Dynamic response and fatigue damage analysis of offshore wind turbines supported by four-pile jacket in clays under typhoons

Wang G., Lu D., Gao Y., Xie Z., Du X.

Wang P., Wang B., Cheng X., Zhao M., Du X.

Wang B., Wang P., Zhao M., Du X.

Wang L., Zhang T., Cheng X., Xu W., Shen Z., Rui S., Tian Y.

Cheng X., Li M., Ma C., El Naggar M.H., Wang P., Sun X.

Tripod pile foundation is pursued as an efficient foundation system for offshore wind turbines (OWTs) installed in deeper waters (20–50m). In this application, the foundations would be subjected to dynamic loads including due to wind, waves, and earthquakes. This paper presents a numerical method for analyzing the dynamic responses of tripod pile foundation installed in clay based on a simplified bounding surface model to capture the clay stiffness degradation. Their behaviors under lateral monotonic, cyclic and seismic loads were investigated, and their bearing mechanism was analyzed. The results revealed that the loading direction significantly affects the ultimate bearing capacity of tripod pile foundation as the foundation capacity results from either a Pile A in tension and a Pile B in compression or vice versa. The evolution of axial force, bending moment and lateral displacement profiles of the Pile A and Pile B with the number of cycles exhibit different characteristics under lateral one-way and two-way cyclic loading. The foundation experiences cumulative rotation angles toward the Pile A side under seismic load due to the lower vertical bearing capacity of the Pile A compared to the Pile B. The tower top experiences the maximum lateral displacement and rotation angle, and the top of tripod support experiences the maximum bending moment. These findings should be considered in the design of OWT tripod pile foundations.

Cheng X., El Naggar M.H., Lu D., Wang P., Tu W.

The p-y method as a simplified analysis tool has been widely used to analyze the behavior of laterally loaded piles. This paper develops a novel cyclic p-y elastoplastic model within the framework of the single-surface bounding surface theory. The model can capture the soil stiffness degradation during cyclic loading by incorporating the cumulative plastic displacement to an interpolation function of the elastoplastic resistance coefficient. The model is relatively simple with only four parameters that can be determined from standard soil properties and stress-strain responses measured in direct simple shear (DSS) tests. The performance of developed model is validated by predicting the cyclic lateral response of piles installed in soft clay during field and centrifuge tests published in the literature. The model can reliably simulate monotonic and cyclic responses of piles under different lateral loading patterns, and capture main characteristics of the pile head load-displacement curve, such as nonlinearity, hysteresis, displacement accumulation and stiffness degradation. It can also predict the evolution of the lateral deflection and sectional bending moment along the pile during cyclic loading.

Qin M., Shi W., Chai W., Fu X., Li L., Li X.

Offshore wind is becoming the way forward for green energy harnessing worldwide. However, frequent typhoons are a major constraint for the development of offshore wind power for some regions in Asia and North America. Typhoons may pose a huge challenge to offshore wind farm development in southern China. In this paper, a fully-coupled analysis was carried out for a 10 MW large-scale monopile offshore wind turbine (OWT) using SIMO-Riflex-Aerodyn (SRA) code. The response characteristics of the OWTs in different typhoon regions are investigated based on the measured typhoon conditions in China. The effect of aerodynamic damping on the response and the load effect is analyzed in detail. Two different distribution methods are used to statistically extrapolate the response value and get the short-term extreme response. Cumulative linear fatigue damage is evaluated by the rain-flow counting method to explore the possible failure modes of large wind turbines during typhoons. The results show that aerodynamic loads play an important role in large monopile OWTs during high wind speeds in parked conditions. The extreme response and fatigue analyses from this study indicate that fatigue is a dominant failure mode for the large OWT tower during typhoons, while buckling is unlikely.

Ma H., Lu Z., Li Y., Chen C., Yang J.

In order to study the effect of typhoons on the accumulated deformation of monopile foundations for offshore wind turbines, a series of 1-g laboratory model tests with a geometrical scale of 1:100 were carried out. Through the horizontal static and cyclic loading tests of a stiff pile embedded in a medium dense sand deposit, the relationship between the accumulated rotation of the pile and the number of loading cycles under different loading conditions was obtained. The results show that the final accumulated rotation is mainly caused by the typhoon load series and is not affected by the loading sequence. Based on these results, a method is presented to predict the accumulated rotation of the monopile foundation during its service life, and a case study of a 6 MW wind turbine supported by a monopile at a water depth of 30 m in sand is conducted by using the method. The results show that the permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons and the contribution of cyclic loading with small amplitudes can be ignored. • A series of model tests have been carried out to investigate the impact of typhoons on the accumulated deformation of monopile of offshore wind turbines. • Typhoon loads can cause a sharp rise in the accumulated rotation, and the final accumulated rotation is mainly caused by the typhoon load series. • The loading sequence has little effect on the accumulated rotation of the monopile. • The permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons.

Cheng X., Wang T., Zhang J., Liu Z., Cheng W.

It is very important to analyze the lateral cyclic response of monopiles for the design of offshore wind turbines (OWTs). A three-dimensional finite element method for lateral cyclic responses of large-diameter monopiles is developed based on a simplified constitutive model of clays that has been successfully encoded into the ABAQUS software package. The applicability of this method is validated by simulating the existing centrifugal model test. The method can predict the nonlinear hysteresis responses of monopiles in clays during cyclic loading, and the evolution of bending moment and lateral deflection profile of monopile with loading cycles. Various lateral cyclic loads with different loading patterns including one-way and two-way loading, symmetric and asymmetric loading, variable-amplitude and constant-amplitude loading are applied to monopiles. The impact of different cyclic loading patterns on lateral responses of large-diameter monopiles is systematically investigated. The research results can provide some reference for the engineering design of large-diameter monopiles under lateral cyclic loads.

Davenport A.G.

Loading of aerodynamically stable suspension bridge by storm winds is considered in 2 parts, viz, static wind loading due to steady component of wind and fluctuating loading due to vertical and horizontal components of gustiness; response to fluctuating loads is determined using statistical concepts of stationary time series; pertinent data on wind structure required in estimating response are suggested; 3300 ft span suspension bridge is analyzed as example; dynamic effects of vertical and horizontal gusts are highly significant.

Cheng X., Du X., Lu D., Ma C., Wang P.

A simple single bounding surface constitutive model is developed to predict the undrained behaviours of saturated clays under cyclic loads. The new model does not involve complex kinematic hardening rules, and it is only required to memorize important stress reverse events; therefore, the simplicity should be the largest advantage of the model. A new interpolation function of an elastoplastic shear modulus is proposed based on bounding surface theories. The evolution of a hardening modulus is described in the deviatoric stress space by the movement and updating of a mapping centre based on the new interpolation function, which enables the model to describe the stress-strain hysteretic responses of clays under cyclic loading. The new model can be regarded as an improvement of some classical one-dimensional soil dynamic models and a generalization in three-dimensional stress space. The model parameters can usually be determined by performing triaxial tests. The model performance has been verified by a comparative analysis on clays subjected to one-way and two-way cyclic loading at different stress levels. The developed model can capture the essential features of behaviours in saturated clay, including reverse plastic flow, evolution of hysteretic loops, accumulation of plastic deformations and soil stiffness degradation. The newly developed constitutive model has been successfully encoded into the ABAQUS software package by the secondary development interface of UMAT. The ability of the model to calculate boundary value problems, such as clay foundations subjected to seismic loads, has been verified to some extent by simulating the seismic responses of homogeneous horizontal sites. • A simple single bounding surface model was developed to predict undrained cyclic behaviors of clays. • The new model can capture the reverse plastic flow and the soil stiffness degradation. • The new model has been successfully encoded into ABAQUS software. • Ability of the model to calculate responses of clay foundations under seismic loads was verified.

Cao G., Chen Z., Wang C., Ding X.

The existing p - y method that is capable of considering soil-structure interaction (SSI) might not be appropriate for large diameter piles widely used in offshore wind turbine structures. In addition, a simplified way is often adopted for the dynamic analysis of OWT, which simplifies or ignore the SSI, blades, ocean environmental loads and operational conditions. Considering these, a new finite element model (FEM) is described for monopile-supported offshore wind turbines, including components of soil reactions, wind and wave combinations, monopile-tower-blades system. This model allows us to simulate the interaction between the large diameter pile and its surrounding soil and reveals more real dynamic responses of OWT under stochastic ocean environmental loads. This paper discusses the difference of the dynamic responses between the OWT model with traditional p-y method and the new model, the numerical analysis shows that traditional p-y method overestimates the top displacements of OWT under stochastic wind and wave loads. Based on the proposed model, the effects of wind-wave combination, wind-wave misalignment angle and operational conditions on the structural responses are further investigated. The combination of 90° wind-wave angle and operating condition is found to be the most unfavorable situation. • A new FE model is described for OWT, and can reveal more real dynamic responses of OWT supported by large diameter pile. • Compared with a new model, the traditional p-y method will overestimate top displacements of OWT under wind and wave loads. • The combination of the 90°wind-wave angle and operating condition should be considered for a safer design of OWT.

Wang H., Ke S.T., Wang T.G., Zhu S.Y.

The typhoon-induced vibration characteristics of large wind turbines are significantly different in different travelling stages of typhoons due to the structural complexity of typhoons. Influences of multi-stage typhoon-induced effects on structural safety of wind turbines have not been studied yet. The objective of this paper is to investigate the vibration characteristics of wind turbines in different stages of the typhoon as well as the influencing rules of the structural design standards. For this purpose, a framework was established for predicting multi-stage typhoon-induced effects of large wind turbines, which includes a new typhoon-induced multi-stage wind field simulation method and an advanced multi-body model for large wind turbines. On this basis, aerodynamic loads and dynamic response of large wind turbines during different travelling stages of typhoon were analyzed systematically based on the blade element momentum, multi-body dynamic methods, spectral analysis and data statistics. The working mechanisms of multi-stage effects on vibration characteristics of the large wind turbine were revealed. Finally, an evaluation method of vibration amplification effects for large wind turbines with considerations to multi-stage effects was established. Research results demonstrate that the proposed method can predict vibration characteristics of large wind turbines considering the multi-stage effects efficiently. The multi-stage typhoon-induced effects can influence the value of peak factor and the extremum of wind-induced force and vibration responses of large wind turbines significantly. Conversely, the wind vibration coefficient of structural design was affected slightly. Instead of using a uniform structural design standard for large wind turbines, the influence rule of multi-stage effects on anti-typhoon safety performance was summarized in this paper.

Wang S., Larsen T.J.

Offshore wind turbines exposed to storm situations are subjected to static and dynamic loads from the same direction over a considerable period of time. Such cyclic loading can potentially result in soil degradation, leading to an undesired permanent rotation of the wind turbine. This paper presents a workflow to predict the permanent accumulated rotation of an offshore monopile wind turbine in sand during an extreme storm event incorporating the use of fully nonlinear irregular waves versus linear waves in current practice. The fully nonlinear irregular waves are realized from a potential flow solver OceanWave3D previously validated at up to near-breaking wave conditions. Given the wave kinematics, the aero-hydro-elastic code HAWC2 is used to calculate horizontal loading and bending moment acting on the embedded pile head. The irregular load series is then decomposed into a set of constant-amplitude load parcels using rainflow counting. Eventually, the permanent accumulated rotation is predicated using the method proposed by LeBlanc et al. (2010b) with Miner’s rule-based superposition. In this paper, a case study of the DTU 10MW wind turbine supported by a monopile at 33 m water depth in sand is presented, where the pile is primarily laterally loaded. The simulation results suggest the importance of taking accumulated rotation into design. The permanent accumulated rotation is primarily decided by soil capacity, loading characteristics and pre-loading history. Furthermore, the results show that wave nonlinearity has only limited influence on the permanent accumulated rotation.

Zuo H., Bi K., Hao H.

In the dynamic analyses of offshore wind turbines subjected to the external vibration sources, the wind turbines are normally assumed in the parked condition and the blades are considered by a lumped mass located at the top of the tower. In reality, the geometrical characteristics and rotational velocity of the blades can directly influence the wind loads acting on the blades. Moreover, the centrifugal stiffness generated by the rotating blades can increase the stiffness and natural frequencies of the blades, which in turn can further affect the structural responses. The lumped mass model, therefore, may lead to inaccurate structural response estimations. On the other hand, monopile, a long hollow steel member inserting into the water and sea bed, is generally designed as the foundation of an offshore wind turbine. The soil-monopile interaction can further alter the vibration characteristics and dynamic responses of offshore wind turbines. In the present study, the dynamic responses of the modern NREL 5 MW wind turbine subjected to the combined wind and sea wave loadings are numerically investigated by using the finite element code ABAQUS. The blades are explicitly modelled and soil-structure interaction (SSI) is considered. The influences of operational condition and rotor velocity on the dynamic behaviours are systematically investigated. It is found that the responses of the wind turbine in the operating condition are much larger than those in the parked condition; SSI can affect the tower vibrations substantially, while it has a negligible effect on the in-plane vibrations of the blades.