This study is focused on the long-term reliability analysis of loads and motions for a utility-scale floating offshore wind turbine supported by a semi-submersible platform situated in the North Sea. The floating wind turbine, which consists of a 5-MW 3-bladed rotor, is assumed to be mounted on the OC4 semi-submersible floating platform. The platform consists of a main column, three offset columns, a cross frame, and pontoons, deployed at a site with a water depth of approximately 200 meters. A mooring system consists of three catenary lines together with the turbine and platform completes the integrated system. Based on the metocean conditions characterizing the selected turbine site, a number of sea states are identified for which coupled-dynamics simulations are carried out using the aeroelastic computeraided engineering (CAE) tool for horizontal-axis wind turbines, FAST. Short-term turbine load and platform motion statistics are established for individual sea states that are analyzed. The long-term global performance analysis yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. This study seeks to assess long-term loads and motions associated with large utility-scale offshore wind turbines that might be deployed on floating platforms for planned offshore wind power development in the U.S.
In wind turbine design, external conditions to be considered depend on the intended site for the planned installation. Wind turbine classes, defined in terms of wind speed and turbulence parameters, cover most sites and applications. In the International Electrotechnical Commission's (IEC's) 61400-1 standard, there is a design load case that requires consideration for ultimate loading resulting from extreme turbulence conditions. Since site-specific wind conditions should not compromise the structural integrity of turbine installations, at some sites where class-based design may not apply, there is sometimes a need to establish extreme turbulence (50-year) levels as part of site assessment by making use of measurements. This should be done in a manner consistent with class-based design where the extreme turbulence model (ETM) provides 50-year turbulence standard deviation (σ) value as a function of the ten minute average hub-height wind speed, V. For one site in Germany and three contrasting terrain sites in Japan, wind velocity data are used to establish 50-year ETM levels. The inverse first-order reliability method (IFORM) is applied with 10 min data for this purpose. Sometimes, as in assessing wind farm wake effects, analysis of turbulence levels by direction sector is important because normal and extreme turbulence levels can vary by sector. We compare ETM levels by sector for the Hamburg, Germany site. The influence of terrain complexity on ETM levels is also of interest; the three sites in Japan have contrasting terrain characteristics—referred to as flat, hilly, and mountainous. ETM levels are compared for these three terrain types. An important overall finding of this study is that site-specific ETM levels can greatly exceed levels specified in the standard for class-based design.
A wind turbine shall be considered as an offshore wind turbine if the support structure is subject to hydrodynamic loading. Foundation design or soil-structure interaction is not clearly outlined in detail in the design standards for wind turbines, IEC 61400-1 (Design Requirements) and IEC 61400-3 (Design Requirements for Offshore Wind Turbines). The general approach in the IEC standards is to follow the API (American Petroleum Institute) design approach for offshore platform foundations. In an API approach, the soil p — y curves are developed for the slender piles which are widely used for offshore oil and gas platforms. The failure mode of these slender piles is the formation of flexural plastic hinges. In contrast, the monopiles supporting offshore wind turbines are typically rigid with 3.5 – 6 m diameter and 30 – 40 m length. The failure mode of these monopiles likely involves rigid motion. Under extreme loading conditions, soil surrounding the monopile may fail because of large deformation due to such motion. Therefore, the API approach developed for slender piles may not be applicable for the monopiles. An improved methodology for analysis and design of a 5-MW offshore wind turbine (OWT) supported by a monopile is presented in this study. The soil-monopile interaction under wind and wave cyclic loads is first studied using advanced finite element analysis (FEA) in ABAQUS. The results of the analyses are utilized to develop a set of p — y curves for the monopile. The FEA-based nonlinear p — y curves are input to SACS through user-defined subroutines. The coupled analysis includes soil-structure interaction and dynamic response of the tower and blades to wind and wave loadings. The fully coupled analysis is carried out using SACS-FAST integrated computational interface. Response of the wind turbine on the monopile is studied in a time domain using frequency domain inputs such as Kaimal spectrum for wind loading (TurbSim, NREL) and JONSWAP spectrum for wave loading in SACS. In a case study, the dynamic response of the 5-MW wind turbine and its monopile foundation obtained using the nonlinear p — y soil curves in the coupled analysis will be compared to the one obtained using an API approach.
The objective of this study is to evaluate fatigue damage and fatigue life of offshore structures using different fatigue calculation methods. Of interest in this study is to identify an efficient method for fatigue calculation of the offshore structures by comparing the Simplified approach, the Spectral approach, and the Fracture Mechanics approach provided in API [3], DNV [5], ISO [7], and ABS [2] guidelines/standards. Fatigue calculations use advanced application tools including SACS [10] and Abaqus [11] coupled with in-house tools. An efficient path in providing safety in structural integrity management is presented in this paper, which can minimize the cost and improve time management for a variety of projects.
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