Development of Design Criteria for Floating Offshore Wind Turbines
Authors: Chen Xiaohong , Yu Qing 
Published: May 2013

This paper presents an overview of the technical challenges in the design of floating offshore wind turbines (FOWTs) and the recent development of design guidelines for FOWTs. Extensive case studies, which evaluated the characteristic load conditions and global responses of FOWTs, are carried out to verify the design criteria. Three design concepts, including a Spar-type, a TLP-type, and a Semisubmersible-type floating wind turbine support structure and their associated stationkeeping systems, are selected for the case studies. Representative operational and extreme storm environmental conditions of the East, West and Gulf of Mexico coastal regions on the US Outer Continental Shelf (OCS) are considered. State-of-the-art simulation techniques are employed for the fully coupled aero-hydro-servo-elastic analysis of the integrated FOWT model. Relative importance of various design parameters as well as its impact on the development of design criteria are evaluated through parametric analyses. The paper is concluded with a brief introduction of the recently published ABS Guide for Building and Classing Floating Offshore Wind Turbine Installation.

Introduction

A significant portion of offshore wind energy resources in the United States are available in water depths greater than 30 meters in the offshore regions near highly populated coastal states. At this and greater water depths, floating offshore wind turbines (FOWTs) could become more economical than bottom-founded designs.

Existing design concepts of floating support structures and stationkeeping systems for FOWTs are mostly developed based on experience from the offshore oil and gas industry, which has witnessed nearly 60 years of designing and operating floating offshore structures. There is a wealth of knowledge about hydrocarbon-related offshore structures installed on the US Outer Continental Shelf (OCS). What makes FOWTs unique, however, is the presence of wind turbines that follow a very different design approach. Strong interactions between the wind turbine, floating support structure and stationkeeping system also pose a great challenge to the design of FOWTs. Economic considerations for typically unmanned FOWTs further require leaner designs, serial production and mass deployment.

For these reasons, it is not technically sound or economically acceptable to transfer existing technologies of hydrocarbon-related offshore structures directly to FOWTs without further calibrations and necessary modifications. To address this, the Bureau of Safety and Environmental Enforcement (BSEE), U.S. Department of the Interior, awarded a research project to ABS in 2011 under its Technology Assessment and Research Program. The project was aimed at conducting a thorough review of existing technologies relevant to FOWT floating support structure and stationkeeping system designs and evaluating global load and response characteristics using the latest simulation methods. A draft design guideline for FOWT floating support structures and stationkeeping systems also was proposed based on the research findings of that project.

This paper presents a summary of the BSEE-funded research (Yu and Chen, 2012) as well as the subsequent development of the ABS Guide for Building and Classing Floating Offshore Wind Turbine Installation (ABS, 2013).