Integrity assessment of an oyster farm mooring system through in-situ measurements and extreme environment modeling

Abstract

Design practices for aquaculture systems in northeastern US estuarine areas have evolved over time based on experience of operators and on project economics in an increasingly competitive market. There has been growing awareness among aquaculture practitioners of the need to evaluate the integrity of their designs under increasing severity and/or frequency of extreme environments, such as winter storms and hurricanes. A parametric study is conducted in this paper to assess the adequacy of mooring ropes for oyster cages under severe environmental conditions, including high tides, storm surges and 100-year extreme wave heights. The test site for this study, comprising of 1080 cages arranged in 24 rows, is located in the upstream estuarine area of the Damariscotta River along the coast of Maine, where significant aquaculture farming takes place. Field measurements were conducted on the main mooring lines at three different locations, by using several load cells and custom-designed data acquisition systems. Environmental data at the site were collected from oceanographic buoys, as well as from local tidal and wind stations. Statistical results of the field data show that the measured mooring tensions are strongly correlated with tide-induced water level variations. While current is not the dominating contributor to mooring tension at high tide, it is a dominant force for rope tension at low to mid tidal level. Also notable is the effect of reduced current on the mooring load as the flow passes through the net cages at the front and back of the farm. The mooring tensions at the front (seaward direction) are observed to be 30–50% larger than the back (landward) loads. In addition, wind gust load also constitutes a significant contribution to mooring tension. At low and high tides, it can add up to 200% and 30%, respectively, to the total mooring load. Formulations commonly used in offshore engineering were applied to evaluate the environmental forcing and the line tensions. By employing recommended inertia and drag coefficients, good agreement with measured data were obtained. These formulations are subsequently used to evaluate the safety factor of the under various scenarios, including larger farm sizes, shorter rope lengths, and effects of extreme storm surge conditions and 100-year return wave height.