Method for operating offshore wind turbine plants based on the frequency of their towers

[0001] Wind power plants, which at present are largely installed on land, are founded on foundation soil of varying strength. If the soil has an adequate bearing capacity flat foundations of concrete are adequate for static and dynamic requirements. If the surface area has inadequate bearing capacities, piles are introduced into the underlying load bearing layers in order to introduce into the soil the wind power plant loads. A design criterion for the structural dimensioning of the tower and foundation parts are the lowest tower natural bending frequencies.

[0002] The exciting frequencies of the rotor must in operation of the plant always have a certain spacing from the aforementioned tower natural frequencies, because otherwise there are dynamic superelevations of the structural loading leading to premature component fatigue failure. The exciting frequencies are the rotor speed and the blade multiple thereof. These dynamic superelevations as a result of resonances must be avoided, in order to achieve the intended mathematical service life of the load-transferring components of a wind power plant. Thus, through the structural dimensioning of the tower and the foundation of land-supported plants, the initial tower natural frequency is conventionally interpreted in such a way that under all operating conditions the exciting frequency must be adequately spaced from the tower natural frequencies.

[0003] In the mathematical interpretation of the necessary natural frequencies account must be taken of the characteristics of the surrounding soil. These soil characteristics influence the rigidity of the foundation fixing and therefore the natural frequencies. In the case of land-supported plants in a first approximation there is no change over a period of time in the fixing rigidities of the foundation. Thus, the natural frequencies of the plant also remain roughly constant over the service life.

[0004] EP 244 341 A1 refers to the occurrence of resonances, which arise on reaching a specific rotor speed. It is proposed that this range be avoided by rapidly passing through it.

[0005] In offshore plants anchored by one or more piles on the ocean bed, as a result of flow changes around the foundation structure, tidal currents or strong wave movements, the piles are flushed out to a greater or lesser extent.

[0006] This phenomenon, known as erosion, has the consequence of the fixing rigidity of the pile changing and consequently so does the tower natural frequency. In addition, the dynamics of the plant also lead to a change in the ocean bed surrounding the pile and therefore to a change in the natural frequency. Moreover, in an offshore wind park, the soil conditions differ at each plant location. As the foundation parts are to have the same construction for cost reasons, the fixing rigidity and therefore the natural frequency differ for each plant. These changes and differences cannot be calculated in advance, are of an arbitrary nature, differ for each plant and are permanently variable over a period of time. As a result, with varying natural frequencies, the plants can be subject to superelevated operating strength loads and premature failure occurs.

[0007] The problem of the invention is to avoid premature failure of the plant, even in the case of plants with varying natural frequencies.

[0008] According to the invention this problem is solved by the features of the single claim. The critical natural frequencies of the plant are constantly determined, changes to the fixing strength are recognized and the forbidden resonance range is correspondingly displaced.

[0009] The critical natural frequency is preferably the natural bending frequency of the overall plant, but can also be the natural frequency of e.g. in particular the rotor blades.

[0010] The critical natural bending frequency is preferably permanently determined using acceleration sensors, strain gauges or path sensors.

[0011] The invention is explained hereinafter relative to the drawings, wherein show:

[0012] FIGS. 1a, 1b & 1c Views of an offshore wind power plant with erosion increasing from a to c.

[0013] FIG. 2 The exciting frequency as a function of the speed and the forbidden speed range as a function of the critical natural frequency of the plants.

[0014] FIG. 3A representation of the dynamic superelevation as a function of the ratio of the exciting frequency to the plant natural frequency.

[0015] FIG. 1a shows the deflection of an offshore wind power plant, where there has been no foundation erosion.

[0016] On reaching the natural bending frequency f1 there is only a relatively limited tower bending. When erosion starts (FIG. 1b) the deflection is more pronounced and the natural frequency f2 is lower than the natural frequency in the case shown in FIG. 1a. In FIG. 1c the erosion is clearly greater, the natural frequency f3 being lower than in the state shown in FIG. 1b and correspondingly the deflection is greater.

[0017] FIG. 2 shows the forbidden speed ranges of a plant as a function of the varying critical natural frequencies f₁ and f₂.

[0018] To avoid the plant being operated in the resonant frequency range, the critical natural frequency of the plant is determined, as is the speed of the rotor where the plant is excited in its critical natural frequency range. This speed range is avoided during the operation of the wind power plant by operating above or below said critical speed range and, if necessary, there is a rapid passage through the critical speed range.

[0019] FIG. 3 shows the dynamic superelevation, which is brought about on reaching given rotor frequencies as a ratio of the exciting frequency fR to the natural frequency f. If this ratio is close to 1, the forbidden range is reached and must be passed through rapidly (a superelevation of the dynamic loads by 10% leads to a 50% service life reduction).

[0020] The natural bending frequency can be determined by acceleration sensors, strain gauges or path sensors. It is important that it is regularly determined, because it changes over a period of time and in particular as a function of increasing erosion.

[0021] It is obvious that the critical natural frequency need not only be that of the overall plant, but also important plant parts and more especially the rotor blades.