a b s t r a c tThe complexity of the loads acting on the offshorewind turbines (OWTs) structures and the significance ofinvestigation on structure dynamics are explained. Test results obtained froma scaledwind turbinemodelare also summarized. Themodel is supported onmonopile, subjected to different types of dynamic loadingusing an innovative out of balance mass system to apply cyclic/dynamic loads. The test results show thenatural frequency of the wind turbine structure increases with the number of cycles, but with a reducedrate of increase with the accumulation of soil strain level. The change is found to be dependent on theshear strain level in the soil next to the pile which matches with the expectations from the element testsof the soil. The test results were plotted in a non-dimensional manner in order to be scaled to predict theprototype consequences using element tests of a soil using resonant column apparatus.  Introduction Harvesting offshore wind energy is a new initia-tive and a promising option for protecting the environment. 38058
Off-shore wind turbines (OWTs) are relatively new structures with nolong term track record of their performance yet they are to beconstructed and meant to produce energy for 25–30 years [1–4].OWTs, due to their slender nature coupledwith irregularmass andstiffness distribution, are dynamically sensitive structures. The firstnatural frequencies of these structures are very close to the forcingfrequencies imposed by the environments and the onboard ma-chinery. Changes of the foundation stiffness under cyclic loadingwill ultimately result in changes to the natural frequency of thestructure. Therefore, the design of foundations and prediction oflong term performance are very challenging. The loading on anOWT is complex and is a combination of static, cyclic, and dynamicloads:(1) the external load produced by the wind and its turbulence,applying approximately one-way cyclic load to the foundation,(2) the external load caused by waves, which is approximatelytwo-way cyclic,(3) the internal load caused by the vibration at the hub level dueto themass and aerodynamic imbalances of the rotor (this load has a frequency equal to the rotational frequency of the rotor (referredto as 1P loading in the literature) and is dynamic in nature),(4) dynamic internal loads on the tower (as shown in Fig. 1) dueto the vibrations caused by blade shadowing effects (referred to as2P/3P in the literature), which is dynamic in nature.Typical natural frequencies of OWTs are in the range of0.3–0.9 Hz [1]. The load frequencies that are close to the naturalfrequency of the turbines can be classified as dynamic load whichrequires special consideration, i.e., the ratio of forcing frequency tonatural frequency (ff/fn) and the damping in the system.A case study Shanghai Donghai Bridge offshore wind farm isone of the first large scale commercial developments [5]. Figure 2shows themain frequencies for a three-bladed 3MWSinovelwindturbine with an operational interval of 8.1–19 RPM (revolutionsper minute).
The 1P lies in the range 0.135–0.316 Hz and thecorresponding 3P lies in the range 0.405–0.948 Hz. The figure alsoshows typical frequency distributions for wind and wave loading.The peak frequency of typical waves is about 0.12 Hz. It is clearfromthe frequency content of the applied loads that the designer ofthe turbine and foundation has to select a systemfrequency whichlies outside this range of frequencies in order to avoid systemresonance and ultimately increased fatigue damage.Three types of designs are possible (see Fig. 2): (1) ‘‘soft–soft’’design, where the target frequency is placed below the 1P freque-ncy range, i.e., less than 0.135 Hz, which is a very flexible structure  and almost impossible to design for a grounded system, (2) ‘‘soft–stiff’’ design, where the target frequency is between 1P and 3Pfrequency ranges and this is the most common in the current off-shore development, (3) ‘‘stiff–stiff’’ design, where target naturalfrequency have a higher natural frequency than the upper limit ofthe 3P band andwill need a very stiff support structure. Det NorskeVeritas (DNV) [6] also specified that the system frequency shouldbe at least ±10% away from operational 1P and 2P/3P frequencies,as indicated by the dotted lines in Fig. 2. Therefore, the availablerange of safe frequency content to place the OWT is narrow.Prediction of OWT’s long term behavior OWTs are subjectedto approximately 107–108cycles of loading in their life timeand two aspects are important with regards to the design offoundations: (1) assessment of the change of soil behavior due toeffect of cycling and its impact on the foundation (this is similar tothe fatigue problem) and (2) dynamic amplification of the responseof the structure over a range of excitation frequencies close tothe natural frequency of the system. This would mean higherdisplacement of the foundation, i.e., higher strain in the soil. Thisis similar to the resonance problem. 单桩海上风力发电机英文文献和中文翻译:http://www.youerw.com/fanyi/lunwen_36963.html