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**8. Acknowledgements** 

**9. References** 


**27** 

*Japan* 

**Application of Wavelet Analysis for the** 

Tomoki Ikoma, Koichi Masuda and Hisaaki Maeda

*Department of Oceanic Architecture and Engineering, College of Science and Technology (CST), Nihon University* 

**Understanding of Vortex-Induced Vibration** 

Oceans are quite important fields for us because many resources lurk there which are oil and gas under seabed, mineral resources, water heat energy and so on. Development of submarine oil has been major in the North Sea and in the Gulf of Mexico. Today, submarine oil has been developed at ultra-deep water fields of offshore of Brazil and West Africa, which does deep over 1000m. Deepest field developed is more than 3000m in water depth of Brazilian seas. Riser system is necessary to develop and to production submarine oil. The riser is a tubing structure which is for drilling and production. Diameter of a drilling riser is greater than 50cm and that of a production riser is about 20cm. The riser is thin rope-like tube in oceans. Therefore, the tubing behaves elastically by marine currents and ocean

These motion behaviors are called as Vortex-Induced Vibration (VIV). VIV of the riser is a complex phenomenon, which is dominated by the natural frequency of the riser system and behavior of vortex shedding around the rider. VIV is very important for structural design of the riser system and the platform of the drilling and the production of submarine oil and so

There are many studies of VIV of the riser and the drilling or the production system including the riser system in the ocean engineering field with numerical approaches, theoretical approaches and model experimental approaches. Behaviors of time variation of VIV obtained from numerical calculations or model experiments using model risers in a water tank include a complicated mechanism so it is not easy to understand them, because the time variation is not steady but transient and chaos. Therefore, we need to understand VIV phenomenon in not only time characteristics but also frequency

For understanding frequency characteristics, we often use a power spectrum with the FFT analysis or others. However, a power spectrum does not inform us time variation of VIV characteristics. Then, the wavelet analysis can be applied to the VIV analysis because we can simultaneously understand the characteristics in time domain and frequency domain.

**1. Introduction 1.1 Marine riser** 

waves and so on.

characteristics.

on.

