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Structural Vibration Control Using Passive Devices
Published in Suhasini Madhekar, Vasant Matsagar, Passive Vibration Control of Structures, 2022
Suhasini Madhekar, Vasant Matsagar
Offshore structures are used for multipurpose operations like oil exploration, drilling, and production. These structures have advanced from very stiff and relatively shallow water structures in the past, to very flexible deep-water structures in the recent years. Offshore structural members are normally designed for significant wave height. Floating structures are influenced by major disturbing forces like waves and wind, while the restoring force is provided by gravity as variable buoyancy. In order to tap untouched resources of oil in deep waters or in hostile environment, there is a growing need for improving the design and construction practice of drilling and production platforms with special prominence on their safety. It is essential that these platforms can withstand the action of dynamic forces due to wind, waves, and earthquakes, for normal operating and extreme conditions. Conventional gravity type platforms in deeper water approach natural frequencies that are within the range of occurrence of everyday wave heights resulting in excessive fatigue-related damage. There are many flexible vertical members in offshore platforms like mooring lines and risers that are exposed to waves. With statistical methods, most probable maximum forces during the lifetime of the structure are calculated using linear wave theory. Statistical approach is used in analyzing the fatigue strength of members and to estimate the dynamic response of offshore platforms.
Waves and offshore engineering
Published in P. Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 2017
P. Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan
The significant wave height, Hs, is used as a design wave height in coastal engineering practice. In the earliest method of wave forecasting, known as the Sverdrup, Munk and Breitshneider (SMB) method (King, 1972) and applicable to deep water, Hs and Ts are related to the fetch F, duration of the wind Tw, and acceleration due to gravity g. The relationship is shown in Fig. 14.13 in non-dimensional form. The TminU/F versus gF/U2 curve gives the minimum time, Tmin, required for the generation of waves of maximum energy for the given fetch and wind speed. If the duration of wind, Tw, is greater than Tmin the generation of the wave is fetch limited. In that case, Hs is found for the given fetch.
Modelling of coastal and nearshore structures and processes
Published in P. Novak, V. Guinot, A. Jeffrey, D.E. Reeve, Hydraulic Modelling – an Introduction, 2010
P. Novak, V. Guinot, A. Jeffrey, D.E. Reeve
Figure 12.6 illustrates the directional spectrum and an idealized directional spreading function. Random waves are often described by the significant wave height Hs and mean period Tm. The significant wave height is the mean height of the largest third of the waves and corresponds (approximately) to the wave height by which an experienced observer would characterize the conditions. The mean wave period is the average wave period taken over a sequence of individual waves.
Bias in estimates of extreme significant wave heights for the design of ship structures caused by neglecting within-year wave climate variability
Published in Ships and Offshore Structures, 2023
Antonio Mikulić, Joško Parunov
To operate safely and efficiently, ships and offshore structures must withstand extreme ocean wave conditions, where significant wave height represents the main variable in the wave environment description. Therefore, the reliable design of marine structures requires a realistic and accurate prediction of extreme significant wave heights expected during their required lifetime (DNVGL-RP-C205 2017). There are two methods of the wave load calculation for the ultimate strength analysis of marine structures, namely the Design Sea State Method (DSSM) and the All Sea State Method (ASSM) (Mansour and Liu 2008). The former method consists of selecting a design sea state and then performing the analysis of wave loads of marine structure for only that short-term sea condition. The latter method considers all sea states with their probability of occurrence, and then the long-term distribution of wave load is computed, enabling the determination of the most probable extreme value. DSSM is often used in the design of offshore structures, while ASSM is currently recommended by the International Association of Classification Societies (IACS Rec. no.34 2000) for analysis of ship structures. However, even in cases when ASSM is used, knowledge of extreme sea states corresponding to long return periods is useful to describe the severity of the wave environment where marine structure operates. The return period of extreme sea states for the design of offshore structures is very often 50, 100, or even 1000 years, while the return period of sea states for ship structural design reads 25 years.
Predicting loads and dynamic responses of an offshore wind turbine in a nonlinear mixed sea
Published in Ships and Offshore Structures, 2021
If we carefully study Figure 5 we can find that in the green wave time series there are two highest waves, each has a wave height of about 10 m (the crest height is about 6 m, and the trough depth is about 4 m). Please note that the wave spectrum based on which this green wave time series were simulated has a significant wave height of 7.5 m (Hs = 7.5 m). The knowledge of the significant wave height can be obtained from: https://en.wikipedia.org/wiki/Significant_wave_height: In physical oceanography, the significant wave height (Hs) is defined as the mean wave height (trough to crest) of the highest third of the waves … … Significant wave height, scientifically represented as Hs, is an important parameter for the statistical distribution of ocean waves. The most common waves are lower in height than Hs. This implies that encountering the significant wave is not too frequent. For example, given that Hs is 10 metres, statistically:1 in 10 will be larger than 10.7 metres1 in 100 will be larger than 15.1 metres1 in 1000 will be larger than 18.6 metres
Significant wave height forecasting using WRF-CLSF model in Taiwan strait
Published in Engineering Applications of Computational Fluid Mechanics, 2021
Jinshan Ma, Honghui Xue, Yindong Zeng, Zhenchang Zhang, Qicong Wang
In recent years, Ali and Prasad (2019), Deka and Prahlada (2012), Duan et al. (2016) and Ni and Ma (2020) combined machine learning method with a decomposition of sea wave observations to predict significant wave height. For example, Ali and Prasad (2019) combined extreme learning machine with empirical value decomposition to forecast the future height. Duan et al. (2016) combined empirical value decomposition with support vector machine for prediction. Deka and Prahlada (2012) combined wavelet decomposition with the neural network for prediction work. Ni and Ma (2020) used the principal component analysis to predict the wave height in the polar region. Many studies using wave decomposition show that different factors will affect the future wave height trend. Firstly, the duration of wind-wave interaction is a crucial factor in forming significant wave height. However, since the parameters of the wave decomposition method are not unique, it is difficult to get better settings. Secondly, wind and wave have a certain scale of coherence in time, which affects the smoothness in a certain time range. These two factors have an important influence on the prediction work.