by Cassandra Burgess
The deployment of marine renewable energy (MRE) devices requires new approaches to offshore mooring and foundations. Mooring and foundations are essential components of MRE device structures, enabling a device to remain in one spot while responding to the movement of tides or waves. Foundations and moorings often account for up to 10% of the total cost of building and maintaining an MRE device; designing a foundation of mooring that can be built at minimal cost requires analyzing the site characteristics, the direction and magnitude of forces on the device, and responses of different technologies to these forces, and the ease of installing and decommissioning the device. These steps present a novel challenge for MRE devices, because such devices have different requirements than traditional offshore technologies. One of the main challenges in applying traditional technologies to this problem is that MRE devices often need to be able to move. This is because the response of the device itself to tides and waves, which is necessary for energy production, is generally linked to the response of the foundation or mooring system. Thus these systems are required to endure loads both horizontally and vertically and to move with the device in some cases. This differs from more common offshore technologies, such as drilling platforms, where movement is not desired. Because of this novel challenge, Karimirad, Koushan, Weller, Hardwick, and Johanning suggest in their 2014 paper that new guidelines need to be developed to aid in the design of MRE device structures.
In addition to the concerns with different loading needs, MRE devices also present challenges in design reliability. These devices are likely to be built in arrays in order to make them economic and to facilitate easy installment and maintenance. However when devices are constructed in arrays, there is a new concern about possible tangling of mooring lines. The authors analyzed several mooring systems, such as taut lines, catenary lines, and surface buoys. In designing these systems it is often desirable to include multiple lines in case one should fail. However, having multiple lines increases the likelihood that the devices will tangle with each other, which could lead to failures in lines, as well as collisions between devices. In addition catenary lines, which are not taught, are often a better choice because their design lowers the peak loading, allowing the mooring to function under more conditions without failing. This is important when considering offshore devices, because the moorings must not fail in the event of large waves that cause greater than normal stresses on the system. Because the lines are not taught, however, there is a possibility that they will become entangled with neighboring lines. These concerns led the authors to consider surface buoys, and different foundation arrangements, in order to handle the increases stresses without raising the risk of line entanglement. Their paper provides a summary of the advantages and disadvantages of each technology examined, and ultimately concludes that new construction guidelines accounting for the increased complexity of MRE device mooring and foundation design are necessary.
Karimirad et al. 2014. Applicability of Offshore Mooring and Foundation Technologies for Marine Renewable Energy (MRE) Device Arrays. The Norwegian Marine Technology Research Institute, DTOcean. [GSSS: Karimirad Koushan]