Nomenclature
COD: Co-direction
DFF: Design fatigue factor
DLC: Design load case
DWR: Design working range
DIA: Design inlet angle
FCS: Fairlead chain stopper
FLS: Fatigue limit states
MBL: Minimum breaking load
MSL: Mean sea level
MUL: Multi-direction
NTM: Normal turbulence model
NSS: Normal sea state
SMP: Submerged mooring pulley
1. Introduction
The installed capacity of offshore wind turbines has been increasing globally, with a current cumulative installation of 75 GW. Among these, floating offshore wind turbines account for approximately 120 MW of the installed capacity, with a planned capacity expected to expand to 8.5 GW by 2030 (GWEC, 2025). In South Korea, the development of floating offshore wind farms with a capacity of over 6 GW is also underway off the coast of Ulsan (Ryu et al., 2022). Achieving these goals necessitates securing the infrastructure and equipment supply chain required for installation. Furthermore, the development of innovative technologies aimed at reducing the levelized cost of electricity (LCOE) for floating offshore wind turbines is essential for improving market competitiveness. Yang et al. (2022) reviewed publicly available floater types and their associated mooring systems. Although innovative mooring systems that can reduce the motion response of floaters have been developed, further research is needed to identify more cost-effective solutions. In response to these challenges, the Korean government has initiated a project to develop a novel disconnectable mooring system that minimizes maintenance costs for floating offshore wind turbines. Campanile et al. (2018) examined mooring system design in intermediate water depths, and Jiang (2025) reviewed the mooring system design process for floating offshore wind turbines. Thus far, most research on mooring systems for floating offshore wind turbines has focused on the system level, whereas studies on individual components at the equipment level are limited. As part of the disconnectable mooring system development project, Lee and Song (2023) conducted structural experiments using scaled models fabricated by 3D printing and validated the results by comparison with numerical results. In their study, composite materials were used, which exhibited different properties from those of the materials intended for actual application. This study examined the structural and fatigue strength of the Fairlead chain stopper (FCS), a key detachable component developed as part of a disconnectable mooring system designed for 10 MW-class floating offshore wind turbines. The mooring system under development consisted of the FCS, submerged mooring pulley (SMP), and chain. An integrated load analysis of the floating offshore wind turbine was required to determine the necessary loads for the structural and fatigue strength evaluation. Therefore, a preliminary design of a 10 MW-class semi-submersible floating structure was conducted, followed by a comprehensive load analysis performed by a collaborating research institution. The structural strength evaluation was based on the minimum breaking load (MBL) of a 147 mm diameter R4S-grade mooring chain. For the fatigue strength evaluation, the anticipated fatigue loads over the design life were calculated. The strength assessment was conducted in accordance with DNV regulations, using the results from structural and fatigue analyses. Numerical analysis for this study was performed using the Abaqus software program (SIMULIA, 2022).
2. General Arrangement
2.1 Floater Design
For the development of a disconnectable mooring system, the floating offshore wind turbine was based on a semi-submersible floating structure paired with the DTU 10 MW offshore wind turbine (Bak et al., 2013). The semi-submersible structure comprised three outer columns and a central column supporting the turbine tower, all interconnected by a lower pontoon and an upper deck, as shown in Fig. 1. The interior of the floating structure was divided into three ballast tanks, with the main specifications listed in Table 1. The system was designed for an installation depth of 150 m. using three mooring lines arranged in a catenary configuration.
2.2 Mooring System
The mooring system used three lines with a catenary configuration, as shown in Fig. 2. The key components of the disconnectable mooring system included the FCS, which featured a bidirectional latch system for easy chain attachment and detachment, and the SMP installed underwater to facilitate tensioning operations. The newly developed FCS enabled safer and faster detachment compared to the existing systems used in floating platforms, significantly reducing the installation and decommissioning times for floating offshore wind turbines. The detachable mooring system developed in this project was designed to accommodate a 147 mm studless R4S grade chain capable of withstanding an MBL of 21,179 kN.
3. Structural and Fatigue Analysis
3.1 Structural Analysis Model – FCS
Fig. 3 presents the FCS, a key component of the developed disconnectable mooring system. The FCS was mounted on the upper section of the outer column of the floating structure, securing the mooring lines to the floating body. One FCS unit weighed 32 t, with the materials and yield strengths for each component detailed in Table 2. The components and their respective functions are outlined below.
3.2 Finite Element Analysis Model
Finite element analysis (FEA) was performed using solid elements (C3D8R), with the material properties defined by an elastic modulus of 206 GPa and a Poisson’s ratio of 0.3. The accuracy of the nonlinear contact analysis was ensured by convergence analysis conducted by varying the element size at the contact interfaces. The equivalent stress results were plotted as a function of the element size, showing that stress convergence occurs at 30 mm or smaller for the plate and 20 mm or smaller for the pin (Figs. 4 and 5). Based on these results, the element size at the contact interfaces was set to 15 mm or less. The final finite element model consisted of 313,621 elements and 418,921 nodes. Fig. 6 presents the generated finite element model, which includes the FCS and a portion of the floater’s support structure.
3.3 Boundary Conditions
The numerical analysis model for strength assessment included the FCS and a portion of the supporting structure. The end of the supporting structure connected to the floater was defined with a fixed boundary condition (Fig. 7(a)). A half-model was used with the symmetry boundary conditions in Fig. 7(b) to minimize the computational cost. A nonlinear analysis incorporating the contact conditions was performed, with the contact surfaces defined at the pin joint and the wall interfacing with the stopper block, as shown in Fig. 8. Table 3 lists the applied friction coefficients. For the pin-connected regions, the friction coefficient between the steel and bushing was applied, while the friction coefficient between steel and steel was applied for the wall surfaces.
4. Structural Safety Assessment
4.1 Load Application
The loads used for the structural strength evaluation were based on the minimum breaking load (MBL) of the mooring chain. The MBL for the 147 mm diameter R4S-grade chain was calculated using Eq. (2) in accordance with DNV-OS-E301 (DNV, 2021a). Fig. 9 shows the design angles at which the loads are applied. In this analysis, a horizontal design working range (DWR) angle of 0° and vertical design inlet angles (DIA) of 10° and 24.5° were considered. The 10° angle represents the minimum requirement set by DNV regulations, while the 24.5° angle, derived from integrated load analysis, is the angle at which the maximum mooring line tension occurs. MBL was applied using the bearing pressure to the chain contact surface. The bearing load was calculated using Eqs. (3)–(5) and applied at each node of the contact surface. The applied direction is the uniaxial load in the direction of chain movement, as shown in Fig. 10.
4.2 Structural Analysis Results
Structural analysis was conducted using Abaqus 6.22, and the final displacements and equivalent stress values were evaluated (SIMULIA, 2022). Fig. 11 presents the resulting displacement distribution, where a maximum displacement of up to 8 mm is observed in the direction of the tensile force acting on the FCS arm. Fig. 12 shows the equivalent stress distribution under each analysis condition, with the maximum equivalent stress of 629 MPa occurring at the load-applied surface of the chain stopper. In addition, high equivalent stress values were observed at the main pin and the FCS support, which appeared to be caused by uneven contact at the support plate because of the bending deformation of the main pin. The FCS arm, where large displacements occurred, showed a relatively uniform distribution of equivalent stress. In contrast, high stress was observed at the intersection of the base plate and wall plate of the FCS housing, where little displacement occurred. The cause of this stress concentration was analyzed during the evaluation process.
4.3 Structural Strength Evaluation
Structural safety evaluation of the FCS was conducted using an allowable stress set to 90% of the yield strength of the material following the DNV-ST-0119 (DNV, 2021b). Table 4 lists the results of the structural analysis, including the maximum equivalent stress and its compliance with the allowable stress criteria. The analysis confirmed that all components satisfied the specified criteria, except for the wall plate of the FCS housing, where the maximum equivalent stress of 291 MPa exceeded the allowable limit of 279 MPa. Fig. 13 presents the stress distribution on the wall plate of the FCS housing. The elevated stress observed at the intersection of the base plate and wall plate was attributed to the use of a fine mesh (20 × 20 × 20 mm) and abrupt geometric transitions, resulting in localized peak stresses. Considering that peak stresses can reach up to 1.7 times the allowable stress, the overall structural strength requirements were still deemed to be satisfied (DNV, 2021c). Therefore, the designed FCS met the structural strength criteria.
5. Fatigue Safety Assessment
5.1 Fatigue Load
Loads with significant influence on fatigue were applied to evaluate the fatigue strength, with a particular focus on the tensile loads induced by the mooring lines. The range of these tensile loads and the number of cycles expected over the design life were derived from the integrated load analysis results provided by a collaborating research institution. The range of tensile loads and their corresponding cycle counts were extracted using the Rainflow counting method from the time series data of mooring line tensions. Fatigue loads corresponding to a 25-year service period were calculated by considering the occurrence probabilities of wind speeds under each design load case. The design load cases, based on IEC 61400-3-2 (IEC, 2019), were selected with reference to the LIFES 50+ project and are summarized in Table 5 (EU, 2015). Table 6 lists the mean tension and corresponding angles of the mooring lines under each design load case. Based on this result, the fatigue loads were calculated for mooring line 2, which exhibited the highest mean tension. Fig. 14 shows the fatigue loads applied to the FLS, where the tension range was divided by 10 kN intervals.
5.2 Screening
The purpose of screening is to select areas relatively vulnerable to fatigue. For this, unit tension of 1kN was applied to the chain, and areas showing high principal stress were selected. The fatigue strength of these identified vulnerable areas was then assessed. Fig. 15 shows the screened zones along with maximum principal stress distributions, and Table 7 lists the maximum (tensile) principal stress and directionality of screened zones. Zone 11 was selected to evaluate fatigue failure at the welded joint of the FCS support structure.
5.4 Fatigue Strength Evaluation
The fatigue strength of the FCS was evaluated to determine its fatigue life and compliance with the design fatigue factor (DFF) requirements. A design life of 25 years was assumed for this evaluation, comprising 20 years under normal operating conditions and five years under non-operating conditions, with a DFF of 10. The fatigue life was calculated using Palmgren–Miner’s linear cumulative damage method, as expressed in Eq. (6) (DNV, 2020). In the case of toe cracks, a stress level higher than the hot spot stress was applied to obtain conservative fatigue assessment results. The hot spot stress is typically determined by extrapolating the maximum principal stress obtained from FEA with a mesh size equal to the plate thickness. In this study, however, a higher stress level was used intentionally to make the results more conservative. A very fine solid mesh with an element size of approximately 15 × 15 × 15 mm, corresponding to approximately 25% of the minimum plate thickness (ranging from 60 to 130 mm), was used to achieve this. The principal stress at the intersection element was extracted and applied for the fatigue safety evaluation. In the case of root cracks, the stress value associated with potential crack initiation in the weld throat of the welded joints was calculated using Eq. (7). Table 9 lists the results of the fatigue strength evaluation. All screening areas exhibited a DFF greater than 100, satisfying the fatigue strength criteria. Nevertheless, the DFF may be reduced if the FCS were operated in a seawater environment.
k: number of stress block
ni: number of stress cycles in stress block i
Ni: number of cycles to failure at a constant stress range Δσi
where the stress components are explained in Fig. 17.
6. Conclusions
This study evaluated the structural and fatigue strength of the FCS as part of the development of a disconnectable mooring system. A basic design of the floating structure was used to facilitate integrated load analysis, ensuring compliance with intact stability regulations that require a safety factor of 1.5 or greater. The loads for the structural and fatigue strength evaluations were derived from integrated load analysis results provided by a collaborating research institution. The structural strength evaluation identified high-stress concentrations at the intersection of the base plate and wall plate of the FCS housing. On the other hand, the structural strength criteria were satisfied by applying the allowable stress based on peak stress.
In conclusion, the designed FCS met the structural and fatigue strength requirements. Nevertheless, the current analysis was limited because it modeled only a portion of the support structure and imposed boundary conditions that may not accurately represent the load transfer mechanisms resulting from the motion response from the floating structure. Future research will involve strength assessments for weight reduction and propose lightweight design solutions through structural optimization.
PDF Links
PubReader
ePub Link
Full text via DOI
Download Citation
Print
