| Offshore wind power has become the first option of new power systems for its abundant resources,less environmental pollution and sustainable use.As offshore wind turbines(OWTs)develop towards the trend of far-reaching sea,taller tower,larger diameter and larger capacity,it is crucial to choose appropriate foundation type and installation technology.Bucket foundation,as one of the most promising supporting structure,holds merits such as strong anti-overturning ability and integrated structure transportation,offering an efficient technique of low-cost and large-scale construction of OWTs.Due to exposure to adverse marine environment and complex loading excitations,the towering wind turbine structure may vibrate greatly during the integrated structure transportation,resulting in an increased risk of instability.Also,abnormal vibration of the structure during the service period may cause damage to the blades,tower barrel and foundation,adversely affecting the operation safety.Determination of the loading information can not only realize online health monitoring,but also provide quantitative parameters for safety assessment and structural optimization design of OWTs.Therefore,it is necessary to carry out load identification of whole life cycle of OWTs.However,load identification during the integrated structure transportation has not been investigated,and the load identification method during the service period has problems of discomfort and accumulated error.The quantitative relationship between the actual load bearing of the structure and the design load is not clear.In this paper,through theoretical derivation,model test,numerical simulation and prototype observation,the load identification research of OWTs supported by bucket foundation was conducted.The vibration of the identification load for the whole life cycle was systematically analyzed.Additionally,a safety evaluation system based on the concept of safety margin was constructed,and the structure optimization design was carried out.(1)A load identification method for the whole life cycle of OWTs was proposed.The multipoint strain method was used to identify the loading acting on the integrated structure during construction,and the uniform arrangement of strain measuring points was suggested,which can efficiently enhance the precision of load identification based on strain in practical engineering.A physical model was utilized to verify the accuracy of the multipoint strain load identification method.Five-point finite difference time-domain method based on wavelet orthogonal operator was proposed for dynamic loading identification of in-service structures,which can overcome the problem of the traditional method that is sensitive dependence on initial conditions and the cumulative error caused by the iterative recursive problem.The improved method was verified with a spring-mass damping model.Compared with the traditional time-domain method,the proposed method has better robustness and noise resistance.Then,using the FAST software,the applicability and accuracy of the proposed approach to operational load identification of OWTs were demonstrated.Therefore,the above method can provide a new accurate,stable and reliable method for load identification of OWTs during construction and service period.(2)Based on prototype observation data during construction,the research on vibration characteristics and load identification of the coupling system of "erection ship-foundation-tower-wind turbine" was carried out,which made up the blank of the research on the integrated transportation of OWTs at home and abroad.Firstly,the vibration behavior of the integrated structure was analyzed.Vibration of a single integral structure during the transportation presented typical forced vibration,and the wave height was the major cause.Both wave height and wind would affect the vibration response of double integrated structures transportation,but the wave height was still the major cause.Under the same wave height,the acceleration at the tower top of a single structure was 3.5 times that of the double structures,which means that the former was more sensitive to the wave height.Secondly,the bending moment of the bottom section of the tower under different working conditions was calculated using multipoint strains.The relationship between the bending moment of the tower bottom section and the environmental factors,as well as its variation with the environmental factors were summarized.The maximum bending moments borne by single and double turbine accounted for 68.9% and 6% of the design load,respectively.Lastly,the integrated transportation proved to be safe,reliable,and worth further promotion.The double-structure transportation was not sensitive to the change of wave height.From the perspective of construction safety,it was recommended to use the transport mode of double turbines under the sea condition with frequent changes of wave height.(3)To make clear the variation of load magnitude borne by the bucket foundation under different operational conditions in service period and the proportion of load components,research on the dynamic characteristics and load composition of offshore wind turbine during operation was carried out.First,the wavelet orthogonal operator five-point difference method was used to identify the dynamic load,and the dynamic load was superimposed with the average load to obtain the total operating load in service period.The operating load showed a non-monotonic variation trend with the environmental factors.The identification load reached the maximum near the rated wind speed,and the maximum identification load in abnormal shutdown was 1.3times of that in normal shutdown.Secondly,the fluctuating load coefficient was put forward,and the variation of fluctuating load coefficient with environmental factors was expounded.The rotor speed changes with the wind speed constantly,and the 3P was close to the natural frequency,and the fluctuating load coefficient fluctuated between 0.2 and 0.5 in the range of 3-7 m/s.Finally,variational mode decomposition was introduced to identify dynamic load of fluctuating wind load,1P and 3P load,revealing the load components and the change in their proportion with operating parameters.In the rated operation stage,three load components accounted for a balanced proportion.In the moderate speed stage,the fluctuating wind load and 3P load were outstanding.Fluctuating wind load took up the largest proportion in the shutdown.(4)The concept of safety margin of OWTs was put forward,and the safety margin of OWTs during construction and service period was studied based on the load identification.The feedback safety margin interval during construction was 1.5～1.86.Based on the safety margin,the safety evaluation system of the integrated structure transportation of the bucket foundation during the construction period was established,and the control standards for the transportation plan and the adjustment of transportation strategy were clearly formulated.The integrated transportation should be carried out under the wave height of no more than 2 m to ensure the safety and stability.For the service period,the relationship between the actual load and the design load was quantified,and the particle swarm optimization algorithm based on the safety margin was performed to optimize the bucket foundation.Through optimization,the amount of steel was reduced by 11.6%,the amount of concrete and rebar was reduced by 7.6% and 7.4%,and the total cost of a single foundation was reduced by at least 465,500 yuan.Finally,the sensitivity of design parameters to foundation stability,stiffness,material consumption and cost was discussed.Compared with the skirt length,the diameter had greater influence on the structural tilt rate,stiffness and cost.Thus,the key points of safety assessment and the direction of OWTs optimization design were clarified. |
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