Task 56 Summary – Offshore Code Comparison Collaboration 7 (OC7)

Objectives

Launched in 2024 under the International Energy Agency (IEA) Wind Technology Collaboration Programme Task 56, the Offshore Code Comparison Collaboration 7 (OC7) is a large-scale international initiative. Continuing the legacy of OC3 through OC6 projects, OC7 seeks to advance physics-based engineering modeling tools through rigorous verification and validation, and to establish best practices for offshore energy systems.

The project is organized into three sequential phases (Figure 1), covering hydrodynamics (Phase I), structural dynamics (Phase II), and aerodynamics (Phase III). Through systematic code comparison and validation against experimental data, OC7 aims to improve the accuracy and reliability of these modeling capabilities. The ultimate objective is to enable offshore designs that are not only more technically robust, but also more cost-effective to develop and deploy at scale.

Participation

The project brings together global collaborators including universities, research institutions, manufacturers, testing laboratories, consultants, and certification bodies from over 14 countries.

OC7 builds on the proven track record of its predecessors, OC3-OC6, which have earned widespread recognition and strong interest throughout the wind energy industry. The current OC7 email list includes more than 250 people.

Progress, Results, and Impact in 2025

The OC7 project completed Phase I, which focused on hydrodynamics, and transitioned into Phase II, centered on structural dynamics. Phase II was organized around two complementary work packages (WP), both built around the VolturnUS-S reference semisubmersible platform.

WP 2.1 focused on member-level load analysis within a floating substructure. A 1:70-scale model of the VolturnUS-S platform, tested by the University of Maine as part of the FOCAL project, provided the experimental dataset used to verify and validate structural dynamic behavior and loading under wave-only conditions. Numerical predictions from 11 academic and industrial partners were compared against physical measurements across a comprehensive set of verification and validation cases. Key findings include a significant influence of hydrodynamic added mass on elastic natural frequencies, with a 20% decrease observed relative to dry conditions. Under regular wave excitation, platform motions and mooring line tensions were reproduced with good accuracy, and member-level loads were predicted with reasonable agreement (Figure 2). Notable underpredictions were identified for potential-flow models that do not account for higher-order effects associated with the instantaneous wetted surface, with torque loading on the downstream pontoon differences reaching 39% at wave frequency.

WP 2.2 applied a global-to-local load mapping approach to the same platform, with contributions from 9 institutions. Loads from an integrated load analysis model were transferred into a detailed finite element model to assess stress distributions and evaluate fatigue performance. This work package addressed a critical, yet largely unstandardized step in structural assessment, establishing a cross-industry benchmark for workflows connecting global load analysis and local finite element analysis. A spectrum of industry practices was evaluated, and the results provide a structured comparison framework highlighting how modeling choices and load transfer techniques influence design confidence. The findings offer practical guidance for reducing uncertainty in global-to-local design workflows.

Both work packages are expected to be formally concluded in 2026, with three publications planned to document and disseminate the findings. Two in-person meetings were held during the year, alongside the OMAE conference in Canada in June and the IOWTC conference in France in October.

Next Steps

Building on the insights from its initial phases, the OC7 project will shift its focus to aerodynamics and wake dynamics in Phase III, scheduled to launch in 2026 and run for one year. This next stage will address the increasing complexity of wind farm layouts and turbine interactions, which are critical for optimizing energy capture and turbine longevity.

Phase III will be structured around two key work packages dedicated to the validation of aerodynamic models against experimental data:

• Work Package 3.1: Validation effort focused on wake development and loads for a downwind turbine, using data from model test campaigns conducted in the Politecnico di Milano wind tunnel under forced motion.
• Work Package 3.2: A second validation effort, focused on wake behavior of a surging rotor tested in the large wind tunnel at the University of Oldenburg.

These initiatives will be conducted in collaboration with the related TURBINIA and JAM tasks, enhancing the project's scope and impact. Through these focused efforts, OC7 will continue its role as a vital hub for global collaboration, driving the advancement of wind energy modeling tools and establishing industry-wide best practices.

References

Three publications from OC7 Phase I were published in the past year:

1. Wang, L., Robertson, et al. (2025). OC7 Phase I Definition Document (Report No. NREL/TP-5000-92862). National Renewable Energy Laboratory. https://www.nrel.gov/docs/fy25osti/92862.pdf
2. Wang, L., Robertson, A., et al. (2025). OC7 phase I: Toward practical sea-state-dependent modeling of hydrodynamic viscous drag and damping. Ocean Engineering, 336, 121745. https://doi.org/10.1016/j.oceaneng.2025.121745
3. Liao, Y., Wang, L., Robertson, A., et al. (2026). OC7 phase I: CFD investigation of viscous forces on rectangular members of semisubmersibles. Ocean Engineering, 357(Part 1), 124908. https://doi.org/10.1016/j.oceaneng.2026.124908

Two publications from OC7 Phase II are currently under review:

• WP 2.1 (Torque conference): OC7 Project Phase II: Code Comparison and Experimental Validation of Hydroelastic Effects and Member-Level Loads in Floating Structures
• WP 2.2 (Wind Energy Science): OC7 Project Phase II: Comparison of Global-to-Local Load Transfer Approaches in Floating Structures