Date of Award

Winter 2016

Project Type

Dissertation

Program or Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

First Advisor

M. Robinson Swift

Second Advisor

Kenneth Baldwin

Third Advisor

Barbaros Celikkol

Abstract

The dynamics of a submersible mussel raft were analyzed using wave tank testing, numerical modeling, and full-scale field tests. When submerged, the raft’s pontoons are flooded, and it is held vertically by lines attached to surface floats and horizontally by a spread mooring. This submerged configuration is used to reduce wave forcing and to avoid contact with floating ice during winters in northern waters. During the prototype design process, numerical modeling in the program Aqua-FE™ indicated that the submerged configuration exhibited significantly less heave (vertical) and pitch (rotational) motion than the surfaced configuration. Subsequent 1/10 Froude-scaled wave tank testing validated those predictions. Full-scale field tests, each about three weeks long, provided 6-degree-of-freedom motion and mooring load data for a variety of wave and current forcing conditions. This data set confirmed that the submerged raft oscillates with significantly smaller heave and pitch amplitudes than the surfaced raft for wave periods of interest (generally between 2 and 8 seconds). The reduced motion of the submerged configuration is attributed to its decreased waterplane area and increased inertia, which reduce the heave and pitch natural frequencies so that they are below the frequencies associated with the highest wave energy. The submerged configuration greatly decreases vertical velocities and accelerations of the mussel rope attachment points, reducing feeding interruptions and mussel drop-off in storms. Numerical models in Aqua-FE™ and OrcaFlex showed good agreement with field measurements of raft motion, particularly for wave periods associated with storm energy at the semi-exposed test site.

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