Extreme hawser tension assessment for FPSO vessel during offloading operation in Bohai bay

The Floating Production Storage and Offloading Unit (FPSO) is an offshore vessel that produces and stores crude oil prior to tanker transport.Robust prediction of extreme hawser tensions during Floating Production Storage and Offloading (FPSO) operation is an important safety concern. Excessive hawser tension may occur during offloading operations, posing an operational risk. In this paper, AQWA has been used to analyze vessel response due to hydrodynamic wave loads, acting on a specific FPSO vessel under actual sea conditions. Experimental validation of numerical results has been discussed as well.This paper advocates methodology for estimating extreme response statistics, based on simulations (or measurements). The modified ACER (averaged conditional exceedance rate) method is presented in brief detail. Proposed methodology provides an accurate extreme value prediction, utilizing all available data efficiently. In this study the estimated return level values, obtained by ACER method, are compared to the corresponding return level values obtained by Gumbel method. Based on the overall performance of the proposed method, it is concluded that the improved ACER method can provide more robust and accurate prediction of the extreme hawser tension.Data declustering issue has been addressed. Paper highlights ability of ACER method to account for a set of varying sea state probabilities, as required in engineering long term statistical analysis.Described approach may be well used at the vessel design stage, while defining optimal vessel parameters that would minimize potential FPSO hawser tension.     

Numerical and experimental investigations into the application of response conditioned waves for long-term nonlinear analyses

The coefficient of contribution method, in which the extreme response is determined by considering only the few most important sea states, is an efficient way to do nonlinear long-term load analyses. To furthermore efficiently find the nonlinear short-term probability distributions of the vessel responses in these sea states, response conditioned wave methods can be used. Several researchers have studied the accuracy of response conditioned wave methods for this purpose. However, further investigations are necessary before these can become established tools. In this paper we investigate the accuracy by comparing the short-term probability distributions obtained from random irregular waves with those from response conditioned waves. We furthermore show how response conditioned wave methods can be fitted into a long-term response analysis. The numerical and experimental investigations were performed using a container vessel with a length between perpendiculars of 281 m. Numerical simulations were done with a nonlinear hydroelastic time domain code. Experiments were carried out with a flexible model of the vessel in the towing tank at the Marine Technology Centre in Trondheim. The focus was on the probability distributions of the midship vertical hogging bending moments in the sea states contributing most to the hogging moments with a mean return period of 20 years and 10 000 years. We found that the response conditioned wave methods can very efficiently be used to accurately determine the nonlinear short-term probability distributions for rigid hulls, but either accuracy or efficiency is to a large effect lost for flexible hulls, when slamming induced whipping responses are accounted for.     

Estimation of nonlinear long-term extremes of hull girder loads in ships

This paper deals with a estimation of long-term extreme value for a given return period, say D=100 yr. In principle, this response is obtained by combining the response in all the sea states. The long-term response for a linear system can be effectively obtained by determining the response for each sea state, specified by the significant wave height, H_s, and the peak period, T_p, in the frequency domain. However, if the response is nonlinear, time domain simulation and a long time series would be required, to limit statistical uncertainty. Therefore, the long-term analysis becomes rather complicated and time consuming. For the long-term analysis, it is crucial to introduce ways to improve the efficiency in the calculation. In this work, it is shown that, the long-term extremes can be estimated by considering only a few short-term sea states. A long-term analysis based on identifying the most important sea state, defined by the coefficient of contribution, using linear analysis is applied. An iteration procedure is thereafter used to find the nonlinear long-term extreme values. It is concluded that only a limited number of sea states is necessary to get an acceptable estimate of the nonlinear D-year response as long as the most important sea states are included, i.e., the sea state with the maximum coefficient of contribution.     

Iterative methodology for response based analysis of an FPSO mooring system

Response based analysis (RBA) has been developed for prediction of extreme N-year return period responses and design metocean conditions of offshore structures. For applying the RBA, the behaviour of the offshore system subjected to a long history of metocean conditions needs to be predicted, and then, the probabilistic analysis is applied to estimate its long-term responses. Due to the large number of analysis cases required, the structural simulation is usually performed either by simplifying the structural model or by using computationally efficient tools, such as frequency-domain (FD) analysis. These approaches usually decrease the accuracy of predictions mainly when they are utilized for nonlinear systems. On the other hand, employing time-domain (TD) simulations leads to more accurate results but it is computationally expensive. Application of RBA for a weathervaning FPSO, which is the subject of the present study, makes TD analysis an essential requirement because of a highly nonlinear behaviour of the system. In the present study, an efficient methodology is proposed that aims at reducing the computational efforts of RBA by joint application of TD and FD simulations in combining the structural and statistical analyses through a single process, such that the number of time-consuming TD simulations is minimized. After initial screening using the results from FD simulations, the methodology identifies the response events (storms) that contribute the most to the N-year response and sets out an iterative process in which only those events that are most important are analysed by fully-coupled TD simulations. Within such events, a similar approach is also applied to intervals (sea states) where only the most contributing intervals are analysed in TD, and the remaining intervals are left for a less accurate FD analysis without sacrificing the overall accuracy. The proposed methodology provides a robust framework for distinguishing between “mild” and “severe” response events, without specifying any predefined limits for the metocean parameters or making a subjective judgement. Although it is developed for the mooring system of a weathervaning FPSO, it should also be applicable to any type of offshore structure and any structural response. This paper is the first part of the study and concentrates on the development of the efficient methodology to optimize the application of RBA to FPSO mooring systems, whilst its detailed application is subject of the second part of the study.     

Efficient estimation of extreme long-term stresses by considering a combination of longitudinal bending stresses

Longitudinal stresses due to combined horizontal and vertical bending moments in ships, corresponding to a return period of 20 years, are estimated by linear response analysis. In principle, the stress should be obtained by combining the stress in all sea states that can occur over a long-term period. A method to determine the desired long-term extreme stress by considering only a few short-term sea states is presented. The sea states have a certain probability of occurrence, and are each identified by a contour line in the (H _s, T _p)-plane. This approach makes it possible to estimate the extreme loads on the vessel in a practical and accurate manner. Moreover, it is shown that the long-term stress can be estimated by combining the individual long-term extreme stresses due to vertical and horizontal bending moments by using the sum-of-squares approach and accounting for the correlation between stresses. It was found that the correlation coefficient can be taken as the largest of the ones calculated along the contour line. It is shown that this correlation coefficient can even be approximated by the normalized phase angle at the wave length where the dominant response has its peak value. A comparison with the results obtained using well-known combination rules is presented. While linear analysis has been used here, it is believed that the approach can be generalized to stresses with nonlinear behavior, and hence represent a significant improvement in calculation efficiency. Received: September 18, 2001 / Accepted: December 18, 2001     

Comparing different contour methods with response-based methods for extreme ship response analysis

Environmental contours are often applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. They facilitate approximate long term analysis of critical structural responses in situations where computationally heavy and time-consuming response calculations makes full long-term analysis infeasible. The environmental contour method identifies extreme environmental conditions that are expected to give rise to extreme structural response of marine structures. The extreme responses can then be estimated by performing response calculations for environmental conditions along the contours.Response-based analysis is an alternative, where extreme value analysis is performed on the actual response rather than on the environmental conditions. For complex structures, this is often not practical due to computationally heavy response calculations. However, by establishing statistical emulators of the response, using machine learning techniques, one may obtain long time-series of the structural response and use this to estimate extreme responses.In this paper, various contour methods will be compared to response-based estimation of extreme vertical bending moment for a tanker. A response emulator based on Gaussian processes regression with adaptive sampling has been established based on response calculations from a hydrodynamic model. Long time-series of sea-state parameters such as significant wave height and wave period are used to construct N-year environmental contours and the extreme N-year response is estimated from numerical calculations for identified sea states. At the same time, the response emulator is applied on the time series to provide long time-series of structural response, in this case vertical bending moment of a tanker. Extreme value analysis is then performed directly on the responses to estimate the N-year extreme response. The results from either method will then be compared, and it is possible to evaluate the accuracy of the environmental contour method in estimating the response. Moreover, different contour methods will be compared.     

非线性混合海况下近海风机塔筒底部载荷的动力响应

本文主要研究在非线性混合海况(即风浪和涌浪组合海况)下,以NREL 5MW_Baseline Monopile近海风机为研究对象,对其塔筒底部(基线)所受到的剪力和弯矩载荷的动力响应进行仿真。在近海风机的时域仿真中,选用了Ochi-Hubble六参数波浪谱,并编制了该谱的程序嵌入到FAST中进行编译。计算过程中,共进行了20次10 min的仿真分析。对于得到的短期载荷,给出了波高程,塔筒底部首尾向剪力和弯矩在线性与非线性不规则波作用下的时程曲线对比图。采用分块最大值法对每一次的短期载荷提取极值,并基于20次仿真所得的极值,给出了塔筒底部首尾向剪力与弯矩在线性与非线性不规则波作用下的超越概率曲线对比图。研究表明,在非线性混合海况下进行近海风机塔筒底部载荷的动力响应研究,计算结果对工程实际应用具有指导意义。     

Uncertainty analysis of extreme mooring loads associated with environmental contours and peak tension distributions

This paper aims to assess the uncertainty on the extreme mooring loads of floating system considering short-term variability. Two environmental contour approaches based on the inverse First and Second Order Reliability Methods are employed to identify critical sea states that may give rise to extreme loads. The uncertainty related to the construction of environmental contours is addressed including significant differences due to marginal distribution fitting, parameter estimation methods and joint models. Three measured datasets are analysed using a known conditional joint distribution and proposed mixed copula model. 3-h time domain numerical simulation for each sea state is conducted and the characteristic extreme responses of mooring lines subjected to design loads are assessed. The uncertainties due to various statistical models including the average conditional exceedance rate method as well as global maxima, peak-over-threshold method combined with Gumbel distribution, Generalized Extreme Value distribution, Generalized Pareto distribution and 3-parameter Weibull distribution are investigated and quantified. It is observed that marginal distributions, joint models and parameters estimation methods have apparent effect on design loads estimation, and the extreme tensions of the semi-submersible platform shows significant difference using various probabilistic models. The results indicate that those epistemic uncertainties should be account for in the reliability analysis or safety factor calibration for mooring systems.     

基于海况参数分析的船体梁载荷预报方法研究

利用三维势流理论在频域内计算载荷频率响应函数,在此基础上对船舶波浪载荷长短期预报方法进行了研究,分析了现有的长期预报方法,并提出只针对特定的一部分短期海况进行分析,以确定船舶在营运寿命期内航行于实际海况中的波浪载荷特征最大值。同时提出了一种基于多项式拟合的波浪载荷预报方法,建立了短期响应的最可能极值关于海浪有义波高与海浪平均向上跨零周期的函数,可以方便地确定响应的特征值。结果表明,提出的方法能够有效进行波浪载荷的预报,可供有关专业人员参考。     

Long-term approach for assessment of sloshing loads in LNG carrier,part II: Grouping method

Assessing the sloshing loads has been a significant issue in designing the cargo containment system (CCS) of a liquefied natural gas carrier (LNGC). The sloshing problem contains numerous physical and technical uncertainties. The long-term approach has been suggested to reduce the possible uncertainties, but it has not been a feasible option owing to the incalculable number of experimental cases. Daewoo Shipbuilding and Marine Engineering Co., Ltd., Hyundai Heavy Industry Co., Ltd., Samsung Heavy Industry Co., Ltd., Korean Register, and Seoul National University conducted extensive sloshing model tests for the 174 K S-LNGC. To consider the long-term approach, Part I describes the differences between short- and long-term approaches based on the guidelines of different classification societies. To address the significant number of experimental conditions required for the long-term approach, a grouping method is investigated (Part II). Although this grouping method is typically used in practical design, it has not been validated, and its applicability is yet to be determined. In Part II, the sea states are classified using 16 different grouping methods. The long-term exceedance probabilities and pressures are obtained individually and then compared with the results obtained from fully considering all sea states. After performing the general sloshing model test of the shipbuilding industry, the sloshing impact pressure of a liquefied natural gas cargo model is measured via 6DoF irregular simulations, where the upper section of the cargo hold structure is considered. Long-term pressures are locally and globally investigated and compared among 16 different sea states and all the sea states. Based on the results, the best grouping method is suggested for the long-term approach.     

Efficient long-term fatigue analysis of deepwater risers in the time domain including wave directionality

Accurate fatigue assessment is a challenging and crucial aspect of riser design. The prediction of the long-term fatigue damage must account for numerous sea states of different wave heights, periods, and directions. Each sea state entails a dynamic analysis, often performed in the time domain owing to the significant nonlinearities. Because of the short-term uncertainties from irregular waves, the simulation duration must be sufficiently long for results to converge. To alleviate the hefty computational cost of long-term fatigue analysis, researchers have proposed efficient methods, but these are not without drawbacks; in particular, wave directionality is commonly neglected. This paper presents an efficient method for long-term fatigue analysis based on time domain simulation, considering wave directionality among other things. The proposed method is based on an enhanced version of control variates to reduce the variance in Monte Carlo simulations (MCS). The control function is constructed by training artificial neural network (ANN) models using existing MCS data. Here, a customized scheme is developed to allow for the situation that the training data and ANN prediction cases have different wave directions. The proposed method is unbiased and provides an error estimate. Simulations are performed on a floating system, and the proposed method is found to improve the efficiency of MCS significantly. Different scenarios such as fixed and random wave directions are compared, confirming that wave directionality is critical and should be included in a long-term fatigue assessment.     

Numerical prediction of wave-induced long-term extreme load effects in a flexible high-speed pentamaran

The theory and procedure established by Wu and Moan in 1996 and 2005 and Wu and Hermundstad in 2002 were applied to a high-speed transatlantic pentamaran containership. Nonlinear time-domain simulations of ship motions and load effects were carried out in different sea states. The simulated responses were validated against model tests with satisfactory results. The short-term probabilities of exceedance were estimated by using different stochastic analysis procedures. The long-term probabilities of exceedance were obtained based on the short-term results. These served as information about loading in a reliability-based design approach. The load effects in a semiprobabilistic design were also calculated at an appropriate probability of exceedance level.     

Practical estimation methods of the design loads for primary structural members of tankers

Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of these design loads have been determined as the standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be weak. Accordingly, it may be difficult to refer and utilize them as the fundamental design concepts for ship designers or the operational guidelines for ship operators. To overcome these difficulties, the authors have developed practical estimation methods of the design loads having transparent and consistent backgrounds to the actual loads acting on primary structural members of tankers. In this paper, the design sea states that closely resemble the actual sea states which are considered as the most severe for hull structures are firstly proposed. Secondly, the practical estimation methods of the design sea states are proposed by parametric studies using the results of series calculation of representative tankers. Thirdly, the practical estimation methods of design regular waves resulting in the same level of stresses with that induced in irregular waves under the design sea states are proposed. Finally, the practical estimation methods of the design loads such as ship motions, accelerations, hull girder bending moments and hydrodynamic pressures that are induced under design regular waves are proposed.     

深水悬链复合锚泊线疲劳损伤计算

以某座Spar平台的锚泊系统为研究对象,首先利用三维绕射理论计算Spar平台主体波浪力,得到平台的总体运动响应时程。再建立复合锚泊线的二维非线性有限元动力分析模型,基于DelVecchio(1992)提出的经验公式,采用迭代的方法计算复合锚泊线的刚度。锚泊线和海床之间的接触作用基于刚性海床假定,基于Morrison公式计算锚泊线的惯性力和拖曳力荷载,根据计算得到的平台主体运动响应时程作为锚泊线顶端输入条件,在时域范围内进行复合锚泊线的动力分析。计算得到中国南海某海域各短期海况条件下复合锚泊线应力的时间历程曲线,采用雨流法对其计数得到对应于各短期海况条件下的疲劳载荷谱。最后根据Miner线性累积损伤模型,对复合锚泊线在长期海况条件下的疲劳损伤进行比较计算。     

Splash zone lowering analysis of a large subsea spool piece

Lifting operation though the wave splash zone is challenging. Careful numerical analysis in the design phase is needed to minimize associated risks. This study addresses numerical modeling and analysis of the splash zone lowering of a large subsea spool. A typical offshore construction vessel is used for the operation. The objective is to compare the effects from different numerical methods and parameters on the allowable sea states and the operability. These methods and parameters include wave short-crestedness, shielding effects from the vessel, wave direction and wave seed number. A coupled numerical model of the spool-vessel system is established in SIMO program, which is a simulation tool for marine operations. Slamming and submergence-dependent loads on the spool during the transient lowering process are calculated. A large number of time-domain simulations has been performed to derive the allowable sea states. The operational criteria for assessment of the sea states include slack sling, snap loads in wires and clearance between spool and the vessel. Operability analysis of the operation at one reference site in the Barent Sea is established using 50-year hindcast data. The influences from different methods on the allowable sea states and the operability are compared and discussed in detail.     

Time series prediction based on LSTM neural network for top tension response of umbilical cables

The umbilical cable is an essential component of offshore oil and gas extraction systems. The severe marine environment poses a high challenge to the safety of the umbilical cable structure during operation. The analysis of an umbilical cable requires complex and resource-demanding finite element time-domain simulations to obtain their nonlinear dynamic response. Therefore, in order to solve the problem of structural safety monitoring and real-time assessment of remaining life of umbilical cables under extreme sea states, there is a great need to predict the dynamic response of umbilical cables quickly and accurately during operation, for ease of making fast decisions for system operation and maintenance before the arrival of extreme sea states. Given the strong nonlinear function-approximation ability of the neural network, this study proposes an efficient method for the prediction of the time series of umbilical cable top tension response based on LSTM (long short-term memory) neural network. We use LSTM neural network and ARIMA (autoregressive integrated moving average) model in a real engineering case for time series prediction of the top tension response of the umbilical cable, and the results of the two models are analyzed and compared, and the efficiency and accuracy of the LSTM neural network model are verified. Furthermore, the hyperparameter, dataset and generalization ability of LSTM model are discussed. The results indicate that feasibility of the tension response prediction of umbilical cables under dynamic load in complex marine environments.     

Extensive study on the design loads used for strength assessment of tanker and bulk carrier structures

ClassNK has undertaken wide-ranging basic research covering many aspects related to the safety of ship structures, including design loads, structural analysis, strength assessment of buckling, collapse, and fatigue, and rational corrosion margins to develop new design standards which have transparency and consistency. Among the wide-ranging basic research, this article summarizes the results of extensive work on the design loads used for strength assessments of tanker and bulk carrier structures. The main aim of the research was to develop practical estimation methods of design loads with rational technical backgrounds relating to the actual loads acting on the primary structural members of tankers and bulk carriers. During this study, we proposed the following methodology. Design sea states that closely resemble the actual sea states which are considered to be the most severe for hull structures. Find practical estimation methods for the design sea states by parametric studies using the results of series calculations on representative tankers and bulk carriers. Find practical estimation methods for design regular waves which will result in the same level of stresses as those induced in irregular waves under the design sea states. We also briefly introduced some practical estimation methods for the design loads, such as ship motions, accelerations, hull-girder bending moments, and hydrodynamic pressures that are induced under design regular waves. The findings in this study have been summarized and implemented in the new design standards for tanker and bulk carrier structures.Updated from the Japanese original which won the 2003 SNAJ prize (J Soc Nav Archit Jpn 2002; 191:195–207; 2002; 191:208–220; and 2002; 192:723–733)     

动力定位中跟踪环境力突变的自适应无迹卡尔曼滤波

无迹卡尔曼滤波可以在状态估计中滤去噪声干扰,已经被广泛应用于动力定位系统中.针对复杂海洋情况下动力定位系统需要准确、及时地估计当前时刻的状态而无迹卡尔曼滤波无法跟踪状态突变的问题,为此文章提出了一种自适应无迹卡尔曼滤波.通过及时判断状态值突变并适当调整后验均方差矩阵,可有效地跟踪船舶状态并减小实际位置与定点位置的偏差.仿真实验证明了算法的有效性.