Date of Award

Winter 2023

Project Type

Dissertation

Program or Major

Ocean Engineering

Degree Name

Doctor of Philosophy

First Advisor

M. Robinson Swift

Second Advisor

M. Robinson Swift

Third Advisor

Igor Tsukrov

Abstract

An experimental seaweed cultivation system was designed for offshore and exposed locations. Novel features were incorporated to address major barriers to the expansion of seaweed, including limited nearshore and sheltered marine locations and risk of marine animal entanglement. A novel modular mooring geometry was developed to improve stability, reliability, and spatial efficiency to achieve an economical and robust structure viable in high wave energy environments. Stiff composite (e.g. fiberglass) rods were used to replace components typically made of rope, including mooring lines and seaweed growth substrate (cultivation lines), as a means to mitigate risk of marine animal entanglement.

This dissertation presents a collection of studies testing and evaluating the farm structural design, its component technologies, and the associated engineering tools and methods. A demonstration farm applying the experimental design concept was designed, deployed, planted with sugar kelp, instrumented, monitored, harvested and recovered at a shallow but exposed location in the Gulf of Maine. Observations indicated structural robustness, with survival and successful kelp growth in several severe winter storms, one with wave heights up to 5.9 m. Noted areas for improvement included kelp attachment strength to the fiberglass growth substrate, and mooring tension management for farm operations and reduced kelp loss.

Instruments for measuring local waves, currents and mooring tensions were deployed on and near the demonstration farm. Response amplitude operators and linear regression models of tension response statistics were calculated from datasets of weeks-long in-situ measurements, providing insight into the behavior of the demonstration farm and its novel mooring geometry. The system was found to exhibit a strong sensitivity to tide level, a desirably mild response to waves with periods of 6 to 15 seconds, a problematic response to high frequency waves with period of 1 to 6 seconds, a relatively low magnitude low frequency response associated with wave group envelopes and a tendency for damaging shock loading when mooring stiffness was elevated.

The demonstration kelp farm was numerically simulated in ocean conditions measured at the deployment site using two models: one defined by design phase assumptions, and another informed by detailed measurements of the deployed system. Engineering relevant results were compared amongst the two model variants, and to in-situ measurements. Results confirmed that when model inputs more accurately reflected reality, especially mooring pretension and kelp biomass, the replicability of in situ measurements improved substantially. For at least one mooring component, differences between simulation results and in situ measurements, in the context of significant tension magnitudes, were within margins typically accepted for aquaculture mooring engineering purposes.

Two versions of a commercial scale kelp farm using the novel mooring geometry were evaluated for their structural behavior and costs: one used conventional nylon ropes throughout (except anchor chain) and the other used fiberglass rods as mooring lines and cultivation lines. Numerical simulations of the two farm variants in design-relevant wave and current conditions (for the Gulf of Maine) indicated substantially higher loads on the composite rod based farm, however, economic analyses using these results indicated only a marginal differences in required structural capital investment.

Using the demonstration farm as a case study, anchoring for exposed seaweed farms was considered from the perspective of anchor type selection, site characterization, anchor design, installation approaches, and risk mitigation methods. During decommissioning of the demonstration farm several of the anchors used, including a novel multi-line / multi-shaft helical anchor, were pulled to failure under measured tensile loads. These measurements were compared to original engineering design targets, revealing that assumptions informed by geophysical surveys and sediment sampling contributed to conservative estimates for anchor holding capacity.

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