Mooring system design approach, a case study for MARMOK-A floating OWC wave energy converter

This paper presents a methodology and a flowchart of steps to take for a, consistent and rapidly convergent design of catenary mooring systems. It is subsequently applied for a floating Oscillating Water Column WEC MARMOK-A developed by Oceantec Energías Marinas, in order to fulfill the technical requirements of such dynamic systems. The approach, based on the catenary equations, considers the water depth as a design scale factor for the mooring system, leading to an equivalent static mooring performance. In general, a mooring system configuration is described by the number and distribution of lines; thus, as a preprocess in the herein described procedure, a database is built for different line lengths. The main advantage of the procedure is that once that, after characterizing a mooring system configuration at a specific water depth with a specific line mass and axial stiffness, the database built can be used for any other water depth with any line mass and axial stiffness, accelerating the design optimization process. Mooring static properties are derived for a given material elastic modulus, lines' mass and water depth. The mean offset and horizontal stiffness are afterwards derived with lines pretension and steady environmental forces (mean wave drift, current and wind) as well as maximum offset and characteristic line tensions. Finally, the process is applied for different lines pretensions to achieve an objective horizontal stiffness of the structure. The introduced procedure is presented through its application to the MARMOK-A device at a 90m depth site moored by means of a Karratu named mooring configuration. Results are presented in terms of total lines mass, device maximum expected excursion and required footprint for different horizontal stiffness and lines mass in order to give an insight of the impact on total plant cost indicators.