An innovative block partition and equivalence method of the wave scatter diagram for offshore structural fatigue assessment

The fatigue assessment plays an increasing role for the offshore structural safety. Many fatigue assessment methods have been developed for this purpose. Among those methods, the time domain method is regarded as the most accurate method but less adopted in practice due to time consuming. In order to improve the efficiency of the time domain method, an innovative block partition and equivalence method of the wave scatter diagram is developed for offshore structural fatigue assessment. After the wave scatter diagram is partitioned into several blocks, the newly developed method, involves determination of the equivalent wave height, wave period and occurrence probability of the representative sea states based on modified energy equivalent principle. The equivalent wave period of the representative sea state is calculated via the spectral moment formula in which the equivalent spectral moments of zero and second order are obtained based on the weighted averaging principle. Combining with the determined wave period, the equivalent significant wave height can be determined by reversing the wave spectrum integral formula, where the equivalent wave energy of a divided block of the wave scatter diagram is modified by introducing a factor to compensate the effect of low- and high-amplitude cycles fatigue damage. The equivalent occurrence probability is equal to the summation of the original sea states’ occurrence probability within the divided block. The developed method has the advantage of preserving the stochastic characteristics of the short term sea states within the divided block during determining the representative sea state. At the same time the newly developed method has no limitation on block partition and can be applied on different offshore structure. Two structural models, a fixed mono-pile platform and a floating semi-submersible platform, are demonstrated in the numerical examples. Results indicate that the newly developed method is robust, computationally affordable, and accurate within engineering expectations.     

Biofouling on mooring lines and power cables used in wave energy converter systems—Analysis of fatigue life and energy performance

This study presents an analysis of a wave energy converter (WEC) system consisting of a buoy, a mooring system, and a power cable connected to a hub. The investigated WEC system is currently under full-scale testing near Runde in Norway. The purpose of the study was to investigate the characteristics of the entire system, primarily with regard to energy performance and the fatigue life of the mooring lines and power cable, considering the effects of marine biofouling and its growth on the system’s components. By means of parametric study, the energy performance and fatigue life of the mooring lines and power cable were investigated considering two mooring configurations, three biofouling conditions, four sea states in a scatter diagram, and three wave and current directions. Hydrodynamic and structural response simulations were conducted in a coupled response analysis using the DNV-GL software SESAM. Energy performance analyses and stress-based rainflow counting fatigue calculations were performed separately using an in-house code. The results show that, for a WEC system which has been deployed for 25 years, biofouling can reduce the total power absorption by up to 10% and decrease the fatigue life of the mooring lines by approximately 20%.     

Fatigue analysis of offshore well conductors: Part II – Development of new approaches for conductor fatigue analysis in clays and sands

This paper, Part II, presents two soil modelling approaches developed specifically for fatigue analysis of well conductors. The first approach uses Winkler springs and can account for soil damping. The second approach is based on continuum soil mechanics and uses the kinematic hardening principles. They focus on cyclic soil behaviour at the steady-state condition. The paper demonstrates appropriateness of the approaches in predicting fatigue damage through comparisons with the centrifuge fatigue lives measured from the Series 1 tests in NC to lightly OC kaolin clay. It also presents and discusses the analyzed data and the fatigue lives measured from the Series 2 (medium-dense sand), Series 3 (NC to lightly OC GoM clay), and Series 4 (OC natural clay) tests. Finally, soil models (based on the first approach) are presented for fatigue analysis of well conductors installed in NC to lightly OC clays, over-consolidated (OC) clays and medium-dense sands, and their ability to predict conductor fatigue damage under wide range of loading conditions is demonstrated. Overall, the accuracy of fatigue life predictions using these novel soil models is very high – generally within about a few percentages of the measured values.     

Temporal significant wave height estimation from wind speed by perceptron Kalman filtering

Significant wave height estimates are necessary for many applications in coastal and offshore engineering and therefore various estimation models are proposed in the literature for this purpose. Unfortunately, most of these models provide simultaneous wave height estimations from wind speed measurements. However, in practical studies, the prediction of significant wave height is necessary from previous time interval measurements. This paper presents a dynamic significant wave height prediction procedure based on the perceptron Kalman filtering concepts. Past measurements of significant wave height and wind speed variables are used for training the adaptive model and it is then employed to predict the significant wave height amounts for future time intervals from the wind speed measurements only. The verification of the proposed model is achieved through the dynamic significant wave height and wind speed time series plots, observed versus predicted values scatter diagram and the classical linear significant wave height models. The application of the proposed model is presented for a station in USA.     

Parametric interpolation method for probabilistic fatigue analysis of steel risers

Slender marine structures such as mooring lines and risers are susceptible to failures due to stress variations coming from environmental actions. Wave, wind and current are random phenomena, and consequently the most adequate methodology to estimate the fatigue damage accumulation on these structures is the probabilistic fatigue analysis. In practice, the estimation of fatigue life requires the numerical simulation of a huge number of loading cases to compute the multi-dimensional integral of the total fatigue damage.This paper presents an efficient approach to compute the long-term fatigue damage of marine structures. The proposed method needs only a few number of numerical simulations to estimate the structure fatigue life. It uses a parametric interpolation procedure to evaluate the fatigue damage for any individual short-term condition (sea state) required in the calculation of the multi-dimensional integral. In this way, the total number of short-term structural analyzes is considerably reduced.The effectiveness and accuracy of the proposed method is compared to the full direct integration by means of two comprehensive examples. The first studied case is an idealized theoretical model of a single-degree-of-freedom (SDOF) system under wave loading, and the second one is a Steel Catenary Riser (SCR) connected to a FPSO (Floating, Production, Storage and Offloading floating unit).     

Enhanced Multi-Layer Fatigue-Analysis Approach for Unbonded Flexible Risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.     

Enhanced multi-layer fatigue-analysis approach for unbonded flexible risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.     

An Integrated Approach to Fatigue Life Prediction of Whole System for Offshore Platforms

The failure of one or even more components usually does not lead to the collapse of the whole structure. Most of the analysis of fatigue is centered on only a single component which the researchers are interested in or much attention should be paid to. However, the collapse of a structure is the result of failure of a series of components in a specific order or path. This paper proposes an integrated approach to fatigue life prediction of whole structural system for offshore platforms, mainly describing the basic principles and prediction method. A method is presented for determining the failure path of the whole structure system and calculating the fatigue life in the determined failure path. The corresponding final collapse criteria for the whole structure system are discussed. A simple method of equivalent fatigue stress range calculation and a mathematical model of structural component fatigue life estimation in consideration of sea wave and sea ice loads are provided. As an application of the propo     

On the Impact of the Assimilation of SARAL/AltiKa Wave Data in the Operational Wave Model MFWAM

As a part of our calibration/validation activities five months of SARAL/AltiKa wave data have been analyzed in this study. A robust quality control procedure using threshold values on signal and retrieved wave heights was implemented before the assimilation. Assimilation runs in the wave model Météo-France (MFWAM) were performed for a long period. The validation of the model outputs was performed with independent wave observations from altimeter and buoy data. The results indicate good performance in terms of bias and scatter index for the significant wave height and the peak wave period. Statistical analyses were performed for different ocean basins (high and intermediate latitudes and tropics). The use of SARAL/AltiKa and Jason-2 wave data combined was also investigated. This leads to further improvements for the analysis and forecast periods. In other respects, the impact of the assimilation of SARAL/AltiKa wave data is discussed for waves under strong wind conditions such as typhoons Fitow and Danas which occurred in early October 2013.     

波频和慢漂组合作用下钢悬链线立管疲劳损伤研究

为了有效地考虑浮体慢漂运动对钢悬链线立管疲劳损伤的影响,提出了波频和慢漂运动组合作用下钢悬链线立管疲劳损伤简化计算的位置组合叠加法。其核心是:基于浮体慢漂运动概率分布选取若干典型慢漂位置,进行波频运动作用下钢悬链线立管动力响应分析;根据钢悬链线立管运动位置变化特征,截取若干慢漂位置对应的波频应力时程叠加到慢漂应力时程上,得到波频和慢漂运动的组合应力时程;编写基于雨流计数法的MATLAB程序处理立管各节点应力,采用海水环境下Do E.E型S-N曲线和Palmgren-Miner累积损伤准则计算立管各节点疲劳损伤。应用位置组合叠加法对某海域500 m水深的立管进行了疲劳分析,并与全耦合法、权重组合叠加法以及波频和慢漂疲劳损伤简单相加法的结果进行了对比,结果表明该方法具有较高的精度和效率。此外,进行了区域设定系数、波浪高度、波浪周期和土壤表面剪切强度等参数对组合作用下立管疲劳损伤的敏感性分析。     

The impact of wave loads and pore-water pressure generation on initiation of sediment transport

The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported.     

An original method for characterizing internal waves

This study consisted in the characterization of internal waves in the south of the Strait of Messina (Italy). The observational data consisted in thermistor string profiles from the Coastal Ocean Acoustic Changes at High frequencies (COACH06) sea trial.An empirical orthogonal function analysis is applied to the data. The first two spatial empirical modes represent over 99% of the variability, and their corresponding time-dependent expansion coefficients take higher absolute values during internal wave events. In order to check how the expansion coefficients vary during an internal wave event, their time derivative, called here changing rates, are computed. It shows that each wave of an internal wave train is characterized by a double oscillation of the changing rates. At the front of the wave, both changing rates increase in absolute value with opposite sign, and then decrease to become null at the maximum amplitude of the wave. At the rear of the wave, the changing rates describe another period, again with opposite sign. This double oscillation can be used as a detector of internal waves, but it can also give information on the width of the wave, by measuring the length of the oscillation, as this information may sometimes be hard to read straight out of the data. When plotting the changing rates one versus another, the resulting scatter diagram puts on a butterfly shape that illustrates well this behaviour.     

Longshore sediment transport estimation using a fuzzy inference system

Accurate prediction of longshore sediment transport in the nearshore zone is essential for control of shoreline erosion and beach evolution. In this paper, a hybrid Adaptive-Network-Based Fuzzy Inference System (ANFIS), Fuzzy Inference System (FIS), CERC, Walton–Bruno (WB) and Van Rijn (VR) formulae are used to predict and model longshore sediment transport in the surf zone. The architecture of ANFIS consisted of three inputs (breaking wave height), (breaking angle), (wave period) and one output (longshore sediment transport rate). For statistical comparison of predicted and measured sediment transport, bias, root mean square error and scatter index are used. The longshore sediment transport rate (LSTR) and wave characteristics at a 4 km-long beach on the central west coast of India are used as case studies. The CERC, WB and VR methods are also applied to the same data. Results indicate that the errors of the ANFIS model in predicting wave parameters are less than those of the empirical formulas. The scatter index of the CERC, WB and VR methods in predicting LSTR is 51.9%, 27.9% and 22.5%, respectively, while the scatter index of the ANFIS model in the prediction of LSTR is 17.32%. A comparison of results reveals that the ANFIS model provides higher accuracy and reliability for LSTR estimation than the other techniques.     

Solitary wave interactions with an array of two vertical cylinders

This paper addresses a numerical investigation of nonlinear waves interactions with an array of two surface-piercing vertical cylinders and the corresponding nonlinear hydrodynamic loads on each individual cylinder. The primary interest of this study is concentrated on the problem of three-dimensional scattering of solitary waves by cylinder arrays and the nonlinear interactions between scattered waves. The theoretical model adopted for simulation is the generalized Boussinesq two-equation model. The boundary-fitted coordinate transformation and multiple-grid technique are utilized here to simplify the computation domain and to facilitate the applications of the boundary conditions on the cylinder surfaces. The velocity potential, free-surface elevation and subsequent evolution of the scattered wave field are numerically evaluated. The hydrodynamic forces on each cylinder during wave impact are also determined. A study of the sheltering effect by the neighboring structures on wave loads is conducted. It is found that the presence of the neighboring cylinder has shown significant influence on the wave loads and the scattering of the primary incident waves. For two transversely arranged cylinders, the transverse force coefficient increases as the separation distance decreases.     

基于谱分析法的超大型浮体疲劳强度分析

运用ANSYS/AQWA对某超大型浮体平台进行水动力响应计算和结构响应分析,在此基础上选择3个典型疲劳热点区域建立有限元子模型,求出各区域的热点应力传递函数。依据谱分析法的疲劳计算流程对危险节点进行疲劳分析,计算疲劳损伤和寿命。分析结果表明,横浪时对平台造成的疲劳损伤最大。横撑与下浮体围壁连接处以及立柱外壳与上箱体底板连接处这两处的疲劳寿命不满足设计要求。建议对该结构连接处进行改进或重新设计。     

Fatigue reliability analysis of mooring system for fish cage

Fish cages in the open sea are exposed to cycle loads due to irregular wave climate during their service life, and thus the fatigue reliability assessment of mooring system should be conducted to ensure the safe operation. The aim of this study is to evaluate the fatigue failure probability of mooring system for fish cage. Numerical simulation of net cage in random waves is performed and the time dependent approach is applied to conduct the fatigue reliability analysis of shackle chains based on S-N curve method. The sensitivity analysis of fatigue reliability of mooring line to the uncertainty of random variables in the fatigue limit state is conducted. In addition, the system reliability for mooring system is analyzed and the effect of the initial pretension and safety factor on system reliability is investigated. The results indicate that a case without the initial pretension on anchor lines is helpful to decrease the failure probability of mooring system and the safety factor of mooring lines in the current regulation is conservative for the system reliability against fatigue damage.     

Wave Uplift Forces on the Bottom of a Circular Cylinder

The analysis of the data of model tests of two large deep wharves and monographic experimental studies show that two aspects are to be improved so as to predict the wave uplift forces on the bottom of a circular cylinder. The first aspect is the uplift pressure distribution on the bottom, and the second is the correct determination of the phase for maximum horizontal wave forces. The second problem has been solved. Synthesizing the results of theoretical analysis and experiments, we suggest a diagram for the determination of the phase when the maximum horizontal wave force appears. On the basis-ef the diagram the simultaneous wave uplift forces can be obtained for the structural stability analysis.     

Wind waves in the coastal zone of the southern crimea: Assessment of simulation quality based on in situ measurements

The study verifies the Black Sea wave model using field data obtained from the Katsiveli research platform. The WAM and mesoscale MM5 and WRF atmospheric models, which are used to calculate the wind field for the wave model, were recently adjusted to the Black Sea region at the Marine Hydrophysical Institute. The results of the work are presented as characteristics of the simulation quality used in world practice in other regions. The scatter index for a significant wave height is 70% in summer and 50% in winter. The values of the scatter index of wave parameters and wind speed appear to be at the same level as in semi-enclosed seas on the northern side of the Mediterranean Sea. It is shown that atmospheric simulation correctly reproduces the interaction between synoptic processes and the mountain range extending alongshore. Error sources in wave simulation are discussed. The most significant drawback is the possibility of mesoscale instability in the atmospheric model without assimilation of observation data within the computational domain.     

Neuro-fuzzy based approach for wave transmission prediction of horizontally interlaced multilayer moored floating pipe breakwater

The ocean wave system in nature is very complicated and physical model studies on floating breakwaters are expensive and time consuming. Till now, there has not been available a simple mathematical model to predict the wave transmission through floating breakwaters by considering all the boundary conditions. This is due to complexity and vagueness associated with many of the governing variables and their effects on the performance of breakwater. In the present paper, Adaptive Neuro-Fuzzy Inference System (ANFIS), an implementation of a representative fuzzy inference system using a back-propagation neural network-like structure, with limited mathematical representation of the system, is developed. An ANFIS is trained on the data set obtained from experimental wave transmission of horizontally interlaced multilayer moored floating pipe breakwater using regular wave flume at Marine Structure Laboratory, National Institute of Technology Karnataka, Surathkal, India. Computer simulations conducted on this data shows the effectiveness of the approach in terms of statistical measures, such as correlation coefficient, root-mean-square error and scatter index. Influence of input parameters is assessed using the principal component analysis. Also results of ANFIS models are compared with that of artificial neural network models.