Date of Award

12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean Engineering and Marine Sciences

First Advisor

Geoffrey Swain

Second Advisor

Stephen Wood

Third Advisor

Austin Fox

Fourth Advisor

Nakin Suksawang

Abstract

Cathodic protection (CP) is required to prevent corrosion of the submerged steel for offshore wind monopile foundations in the marine environment. The design of these systems depends on the environmental conditions, including temperature, salinity, pH, dissolved oxygen, depth, micro- and macro-organisms, and water flow rates. The standards utilized to design cathodic protection systems consider only some of these conditions. This research identified the gaps in knowledge for the design of corrosion prevention systems for offshore wind monopile foundations and investigated the changes in cathodic current densities due to macro-galvanic cells, fouling communities and corrosion products established on uncoated vertical steel surfaces. Steel panel arrays were immersed in seawater for nine weeks at Florida Tech’s test site at Port Canaveral, FL. These included three different zones; intertidal, submerged and buried zones. Two arrays were used for each experiment, and they were exposed to semidiurnal tides. The first field experiment to determine the effects of macro-galvanic cells on cathodic current densities included one array which was freely corroding, while the other was cathodically protected with an impressed current cathodic protection (ICCP) system. The second field experiment to determine the effects of fouling and corrosion products on cathodic current densities included one array with preexisting fouling and corrosion products while the other was clean. The arrays utilized for the second experiment were cathodically protected with ICCP systems. The seawater temperature, salinity and pH were recorded. The potentials of individual panels, the current flow between panels, and the total current output from ICCP systems were measured. The fouling and corrosion products were visually assessed via underwater photos. At the end of the immersion, EDX analyses were performed on surface products. The literature review found that coating systems were optional for external submerged surfaces and were not required for any of the internal surfaces. Furthermore, the monopile foundations may not receive any cathodic protection for up to two years after being deployed in seawater. The delay between the deployment and the application of any cathodic protection allows for fouling communities and corrosion products to establish on steel surfaces. This research confirmed the presence of macro-galvanic cells on uncoated vertical steel structures placed in the marine environment. These develop due to the higher partial pressure of oxygen in the intertidal zone, which accelerates corrosion in areas of low partial pressure of oxygen. The high current demand at the intertidal zone was shown to increase the initial and mean cathodic protection current demand of the structure. This demonstrates the importance of maintaining a protective barrier at the splash and the intertidal zone to reduce the cathodic protection current demand. It was also shown that the condition of the steel surface prior to the application of cathodic protection influenced the cathodic protection current demand. Steel surfaces that were left to foul and corrode prior to the application of ICCP required higher initial and mean current densities to prevent corrosion. The findings from this research emphasize the importance of well-designed protective coatings in the splash, tidal, and submerged zones of the structure and ensure that the cathodic protection system is applied as soon as possible after the immersion of the structures.

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