Offshore Code Comparison Collaboration (OC3) for IEA Wind Task 23 Offshore Wind Technology and Deployment

Abstract

The vast offshore wind resource represents a potential to use wind turbines installed offshore to power much of the world. Design standardization is difficult, however, because offshore sites vary significantly through differences in water depth, soil type, and wind and wave severity. To ensure that offshore wind turbine installations are cost effective, the use of a variety of support structure types is required. These types include fixed-bottom monopiles, gravity bases, and space-frames-such as tripods and lattice frames ("jackets")-and floating structures. In this context, the offshore wind industry faces many new design challenges.Wind turbines are designed and analyzed using simulation tools (i.e., design codes) capable of predicting the coupled dynamic loads and responses of the system. Land-based wind turbine analysis relies on the use of aero-servo-elastic codes, which incorporate wind-inflow, aerodynamic (aero), control system (servo), and structural-dynamic (elastic) models in the time domain in a coupled simulation environment. In recent years, some of these codes have been expanded to include the additional dynamics pertinent to offshore installations, including the incident waves, sea current, hydrodynamics, and foundation dynamics of the support structure. The sophistication of these aero-hydro-servo-elastic codes, and the limited data available with which to validate them, underscore the need to verify their accuracy and correctness. The Offshore Code Comparison Collaboration (OC3), which operates under Subtask 2 of the International Energy Agency (IEA) Wind Task 23, was established to meet this need.To test the newly developed codes, the main activities of OC3 were to (1) discuss modeling strategies, (2) develop a suite of benchmark models and simulations, (3) run the simulations and process the simulation results, and (4) compare and discuss the results. These activities fell under broader objectives including:• Assessing the accuracy and reliability of simulations to establish confidence in their predictive capabilities • Training new analysts to run and apply the codes correctly • Identifying and verifying the capabilities and limitations of implemented theories • Investigating and refining applied analysis methodologies • Identifying further research and development (R&D) needs.Such verification work, in the past, led to dramatic improvements in model accuracy as the code-to-code comparisons and lessons learned helped identify model deficiencies and needed improvements. These results are important because the advancement of the offshore wind industry is closely tied to the development and accuracy of system-dynamics models.The OC3 project was performed through technical exchange among a group of international participants from universities, research institutions, and industry across the United States of America, Germany, Denmark, the United Kingdom, Spain, the Netherlands, Norway, Sweden, and Korea. Moreover, most of the aero-hydro-servo-elastic codes developed for modeling the dynamic respon...

Seismic analysis of offshore wind turbines on bottom-fixed support structures

“…It is observed that the profiles in figure 9, with positive and negative deflections (the latter are slightly visible, for instance, in pile #1 of the Tripod), are in accordance with typical deflection profiles of flexible piles constrained by lateral springs and supporting structures under dynamic lateral loads [37]. For comparison, figure 9 also includes (i) the maxima lateral deflections due to wind-wave loads only (no earthquake loads), for the environmental states considered in load cases LC1-LC2-LC3; (ii) the lateral deflections at which the soil resistance forces attain, with a tolerance of 10 −2 , the maxima asymptotic values given by the p − y API curves [26] for the considered sandy soil [18,19], at various depths along the piles; note that these lateral deflections can be taken as indicators of a significant nonlinear soil response, because p − y curves deviate from linearity also for relatively small soil displacements [26]. Figure 9 shows that earthquake loads cause a considerable increase of lateral deflections with respect to corresponding values without earthquake loads.…”

“…In particular, the study is carried out on the NREL 5 MW HAWT, as mounted on two typical steel support structures for transitional water depths, one with a tripod and one with a jacket, both resting on pile foundations. Wind and wave environmental states, water depth and soil profile are set in agreement with similar theoretical studies on offshore HAWTs [18,19]. Consistently with the approach followed for land-based HAWTs [9][10][11], the seismic response is investigated for a set of real earthquake records taken from existing databases [20,21], with different frequency content and intensity.…”

“…not operating. Consistently with similar studies [18], wave periods T p and significant heights H s have been chosen as those associated with the selected wind speeds V hub in typical offshore environmental states, as for instance some encountered in the Pacific Ocean [28].…”

Operational Modal Analysis of a Spar-Type Floating Platform Using Frequency Domain Decomposition Method

“…Among floating concepts under study, the spar, consisting of a slender hollow cylinder, placed in vertical position and ballast-stabilized, seems to be particularly appropriate for deep waters (above 100 m). Spar prototypes are involved in projects such as UMaine-Hywind [4], Hywind [7], and OC3-Hywind [8,9]. Insight into the wind-wave response of a spar floating wind turbine has been provided by numerical studies-see e.g., Karimirad and Moan [10].…”

Development of a comprehensive database of scattering environmental conditions and simulation constraints for offshore wind turbines

“…Literature values partly differ significantly. To reduce the effects of initial transients, the first 20, 30, or 60 s are discarded, for example (Vemula et al, 2010;Jonkman and Musial, 2010;Hübler et al, 2017), and simulation lengths of 10 min and 1 h are common practice (Jonkman and Musial, 2010;Popko et al, 2012;Cheng, 2002). However, longer simulation lengths are partly used as well, especially in the oil and gas industry or for floating substructures (DNV, 2013).…”