| Research Abstract |
As there is not so many shallow water areas appropriate for offshore wind turbine (OWT) in Japan, as well as economical disadvantage such as the magnitude of variability of sea conditions, it is necessary to develop substructures i.e. tower and sub-sea base, which should be safe, economic and having high fatigue resistance. To do this, we conducted to 1) propose appropriate sub-sea base corresponding to the soil condition, water depth, and wind and wave conditions, 2) to make clear the dynamic characteristics of OWT consisted of blades and a nacelle, a supporting tower and sub-sea base subjected to wave and winds, and 3) to establish fatigue analysis methodology to evaluate the endurance of OWT. Actually in 2001, we carried out as follows,
1) Survey of horizontal and vertical axis wind turbine possible to use in offshore,
2) Study for OWT on application of fixed sub-sea base structures such as a mono-pile, a caisson and a jacket along with floating base
3) For horizontal wind turbine to b
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e feasible for OWT with large Mega Watt capacity, time domain dynamic structural analysis program based of modal analysis together with the fluctuating wind and wave loadings generator routines, indicating the effect of the sub-sea base property to the vibration characteristics of OWT.
4) Survey of counting method such as the rain flow
In 2002, we also conducted as follows,
1) Development of counting program that calculates numbers of cycle and stress ranges,
2) Application for simulated stress histories with annual combined occurrence of waves and winds,
3) Fitting the analyzed relation of stress ranges and numbers to a two- parameter-Weibull distribution and evaluation fatigue damage by Miner rule.
In detail, the counting program is based on three kinds of counting methods i.e. rain flow, range pair and ordinarily zero up cross. About 2), assuming that a 500kW offshore turbine set a water of 15m, we conducted the modal analyses against fluctuating wind and wave loadings during twenty years, then stress time histories were analyzed to get cumulative numbers of stress ranges. Finally, we compared the difference of counting methods, including evaluation of applicability of Weibull distribution method for fatigue life estimations. Less
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