1. Introduction
Owing to environmental issues, including global warming caused by greenhouse gas emissions and the depletion of fossil fuels, research has been actively conducted on renewable energy technologies, such as solar, wind, tidal, and wave energy, to achieve carbon neutrality (Schwartz et al., 2010). Offshore wind power industry is actively engaged in technological development in many countries. Offshore wind turbines can avoid the complaints of onshore wind turbines (e.g., spatial constraints, noise, and visual interference) and utilize high-quality wind and it is possible to increase turbine sizes. As these benefits become more evident as the distance from the coast increases, the design and commercial operation of floating offshore wind turbines have attracted significant attention in recent years (Ivanov et al., 2023). However, the position of the floater of a floating offshore wind turbine may change owing to various environmental loads such as wind, waves, and tidal currents. Therefore, a reliable mooring system is required to limit these changes, which also significantly affects the lifespans of floating offshore wind turbines.
In general, the mooring system for maintaining the position of the floater consists of various elements, including anchors, mooring lines, buoys, and clump weights. In this study, Clump weight (CW) is introduced and will be referred to as CW hereafter. When a CW is applied to catenary mooring system, it can increase the restoring force of the mooring system (Yuan et al., 2014) and significantly impact the motion response of the floater, depending on its position (Liu., 2019). Moreover, the attachment of a CW to a single mooring line can effectively reduce the horizontal motion response of a floater (Lopez-Olocco et al., 2022). Such additional devices attached to the mooring line can achieve their intended purpose of reducing the tension or increasing the restoring force. However, depending on their arrangement, they can cause repeated contact with the seabed or excessive tension. Therefore, this arrangement should be examined thoroughly in advance.
Lee et al. (1991) conducted research on the effects of the seabed contact of a mooring line applied to a guyed tower on the dynamic behavior and stability of the structure. Song et al. (2013) examined the changes in tension and irregular vibrations caused by contact between the mooring line and seabed. As the weight of the CW attached to the mooring line increases, the design efficiency is improved by the significant mooring line length reduction effect. However, a snapping phenomenon occurs. The snapping phenomenon causes excessive tension in the mooring line, which can damage the mooring line (Jung et al., 2012).
Hence, this study aims to numerically analyze the dynamic effects of the contact between the CW and the seabed on the mooring tension and motion response of a floater. The CW was placed at two different positions on the mooring line of a 15 MW semi-submersible floating offshore wind turbine. Two cases were set up: one with continuous CW contact with the seabed and one without contact. Under these conditions, the effects of the CW-seabed interaction on the dynamic responses of the floater and mooring system were analyzed by simulating extreme environmental conditions with the colinear wind, waves, and currents. After the difference depending on the contact of the CW with the seabed was examined, the effects of reducing mooring tension fluctuations via the distribution of the weight of the CW were quantitatively examined. Based on these results, it was confirmed that the optimal arrangement of the CW can reduce the fatigue load of the mooring system of a floating offshore wind turbine.
2. Numerical Analysis
The mooring line, which is a cable connected between a fixed point on the seabed and a floating offshore platform, provides a restoring force to maintain the position of the structure against external forces (e.g., winds, waves, and currents). That is, the tension in the mooring line significantly affects the motion of the floater. Similarly, the motion of the floater affects the tension of the mooring line. These results indicate that an integrated analysis that uses a floater-mooring coupled model is required, rather than requiring separate analyses of the behavior of the floater and mooring line. In this study, the added mass, radiation damping, and wave exciting force coefficients were calculated in the frequency domain using a three-dimensional panel method based on linear potential theory. Based on the calculated hydrodynamic coefficients, a time-domain dynamic analysis was conducted using OrcaFlex, which is a commercial software program for floater–mooring coupled analysis. In addition, the displacement of the floater and tension of the mooring line were calculated over time. Fig. 1 shows the frequency- and time-domain numerical simulation processes.
2.1 Numerical Model
This section presents the specifications and motion characteristics of the offshore wind turbines used in this study. The reference coordinate system for an offshore wind turbine is shown in Fig. 2. This system was used to define the position and motion characteristics of each component.
The floating offshore wind turbine model used in this study was the IEA 15 MW reference wind turbine (RWT) model, as shown in Fig. 2 (Gaertner et al., 2020). This model was developed to provide standardized criteria for the evaluation and prediction of the performance, cost, and technological development of large-scale offshore wind turbines. This model has been widely used in various studies and development, including the design, simulation, and economic analysis of floating wind turbines. Table 1 provides detailed information on the rotor and tower specifications.
Offshore wind turbines installed in deep water use floater to support superstructures, such as rotors and towers. The UMaine VolturnUS-S was selected as the semi-submersible floater for this study (Allen et al., 2020). The floater consists of three external columns with respect to the central column, and the columns are connected using three pontoons at the bottom, as shown in Fig. 3. The external columns were connected to the central column using three braces at the top. The floater had a total height of 35 m, draft of 20 m, and freeboard of 15 m.
A new damping model for the floater was constructed and verified by improving the damping model provided in the UMaine Volturn US–S report. The original damping model adopted quadratic damping, which is proportional to the square of the floater’s velocity. On the other hand, the damping model of the floater used in this study combined the quadratic damping with the Morison drag to simulate the drag load generated by the flow of fluid particles, even in the absence of floater motion. The damping and drag coefficients were determined such that the natural period and damping ratio of the combined damping model were identical to those in the IEA 15 MW RWT UMaine VolturnUS-S report. Fig. 4 shows a comparison of the freedecay simulation results of the surge, heave, and pitch for each damping model.
To verify the RAO of the floater motion according to the application of the damping model, the heave and pitch RAO presented in the VolturnUS-S report were expressed as black solid lines together with the RAO of the quadratic damping model constructed for comparative purposes under the same conditions as those described in Fig. 5. The RAO of the combined damping model used in this study was also expressed to compare the motion responses. As shown in Fig. 5, the RAO presented in the report and the response of the model that applied only quadratic damping in the same manner as that described in this study, were almost identical near the heave and pitch natural frequencies (0.30 and 0.22 rad/s, respectively). Moreover, the combined damping model applied in this study was almost identical to the model presented in the report in terms of the damping performance.
Based on the UMaine Volturn US-S report, a numerical model of the mooring system was constructed as catenary mooring line at a depth of 200 m. The layout of the mooring line is shown in Fig. 6. The seabed used in this study was constructed as a linear spring model that subsided by approximately 14 mm when a CW with a 75 t weight was placed on it. To simplify the analysis, the seabed damping and friction forces acting on the mooring lines and CW were neglected, minimizing seabed interaction effects. In all simulation cases, the same seabed model was used to fairly compare the effects of CW contact with the seabed and the distribution of the CW. Table 2 lists the properties of the mooring lines. The anchor radius and mooring line length were 837.70 and 850 m, respectively. The nominal diameter of the studless chain mooring line was 165 mm. In addition, in accordance with DNV regulations, the drag coefficients of the mooring line were set to 2.4 and 1.15 in the longitudinal and lateral directions, respectively (DNV, 2021).
2.2 Environmental Conditions
The design load case (DLC) is a concept that represents a combination of various environmental loads to examine the structural stability of wind turbines. The DLC is used to evaluate the dynamic responses of wind-turbine structures under various load conditions. In this study, the dynamic responses of a floater and mooring system were examined by simulating extreme environmental conditions (DLC 6.1) for 3 hours (10,800 s). The environmental load was set to head toward the floater along mooring line 1 (ML1), which is in the +x direction of the coordinate system, as shown in Fig. 6. This is a conservative condition for the maximum load on the ML1 mooring line. The environmental conditions are listed in Table 3. As the turbine generator did not operate under extreme environmental conditions, the analysis was conducted by setting the blade pitch angle to 90° to minimize the load on the blades. A turbulence model was created for wind conditions based on the engineering sciences data unit (ESDU) spectrum. The current speed distribution by depth was constructed using a power law based on the current speeds of the sea surface and seabed. In addition, the energy distribution of the irregular waves is represented as a spectral density, as shown in Fig. 7.
3. Results and Discussion
3.1 Analysis of the Results According to the Contact Between CW and the Seabed
In Table 4, a dynamic analysis was conducted by attaching a 75 t CW to the weather-side mooring line to analyze the effect of the contact of the CW with the seabed during the dynamic behavior of the floater. The CW was not attached to the lee-side mooring line of the floater because no ultimate load was applied under the environmental conditions used for the analysis. The CW was positioned along the anchor direction from the touchdown point of the weather-side mooring line, which was initially in static equilibrium without the CW, to ensure that the dynamic analysis began from the same equilibrium state. During the dynamic analysis, a position highly likely to cause repeated contact by the CW (Case 2) and a position with no contact (Case 1) were selected as the attachment positions of the CW, as shown in Fig. 8. The selected CW positions were 490 m and 350 m from the anchor in Cases 1 and 2, respectively.
Table 5 lists the statistical values of the dynamic responses of floaters under extreme environmental conditions. The tendencies of the surge, heave, and pitch, which were most significantly affected by the direction of the environmental loads, were compared. The maximum surge and heave values of Case 2 increased by 4.7% and 5.1%, respectively, compared with those of Case 1. The maximum pitch increased by 7.8%. The difference in the surge between Cases 1 and 2 was caused by the difference in the mooring restoring force, depending on the CW position. To examine this difference, a surge offset test was conducted on the mooring line, and the results are shown in Fig. 9. In the surge offset test, the restoring force was higher within the maximum and minimum ranges of the surge (within the rectangular dotted line) for Case 1, in which the concentrated load of the CW was closer to the touchdown position, compared with that of Case 2. This reduces the average surge behavior in Case 1. The differences in the maximum values of the heave and pitch between Cases 1 and 2 were judged to be the floater response results because of the mooring tension fluctuations caused by the contact of the CW with the seabed.
Fig. 10 compares the partial time-series data from the 3 hours analysis of the mooring tension near the fairlead of the weather-side mooring line according to the contact of the CW with the seabed. In Table 6, the mean mooring tension at the fairlead in Case 2 decreased by 2.9% compared with that in Case 1. However, the maximum value and standard deviation increased by 10.1% and 35.7%, respectively. To examine whether these tension fluctuations were caused by contact of the CW with the seabed, the vertical displacement of the CW was examined. In Case 2, the repeated contact of the vertical position of the CW with the seabed was confirmed, and it was found that the tension fluctuations of the mooring line significantly increased when the CW moved near the seabed (gray sections).
Fig. 11 shows a comparison of the spectral density of mooring line tension at the fairlead. When the CW was in continuous contact with the seabed, as in Case 2, significant mooring tension fluctuations occurred at the fairlead. The high-frequency tension response of Case 2 observed in Fig. 10 is thought to be the response of the axial vibration of the mooring line caused by the contact of the CW with the seabed and its detachment. However, this was the numerical response observed in the analytical model of the mooring line, and its occurrence in an actual physical environment remains uncertain. Therefore, this high-frequency response is excluded from the discussion.
Previous analyses of irregular waves were based on a single random seed. Two additional random seeds were included to ensure the generalized results. Table 7 shows the results of the comparison of the mooring tensions with different random seeds. The maximum and minimum values of mooring tension varied depending on the random seeds. However, the rates of increase and decrease for Cases 1 and 2 are consistent with previous results, confirming that it is possible to generalize the research results.
3.2 Analysis of Seabed Contact Results According to the CW Distribution
In this section, the characteristics of the mooring line tension fluctuations are examined when the weight of the CW is distributed in Case 2 with repeated contacts between the CW and the seabed, as shown in Table 8. In Fig. 12, a 75 t CW was placed 350 m from the anchor of the mooring line in Case 2. In Cases 2-1 and 2-2, three 25 t CWs and five 15 t CWs were placed at 10 m intervals, respectively. The restoring stiffness of the mooring was maintained as similar as possible across the cases by applying the same total weight of CW to the front mooring line in each case. Through the analysis of these additional cases, the effects of the CW distribution on the floater motion response and mooring line tension were compared and analyzed under extreme environmental conditions.
Table 9 compares the statistical values of the surge, heave, and pitch that exhibited significant responses under extreme environmental conditions to examine the difference in the floater motion response depending on the distribution of the CW. There were no significant differences in the mean and standard deviation values of the surge, heave, and pitch. These results indicate that even the distribution of the CW causes no significant difference in the floater response, as the restoring force characteristics of the mooring line remain almost the same.
Figs. 13 and 14 compare the changes in mooring tension for each case through the time series and the response spectrum when the CW was distributed. In Fig. 13(a), the sections where the CW moved near the seabed (gray sections) were confirmed through the vertical position of the distributed CW. Based on these results, the tendency of the CW distribution to decrease the mooring line tension fluctuations is confirmed in the gray sections of Fig. 13(b). In the other sections, the tension change caused by the CW distribution was negligible. When the statistical values of the mooring tension in Table 10 were examined, the standard deviations of Cases 2-1 and 2-2 decreased by 2.9% and 4.5%, respectively, compared with Case 2. The maximum tensions decreased by 0.9% and 1.2%, respectively. These results imply that the distribution of the CW can reduce the tension fluctuations and maximum tension caused by contact with the seabed. The effect was even greater when many CWs with small weights were arranged than when only a few CWs with large weights were arranged.
4. Conclusion
This study analyzed the effects of the contact of a clump weight (CW) attached to a floating offshore wind mooring system with the seabed on the dynamic response of the floater and mooring line tension fluctuations under extreme environmental conditions. To analyze the influence of the contact of the CW with the seabed, a CW with a 75 t weight was attached to the mooring line, and tension fluctuations were examined according to its contact with the seabed. When the CW was in repeated contact with the seabed, the tension fluctuation (standard deviation) increased by 35.7%. An increasing tendency of the tension fluctuations was also observed in the results of the analysis conducted using different random seeds under irregular wave conditions. These results imply that the large tension fluctuations in the mooring line caused by contact of the CW with the seabed may have a negative impact on the lifespan of the mooring line. To reduce this negative impact, cases in which the weight of the CW was distributed were analyzed. After creating cases by attaching one 75 t CW, three 25 t CWs, and five 15 t CWs, respectively, the behavior of the floater and tension of the mooring line were analyzed under extreme environmental conditions. The tension fluctuations (standard deviations) decreased by 2.9% and 4.5% when the weights were distributed to three and five CWs, respectively. However, in these cases, the floater behaviors were almost identical. These results suggest that strategically distributing the clump weight (CW) can effectively mitigate mooring line fatigue by reducing tension fluctuations, while maintaining the dynamic characteristics of the floater. These results also indicate that both the motion stability and structural reliability of the floating offshore wind turbine system can be improved by optimizing the arrangement of the mooring system components during the design stage. In the future, it will be necessary to quantitatively analyze the changes in the fatigue life of the mooring line owing to the contact of the CW with the seabed and distribution of the CW.
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