Jade Chung

Date of Award

8-2012

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Melissa Landon Maynard

Second Committee Member

Thomas C. Sanford

Third Committee Member

William G. Davids

Abstract

Over the past decade a number of Federal and State policies and programs have promoted the development of the wind energy industry, including the establishment of offshore wind. A strategy by the Department of Energy set objectives of reducing cost and reducing time to deployment through specific deliverables such as innovative anchor and mooring design for floating offshore systems and hardware design concepts including turbine array grids. This research program proposes and investigates use of suction caissons as combined anchors to resist line loads from multiple platforms as an efficient solution for anchoring a network of wind turbine platforms.

Suction caissons are a ‘mature’ anchor technology in the offshore oil and gas industry, yet there is minimal experience with application for offshore wind platforms. Established design methods, standards and recommended practices from the oil and gas industry, serve as a starting point for further adaptation. Considerations of the differences in conditions (e.g. loads, risk, failure and serviceability tolerances) between the two applications, is important for developing efficient anchor design suited to offshore wind platforms. A physical modelling program was developed to investigate the behaviour of caissons subjected to orthogonal cyclic and post-cyclic monotonic line loads, compared to the behaviour of single line loaded caissons.

Modeling was performed in a geotechnical centrifuge in order to simulate in-situ stress profile at model scale, as stresses are critical to soil and foundation behavior. Load tests were performed on a model suction caisson anchor installed by jacking into normally consolidated kaolin clay (in-flight). Baseline tests were performed with single line loading for comparison to the multi-line loading tests. Line loads were applied in orthogonal directions for the multi-line load tests. The effect of varying cyclic mean load and cyclic load amplitude was also investigated. Comparison of test results was based on line displacement, applied line load, caisson rotation and internal pore pressure at the underside of the caisson cap.

Centrifuge test results appear to indicate that the line load-displacement response during monotonic loading is similar for the multi-line and the single line loaded suction caisson anchors. The post-cyclic peak monotonic line load resistance provided by the caisson loaded in multiple directions was greater than the resistance provided by the caisson loaded in a single direction (accounting for the total resultant load applied). For all selected load cases, the accumulated permanent displacements during the cyclic loading did not result in a displacement (serviceability) failure of the suction caisson nor contribute significantly to the displacement correlating to the peak line load resistance of the caisson.

Test results indicate that the resistance capacity of a given caisson is not reduced by applying line loads in multiple directions, when considering the resistance to the total resultant load. Test observations appear to support conceptualizations of a modified “zone of influence” (active/passive earth pressure wedges) due to the changing load orientation from resolving multiple out of phase line loads.

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