Fatigue Damage Of Mooring Lines Research Articles

Application of an active learning method for cumulative fatigue damage assessment of floating wind turbine mooring lines

Long-term cumulative fatigue damage of mooring lines is crucial for the design of floating wind turbine structures (FWTs). Although many efforts are carried out for the offshore floating platforms, there still needs to be an efficient approach for assessing the long-term fatigue damage of the floating wind turbine mooring lines due to the complex loading in FWTs. An active learning approach named AK-DA (Adaptive Kriging Damage Assessment) was recently proposed for the cumulative fatigue damage assessment of wind turbine structures. However, in the original work, the AK-DA approach was only tested on fatigue damage assessment of a 5MW wind turbine tower with monopile support structures. It is unclear whether it is applicable to other parts of the wind turbine system, especially considering the complex loading of FWTs. Therefore, in this work, the AK-DA approach is used to assess the cumulative fatigue damage of mooring lines. The Gaussian process regression (Kriging) model is used to predict the fatigue damage of the mooring line under different wind-wave cases. The cumulative fatigue damage of the mooring lines in the IEA 15MW semi-submersible wind turbines is assessed with the AK-DA approach. The numerical simulation results show that the AK-DA approach can efficiently and accurately estimate the cumulative fatigue damage of the mooring lines. Compared to the traditional simulation approach, the AK-DA approach can increase efficiency by more than 45 times, and the absolute error is less than 1%. This active learning approach could serve as a helpful tool for offshore mooring system designers, facilitating the cumulative fatigue damage assessment during the design process.

Effect of mooring line layout on the loads of ship-shaped offshore installations

An offshore mooring system stations a ship-shaped offshore installation in place while withstanding incoming loads from the marine environment with short-term and long-term uncertainties. This study aims to develop a novel framework for analysing the loads on floating systems, namely mooring line tension, mooring line fatigue damage, and hull bending moment, as a function of the mooring layout design variables and environmental random variables. The nonlinear influence of those variables is assessed by means of advanced techniques using response charts, response divergence charts, and Sobol's total-effect sensitivity indexes. The developed procedure includes a probabilistic selection of mooring scenarios, station-keeping numerical analyses, and metamodel selection to define input loads. An example of a hypothetical floating production storage and offloading (FPSO) unit with taut legs in the Gulf of Mexico illustrates the procedure. The details of the computations are documented, and the findings show that the mooring line top-tension has a high total-effect index for the wave-induced bending moment and the total mooring line tension, whereas the fatigue damage is mostly affected by the chain diameter. The results of this research offer useful insights to designers and propose the use of a surrogate model to be used in the reliability-based design of mooring systems.

Effect of wave inhomogeneity on fatigue damage of mooring lines of a side-anchored floating bridge

Mooring systems are important structural components of very long floating bridges. They effectively limit the transverse motions of the bridge under environmental loads. They also add viscous hydrodynamic damping to the entire system. The safe and economical design of mooring systems is thus important but also challenging especially when the wave conditions are inhomogeneous. In this paper, a computational study is carried out to investigate the responses of the mooring lines for a 4.6 km long fjord crossing floating bridge accounting for inhomogeneous wave conditions. Based on the structural responses, this study also attempts to evaluate the fatigue damage in the mooring lines by using different fatigue analysis methods. The accuracy of the spectral methods is examined by comparison with the conventional rainflow cycle counting algorithm. Numerical studies are conducted to obtain an indication of the effect of various wave inhomogeneities on the fatigue damage in the mooring lines.

Assessment of fatigue damage of mooring line for fish cage under wave groups

Wave groups may have a great impact on the dynamic response of mooring line for offshore structures. Thus, it is essential to investigate the fatigue damage of mooring line for fish cage system under wave groups. Based on the wave envelope spectrum, the kinematics of wave groups for the whole wave field can be modeled and the hydrodynamic behaviors of mooring line for fish cage under wave groups are analyzed. And using the rainflow counting method and Miner's rule, the long-term fatigue damage assessment of mooring line for fish cage under wave groups is conducted. The influence of the intensity of wave groups on the mooring line tension, stress range histogram, fatigue damage plot and the short-term and long-term fatigue damage of mooring line is investigated. The results indicate that the fatigue damage of mooring line is significantly sensitive to the wave group factor of height. This demonstrates that the high wave groups will greatly affect the fatigue damage of mooring line and the fatigue life of mooring system for fish cage. Thus the analysis of the action of wave groups will help to understand the fatigue damage of mooring lines for offshore structures in the realistic seas.

An improved lumping block equivalent method for predicting fatigue damage of mooring system for fish cage

The lumping block method is usually adopted to reduce the sea states in the wave scatter diagram for the practical engineering fatigue design and the equivalent parameters in the lumping blocks are of vital importance for accurately predicting the long-term fatigue damage. In this study, a comparison of the mooring-line fatigue damage calculated by the existing lumping block equivalent methods is conducted and an improved lumping block equivalent method is proposed. The fatigue damage for the original sea states is calculated as a benchmark. A series of dynamic analysis of the fish cage system for the equivalent sea states obtained by six existing equivalent methods are carried out to estimate the fatigue damage of mooring lines. However, the existing equivalent methods are unable to achieve the accurate estimation of the fatigue damage of mooring system. Then, the improved lumping block equivalent method through selecting the more severe significant wave period for each mooring line is proposed based on the slightly conservative principle for the individual block and the relationship between the fatigue damage and the significant wave period. The results indicate that the proposed improved equivalent method is robust and efficient for accurately evaluating the fatigue damage of mooring system.

Fatigue damage distribution and reliability assessment of grid mooring system for fish cage

This paper investigates the evolution of fatigue damage distribution with the loading cycle and predicts the annual fatigue damage distribution and system reliability of the grid mooring system under fatigue limit state for fish cage, employing the generalized probability density evolution method (GPDEM). A series of the hydrodynamic analysis for the grid mooring system of fish cage are performed to obtain the fatigue damage of the grid mooring system considering the long term environmental loading. The GPDEM equation is solved to determine the evolution of probability distribution of fatigue damage in term of the loading cycles, and the probability distribution of the annual fatigue damage of mooring lines is evaluated through the combination of the GPDEM and the virtual stochastic process model. In addition, the system reliability assessment of the grid mooring system under fatigue limit state is performed by the equivalent extreme-value event method. The results indicate that the probability distribution of the fatigue damage of the mooring component becomes more asymmetrical and platykurtic with the increase of the loading cycle and the annual fatigue damage distribution of the components in the grid mooring system can be estimated by the GPDEM from a stochastic point of view without any approximations or any asymptotic distributions. And it is necessary to conduct the uncertainty quantification of the Miner's sum at failure for the system reliability prediction of the grid mooring system under fatigue limit state to ensure the safety of fish farm.

Ultimate structural and fatigue damage loads of a spar-type floating wind turbine

ABSTRACTThis study addresses the ultimate structural and fatigue damage loads of a spar-type offshore floating wind turbine under joint excitations of wind and wave. Aero-hydro-servo-elastic coupled analysis is performed in time-domain to capture the dynamic responses of the floating wind turbine. Based on the mean up-crossing rate method, the short-term ultimate structural load is estimated. The cumulative fatigue damage load is computed with the S-N method. It is shown that the low-level ultimate load is mostly influenced by wind forces whereas the high-level ultimate load is more closely related to wave forces. The wave excitations dominate the fatigue damage at tower top and tower base, whereas the mooring line fatigue damage is more sensitive to the wind forces.

Investigation on the probabilistic distribution of mooring line tension for fatigue damage assessment

This paper is to investigate the applicable probability density functions (PDFs) for the short- and long-term mooring line tension amplitudes to be used for accurately assessing the mooring line fatigue damage with the spectral method. Statistical characteristics of the mooring line tensions were investigated, in which kurtosis and skewness were computed and compared to validate the non-normality of the line tension response. Several PDFs, including Rayleigh, GEV, and Gamma distributions, are used to express the short-term statistical distribution of the mooring line tension amplitudes. Results indicate that the Gamma distribution can give much better fitting than the Rayleigh and GEV distributions. In order to identify the long-term distribution of the tension amplitudes, the Gamma distribution is modified by bringing in a location parameter. Compared with the generally used Weibull distribution, the modified Gamma distribution can characterize the long-term PDF of the tension amplitudes much better. Moreover, the modified Gamma distribution can give much more accurate assessment of the mooring line fatigue damage than the Weibull distribution.

An investigation on low frequency fatigue damage of mooring lines applied in a semi-submersible platform

Assessing the fatigue life of mooring systems is important for deep water structures. In this paper, a comprehensive fatigue analysis is conducted on the mooring lines applied in a semi-submersible platform with special focus on the low frequency (LF) fatigue damage. Several influential factors, including water depth, wave spectral parameters, and riser system, are considered. Numerical simulation of a semi-submersible platform with the mooring/riser system is executed under different conditions, and the fatigue damage of mooring lines is assessed by using the time domain analysis method as a benchmark. The effects of these factors on the mooring line tension and the fatigue damage are investigated and discussed in detail. Research results indicate that the LF fatigue damage only accounts for a very small portion of the total damage, although the LF components dominate the global motion response and the mooring line tension of the semi-submersible platform. However, it is demonstrated that the LF fatigue damage is clearly affected by the influential factors. The increase in water depth and spectral peak periods, and the existence of risers can weaken the contribution of the LF components to the mooring line fatigue damage, while the fatigue damage due to the LF components increases with the increase of significant wave height.

高レイノルズ数域でのSpar構造物のVIM特性

FOWTs, i.e. Floating Offshore Wind Turbines, are recently planned to be set in the offshore, where bottom-mounted type's one is not feasible, to generate natural resource electricity. One of the ways for FOWTs' application is that a wind turbine facility is mounted on a slender column type floater to keep buoyancy and stability in the sea, that is called as Spar type. In this case, the characteristic of VIM, i.e. Vortex Induced Motion, on the column type floater is concerned about to keep mooring safety of the FOWT. Then VIM characteristic on the column type floater was investigated using a huge experimental model in supper critical Reynolds number at the large towing tank in the National Maritime Research Institute, Japan. The results of the experiment were compared with ones of small similar figure models under lower Reynolds number, and the VIM amplitudes against reduced velocity parameter were clearly understood. Using these results the amplitude distribution relating to duration time for the huge experimental model is investigated having the effect on fatigue damage of mooring lines. These unique and precious results on the column type floater have important role for safety assessment on FOWT mooring lines.

부유식 해양플랜트 다점 계류라인의 피로손상모델 비교

The mooring lines of a floating type offshore plant are known to show wide banded and bimodal responses. These phenomena come from a combination of low and high frequency random load components, which are derived from the drift-restoring motion characteristic and wind- sea, respectively. In this study, fatigue models were applied to predict the fatigue damage of mooring lines under those loads, and the result were compared. For this purpose, seven different fatigue damage prediction models were reviewed, including mathematical formula. A FPSO (floating, production, storage, and offloading) with a <TEX>$4{\times}4$</TEX> spread catenary mooring system was selected as a numerical model, which was already installed at an offshore area of West Africa. Four load cases with different combinations of wave and wind spectra were considered, and the fatigue damage to each mooring line was estimated. The rain flow fatigue damage for the time process of the mooring tension response was compared with the results estimated by all the fatigue damage prediction models. The results showed that both Benasciutti-Tovo and JB models could most accurately predict wide banded bimodal fatigue damage to a mooring system.

Mooring line fatigue damage evaluation for floating marine energy converters: Field measurements and prediction

The vision of large-scale commercial arrays of floating marine energy converters (MECs) necessitates the robust, yet cost-effective engineering of devices. Given the continuous environmental loading, fatigue has been identified as one of the key engineering challenges. In particular the mooring system which warrants the station-keeping of such devices is subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves.To ensure the integrity of the mooring system the lifecycle fatigue spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated.This paper describes an approach where load measurements from large-scale field trials at the South West Mooring Testing Facility (SWMTF) are used to calculate and predict the fatigue damage. The described procedure employs a Rainflow cycle analysis in conjunction with the Pålmgren–Miner rule to estimate the accumulated damage for the deployment periods and individual sea states.This approach allows an accurate fatigue assessment and prediction of mooring lines at a design stage, where field trial load measurements and wave climate information of potential installation sites are available. The mooring design can thus be optimised regarding its fatigue life and costly safety factors can be reduced. The proposed method also assists in monitoring and assessing the fatigue life during deployment periods.

Fatigue damage induced by nonGaussian bimodal wave loading in mooring lines

Catenary mooring lines are typically subjected to bimodal loads, comprising of a wave frequency (WF) component due to the first-order wave forces and a low frequency (LF) component induced by the second-order wave forces. For moored vessels, the LF forces due to current and wind also play a role. Only dynamic wave loads are considered herein, while current and wind loads are modeled as constant forces. Because of the nonlinearities of the mooring line characteristics, the dynamic line tension and the second-order responses, both the WF and LF line tensions are in principle nonGaussian. These facts make it difficult to estimate the combined fatigue damage of mooring lines in the frequency domain. A fatigue combination rule based on the Jiao and Moan’s theory has been extended to cover the nonGaussian case. The purpose of this paper is to improve and validate the frequency-domain method by time-domain analysis based on a simplified, but accurate mechanical model of the dynamic line tension. Improvements on the LF and combined fatigue damage estimation have been made by considering the nonsymmetrical property of the LF line tension distribution. Both the WF and LF mooring line tensions due to wave loading have been simulated in the time domain for different sea states and the combined fatigue damage has been estimated by using the rainflow cycle counting algorithm. The accuracy of the frequency-domain method for estimating the bimodal nonGaussian fatigue damage of mooring lines has been verified by the time-domain simulations and is considered to be acceptable.