slow drift motion
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Author(s):  
Dong-Hyun Lim ◽  
Yonghwan Kim

In this study, a new design wave analysis method for estimating the extreme slow-drift motion of floating offshore structures is introduced. Here, the design wave refers to the irregular incident wave of short duration that induces the extreme response of desired return period. The present method is composed of following four steps: linearization of the dynamic system, probabilistic analysis of the Volterra series, generation of the design waves, and the fully-coupled nonlinear time-domain simulations. In generating the design waves, the conditioning of the most likely extreme response profile is suggested. The method was applied to a deep-water semi-submersible platform, and the results appeared to be promising compared to the full-length nonlinear simulations.


Author(s):  
Bo-Woo Nam ◽  
Sa Young Hong ◽  
Hyun Joe Kim

Wave-in-deck impacts in extreme waves can cause a serious damage on both fixed and floating offshore structures. In particular, tension leg platform (TLP) can be exposed to frequent wave-in-deck impact events due to good vertical motion performances as well as wave amplifications inside columns. In this study, the wave-in-deck impact event as well as slow-drift motion response of a tension leg platform are numerically investigated. First, the experimental observations of the wave-in-deck events are suggested based on the model tests of a TLP, where the extreme waves of 100-year and 1,000 year return periods were tested. Both weak and strong wave-in-deck events are described in detail. Then, time-domain simulations were carried out to predict the motion responses of the TLP and wave-in-deck events. Appropriate numerical modeling is suggested to simulate the TLP motion responses. The numerical prediction results are directly compared with the model test data. Discussion is made on the appropriate numerical modeling for the prediction of the wave-in-deck impact event.


Author(s):  
Richard C. Lupton ◽  
Robin S. Langley

As offshore wind turbines are installed in deeper water, interest is growing in floating wind turbines because, among other reasons, they may become cheaper than fixed-bottom turbines at greater depths. When analysing floating wind turbines, linear diffraction theory is commonly used to model the hydrodynamic loads on the platform. While it well known that slow drift motion due to second-order loads can be important for other floating offshore platforms, it has not yet been established how important such effects are for floating wind turbines. In this paper we aim to give a general result by developing approximate closed-form expressions to estimate the second-order slow drift motion of platforms of different sizes. The values are bench-marked against a typical calculation of the slow-drift response of a platform. The results show that floating wind turbines, which tend to have smaller dimensions than other floating structures, may be expected to show smaller slow-drift motions.


Author(s):  
Bernard Molin ◽  
Fabien Remy ◽  
Yanan Liu ◽  
Marie-Christine Rouault

An experimental campaign is reported on the slow-drift motion of a rectangular barge moored in irregular beam seas. The 24 m long false bottom of the basin is raised and inclined at a slope of 5%, from 1.05 m below the free surface to 0.15 m above. The barge is moored successively at 4 different locations, in water-depths ranging from 54 to 21 cm. The measured slow-drift component of the sway motion is compared with state-of-the-art calculations based on Newman approximation. At 54 cm depth good agreement is obtained between calculations and measurements. At 21 cm depth the Newman calculation exceeds the measured value. When the flat bottom setdown contribution is added up, the calculated value is 2 to 3 times larger than the measured one. A second-order model is proposed to account for the shoaling of a bichromatic sea-state propagating in decreasing water-depth. Application of this numerical model to the scale-model tests shows that in shoaling conditions the setdown contribution to the slow-drift excitation can counteract and not necessarily add up to the Newman component.


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