Probabilistic vulnerability assessment of floating offshore wind turbines under typhoon hazards considering long-term corrosion–fatigue degradation

Reliability and fragility assessment of offshore floating wind turbine subjected to tropical cyclone hazard

Floating offshore wind turbines (FOWTs) face significant challenges during super typhoons, especially in typhoon-prone regions like China’s southeastern coastline. Effective disaster mitigation and load reduction require accurate characterization of FOWT dynamics under extreme typhoon conditions, yet the underlying response mechanisms remain unresolved. This study investigates the dynamic response of a 5[Formula: see text]MW OC4-DeepCwind semi-submersible FOWT during mooring line failure induced by super typhoons. The proposed methodology offers three key contributions: (1) A method called “Typhoon-Wind-Field and Wave-Stage Coupling Model” is proposed, enabling detailed modeling of three-dimensional wind field and wave field throughout the entire typhoon process; (2) identification of mooring line failure risks under typhoon conditions and analysis of resulting impacts on critical turbine components; (3) examination of both shutdown and disconnected turbine states, a scenario rarely studied in prior work. The results provide insights into typhoon-induced failure risks and support the design of more resilient FOWTs.

This paper presents an overview of the technical challenges in the design of floating offshore wind turbines (FOWTs) and the recent development of design guidelines for FOWTs. Extensive case studies, which evaluated the characteristic load conditions and global responses of FOWTs, are carried out to verify the design criteria. Three design concepts, including a Spar-type, a TLP-type, and a Semisubmersible-type floating wind turbine support structure and their associated stationkeeping systems, are selected for the case studies. Representative operational and extreme storm environmental conditions of the East, West and Gulf of Mexico coastal regions on the US Outer Continental Shelf (OCS) are considered. State-of-the-art simulation techniques are employed for the fully coupled aero-hydro-servo-elastic analysis of the integrated FOWT model. Relative importance of various design parameters as well as its impact on the development of design criteria are evaluated through parametric analyses. The paper is concluded with a brief introduction of the recently published ABS Guide for Building and Classing Floating Offshore Wind Turbine Installation. Introduction A significant portion of offshore wind energy resources in the United States are available in water depths greater than 30 meters in the offshore regions near highly populated coastal states. At this and greater water depths, floating offshore wind turbines (FOWTs) could become more economical than bottom-founded designs. Existing design concepts of floating support structures and stationkeeping systems for FOWTs are mostly developed based on experience from the offshore oil and gas industry, which has witnessed nearly 60 years of designing and operating floating offshore structures. There is a wealth of knowledge about hydrocarbon-related offshore structures installed on the US Outer Continental Shelf (OCS). What makes FOWTs unique, however, is the presence of wind turbines that follow a very different design approach. Strong interactions between the wind turbine, floating support structure and stationkeeping system also pose a great challenge to the design of FOWTs. Economic considerations for typically unmanned FOWTs further require leaner designs, serial production and mass deployment. For these reasons, it is not technically sound or economically acceptable to transfer existing technologies of hydrocarbon-related offshore structures directly to FOWTs without further calibrations and necessary modifications. To address this, the Bureau of Safety and Environmental Enforcement (BSEE), U.S. Department of the Interior, awarded a research project to ABS in 2011 under its Technology Assessment and Research Program. The project was aimed at conducting a thorough review of existing technologies relevant to FOWT floating support structure and stationkeeping system designs and evaluating global load and response characteristics using the latest simulation methods. A draft design guideline for FOWT floating support structures and stationkeeping systems also was proposed based on the research findings of that project. This paper presents a summary of the BSEE-funded research (Yu and Chen, 2012) as well as the subsequent development of the ABS Guide for Building and Classing Floating Offshore Wind Turbine Installation (ABS, 2013).

The station keeping ability of a platform is the most fundamental guarantee to securing the dynamic stability of a Floating offshore wind turbine (FOWT) when being subjected to complex marine environment. These are usually evaluated in terms of rigid body dynamic response of a floating platform which supports whole FOWT. To investigate the dynamic response of array of FOWT, we put forward the concept of a large FOWT farm which consists of nine spar-type wind turbine array and meanwhile shares mooring line system by connecting those FOWT to one another. The whole FOWT is established based on OC3-Hywind spar buoy platform with NREL 5MW wind turbine. Considering effects of wind, wave and current loads, the dynamic response of the proposed FOWT was conducted based on FEM software ANSYS AQWA under both operational condition and survival condition. Besides, comparisons of dynamic response between the original single FOWT and the proposed FOWT farm were also made. Several interesting conclusions were gained through the investigation. For the original single FOWT, RAO in surge, heave and pitch are concentrated in low frequency, while the peak value of frequency is about 0.2 rad/s. In addition, FOWT (P1, P3, P7 and P9) located on the four vertices of 3×3 have higher sway motion which range from about -0.15 m~ 0.15 m, while sway motion of the rest can be neglected. With the worsening of environmental condition, surge motion of the original single FOWT increases significantly, while it’s not significant for the proposed FOWT farm. At the same time, statistic values of pitch motion of the proposed FOWT farm are quite close to those of the original single FOWT. While, under the survival condition, range of pitch motion of the proposed FOWT farm is significantly smaller than that of the original single FOWT. In all, the results verify the effectiveness of the proposed FOWT farm.

The safety and reliability margin of offshore floating wind turbines need to be higher than that of onshore wind turbines due to larger environmental loads and higher operational and maintenance costs for offshore wind turbines compared to onshore wind turbines. However rotor cyclic loads coupled with 6 DOFs motions of the substructure, amplifies the fatigue damage in offshore floating wind turbines. In general a lower fatigue design factor is used for offshore wind turbines compared to that of the stationary oil and gas platforms. This is because the consequence of a failure in offshore wind turbines in general is lower than that of the offshore oil and gas platforms. In offshore floating wind turbines a sub-system fault in the electrical system and blade pitch angle controller also induces additional fatigue loading on the wind turbine structure. In this paper effect of selected controller system faults on the fatigue damage of an offshore floating wind turbine is investigated, in a case which fault is not detected by a fault detection system due to a failure in the fault detection system or operator decided to continue operation under fault condition. Two fault cases in the blade pitch angle controller of the NREL 5MW offshore floating wind turbine are modeled and simulated. These faults include: bias error in the blade pitch angle rotary encoder and valve blockage or line disconnection in the blade pitch angle actuator. The short-term fatigue damage due to these faults on the composite blade root, steel low-speed shaft, tower bottom and hub are calculated and compared with the fatigue damage under normal operational conditions considering same environmental conditions for both cases. This comparison shows that how risky is to work under the fault conditions which could be useful for wind turbine operators. The servo-hydro-aeroelastic code HAWC2 is used to simulate the time domain responses of the spar-type offshore floating wind turbine under normal and faulty operational conditions. The rain-flow cycle counting method is used to calculate the load cycles under normal operational and fault conditions. The short term fatigue damage to the composite blade root and steel structures are calculated for 6-hour reference period. The bi-linear Goodman diagram and a linear SN curve are used to estimate the fatigue damage to the composite blade root and the steel structures respectively. Moreover the fatigue damage for different mean wind speeds, sea states and fault amplitudes are calculated to figure out the region of wind speeds operation with the highest risk of damage.

Offshore floating wind turbines (OFWTs) are expected as the future of wind energy. However, the analysis of OFWTs is much more complicated than that of fixed-bottom wind turbines. The simulators currently in use, based on the classic blade element momentum (BEM) theory, will be inadequate while used on OFWTs because of the unsteady motions induced by wind and waves. Thus, simulators with advanced approaches are of necessity for analyzing OFWTs. In this work, a coupled aero-hydrodynamic simulator in MATLAB/Simulink is developed, for simulating the response and aerodynamic performance of OFWTs under wind and waves in the time domain. For aerodynamics, the code uses an unsteady BEM model or the free vortex wake method (FVM) to calculate the aerodynamic loads and performance of the wind turbine. For hydrodynamics, a linearized classic marine hydrodynamic model, based on the frequency-dependent parameters obtained from the code of WAMIT, is employed to calculate the hydrodynamic loads of the platform by solving the hydrostatic, diffraction and radiation problems with fluid-memory effect. Furthermore, Morison’s equation and the strip theory are applied to calculate the nonlinear viscous drag for improving the quality of the model. Finally, a series of cases with different wind and wave conditions is tested on a sample model combining “NREL offshore 5-MW baseline wind turbine” with “OC3-Hywind platform”. The results show that the simulator is able to predict the response and aerodynamic performance of OFWTs under wind and waves. Moreover, the FVM is more suitable for analyzing the aerodynamic performance of OFWTs than the unsteady BEM model because of its higher fidelity and less limitations.

Wind power generation has been paid much attention over the years as a countermeasure against global warming. Especially in recent years, researches and developments have also been made on Floating Offshore Wind Turbine (FOWT) in relatively deep offshore. Unlike Bottom-mounted Offshore Wind Turbine (BOWT), the motion characteristics of FOWT is complicated owing to coupled response of wind turbine and floating platform motions since the FOWT system is not fixed to the seabed. Due to these complexities, negative damping is one of the major problems reported for SPAR type FOWT moored by catenary chains. Negative Damping, in which the natural periodic motion is excited by blade pitch control employed for keeping the power generation constant, has to be addressed. In this paper, we discuss the negative damping of TLP type FOWT with a series of dedicated experiments. We manufactured a 1/100th-scale model TLP type FOWT model with a primary control system of the blade pitch angle for a geometrically scaled model of the 5MW wind turbine based on the NREL. At first, we formulated the mechanism for occurrence of Negative Damping and derived the conditions under which unstable fluctuations of the floating platform occurs using the motion equation. After that, we conducted scale model tank tests in wind alone and confirmed the phenomenon wherein the fluctuation of the floating platform does not converge. Finally, how dangerous such coupled motion of wind turbine and floating platform would be for real-scale FOWT is discussed.

The floating offshore wind turbine (FOWT) market is currently dominated by single unit demonstration projects and first pre-commercial wind farms such as the Hywind Scotland wind farm. Today’s FOWT substructure design process is therefore not yet at a level of industrialization comparable to fixed-bottom substructure design methodologies, where standardized design and realization procedures are well established throughout the industry. Aligned with work performed in the Ramboll and University of Stuttgart-led work packages “Concept Industrialization” and “Design practice” of the European H2020 LIFES50+ project, the ambition of this paper is to define state-of-the-art fixed-bottom and floating design methods and based on these identify key differences through comparative analysis. In the first part of the paper the scope and selected details of today’s industrialized design process for fixed-bottom substructure design is presented, including e.g. load iteration schemes and applied numerical methods, risk assessment incorporation, optimization, and cost modelling. In the second part, the key elements of this industrialized procedure which are different to FOWT design are identified and described: This is done based on a review of the current FOWT common design practice, where elements are identified that are unique and/or different for FOWTs — this includes e.g. the requirement to adapt the controller to the specific platform behaviour, as well as a tower and/or selected rotor-nacelle assembly (RNA) component redesign, and also includes differences in terms of defining and performing load case simulations. Another observation that is described relates to the floating specific required numerical methodologies applied for the detailed structural and mooring design, where challenges exist regarding the interface between coupled global loads analyses tools and detailed structural, mooring, and geotechnical tools. A further key item discussed in this respect is the industry-common load exchange practice for fixed-bottom design, where only a limited data exchange between WT manufacturers and platform designers is done; a procedure that is challenging to be applied for FOWTs. Compared with fixed-bottom design, the consideration of manufacturability, fabrication constraints, serial production, design complexity reduction, assembly, supply chain, installation, geotechnics, O&M and risk is often limited and these items will also be briefly discussed. Overall the paper is intended as a technical review of existing fixed-bottom design procedures and, facilitated through the comparative analysis with these established design methodologies, identifies and presents the key differentiating design elements and challenges for an industrialized FOWT design. The content of the paper will provide input for the focused development of design processes for industrialized detailed design of FOWTs to guarantee the demanded technology-readiness and manufacturing-readiness levels (TRL and MRL) and ultimately improve their CAPEX and OPEX by applying industrial design procedures. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 640741 (LIFES50+ project, www.lifes50plus.eu).

Offshore wind energy resources in Japanese EEZ (Exclusive Economic Zone) are now considered to be huge. In order to utilize the huge amount of energy located in relatively deep water areas, Ministry of the Environment, Japan has kicked-off the demonstration project on floating offshore wind turbine (FOWT). The project will continue for six years beginning from 2010fy to 2015fy. In the project, two FOWTs have been installed. The first FOWT mounts a 100kW wind turbine of downwind type, and the length dimensions are almost half of the second FOWT (so called as 1/2 scale model). The second FOWT mounts a 2MW wind turbine of downwind type, and called as a full scale model. The FOWTs consist of PC-steel hybrid spar (which is cost-effective) and are moored by three mooring chains. The half scale model was installed at the site on 11 June 2012 as the first grid-connected FOWT in Japan. The half scale model was attacked by very severe typhoon Sanba (1216), the greatest tropical typhoon in 2012 in the world. The behavior during the typhoon attack, including the measured environmental data and the FOWT responses is introduced. The installation of the full scale model has also successfully been made. The installation at the site completed on 18 October 2013; as the first multi-megawatt FOWT in Japan. The installation procedures and current status are also presented.

A fully-coupled analysis of the spar-type floating offshore wind turbine with bionic fractal heave plate under wind-wave excitation conditions

Numerical investigation of aerodynamic responses and wake characteristics of a floating offshore wind turbine under atmospheric boundary layer inflows

The northern Taiwan Strait, characterized by deep waters and high wind energy density, presents significant potential for developing floating offshore wind turbines (FOWTs). However, the region is prone to typhoons, with substantial variations in wind speed and direction during typhoon eye passages, posing challenges to FOWT safety and performance. This study investigates the serviceability of a 10 MW FOWT installed offshore of Hsinchu under typical wind and wave conditions during the eye of Typhoon SOULIK. Wind and wave data were sourced from the ERA5 reanalysis database. Simulations were conducted using OrcaFlex 11.4c, which enables fully coupled dynamic analysis of the entire FOWT system, including the mooring system, platform, tower, turbine, and nacelle, facilitating accurate predictions of system behavior in complex offshore environments. This study evaluated scenarios of maximum wind speed, significant wave height, wind–wave misalignment, and minimum wind speed during typhoon eye passage, considering both idle and power production modes in accordance with IEC TS 61400-3-2 requirements. The results indicate that platform yaw motion exceeds IEC limits during typhoon events, particularly in power production mode. This highlights the need for reducing platform motion. It is recommended to further develop control strategies or implement an active control system for the platform to ensure operational reliability. This research provides critical insights into FOWT design and operational challenges in typhoon-prone regions.

Floating offshore wind turbines (FOWTs) contribute to an emerging green energy technology, by exploiting higher and consistent wind speeds above the ocean. There are several challenges facing the design of mooring system of FOWTs, including installation costs, stability of light-weight minimalistic platforms, and shallow depths (50–300m). The extreme tension in mooring lines of a light displacement platform in shallow-water is dominated by snap loads. This is because light pre-tension requirements in the line may be insufficient to prevent the mooring lines from being exposed to wave motion induced slack and shock events. In this paper, we present a comparative analysis of a semi-submersible based FOWT exposed to a 100-year storm condition, based on model test data and numerical simulations of well-known industry standard software. The data was obtained from a 1/50th-scale FOWT with the wind turbine modeled after the NREL 5MW wind turbine. The software, OrcaFlex, was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic load for FOWT analyses. The numerical simulation of the moored FOWT in a 3-hour storm was executed in both the frequency-domain and the time-domain to determine the dynamic behavior of the platform and mooring system, respectively. Snap–type impact events were observed in both test data and numerical simulation. Tension maxima were fitted into extreme value distributions and comparisons are made between simulated and measured data. It is seen that snap events follow a different exceedance probability distribution compared to the cycle-to-cycle tension maxima.

FOWTs (Floating Offshore Wind Turbine) are feasible renewable devices to harness the wind energy in the near future. However, because of the complicated interactions among wind turbine, mooring system and the hull, the motion of a FOWT under the impact of severe wind, wave and current has not been well studied yet. This research focuses on the coupled numerical analysis of a FOWT. A numerical code COUPLE-FAST is developed by integrating two existing codes, namely, COUPLE and FAST, to carry out the task. In this study, a particular FOWT model is chosen for the numerical simulation, which consists of a NREL 5-MW baseline wind turbine and OC3-Hywind Spar. Although the numerical simulation is limited to this particular type of FOWTs, the results and related code (COUPLE-FAST) may be helpful to the design of FOWTs in the future.

Offshore wind energy resources in Japanese EEZ (Exclusive Economic Zone) are now considered to be huge. In order to utilize the huge amount of energy located in relatively deep water areas (water depth range: 50 – 300m), Ministry of the Environment, Japan has kicked-off the demonstration project on floating offshore wind turbine (FOWT). The project will continue for six years beginning from 2010fy to 2015fy. In the project, two FOWTs have been installed. The first FOWT mounts a 100kW wind turbine of downwind type, and the length dimensions are almost half of the second FOWT (so called as half scale model). The second FOWT mounts a 2MW wind turbine of downwind type, and called as full scale model. The FOWTs consist of PC-steel hybrid spar (which is cost-effective) and are moored by three mooring chains. The half scale model was installed at the site on 11th June, 2012 as the first grid-connected FOWT in Japan. The half scale model was attacked by very severe typhoon Sanba (1216), the greatest tropical typhoon in 2012 in the world. The behavior during the typhoon attack, including the measured environmental data and the FOWT responses, is described. The behavior during power production is also described. The installation of the full scale model has successfully been made; the opening ceremony was held on 28th October, 2013 as the first multi-megawatt FOWT in Japan. The installation procedures are briefly mentioned.