Method for controlling oscillation absorption stand for doppler lidar

Abstract

To provide a method for controlling an oscillation absorption stand for a Doppler Lidar, capable of reducing oscillation of a Doppler Lidar itself to realize highly accurate wind condition observations.SOLUTION: In a control method for reducing oscillation of an oscillation absorption stand that is installed on an offshore platform and equipped with a Doppler Lidar for observing a wind condition, the oscillation absorption stand is installed so as to be able to oscillate relative to the offshore platform. The control method controls the oscillation of the oscillation absorption stand by applying a control torque to the oscillation absorption stand, using a variable damper.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for controlling a motion absorbing stand for a Doppler LIDAR for observing wind conditions on the ocean. [Background technology]

[0002] The installation of a wind power generation facility requires one year of observation data to thoroughly investigate wind conditions at the proposed installation site, and wind observation towers are used for much of this measurement. As wind observation projects expand, plans for offshore wind power generation facilities are currently progressing rapidly along the coast of Japan, but if the installation areas continue to expand in the future, there are concerns that the waters will become deeper and that the foundation work for installing the wind observation facilities will become more difficult.

[0003] As an observation method that is easy to install and has little impact on the environment, a method of mounting a laser-type wind direction and speed meter (Doppler LIDAR) on a floating platform is being considered. However, this method requires that the motion (sway and tilt) of the floating platform caused by tides, wind, and waves affect the measurement accuracy of the Doppler LIDAR, and it is therefore necessary to reduce these motions. Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the above-mentioned method, the motions (sway and tilt) of the floating platform due to tides, wind, and waves affect the measurement accuracy of the Doppler LIDAR, so it is necessary to reduce these motions.

[0005] The present invention has been made in response to such demands, and has an object to provide a control method for a motion absorbing stand for a Doppler LIDAR that can reduce the vibration of the Doppler LIDAR itself and enable highly accurate wind condition observation. [Means for solving the problem]

[0006] The object of the present invention is achieved by a control method for reducing the vibration of a vibration absorption base that is installed on an offshore platform and equipped with a Doppler LIDAR for observing wind conditions, the vibration absorption base being installed so as to be able to swing relative to the offshore platform, and the control method controlling the vibration of the vibration absorption base by applying a control torque to the vibration absorption base by a variable damper. Effect of the Invention

[0007] According to the present invention, it is possible to provide a control method for a motion absorbing stand for a Doppler LIDAR that can reduce the vibration of the Doppler LIDAR itself and achieve highly accurate wind condition observation. [Brief description of the drawings]

[0008] [Figure 1] 1 is an image of an actual spar buoy with attitude stabilization device on which the motion absorption stand according to the present invention is mounted, and a schematic diagram thereof. [Diagram 2] Left: Schematic diagram of the spar buoy and motion absorption platform, and right: Model of the single-pendulum type motion absorption platform (including Doppler LIDAR) simplified to a single pendulum. [Diagram 3] This is a model related to the skyhook damper theory. [Figure 4] Simplified model of a single pendulum type motion absorber with a skyhook damper (left) and an active damper controlled by the skyhook damper theory (right). [Diagram 5] This is a time series data of the behavior of a single pendulum type motion absorber with a linear damper induced by the sinusoidal motion of the spar buoy. [Figure 6] This is a time series data of the behavior caused by sinusoidal motion of a spar buoy of a single pendulum type motion absorber with an active damper controlled by the skyhook damper theory. [Figure 7] 13 is a graph showing the motion amplitude magnification of a simple pendulum type motion absorber stand. [Figure 8]This is a time series data of the behavior caused by the random motion of a spar buoy of a single pendulum type motion absorber with a linear damper or an active damper controlled by the skyhook damper theory. [Figure 9] This is a comparison diagram of simplified models of a single pendulum type motion absorption platform (left) and a counterweight type motion absorption platform (right). [Figure 10] This is a time series data of the behavior induced by sinusoidal motion of a spar buoy of a counterweight type motion absorber with linear or active dampers controlled by the skyhook damper theory. [Figure 11] 13 is a graph showing the motion amplitude magnification of a counterweight type motion absorption platform. [Figure 12] 1 is a graph showing the vibration amplitude of a counterweight type vibration absorbing stand. [Figure 13] This is a time series data of the behavior of a spar buoy of a counterweight type motion absorber with an active damper controlled by the skyhook damper theory due to irregular motion. [Figure 14] This is time series data of the behavior of a spar buoy of a non-rebounding counterweight type motion absorber equipped with a linear damper based on the skyhook damper theory or an active damper and an active spring based on the skyhook spring theory, due to irregular motion. [Figure 15] This is a simplified model of a counterweight type motion absorption stand mounted on a discus type buoy. [Figure 16] This is time series data on the behavior of a discus-type buoy in irregular waves. [Figure 17] This is time series data of the behavior of a non-rebound counterweight type motion absorber equipped with a linear damper based on the skyhook damper theory or an active damper and an active spring based on the skyhook spring theory, caused by the irregular motion of a discus type buoy. [Figure 18]This is time series data of the behavior of a non-rebounding counterweight type motion absorber with an active damper and spring based on the skyhook theory at a control frequency different from that in Figure 17 due to the irregular motion of a discus-type buoy shown in Figure 16. [Figure 19] Figure 18 shows the time series data of the behavior of a non-rebound counterweight-type motion absorber with an active damper and spring based on the skyhook theory with a control frequency of 1 Hz and a control delay of one step. [Figure 20] The time series data in Figure 17 is time series data of the damping torque and restoring torque of a non-returning counterweight type motion absorber equipped with an active damper and spring based on the Skyhook theory. [Figure 21] This is time series data on the behavior of a non-rebounding counterweight-type motion absorber equipped with an active damper and spring based on the Skyhook theory in rough seas (significant wave height 9m, significant wave period 12 seconds). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The control method for the motion absorption mount for a Doppler LIDAR according to the present invention will be described below. The motion absorption mount for a Doppler LIDAR according to the present invention is broadly divided into two types: a single pendulum type motion absorption mount and a counterweight type motion absorption mount which is an improved version of the single pendulum type.

[0010] First, we will explain the single pendulum type motion absorption stand. Figure 1 shows an image of the actual spar buoy with attitude stabilization device and its schematic diagram. A Doppler LIDAR is mounted on the upper structure of the actual spar buoy, and in order to reduce the shaking of this Doppler LIDAR itself, the Doppler LIDAR is placed on top of the single pendulum type motion absorption stand and then mounted on the upper structure.

[0011] The left side of Figure 2 shows an outline of the spar buoy and the motion absorption platform, and the right side shows a model of the single pendulum type motion absorption platform (including the Doppler LIDAR) simplified to a single pendulum. For simplicity, we assume a unidirectional long-crested wave, and only consider the motion in the vertical plane including the wave direction. In the figure, M is the mass of the motion absorption platform including the Doppler LIDAR, l1 is the distance from the mooring point at the bottom end of the spar buoy to the fulcrum of the single pendulum, l2 is the distance from the fulcrum of the single pendulum to the centre of gravity of the motion absorption platform, and Tc is the control torque. θ1 is the vertical sway of the spar buoy, and θ2 is the sway of the motion absorption platform (angle from the vertical above). Note that a state in which the Doppler LIDAR is not swaying means that θ2 = π continues to hold.

[0012] The specifications of the single pendulum type motion absorber are shown in Table 1. In order to avoid synchronization with the external force period, the natural period satisfies the shape constraints and is outside the spar buoy's motion period of 3 to 12 seconds (determined based on the wave data of the sea area where the spar buoy is installed).

[0013] [Table 1]

[0014] The equation of motion for the simple pendulum type motion absorber is explained below. If we assume that the floating platform is sufficiently heavy compared to the motion absorber including the Doppler LIDAR and that the effect of the Doppler LIDAR motion on the floating platform motion can be ignored, then the magnitude of the gravitational acceleration is g, and the equation of motion for the simple pendulum type is given by Equation 1 below. TIFF2025002235000003.tif23165Here, θ1 and its time derivative (θ1') are the input values. By rearranging Equation 1 with respect to θ2" to obtain Equation 2, TIFF2025002235000004.tif38169The equation of motion was solved using the Euler method. The initial values ​​of θ2 and θ2' were set to 0.97π (approximately 5 degrees from the vertical downward direction) and 0.

[0015] The control torque is a linear damper in Eq. 3 and a variable damper according to the skyhook damper theory. TIFF2025002235000005.tif15141Here, c is the linear damping coefficient. The skyhook damper theory is a theory in which an object that receives displacement excitation via a linear spring, as in the model on the left of Figure 3, is suspended (skyhook) from an imaginary ceiling by a linear damper, and a similar damping force is reproduced by the variable damper in the model on the right of Figure 3. Note that ZA and ZB in Figure 3 correspond to θ1 and θ2 in Figure 2, respectively.

[0016] In addition, if we use the mass M, spring constant k, linear damping coefficient csky, variable damping coefficient csemi, vertical displacement ZB of the mass point, and vertical displacement ZA due to displacement excitation in Figure 3, the equation of motion for the left model in Figure 3 is Equation 4, and the equation of motion for the right model is Equation 5. Therefore, to simulate the damping force of Equation 4 using the variable damping coefficient csemi, csemi should be expressed as Equation 6. TIFF2025002235000007.tif41154Note that when (Z'B-Z'A)→0, csemi→∞, so an upper limit must be set for csemi, and that upper limit is csky. Also, when csemi< 0, csemi=0.

[0017] Figure 4 shows the application of the skyhook damper theory to a simple pendulum type. In this figure, instead of the imaginary ceiling in Figure 3, an imaginary vertical wall containing the axis of rotation of the simple pendulum is assumed. The equation of motion for the simple pendulum type is Equation 1, which is obtained by substituting the variable damping coefficient csemi in Equation 7 for the control torque Tc. As can be seen from this equation, the application of the skyhook damper requires measurement of θ2″. TIFF2025002235000008.tif24169

[0018] First, assuming that the pitching motion of the spar buoy is a sinusoidal motion, the motion reduction effect of the motion absorber is confirmed. The pitching amplitude θ1a of the spar buoy is set to 7.5 degrees, and its motion period 2π / ω1 is set to 3 to 12 seconds (determined based on the wave data of the sea area where the spar buoy is installed) in 1 second increments. The calculation time step is set to 0.05 seconds, and the calculation time is set to 50 seconds. The skyhook damper is applied to each step without control delay.

[0019] Figure 5 shows the calculation results when the motion period of the spar buoy is 5 seconds and the linear damping coefficient c is 0, 10, and 50 Nms. When c = 0, the amplitude of the motion absorber is larger than that of the spar buoy, and the components of the motion period of the spar buoy and the natural period of the motion absorber overlap. When the linear damping coefficient is increased, the components of the natural period of the motion absorber almost disappear at c = 50, but the components of the motion period of the spar buoy are almost unchanged. Therefore, the linear damping coefficient csky based on the skyhook damper theory is set to 0, 10, and 50 Nms instead of the linear damping coefficient, and the upper limit of each variable damping coefficient csemi is set to 0, 10, and 50 Nms. The results are almost the same as those of the linear damping coefficient.

[0020] In order to evaluate the validity of the above calculation results, the inclination angle θ2QS of the motion absorber caused by the acceleration acting on the motion absorber at the moment of maximum displacement of the spar buoy under quasi-static assumptions was calculated using Equation 8, and the amplitude magnification was calculated by dividing the amplitude θ2a of the motion absorber by this angle. TIFF2025002235000009.tif29135

[0021] Figure 7 shows the amplitude magnification with ω1 / ω2n on the horizontal axis. ω2n is the natural circular frequency of the motion absorber base shown in Table 1, and the amplitude θ2a on the vertical axis is half the maximum and minimum width of the waveforms in Figures 5 and 6. The natural period of the motion absorber base is shorter than the motion period of the target spar buoy, and since the amplitude magnification is approximately 1, the damping effect is sufficient, and in order to further reduce motion, it is necessary to lengthen the natural period of the motion absorber base.

[0022] Next, the motion reduction effect of the motion absorber was confirmed even when the pitching motion of the spar buoy was irregular. For the pitching motion θ1 of the spar buoy, the results of a tank test using a small similarity model were converted to the actual size according to Froude's law of similarity. The tank test was carried out in the Osaka Municipal University ship test tank, and a spar buoy equipped with a shallow water attitude stabilization device, which was tension-moored at one point, was used. The spectrum of irregular waves was Bretschneider-Mitsuyasu type, and the significant wave height was 7.11 m and the significant wave period was 10.2 seconds converted to the actual size in order to measure the motion in relatively rough sea conditions. In the numerical simulation, the tank test results were converted to the actual size, so the calculation time step was 0.120 seconds and the calculation time was 290 seconds. The skyhook damper was applied to each calculation step without any control delay.

[0023] The calculation results using the linear damping coefficient and the variable damping coefficient are shown in Fig. 8. Although the vibration of the vibration absorber base is smaller by applying a damping force than the result of c = 0, it is still larger than the motion of the spar buoy.

[0024] Based on the results of the study on the single pendulum type motion absorber, we considered lengthening the natural period of the motion absorber. In the case of the single pendulum type, in order to lengthen the natural period, it is necessary to lengthen l2 according to the formula in Table 1. However, even if l2 is set to 30 m, which is equal to the total length of the spar buoy, the natural period is only about 11 seconds, making this difficult to achieve. Therefore, we considered the counterweight type motion absorber shown in Figure 9. As with the single pendulum type, if we consider that the effect of the motion of the motion absorber, including the Doppler LIDAR, on the motion of the floating platform can be ignored, the equation of motion for the counterweight type is expressed as Equation 9, using the letters in Figure 9. TIFF2025002235000010.tif43168, where TIFF2025002235000011.tif28157. Therefore, if we consider the counterweight type to be stable when M2 is at the bottom, the natural period is The result is TIFF2025002235000012.tif28132.

[0025] The main specifications of the counterweight type are shown in Table 2. First, the motion of the spar buoy is assumed to be sinusoidal, and the motion reduction effect of the motion absorber is confirmed. The calculation conditions are the same as those for the case where the vertical motion of the spar part is sinusoidal as described above. The linear damping coefficient c is set to 10 Nms, the linear damping coefficient csky based on the skyhook damper theory is set to 0 and 10 Nms, and the upper limit of the variable damping coefficient csemi is set to 0 and 10 Nms. The calculation results (time series data) are shown in Figure 10, the amplitude magnification is shown in Figure 11, and the motion amplitude is shown in Figure 12. In Figure 10, when csky = 0, the motion period of the spar buoy and the natural period component of the motion absorber overlap, but the half amplitude is about 3 deg. When c = 10 and csky = 10, the natural period component of the motion absorber disappears, and when c = 10, the half amplitude is about 1.5 deg, and when csky = 10, the half amplitude is about 0.5 deg. Also, from Figures 11 and 12, it can be seen that when the motion period is longer at c = 10, the oscillation becomes larger than when csky = 0, but on the other hand, when csky = 10, the amplitude magnification is 0.1 or less and the motion amplitude is also 0.5 deg or less, which is a sufficient effect.

[0026] Furthermore, the motion reduction effect of the motion absorber stand will be confirmed when the pitching motion of the spar buoy is irregular. The calculation conditions are the same as when the pitching motion of the spar section is irregular as described above, and the calculation results are shown in Figure 13. As in the case when the motion of the spar buoy is sinusoidal, when csky = 0, the motion period of the spar buoy and the natural period component of the motion absorber stand overlap, but the half amplitude is about 0.6 deg, and when csky = 10, the natural period component of the motion absorber stand disappears and the half amplitude is about 0.4 deg, confirming a sufficient motion reduction effect.

[0027] [Table 2]

[0028] The effectiveness of the counterweight type motion absorption stand was confirmed in the above. However, it was also found that it is difficult to completely stop motion. The reason for this is that the counterweight type motion absorption stand has a restoring force as part of its mechanism, and so the forcing force of the second term in Equation 9 acts on it. Therefore, we considered setting this restoring force to zero, thereby zeroing the forcing force of the motion absorption stand caused by the movement of the spar buoy. The main details are shown in Table 3.

[0029] [Table 3]

[0030] In this case, if the motion absorber platform is displaced from the vertical downward direction by some disturbance, it cannot return to the vertical downward direction because it has no restoring force. To solve this problem, we decided to measure θ2 at each moment and actively apply a restoring force. The equation of motion in this case is Equation 12. TIFF2025002235000015.tif17166Here, Tc is the damping torque by the linear damper or skyhook damper, and Tk is the restoring torque by the skyhook spring. The skyhook spring is a spring that replaces the variable damping coefficient of the skyhook damper with a variable restoring coefficient and multiplies it by θ2 to provide a restoring force, which can be expressed by Equation 13. TIFF2025002235000016.tif18169Note: The coefficient of stability is the same as above (ksky = 1.31), and the upper limit of ksemi is ksky. In the calculation, the irregular motion data for θ1 was only 290 seconds long, so the irregular motion data was repeatedly connected. In addition, other calculation conditions were the same as when the spar buoy's pitching was sinusoidal motion, and the skyhook damper and skyhook spring were applied to each step without control delay.

[0031] The upper diagram in Figure 14 shows the pitching motion of the spar buoy in irregular waves, the middle diagram shows the calculation results of the motion of the motion absorber base when a linear damper is used, and the lower diagram shows the calculation results of the motion of the motion absorber base when a skyhook damper is used. When Tc is a linear damper, the motion of the motion absorber base does not subside even if the coefficient is increased, but by using a skyhook damper and increasing the coefficient, the motion can be reduced to almost zero.

[0032] Next, we will explain the application of the counterweight type motion absorber that has no restoring force and uses the above-mentioned skyhook spring to a discus type buoy, which is widely used in wave observation buoys, etc. Figure 15 shows an outline diagram and coordinate system of a discus type buoy and a counterweight type, and their main specifications are shown in Table 4. Unlike the above-mentioned spar buoy, a discus type buoy is loosely moored, and even if the motion of a discus type buoy is limited to a vertical plane, it has three degrees of freedom: forward and backward rocking, up and down rocking, and pitching.

[0033] [Table 4]

[0034] As mentioned above, if we assume that the effect of the motion of the motion absorber platform including the Doppler LIDAR on the motion of the floating platform can be ignored, the equation of motion for the counterweight type mounted on the discus type buoy is expressed as Equation 14 using the letters in Figure 15. TIFF2025002235000018.tif58168If there is no restoring force, the above formula 12 is used. Here, Tc is the linear damper or the skyhook damper of formula 7, and Tk is the skyhook spring of formula 13. Note that the application of the skyhook damper and skyhook spring requires the measurement of θ2 and θ2', respectively.

[0035] Figure 16 shows the results of a tank test of a discus-type buoy in irregular waves, converted to the actual size according to Froude's law of similarity. The tank test was conducted in the Osaka Municipal University ship test tank, and a catenary-moored discus-type buoy was used. The irregular wave spectrum was Bretschneider-Mitsuyasu type, and the machine-equivalent significant wave height was 4.5 m and significant wave period was 12 seconds. Since the motion data was short, the data was repeatedly connected, and other calculation conditions were the same (see below for the influence of control frequency, control delay, and wave height on the motion reduction effect, and the possibility of control torque generation). The upper figure of Figure 17 shows the calculation results of the motion of the motion absorber when a linear damper is used, and the lower figure shows the calculation results when a skyhook damper is used. When Tc is a linear damper, the motion of the motion absorber does not subside even if the coefficient is increased, but when it is a skyhook damper, the motion of the motion absorber can be reduced to almost zero by adjusting the coefficient.

[0036] In the above, in order to improve the continuous measurement capability and measurement accuracy of the offshore wind observation system, we considered placing the Doppler LIDAR on a motion absorbing stand to reduce the vibration of the sensor itself, developed a model for it, and confirmed its validity through numerical simulation.

[0037] First, a single pendulum type motion absorber was considered as a motion absorber, and the motion reduction performance was evaluated when this motion absorber was installed on a spar buoy with attitude stabilization device for shallow waters. A linear damper and a skyhook damper were applied to reduce motion, but the motion of the motion absorber in regular and irregular waves was not smaller than the motion of the spar buoy. However, an evaluation based on the amplitude magnification showed that the skyhook damper was effective in reducing motion. The reason for this result is thought to be that the natural period of the motion absorber cannot be made sufficiently long compared to the period of the external force due to the single pendulum type mechanism.

[0038] Next, a counterweight type vibration absorber was considered to lengthen the natural period of the vibration absorber. This counterweight type had a vibration reduction effect compared to the simple pendulum type even without applying a damping force, but the motion amplitude increased in proportion to the vibration frequency. When a linear damper was applied, the vibration was significantly reduced in the high vibration frequency range, but the results were inversely proportional to the vibration frequency. Furthermore, by applying a skyhook damper, the motion amplitude was reduced regardless of the vibration frequency, and a sufficient vibration reduction effect was confirmed even during irregular motion, but it is difficult to completely stop the vibration. Finally, it was confirmed that the vibration could be reduced to almost zero by using a skyhook damper with the restoring force set to zero and applying a skyhook spring to maintain the vertical downward position.

[0039] By applying a skyhook spring and skyhook damper to a counterweight-type motion absorber that has no restoring force, it was confirmed that a sufficient motion reduction effect was achieved even for a discus-type buoy.

[0040] Here, the control frequency of the control torque given to the motion absorption stand is 0.071 seconds (approximately 14 Hz) converted to an actual machine, which may be too high when implemented on an actual machine. Therefore, the motion measurement results of the discus-type buoy in Figure 16 were thinned out to confirm the motion reduction effect when the motion sampling frequency was converted to 4 Hz and 1 Hz equivalent to an actual machine. Other calculation conditions were the same as when a counterweight-type motion absorption stand was applied to a discus-type buoy (ksky = 1.31, csky = 2). From the results shown in Figure 18, it can be confirmed that there is no difference in the motion reduction effect within the range of motion sampling frequencies under consideration.

[0041] In addition, the control delay of the skyhook damper and skyhook spring is not taken into consideration. However, in reality, calculations are performed after the motion sampling, and then the actuator is operated, so a control delay occurs. Therefore, the motion sampling data obtained by thinning out the data from the motion measurement results of the discus-type buoy in Figure 16 and setting the motion sampling frequency to 1 Hz in actual equipment was used to perform calculations by applying a one-step delay to the control torque of the skyhook damper and skyhook spring. It can be seen from Figure 19 that, although the control delay affects the motion reduction effect, a sufficient motion reduction effect can be obtained by increasing the damping coefficient of the skyhook damper csky = 2 from 3.

[0042] Additionally, in order to confirm the feasibility of the motion absorption stand, time series data of the control torque (damping torque and righting torque) for a discus-type buoy without the above-mentioned righting force is shown in Figure 20. From this figure, it can be confirmed that the maximum control torque is less than 0.1 Nm even when both torques are added together, and that this can be achieved with a commercially available DC motor.

[0043] In addition, in the measurement of the motion of the discus-type buoy in irregular waves mentioned above, the significant wave height of the incident waves was set to 4.5 m (significant wave period 12 seconds, Bretschneider-Mitsuyasu type spectrum) due to the wave-making performance of the Osaka Municipal University Ship Testing Basin. However, since this wave height is low for wave heights in rough weather, we made the bold assumption that the motion of the discus-type buoy in irregular waves is proportional to the significant wave height, and calculated the motion at a significant wave height of 9 m. We then applied the same control (ksky = 1.31, csky = 2) to confirm the motion reduction effect. The results shown in Figure 21 show that the motion damping time of the motion absorber is longer than that of Figure 17, but it can be confirmed that the motion reduction effect can be obtained.

Claims

[Claim 1] A control method for reducing vibrations of a vibration absorption platform installed on an offshore platform and equipped with a Doppler LIDAR for observing wind conditions, comprising the steps of: The motion absorbing platform is installed on the offshore platform so as to be capable of swinging thereon, A control method for controlling the vibration of the vibration absorbing stand by applying a control torque to the vibration absorbing stand using a variable damper.

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