less than ten days of on-water work at the deployment site with expensive lift and barge assets. Deployment of the OCGen® system took place in two days with the majority of the assembly work conducted in protected near shore environments. Actual deployment of the system at the site took approximately six hours.
A further advantage of the OCGen® anchor system was the reduced environmental impact of the installation of the two concrete anchors in comparison to the steel framed bottom support structure with concrete piles for TidGen® Power System. Whereas installing a piled structure for the TidGen® device required hydroacoustic monitoring and observations of endangered species and marine mammals, it was determined by regulators that the tensioned mooring system required no such monitoring.