Session: 09-02-07 Wind Energy: Fatigue Analysis

Submission Number: 180138

Co-Validation of Load Transfer Approaches for Floating Offshore Wind Turbines 

This paper presents the results of a collaborative research effort between Japan Marine United Corporation (JMU), NIHON SHIPYARD CO., LTD.(NSY) and Ramboll.

JMU and NSY are shipbuilding companies with decades of experience in naval architecture. Drawing on this expertise, we have been engaged for many years in the development of floating substructures (floaters) for offshore wind turbines.

Ramboll is an independent engineering consultancy and the leader of the structural design subtask in the IEA OC7 Code Comparison project and has broad insight into the latest approaches.

In designing the floater, integrated load-coupled analysis (ILA), incorporating aero-hydro-servo-elastic interactions, is indispensable to simulate the global dynamic interactions among the floater, tower, mooring system, and controller. However, because the ILA model is simplified to reduce computational cost, it can only capture global responses and is insufficient for evaluating local member strength required for certification-level verification. Accordingly, a more detailed structural analysis using refined finite element models is required in the subsequent design phase.

A key challenge in structural analysis is ensuring the accuracy of the Load Transfer approach, which transfers global response data into detailed structural models. This approach involves mapping loads obtained from the ILA, together with reconstructed hydrodynamic pressures on the hull, onto the structural finite element (FE) model while preserving global equilibrium. Ensuring consistency between models of different fidelity requires careful adjustment under reasonable assumptions; otherwise, transfer errors may misestimate local stresses, affecting fatigue life predictions and ultimate strength checks.

Moreover, with the continuing upscaling of turbine capacity and cost-driven optimization of floater configurations, the structural flexibility of floaters has become increasingly significant, making Load Transfer even more challenging. Although floaters were traditionally modeled as rigid bodies, their elastic response strongly influences the stress states of local members. It is therefore essential to incorporate this elastic behavior appropriately into structural analysis models. As a result, the Load Transfer approach is becoming increasingly important for ensuring the structural integrity of floaters.

However, current Load Transfer approaches remain highly dependent on the floater concept and designers’ discretion and have not been systematically established. Limited full-scale data introduce uncertainties in reliability, as results may vary depending on the applied approach even under identical conditions. These uncertainties complicate certification, lead to excessive design margins, and increase both costs and the risk of delays in commercialization. Therefore, establishing a validated and reliable Load Transfer approach is urgently required.

JMU and NSY are developing a semi-submersible floater and ultimately aim to establish a reliable structural analysis methodology, including validation against full-scale measurement data.

As the first step toward this goal, this paper presents the results of a code-to-code comparison of two different Load Transfer approaches. Although previous studies have reported results based on individual approaches, side-by-side validation comparing different approaches under common conditions has rarely been conducted. Differences in theoretical background and model implementation often lead to discrepancies between approaches. Code-to-code comparison provides an effective way to objectively assess their accuracy and limitations and to identify directions for further improvement.

This study was conducted to reduce uncertainties in structural analysis results arising from the use of independently developed Load Transfer approaches that have not yet been comprehensively established.

In the study, the Load Transfer approaches developed and applied independently by JMU/NSY and by Ramboll were compared, which are among the currently most advanced Load Transfer processes.

The two approaches were applied to structural analyses using the same inputs and the resulting structural responses were compared.

As a result, despite differences in theory and implementation, the two approaches produced consistent responses. This demonstrates the reliability of both approaches and provides strong evidence that agreement between independent approaches enhances confidence in their validity. The comparative validation also clarified the required accuracy, applicable range, and areas for improvement of Load Transfer approaches.

These findings contribute to improving their fidelity and offer insights for future standardization, certification, and design optimization of floating offshore wind turbines.

Presenting Author: Shu Kuwada NIHON SHIPYARD CO., LTD.

Presenting Author Biography: Shu Kuwada is a structural engineer at Nihon Shipyard Co., Ltd.
He has been engaged for several years in the development of floating substructures.
His expertise includes finite element analysis, fatigue and ultimate strength assessment of floating substructures.

Authors:

Shu Kuwada NIHON SHIPYARD CO., LTD.
Ryo Matsuoka NIHON SHIPYARD CO., LTD.
Haruki Yoshimoto Japan Marine United Corporation
Michael Karch Ramboll
Denis Matha Ramboll