The farming of brown macroalgae of order Laminariales (kelp) has grown tremendously, especially in temperate waters of the United States like in the Gulf of Maine. As sites move from protected near shore waters to more exposed conditions, engineered approaches will be necessary to assess mooring components for fatigue loads. In this study, a set of comprehensive measurements of forcing, biomass and mooring line response were obtained from a kelp farm in the Gulf of Maine exposed to northeast conditions to assess cyclical tensions. Wave and current profile forcing parameters were measured with two Acoustic Doppler Current Profilers. Mooring line tension was measured on each of the two mooring legs with custom made load cell instruments. The mooring leg components examined in this study consisted of 25 mm (1 inch), 3-strand nylon rope connected to the load cells. From the field study, a relationship is presented correlating the waves, currents, and biomass to the cyclical tensions in the nylon mooring rope. Both operational and extreme wave conditions for periods of maximum biomass were estimated by comparing in-situ measurements to those obtained from the National Data Buoy Center station 44007. The fatigue life of the nylon mooring rope was then examined by estimating the number of cycles to failure with techniques described in Huang and Pan (2010), Mandell (1987), and Leeuwen (1981), that included extreme tension values from St. Gelais et al. (2022).
References:
Huang, C-C and J-Y Pan. 2010. Mooring line fatigue: A risk analysis for an SPM cage system. Aquacult. Eng. 42:8-16. http://doi.org/10.1016/j.aquaeng.2009.09.002.
Mandell, J.F. 1987. Modeling of marine rope fatigue behavior. Textile Res. J. 57 (6): 318-330. https://doi.org/10.1177/004051758705700602.
St-Gelais, A.T., Fredriksson, D.W., Dewhurst, T., Miller-Hope, Z.S., Barry Costa-Pierce, B-A, Johndrow, K. 2022. Engineering a low-cost kelp aquaculture system for community-scale seaweed farming at nearshore exposed sites via user-focused design process. Front. Sustain. Food Syst. 6: 848035. https://doi.org/10.3389/fsufs.2022.848035.
Van Leeuwen, J.H. 1981. Dynamic behavior of synthetic ropes. Proc. Offshore. Tech. Houston, TX. Paper 4003. pp.453-464. https://doi.org/10.4043/4003-MS.