the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2026
OC7 Project Phase II: Comparison of Global-to-Local Load Transfer Approaches in Floating Structures
Abstract. Global-to-local load transfer remains a critical – yet largely unstandardized – step in the structural assessment of floating structures. This paper presents the results of package WP2.2 from the OC7 project Phase II, establishing a cross-industry benchmark for the workflows connecting global performance analysis (based on integrated loads analysis, ILA) and the local structural assessment (based on finite-element analysis, FEA). The study evaluates a spectrum of industry practices, including sequential approaches with the FEA following the ILA, fully integrated time-domain approaches with hydro-structural coupling, and simplified ILA-only approaches. Using the VolturnUS-S reference semi-submersible, the models were first harmonized through mass/inertia, static, and modal verifications. Structural responses were then compared across three primary scenarios: topside-only excitation, irregular waves, and combined wind/wave loading. The results establish a structured comparison framework, highlighting how specific modelling choices and load transfer techniques directly influence confidence in design processes. The findings offer practical guidance to reduce uncertainty in "global-to-local" design workflows.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science.
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Status: open (until 02 Jun 2026)
- RC1: 'Comment on wes-2026-78', Anonymous Referee #1, 12 May 2026 reply
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RC2: 'Comment on wes-2026-78', Anonymous Referee #2, 20 May 2026
reply
This manuscript presents a comparative study of numerical approaches for load-to-stress analysis of floating offshore wind turbines (FOWTs). Using the same configuration, different approaches were first calibrated in terms of model properties and rigid-body motions, and were subsequently applied to compare stress responses within the FOWT under various loading conditions.
The key findings include:
- Structural dynamics can influence stress responses and, subsequently, fatigue damage estimation. The influence varies under different dominant loading conditions, such as wind- or wave-dominated cases.
- The consideration of nonlinear wave pressure introduces additional loads into the floater depending on the floater geometry, thereby affecting the stress responses compared with the linear wave-pressure assumption based on the mean water level.
- The unit-load reconstruction approach demonstrates accuracy comparable to that of the direct calculation approach, indicating its potential for future application in industrial practice.
- In addition, the authors highlight the importance of global load calculation, as it provides a fundamental basis for global-to-local load transfer approaches. The accuracy of the final local stress calculation is highly influenced by the global load assumptions and their accuracy.
It is evident that the objective of this study is not to identify the most accurate approach, given the limited availability of validation data, but rather to investigate key factors that may affect load-to-stress analysis and help reduce associated uncertainties. The comparisons are carefully designed, clearly documented, and well presented. The observations and discussions are meaningful and provide a valuable basis for future research.
Therefore, this manuscript is recommended for publication. Nevertheless, some refinements to the discussion of the results may help readers better understand the key findings. The following aspects could be further considered:
- The stress comparison figures in Section 7, including Figures 11, 13, 14, 15, 16, and 17, contain a substantial amount of useful information. In the statistical plots shown in the middle of these figures, the use of colours could be reconsidered to distinguish different modelling assumptions or techniques, such as rigid versus flexible models, linear versus nonlinear wave pressure, or shell versus beam modelling. This may provide a clearer visual comparison of the effects of different modelling choices. At present, the colours represent different model IDs, which are already distinguished by their positions in the figures.
- The authors provide observations and discussions for each loading case. It would be beneficial to include a summary subsection, for example Section 7.4, to synthesize the main findings. In particular, the variations in fatigue damage index under different modelling techniques and loading conditions may indicate conservative or unconservative estimations arising from different sources. A concise table or summary paragraph outlining the trends associated with the selection of different modelling techniques would better guide future research and industrial applications.
- The authors observe that the rigid and flexible models exhibit different trends in fatigue damage index when aerodynamic loads are considered in addition to wave loads. Furthermore, when the wave height increases from LC6.X to LC7.X, the trend for the flexible model appears to change. This behaviour may be related to nonlinear hydrodynamic effects. Considering the increasing size of FOWTs and the growing need to account for floater flexibility, this observation is highly relevant to future studies. Further discussion or additional information of this point would be valuable.
Overall, the manuscript addresses an important topic in the numerical analysis of floating offshore wind turbines. The work is well organized and provides meaningful insights into the influence of modelling choices on load-to-stress analysis.
Citation: https://doi.org/10.5194/wes-2026-78-RC2
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The paper represents an important step toward addressing a critical gap in the literature concerning global-to-local load transfer methodologies for floating offshore structures. The authors tackle this problem through a comprehensive comparison of the main approaches currently adopted in academia and industry.
The study analyses three principal modelling strategies. The first is the sequential workflow, which is generally considered current industrial practice, where the global integrated load analysis (ILA) is performed first and the resulting loads are subsequently transferred to a structural finite element model (FEA). The second is the integrated workflow where the global analysis and the structural FEA are solved through a strongly coupled hydro-structural interaction. The third is a simplified global analysis approach based on beam-element modelling.
The comparison is carried out using stresses evaluated on selected structural panels as the primary metric. The assessment includes comparisons of time histories, mean values, standard deviations, and fatigue-related indicators obtained through rainflow cycle counting and simulated fatigue damage evaluation.
The numerical results reinforce several important conclusions: (i) they highlight the critical importance of accurately predicting the global dynamic response, since any inaccuracy at the global level inevitably propagates into the local structural assessment; (ii) the study demonstrates the relevance of hydroelastic coupling effects: hull elasticity contributes significantly to the structural response, and rigid-body assumptions may lead to non-negligible deviations; (iii) the simplified beam-element approaches appear to have limited applicability for detailed structural assessment, especially in high stress-concentration regions like junctions; (iv) load superposition methods based on pressure linearisation provide results reasonably consistent with those obtained from fully coupled time-domain structural simulations.
The main limitation of the paper, however, is the absence of experimental validation. The conclusions are based entirely on numerical cross-comparisons, without comparison against experimental measurements of panel stresses or other physical reference data. As also acknowledged by the authors, this prevents a direct assessment of the accuracy of the investigated methodologies and partially limits the scientific strength of the conclusions.
Nevertheless, despite this limitation, the paper provides a valuable and timely contribution to the field. The breadth of the benchmark exercise, the quality of the comparisons, and the practical relevance of the findings make the work scientifically significant and deserving of publication.
The paper may be accepted in its current form. However, I would like to offer the following considerations that, if deemed relevant by the authors, could further strengthen the manuscript: