ENGINEERING ANALYSIS OF A MUSSEL BACKBONE SYSTEM FOR AN OPEN-OCEAN SITE

The sustainability of open-ocean shellfish farms depends on an accurate understanding of the system's behavior in waves and currents. Several engineering designs were tested using numerical models for farming systems proposed for the Ventura Shellfish Enterprise (VSE) project to be located in federal waters outside Ventura Harbor, California.  This modeling was conducted to address regulatory concerns about the suitability of farm engineering to withstand storms.

Extreme wave, wind, and current values were quantified for 100-year and 20-year return periods at the selected project location by fitting Gumbel distributions to historical data for the site. The dynamic behavior of the system was quantified under the extreme storm conditions using a hydro-/structural dynamic finite element approach. The engineering approach considered four factors: Survival (minimum required capacities of lines and anchors); operations (force required to lift the backbone for maintenance and harvesting, installability, and navigability); performance (RMS accelerations of mussel ropes as a proxy for mussel drop-off; ability to facilitate predation avoidance); and budget (minimum required component sizes and availability of components). Multiple design alternatives were compared based on those four factors. Additionally, a theoretical limit for the percentage of mussel weight that should be supported by submerged buoyancy was established as a function of incident current speed. This limit is based on the force balance between the wet weight of the mussel ropes and the vertical component of normal drag on the mussel ropes as they lay back at an angle due to the incident current. For the maximum expected mussel growth considered in this application, it was found that the maximum submerged buoyancy should be limited to two-thirds of the wet weight of the mussel droppers.