The structural verification of concrete substructures for floating wind turbines remains a significant engineering challenge for projects progressing beyond the conceptual design stage. While the existing design codes outline the required verifications, they often lack clear guidance on their practical application.
One example is the handling of stress directionality in FE shell elements, where the codes might be ambiguous and can lead to a misinterpretation of watertightness requirements. These requirements significantly impact the required concrete wall thickness, affecting mass estimates and the overall design. Such misinterpretation, especially in the early design stages, could lead to inappropriate or oversimplified and unconservative load assumptions, resulting in an unsafe design or increasing project risks by too optimistic mass and cost estimates. Conversely, such misinterpretation could result in overly conservative load assumptions, jeopardizing the bankability and commercial viability of a technology or project, as the impact could be a significant increase in fabrication costs.
Furthermore, many of the required structural verifications are only inadequately supported by existing commercial software, which further increases the engineering challenge. To address this challenge, Ramboll has developed an efficient methodology and toolchain for the structural analysis of concrete structures that can be applied to various design codes and captures all relevant structural effects, such as stress bi-directionality and contribution of reinforcement to structural resistance. Over the past two years, this methodology has been applied to commercial projects at different design stages and for different concrete substructure types. This presentation will provide an overview of the methodology and share insights from its application. Furthermore, it will emphasize the importance of an effective structural design process as part of a project's de-risking strategy.

The design of monopiles for offshore wind turbines is driven by the fatigue limit state; the wave frequency being typically almost in resonance with the first natural frequency of the turbine. While in operating conditions the oscillations induced by wave loading in fore-aft direction are typically well damped by aerodynamic damping, the excitation of vibrations in side-side direction or during standstill can lead to larger fatigue loads and thus a reduced lifetime of the substructure. Offshore wind turbines are thus often equipped with active and passive vibration mitigation devices, such as active tower dampers or tuned mass dampers, to reduce these otherwise low damped vibrations.

As part of the project FOOS, the feasibility and performance of active tower dampers as well as passive vibration mitigation devices, for offshore wind turbines in the 15+ MW range are to be investigated with a special focus on the upcoming Princess Elisabeth Zone in the Belgian North Sea. As no detailed designs for offshore wind turbines in this Zone exist yet, commonly available reference turbines, IEA 15 MW and IEA 22 MW, are adapted to the conditions in the Princess Elisabeth Zone

In this contribution, the adaptation of the IEA 15 MW and IEA 22 MW reference conditions to the conditions in the Princess Elisabeth Zone is detailed. Two reference locations in the Princess Elisabeth Zone are introduced and the metocean and soil conditions characterised. A set of reduced DLC tables based on IEC 61400-3-1 is derived from the metocean conditions for the Belgian offshore wind zone. The monopile foundations of both reference turbines are modified to the conditions of the Princess Elisabeth Zone and an ULS assessment is performed to verify the structural integrity of the monopiles as well as the soil capacity and find a feasible design without running a full detailed design. Moreover, a FLS assessment based is performed to verify the wall thickness of the monopiles. This FLS assessment will also provide a benchmark for the later planned investigation of the feasibility and effectiveness of active and passive damping devices for 15+ MW offshore wind turbines.

The utilization of wind power as a means of energy production offers significant potential for greenhouse gas reduction. While suitable locations for onshore or bottom-fixed offshore wind turbines are limited, the potential for floating offshore wind turbines (FOWTs) is substantial. Several types of floaters and moorings for FOWTs have been proposed and are already in operation. Floaters include spars, semi-submersibles, and barges, while moorings include catenaries, taut, and tension legs. Tension leg mooring platforms (TLPs) have many advantageous properties and occupy an area roughly the same size as the platform itself. In contrast, catenary or taut mooring systems require an area several times the water depth. These properties have proven advantageous for various maritime activities, such as fishing and shipping.
This study proposes a TLP floater with a hybrid concrete and steel structure to enable efficient, large series production. Concrete center columns are constructed on seaside sites. Meanwhile, the pontoons are manufactured in factories by rolling steel plates. Then, they are transferred to the seaside sites and integrated with the center columns. The tendon, or mooring line, are composed of high-strength polyethylene fiber. The TLP is developed by Obayashi Cooperation under a grant from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
Numerical simulations of the TLP floater with 15 MW wind turbine are conducted, and the results are compared with those of a semi-submersible with catenary mooring. To provide a basis for comparison, numerical simulations of a semi-submersible floater of the UMaine Volturn US-S platform are also conducted. In both cases, a 15MW wind turbine provided by the National Renewable Energy Laboratory (NREL) is employed. A commercial program known as OrcaFlex is employed for the numerical simulations. A comparative analysis and subsequent discussion are conducted on the motion of the platform, including pitch and roll motions, mean, fluctuating and maximum bending moments at the tower base, and power production.
Compared to the semi-submersible platform with catenary mooring, the TLP shows significantly reduced pitch and roll motions. The mean and maximum of bending moment at the tower base are reduced, but the fluctuating bending moment slight increases. Furthermore, the reduced rotor inclination increases power production.

Corrosion-induced degradation remains a major challenge in offshore wind energy, impacting reliability, safety, and operational expenditure throughout asset lifecycles. As the sector moves toward larger turbines and deeper installations, substructures are becoming more complex, combining dissimilar materials and exposed geometries that accelerate degradation under marine conditions. To ensure long-term structural integrity, predictive and digitally integrated assessment methods are needed.
This study presents a digital corrosion modelling framework supporting integrity management and maintenance optimisation for offshore wind substructures, with focus on lightweight aluminium components coupled to steel assemblies. Aluminium integration in next-generation floating wind turbines (WT) improves strength-to-weight ratio and corrosion resistance. However, dissimilar assembly is prone to galvanic corrosion, threatening durability. Field observations show severe aluminium corrosion when in electrical contact with stainless steel, prompting updates to NORSOK standards. While NORSOK M-001:2014 permitted such contact, this is no longer acceptable in the 2021 revision. Section 6 of NORSOK M-102 details bolted aluminium-stainless steel joints, but gaps persist regarding frictional effects, mechanical degradation, and corrosion product accumulation.
New predictive tools developed at SINTEF under the Norwegian Centre for Research-based Innovation (SFI BLUES - Floating Structures for the Next Generation of Ocean Industries) assess corrosive environmental effects on material degradation and structural integrity. They build on finite element simulations integrating multi-physics corrosion modelling. The workflow links electrochemical kinetics, degradation mechanisms, and meso-scale structural behaviour to predict how corrosion influences strength and stiffness over time. A case study on aluminium-stainless steel (Al-SS) bolted joints demonstrates the model’s capability. Using experimentally derived parameters from cyclic salt spray testing, simulations predict potential, current density, and mass loss across joint interfaces. Results show galvanic coupling produces up to six times greater aluminium mass loss than aluminium-aluminium (Al-Al) configurations. A significant increase in force for Al-Al assembly compared to Al-SS and uncorroded cases highlights corrosion products’ contribution to frictional response. This was captured by the models, demonstrating realistic corrosion behaviour and the importance of preventive design and material selection to mitigate galvanic effects.
This research contributes to the New Methods and Tools priority area for offshore wind by delivering a scalable, physics-based framework that connects local corrosion processes with system-level digital models. It enables the estimation of the effects of corrosion (material degradation and corrosion products) on the stiffness and stress states providing realistic data to evaluate ultimate stress state and fatigue stress state. By integrating with digital twin environments, it has the potential to enable condition-based maintenance rather than periodic inspection routines, improving both cost efficiency and sustainability of WT farms. It also lays the groundwork for developing standards that account for corrosion-induced mechanical effects in multi-material structures, supporting Europe’s 2050 vision for sustainable, resilient, and cost-effective offshore energy systems.

Keywords:
Offshore wind, corrosion modelling, aluminium-steel joints, digital framework, maintenance optimisation, structural integrity, finite element simulation, sustainability.

Wave and wind-induced motions are a known problem for floating wind platforms. As offshore wind systems transition from fixed foundations to floating foundations in deep-water applications, the entire structure becomes subjected to wave and wind-induced motions. These motions cause undesired effects such as an increase in fatigue and a decrease in power production, while also impacting installation, maintenance, operation and lifespan.

Different approaches have been proposed in motion reduction/stabilisation strategies, such as including appendages to the platform, blade pitch control, and hybridisation with wave energy converters. The latter is presented as an option to reduce the levelized cost of energy while exploring synergies between wind and wave.

The use of gyroscopic mechanisms is a well-known engineering solution in a plethora of applications. In naval engineering, control moment gyroscopes (CMG) are used in active stabilisation of boats, as can be seen in products by SEAKEEPER in the USA or GyroMarine in Italy. Gyroscopic devices can also be seen in the context of wave energy, in which the gyroscopic effect is exploited to extract energy from ocean waves.

This paper proposes a new hybridisation concept in which a dual-purpose gyroscopic mechanism with a PTO is contained within the floating platform structure. This proposal builds on existing knowledge of gyroscopic type WECs, proposing a mechanism that can be used either as a stabilising agent or a wave energy converter, depending on a control decision. The proposal integrates the gyroscopic device within the floating wind structure, eliminating the need for external vessels and reducing the complexity of the hybrid system.

The gyroscopic mechanism can be described as a CMG whose gimbal can either be motorised (CMG mode) or act as a generator (WEC mode). In such a scenario, a closed-loop control process dictates the mechanism mode of operation, either consuming energy to help with the stabilisation task or generating energy from the wind and wave-induced motion. Given this description, the gyroscopic device is henceforth referred to as the CMG-WEC.

The OC3-Hywind platform is chosen as the vessel in which the CMG-WEC is implemented. The vast amount of publicly available information about the platform allows for ease in reproducing its dynamical model, and consequently its dynamical behaviour. The dynamics of the CMG-WEC are obtained from first principles, and the equations of motion (EoM) for the complete OC3-Hywind+CMG-WEC system are found using Lagrangian mechanics. The proposed semi-analytical model makes use of outputs from Boundary Element Method (BEM) hydrodynamics solvers to characterise fluid forces acting on the system, as well as linearised mooring models from OrcaFlex.

A closed-loop control law is implemented in the system in order to investigate the possible impact of the proposed CMG-WEC device on the dynamic behaviour of the OC3-Hywind platform. Preliminary results show a considerable reduction in pitch motion amplitude for monochromatic sea conditions under state-feedback control.