Mooring System Considerations for Renewable Energy Standards

Abstract

Marine renewables have made great strides in recent years. The IEC, ABS, and DNV GL continue to generate standards and recommended practices in an effort to formulate approved processes as the renewable products make their way offshore and into the market. There are many similarities in some of the processes and designs when compared to oil and gas structures, especially when it comes to moorings. However, many design areas are uniquely related to renewables, even within the same field of energy conversion (e.g. multiple types of wave energy converters). As more renewable systems are installed, the standards will continue to transition from philosophical to more prescriptive recommendations. One area in which the lines are blurred between oil and gas and renewable industries is mooring systems. The interdependency between the mooring and power generation systems plays a crucial role early in the design phase. Modeling marine energy converters and the mooring system can be complex due the variability of moving parts, and without proper attention, it may be easy to underestimate the loads and fatigue cycles to which moorings will be exposed. Moorings for these structures should incorporate existing standards and recommended practices to ensure safety and reliability. Inspection, maintenance, repair, and replacement should also be considered. As the renewable industry continues to move forward from scaled prototypes to farms of devices, the oil and gas supply chain will contemplate when to become involved from a financial and resource perspective. However, there are still hurdles within the US authorization bodies like BSEE, BOEM, FERC, NOAA, USCG, etc. to overcome. This paper addresses the existing mooring related standards and delineates areas that need further refinement or conservatism as the renewable industry moves forward with the installation of offshore energy converters.

Design and Analysis of a Mooring System for a Wave Energy Converter

“…Extensive experiences on mooring designs in the offshore oil and gas industry [3] have provided useful references to mooring system design of WECs; however, a detailed discussion of how existing mooring standards from the oil and gas industry fitted in the application of station-keeping of WECs can be found in [4,5]. The main differences between WECs mooring systems compared to those from other offshore applications are discussed in [6], such as enabling resonance at the dominant wave frequency of target site, installation in shallow water (generally less than 100 m), accounting for PTO characteristics, enabling converters to be densely deployed for a WECs farm, a lower return period and relatively high cost.…”

“…For the motion-independent devices (e.g., OWC), the mooring design is similar to conventional oil and gas platforms, i.e., the resonant period of mooring should be designed outside the wave period to reduce dynamic loads. On the contrary, for the motion-dependent device, the mooring system needs to be designed to make the resonant period of energy extraction primary modes match the classical wave period of the target site as close as possible [6].…”

Dynamics of The FKT System with Different Mooring Lines

“…Recently, Cribbs et al [17] have surveyed the existing mooring standards. They pointed out that modelling mooring dynamics can be complicated and the loads and fatigue cycles of a mooring system can easily be underestimated due to unexpected marine environments.…”