Study of Stress-Monitoring Method in Supporting Structure of Floating Offshore Wind Turbine
- Shunka C. Hirao (National Maritime Research Institute) | Yosuke Anai (National Maritime Research Institute) | Shigesuke Ishida (National Maritime Research Institute) | Shunji Inoue (National Maritime Research Institute)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- September 2016
- Document Type
- Journal Paper
- 219 - 224
- 2016. The International Society of Offshore and Polar Engineers
- fatigue life, motion, Monitoring, supporting structure, load, stress, floating offshore wind turbine
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- 33 since 2007
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This paper presents a new stress-monitoring method in the supporting structure of a floating offshore wind turbine (FOWT). Though monitoring many points is preferable when making the maintenance plan and residual fatigue life prediction of the structure, measurement of these quantities is not realistic. Instead of direct measurement, the authors propose a method of estimation from motions that are easier to measure. The feasibility of the method was confirmed by a tank test with a spar-type FOWT.
One of the problems with floating offshore wind turbines (FOWTs) is accessibility. To reduce the life cycle cost, maintenance work should be minimized. One solution for this problem is monitoring. As for structure, if load and stress are monitored with enough density in time and space, maintenance work can be conducted with a reasonable plan based on the residual fatigue life prediction. However, measurement of stress (strain) at many points of the floater, even if it is concentrated on the key points, is not so easy. This paper proposes a simple monitoring method using the motion data and the transfer function between motion and stress.
In this paper, the linearity among motion, load, stress, and strain is assumed. The relation between load and motion can be calculated by a hydrodynamic code or by tank tests. The relation between these two quantities and stress (strain) can be evaluated by structural analysis, e.g., the finite element method (FEM). Based on these assumptions and procedures, the authors propose a practical method of monitoring the internal forces of the structure from the motions of the floater.
Comparison between this monitoring method and others is necessary. There are two patterns of common monitoring methods with these kinds of measured items. Table 1 shows three monitoring methods and Fig. 1 shows the flow of monitoring.There are two or more problems with method 1. It is difficult to measure stress with many strain gauges on many monitoring points, and the life of strain gauges is too short for the lifetime of the structure. The suggested method was developed to settle these problems. With the suggested method, it is possible to measure the stress on monitoring points with only the motion data of the floater. It is easy to measure the motion (e.g., by GPS and gyro) of the floater in the long term.
|File Size||3 MB||Number of Pages||6|