Date of Award

Spring 2011

Project Type

Dissertation

Program or Major

Ocean Engineering

Degree Name

Doctor of Philosophy

First Advisor

Igor Tsukrov

Abstract

Methods to effectively predict system response in marine settings are critical in the engineering design process. The high energy ocean environment can subject structures to large wave and current forces, causing complex coupled motions and loads. This research focused on the development of effective methods to predict flexible system response and the structural integrity of marine High Density Polyethylene (HDPE) components. Numerical modeling tools were developed to analyze and design flexible structures in open ocean environments. Enhancements to the University of New Hampshire's Aqua-FE finite element computer program were performed, including expansion of the element library to include spherical geometries and implementation of various hydrodynamic effects such as Stokes 2nd order waves and water velocity reduction due to component shadowing. Two case studies, involving laboratory and field experiments, were performed evaluating the software modifications and examining the response of flexible systems in various environmental conditions. Practical applications of the numerical model are presented, focusing on the design, analysis and deployment of a submerged grid mooring 10 km from Portsmouth, NH. The system was recovered after a seven year deployment and inspected. The numerical model proved to be a valuable engineering tool for investigating a system's motion dynamics and mooring tension response in marine environments. High density polyethylene is a primary structural component for marine systems such as fish containment, wave attenuators and marine defense barrier systems. The fundamental engineering issues with the compliant HDPE material are associated with how the material changes its stiffness and strength depending upon the service life, load rate and temperature. Structural modeling techniques were developed to determine effective methods of analyzing marine systems constructed of HDPE. This included the investigation of the mechanical behavior of new and environmentally fatigued HDPE specimens, obtained from commercial fish farms, at different strain rates and validation of the modeling approach with laboratory experiments. The operational limits, loads and modes of a failure of the HDPE cage frame were estimated, providing valuable information on the survivability of these large, flexible systems in offshore environments.

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