Date of Award

Fall 12-15-2017

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Krish P. Thiagarajan

Second Committee Member

Lance Manuel

Third Committee Member

Andrew J. Goupee

Additional Committee Members

Vincent Caccese

Melisa E. Landon

Abstract

For offshore wind industry, exploiting the dynamic behaviors of mooring lines is of increasing importance in floating offshore wind turbine (FOWT) mooring system design. Currently, the design philosophy for structures and moorings is based on principles and practices adopted in offshore oil and gas, including mooring systems that are optimized for applications in deeper waters. However, the design of FOWT mooring systems is facing several challenges, including installation costs, the stability of light-weight minimalistic platforms, and shallow water depths (50-300m). The extreme tension in mooring lines of a light displacement platform in shallow water is dominated by snap loads. Hence the extreme tension values of the lines are one of the most important factors to consider for safe design of permanently FOWT mooring system. Thus, there is also a need for dynamic performance assessment of FOWT mooring systems using numerical modeling while considering the extreme events on the mooring lines. The overall goal of the study is to investigate the extreme dynamic tension of mooring systems of a FOWT and propose a long-term distribution model which could improve the current approach to design tension of a mooring line. This dissertation advances the current state-of-the-art with respect to mooring line design in the offshore wind industry by considering three interrelated issues. The first is to fundamentally understand the underlying physics of extreme dynamic tension of a mooring system. To reach the goal, the factors can influence the snap events on vertical hanging cable system are investigated. In this issue, the nonlinearity of bilinear cable stiffness, hydrodynamic drag force as well as the pretension of the cable are considered. Therefore, the present work finds that potential exists for snap type impact to affect the mooring system of the FOWT. Here, the DeepCWind semi-submersible floating wind system proposed by phase II of Offshore Code Comparison Collaboration Continuation (OC4) project, is selected for this study. There is a reasonably strong correlation between the tension spikes and wave motion, as well as the vertical motion of the fairlead. Third, to estimate the extreme mooring line tension due to snap loads, a composite Weibull distribution is proposed. The model is composed of two Weibull distributions with different characteristics on either side of a transition tension value, and whose parameters is estimated from test data. The proposed distribution model has a good fit to the measured tension data, particular in the extreme range. Moreover, the research addresses the simulation of long, time-dependent mooring tension sequences by coupled AQWA-OrcaFlex-FAST. A number of comparisons between numerical modeling predictions and test data are done and have good quality results validation. Therefore, the focus is shifted to a probabilistic examination of snap load frequency and magnitude both in experimental data and in the simulation. The CWD model of the numerical data is also compared with the CWD model of test data.

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