The Influence of the Mooring System on the Motions and Stability of a Semi-Submersible Floating Wind Turbine

2015 ◽  
Author(s):  
Fons Huijs
Keyword(s):  
Wind Turbine ◽  
Low Frequency ◽  
Considerable Effect ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Mooring System ◽  
Mooring Lines ◽  
System A ◽  
Turbine Design

One of the main aspects of a floating offshore wind turbine design is its mooring system, which can strongly influence the floater stability and motions. This is illustrated by considering two catenary mooring systems for the same semi-submersible. The main difference between the two systems is the position of the connection points of the mooring lines on the floater, the so-called fairleads. The philosophy is that the design can be improved by shifting the fairleads to the highest feasible level. For both mooring systems, the floater motions and stability are assessed. Stability curves are derived, taking both the effect of hydrostatics and the mooring system into account. Floater motions are analyzed using both uncoupled frequency domain calculations and coupled aero-hydro-servo-elastic time domain simulations. The mooring system is found to have a considerable effect on the floating stability. The effect on the motions is less profound for the considered mooring systems and limited to the low frequency range. Mooring line tensions are however significantly affected by the fairlead position. It is concluded that, with a well-designed mooring system, a smaller and thus less expensive floater can be used while still meeting the requirements in terms of stability and maximum motions. In addition, the mooring lines may be lighter as well.

scholarly journals Alternative Mooring Systems for a Very Large Offshore Wind Turbine Supported by a Semisubmersible Floating Platform

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Jinsong Liu ◽  
Lance Manuel
Keyword(s):  
Wind Turbine ◽  
Integrated System ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
System Response ◽  
Mooring System ◽  
Floating Platform ◽  
Mooring Lines ◽  
Wind Speeds

As offshore wind turbines supported on floating platforms extend to deep waters, the various effects involved in the dynamics, especially those resulting from the influence of moorings, become significant when predicting the overall integrated system response. The combined influence of waves and wind affect motions of the structure and induce tensile forces in mooring lines. The investigation of the system response under misaligned wind-wave conditions and the selection of appropriate mooring systems to minimize the turbine, tower, and mooring system loads is the subject of this study. We estimate the 50-year return response of a semisubmersible platform supporting a 13.2 MW wind turbine as well as mooring line forces when the system is exposed to four different wave headings with various environmental conditions (wind speeds and wave heights). Three different mooring system patterns are presented that include 3 or 6 mooring lines with different interline angles. Performance comparisons of the integrated systems may be used to define an optimal system for the selected large wind turbine.

scholarly journals Dynamic Response for a Submerged Floating Offshore Wind Turbine with Different Mooring Configurations

2019 ◽  
Vol 7 (4) ◽  
pp. 115 ◽  
Author(s):  
Yane Li ◽  
Conghuan Le ◽  
Hongyan Ding ◽  
Puyang Zhang ◽  
Jian Zhang
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Intermediate Water ◽  
Mooring System ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbine

The paper discusses the effects of mooring configurations on the dynamic response of a submerged floating offshore wind turbine (SFOWT) for intermediate water depths. A coupled dynamic model of a wind turbine-tower-floating platform-mooring system is established, and the dynamic response of the platform, tensions in mooring lines, and bending moment at the tower base and blade root under four different mooring configurations are checked. A well-stabilized configuration (i.e., four vertical lines and 12 diagonal lines with an inclination angle of 30°) is selected to study the coupled dynamic responses of SFOWT with broken mooring lines, and in order to keep the safety of SFOWT under extreme sea-states, the pretension of the vertical mooring line has to increase from 1800–2780 kN. Results show that the optimized mooring system can provide larger restoring force, and the SFOWT has a smaller movement response under extreme sea-states; when the mooring lines in the upwind wave direction are broken, an increased motion response of the platform will be caused. However, there is no slack in the remaining mooring lines, and the SFOWT still has enough stability.

scholarly journals Proposal and Performance Study of a Novel Mooring System with Six Mooring Lines for Spar-Type Offshore Wind Turbines

2021 ◽  
Vol 11 (24) ◽  
pp. 11665
Author(s):  
Shi Liu ◽  
Yi Yang ◽  
Chao Wang ◽  
Yuangang Tu
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Offshore Wind Turbine ◽  
Mooring System ◽  
Mooring Lines ◽  
Included Angle ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines

Spar-type floating offshore wind turbines commonly vibrate excessively when under the coupling impact of wind and wave. The wind turbine vibration can be controlled by developing its mooring system. Thus, this study proposes a novel mooring system for the spar-type floating offshore wind turbine. The proposed mooring system has six mooring lines, which are divided into three groups, with two mooring lines in the same group being connected to the same fairlead. Subsequently, the effects of the included angle between the two mooring lines on the mooring-system’s performance are investigated. Then, these six mooring lines are connected to six independent fairleads for comparison. FAST is utilized to calculate wind turbine dynamic response. Wind turbine surge, pitch, and yaw movements are presented and analyzed in time and frequency domains to quantitatively evaluate the performances of the proposed mooring systems. Compared with the mooring system with six fairleads, the mooring system with three fairleads performed better. When the included angle was 40°, surge, pitch, and yaw movement amplitudes of the wind turbine reduced by 39.51%, 6.8%, and 12.34%, respectively, when under regular waves; they reduced by 56.08%, 25.00%, and 47.5%, respectively, when under irregular waves. Thus, the mooring system with three fairleads and 40° included angle is recommended.

Parametric Study of Catenary Mooring System for a Semisubmersible Floating Wind Turbine in Intermediate Water Depth

2020 ◽  
Author(s):  
Jiawen Li ◽  
Qiang Zhang ◽  
Jiali Du ◽  
Yichen Jiang
Keyword(s):  
Wind Turbine ◽  
Water Depth ◽  
Offshore Wind ◽  
Design Parameters ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Reference Structure ◽  
Intermediate Water ◽  
Mooring System ◽  
Floating Wind Turbine

Abstract This paper presents a parametric design study of the mooring system for a floating offshore wind turbine. We selected the OC4 DeepCwind semisubmersible floating wind turbine as the reference structure. The design water depth was 50 m, which was the transition area between the shallow and deep waters. For the floating wind turbine working in this water area, the restoring forces and moments provided by the mooring lines were significantly affected by the heave motion amplitude of the platform. Thus, the mooring design for the wind turbine in this working depth was different from the deep-water catenary mooring system. In this study, the chosen design parameters were declination angle, fairlead position, mooring line length, environmental load direction, and mooring line number. We conducted fully coupled aero-hydro dynamic simulations of the floating wind turbine system in the time domain to investigate the influences of different mooring configurations on the platform motion and the mooring tension. We evaluated both survival and accidental conditions to analyze the mooring safety under typhoon and mooring fail conditions. On the basis of the simulation results, this study made several design recommendations for the mooring configuration for floating wind turbines in intermediate water depth applied in China.

Estimation of Risk of Progressive Drifts in a Wind Farm Caused by Collision of Drift Ship

2015 ◽  
Author(s):  
Eiji Hirokawa ◽  
Hideyuki Suzuki ◽  
Shinichiro Hirabayashi ◽  
Minon Muratake
Keyword(s):  
Wind Turbine ◽  
Wind Farm ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Mooring System ◽  
Environmental Load ◽  
Collision Process ◽  
Tank Experiment ◽  
Estimation Of Risk

In off-shore wind turbine, it is difficult to determine the risk of accident caused by the mooring destruction through experiment. In this paper, the authors discuss the risk, with the case of a drifting ship wanders into the wind farm. In the design of a floating offshore wind turbine (FOWT), drift of a FOWT is considered as a serious failure mode and the mooring system must be designed to avoid the failure. The failure of mooring line is not initiated just by extreme environmental load but can be initiated by collision with a drifting ship, which enters the wind farm. This phenomenon is difficult to investigate by a tank experiment. So far, little knowledge exists about the phenomenon. In this research, a simulator to reproduce the collision process of a FOWT and a drift ship and a progressive drift of FOWTs in a wind farm was developed. Using this simulator and statistics of drift incidents of a ship, a procedure to evaluate risk of progressive drifts in a wind farm was established. In that case, second accident that a wind turbine which has started drifting caused by the drifting ship collides with one another wind turbine is expected. As a result, the risk mainly depends on the risk of drifting caused by a large displaced ship. In addition, the risk partly depends on the arrangement of wind farm.

scholarly journals Numerical Analysis of a Catenary Mooring System Attached by Clump Masses for Improving the Wave-Resistance Ability of a Spar Buoy-Type Floating Offshore Wind Turbine

2019 ◽  
Vol 9 (6) ◽  
pp. 1075 ◽  
Author(s):  
Zhenqing Liu ◽  
Yuangang Tu ◽  
Wei Wang ◽  
Guowei Qian
Keyword(s):  
Wind Turbine ◽  
International Energy Agency ◽  
Wave Resistance ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Induced Response ◽  
Mooring Lines ◽  
Energy Agency

The International Energy Agency (IEA), under the auspices of their Offshore Code Comparison Collaboration (OC3) initiative, has completed high-level design OC-3 Hywind system. In this system the wind turbine is supported by a spar buoy platform, showing good wave-resistance performance. However, there are still large values in the motion of surge degree of freedom (DOF). Addition of clump masses on the mooring lines is an effective way of reducing the surge motion. However, the optimization of the locations where the clump masses are added is still not clear. In this study, therefore, an in-house developed code is verified by comparing the results of the original OC3 model with those by FAST. The improvement of the performance of this modified platform as a function of the location of the clump masses has been examined under three regular waves and three irregular waves. In the findings of these examination, it was apparent that attaching clump masses with only one-tenth of the mass of the total mooring-line effectively reduces the wave-induced response. Moreover, there is an obvious improvement as the depth of the location where the clump masses mounted is increased.

scholarly journals Proposal of a Novel Semi-Submersible Floating Wind Turbine Platform Composed of Inclined Columns and Multi-Segmented Mooring Lines

Energies ◽  
2019 ◽  
Vol 12 (9) ◽  
pp. 1809 ◽  
Author(s):  
Zhenqing Liu ◽  
Qingsong Zhou ◽  
Yuangang Tu ◽  
Wei Wang ◽  
Xugang Hua
Keyword(s):  
Wind Turbine ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Analysis Tool ◽  
Mooring Lines ◽  
Offshore Area ◽  
Floating Wind Turbine ◽  
Inclined Angle ◽  
Heave Motion

The semi-submersible floating offshore wind turbine has been studied in detail due to its good stability. However, the occurrence of typhoons are very frequent in China’s offshore area, putting forward a higher requirement for the stability of the floating wind turbine system. By changing the connection mode of the mooring line as well as the structural form of the platform based on the original OC4 model, two groups of models were examined by an in-house developed code named as the Analysis Tool of Floating Wind Turbine (AFWT). The influence of the arrangement of the mooring lines and the inclination angle of the upper columns on the motion response were clarified. It was found that the surge motion of the platform would be obviously decreased by decreasing the length of the upper segments of the mooring lines, while the heave motion of the platform would be significantly decreased as increasing the inclined angle of the columns. Therefore, a new model integrating the optimized multi-segmented mooring lines and the optimized inclined columns was proposed. The examinations showed that compared with the response motions of the original OC4 semi-submersible model, the proposed model could reduce both the surge and heave motions of the platform effectively.

Snap Loads on Mooring Lines of a Floating Offshore Wind Turbine Structure

2014 ◽  
Author(s):  
Wei-ting Hsu ◽  
Krish P. Thiagarajan ◽  
Matthew Hall ◽  
Michael MacNicoll ◽  
Richard Akers
Keyword(s):  
Wind Turbine ◽  
Impact Load ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Special Focus ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbine ◽  
Marine Applications

There are a number of design challenges facing mooring systems of floating offshore wind turbine (FOWT) platforms in an offshore environment. Some unique aspects of the FOWT industry should be considered when examining applicability of established offshore mooring practices. Important among these are: economy and cost effectiveness; light weight minimal platforms; and water depths ranging from 50–300 m. A lighter displacement platform in shallow water, supported by lines with light to moderate pre-tension can result in a higher probability of slack line events and hence snap loads during re-engagement. Such loads can result in shock on the line material and considerably reduce the fatigue life. Such events have the potential to occur in various sea states, and not necessarily limited to extreme conditions. These conditions will be dependent on structure resonant motions, which are influenced by wind loads and moments, wave conditions and mooring line properties. Model tests of typical concepts for FOWT reported in literature have shown occasional slack line episodes. This paper is a review of literature on snap load occurrence in marine applications, including lifting and lowering operations, ROV and diving bell operations. This paper presents a case study of a FOWT. Special focus is on mooring systems which are affected by impact load conditions. Criteria are reviewed and consequences are documented.

scholarly journals DeepCwind Semi-Submersible Floating Offshore Wind Turbine Platform with Nonlinear Multi-Segment Catenary Mooring Line and Intermediate Buoy

2021 ◽  
Author(s):  
mohammad motallebi ◽  
Hassan Ghassemi
Keyword(s):  
Adverse Effects ◽  
Wind Turbine ◽  
Water Depth ◽  
Offshore Wind ◽  
Correct Selection ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbine ◽  
Selection Of

In this paper, with the purpose of improving the mechanical behavior of DeepCwind semi-submersible floating offshore wind turbine (FOWT) platform mooring lines, nonlinear catenary cables of platform are divided into multi-segments and intermediate buoy. The mathematical formulations of the dynamic equation acted on the cable with buoys are described. Present study is employed to the OC4-DeepCwind semi-submersible FOWT platform. It is designed for 200-meter water depth with mooring lines consist of three catenary steel chain cables that have an angle of 120 degrees to each other. The dynamic response of multi-segment catenary mooring line with different buoys radiuses and different positions along the cables were investigated. The full-scale platform was modeled in ANSYS-AQWA software and the simulations are performed in harsh offshore. The tension, strain, anchor uplift, cable uplift for different buoy radiuses and its position along cable are presented and discussed. Moreover, platform motions at three directions (surge, heave and pitch) are also analyzed. It is concluded that by correct selection of the buoy volume and position along cable, the tension of the cable may be reduced up to 45%. By incorrect selection of the buoy, the results will cause adverse effects.

Prediction of Extreme Tensions in Mooring Lines of a Floating Offshore Wind Turbine in a 100-Year Storm

2015 ◽  
Author(s):  
Wei-ting Hsu ◽  
Krish P. Thiagarajan ◽  
Michael MacNicoll ◽  
Richard Akers
Keyword(s):  
Numerical Simulation ◽  
Numerical Simulations ◽  
Wind Turbine ◽  
Test Data ◽  
Green Energy ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Mooring System ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbines

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.

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