A nonlinear vortex induced vibration model of marine risers

With the exploitation of oil and gas in deep water, the traditional vortex induced vibration (VIV) theory is challenged by the unprecedented flexibility of risers. A nonlinear time-dependent VIV model is developed in this paper based on a VIV lift force model and the Morison equation. Both the inline vibration induced by the flow due to vortex shedding and the fluid-structure interaction in the transverse direction are included in the model. One of the characteristics of the model is the response-dependent lift force with nonlinear damping, which is different from other VIV models. The calculations show that the model can well describe the VIV of deepwater risers with the results agreeing with those calculated by other models.     

Investigation of a new vortex-induced vibration suppression device in laboratory experiments

In order to mitigate vortex-induced vibration (VIV) of marine risers, especially to eliminate the phenomenon of frequency ’lock-in’, a new suppression device of crescent-shaped flow spoiler was designed with seven different layout schemes. VIV model tests with six flow levels were conducted in a large wind-wave-current flume. In all cases, vibration responses in both in-line and cross-flow cases were measured. With the installation of suppression devices vibration frequency evolution of a riser was analyzed by Morlet wavelet transform. The principle of VIV suppression was interpreted through vibration characteristics. Fatigue life of the riser was calculated by the Palmgren-Miner rule. Compared with a bare riser, vibration of an outfitted riser with suppression devices disturbed the steady flow, the vibration amplitudes in the two flow directions were reduced, and the riser fatigue life was improved.     

CFD-based numerical analysis of a variable cross-section cylinder

Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration (VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation (LES) is conducted for studying the fluid-structure interaction. The vortex shedding in the wake, the motion trajectories of a cylinder, the variation of drag and lift forces on the cylinder are analyzed. The results show that the vortices of variable cross-section cylinder are chaotic and are varying along the cylinder. In places where cross-sections are changing significantly, the vortices are more irregular. The motion trail of the cylinder is almost the same but irregular. The drag and lift coefficients of the cylinder are varying with the changes of diameters.     

Deep-water riser fatigue monitoring systems based on acoustic telemetry

Marine risers play a key role in the deep and ultra-deep water oil and gas production. The vortex-induced vibration (VIV) of marine risers constitutes an important problem in deep water oil exploration and production. VIV will result in high rates of structural failure of marine riser due to fatigue damage accumulation and diminishes the riser fatigue life. In-service monitoring or full scale testing is essential to improve our understanding of VIV response and enhance our ability to predict fatigue damage. One marine riser fatigue acoustic telemetry scheme is proposed and an engineering prototype machine has been developed to monitor deep and ultra-deep water risers’ fatigue and failure that can diminish the riser fatigue life and lead to economic losses and eco-catastrophe. Many breakthroughs and innovation have been achieved in the process of developing an engineering prototype machine. Sea trials were done on the 6th generation deep-water drilling platform HYSY-981 in the South China Sea. The inclination monitoring results show that the marine riser fatigue acoustic telemetry scheme is feasible and reliable and the engineering prototype machine meets the design criterion and can match the requirements of deep and ultra-deep water riser fatigue monitoring. The rich experience and field data gained in the sea trial which provide much technical support for optimization in the engineering prototype machine in the future.     

随机振动系统的时间域分析方法

通过对随机激励下的单自由度和多自由度线性振动系统的激励与响应信号进行分析处理,从而在时间域内用确定性信号建立起了振动系统输入与输出之间的关系。     

地震动输入方向对结构扭转振动的影响

选取2009年姚安MS6.0、2014年鲁甸MS6.5地震的水平结构强震记录进行谱分析,结果均显示17 Hz、2.1 Hz为该结构第一、二振型的自振频率,且第二振型出现扭转现象。对比两次强震记录发现,鲁甸MS6.5地震作用下结构的扭转振动远超姚安MS6.0地震。利用SAP2000建立该结构的有限元模型,分别以实际入射角度输入场地强震记录并进行时程分析,其结果与强震记录谱分析结论一致。由于输入地震动的强度、频谱相近,因此判断其输入方向是影响结构平面扭转振动的主要原因。     

Current Model Analysis of South China Sea Based on Empirical Orthogonal Function(EOF) Decomposition and Prototype Monitoring Data

Environmental load is the primary factor in the design of offshore engineering structures and ocean current is the principal environmental load that causes underwater structural failure. In computational analysis, the calculation of current load is mainly based on the current profile. The current profile model, which is based on a structural failure criterion, is conducive to decreasing the uncertainty of the current load. In this study, we used prototype monitoring data and the empirical orthogonal function(EOF) method to investigate the current profile in the South China Sea and its correlation with the design of underwater structural strength and the dynamic design of fatigue. The underwater structural strength design takes into account the size of the structure and the service water depth. We propose profiles for the overall and local designs using the inverse first-order reliability method(IFORM). We extracted the characteristic profile current(CPC) of the monitored sea area to solve dynamic design problems such as vortex-induced vibration(VIV). We used random sampling to verify the feasibility of using the EOF method to calculate the CPC from the current data and identified the main problems associated with using the CPC, which deserve close attention in VIV design. Our research conclusions provide direct references for determining current load in this sea area. This analysis method can also be used in the analysis of other sea areas or field variables.     

确定板块运动学模型的台站选取

国际地球参考架(ITRF)和板块运动模型的台站选取标准都具有随意性，直接影响地球参考架和板块运动模型的精度和可信度。提出了一种确定刚体板块内台站的新方法，即从ITRF2000速度场利用7参数转换得到的各台站运动速度残差，使其向刚性板块运动逼近，通过多次循环，从而确定各板块的刚性台站，可更好地定向地球参考架和建立高精度板块运动模型。     

Fatigue life assessment of top tensioned risers under vortex-induced vibrations

The fatigue life of top tensioned risers under vortex-induced vibrations (VIVs) with consideration of the effect of internal flowing fluid on the riser is analyzed in the time domain. The long-term stress histories of the riser under VIVs are calculated and the mean stresses, the number of stress cycles and amplitudes are determined by the rainflow counting method. The Palmgren-Miner rule for cumulative damage theory with a specified S-N curve is used to estimate the fatigue life of the riser. The corresponding numerical programs numerical simulation of vortex-induced vibrations (NSVIV) which can be used to calculate the VIV response and fatigue life of the riser are compiled. Finally the influences of the riser’s parameters such as flexural rigidity, top tension and internal flow velocity on the fatigue life of the riser are analyzed in detail and some conclusions are drawn.     

Dynamic response analysis of a floating mooring system

An innovative floating mooring system with two or more independent floating mooring platforms in the middle and one rigid platform on each side is proposed for improving efficiency and safety in shallow water. For this new system, most of collision energy is absorbed through the displacement of floating platforms. In order to illustrate the validity of the system, a series of model tests were conducted at a scale of 1:40. The coupled motion characteristics of the floating mooring platforms were discussed under regular and irregular waves, and the influences of wave direction and other characteristics on dynamic response of the system were analyzed. The results show that the mooring system is safest at 0° of wave incident angle, whereas the most dangerous mooring state occurs at 90° of wave incident angle. Motion responses increase with the increase of wave height, but are not linearly related to changes in wave height.     

九寨沟MS7.0地震强地震动模拟及参数敏感性分析

对2017年九寨沟M_S7.0地震进行强地面运动模拟,应用复合震源模型建立特定圆形子震分布的断层破裂运动学模型,对地震矩、应力降、破裂速度、子断层尺寸和震源深度等参数进行敏感性分析,得到一组合理的震源参数,然后基于复合震源模型计算得到该地区地震动,最后与观测的峰值加速度(PGA)、反应谱和PGA的空间分布特征进行对比。结果表明,该模型整体上可以再现各台站地震动强度特征,验证了复合震源模型计算强地面运动的有效性,同时敏感性规律也为复合震源模型计算缺乏强震记录区域的地震动参数的变化区间提供了依据。     

Modeling and simulating for TSHD’s swell compensator by ADAMS

CHC Trailing suction hopper dredgers (TSHD) have been widely used in dredging industry. In order to simulate the dredging process accurately, a mathematical model for a swell compensator used in TSHD is proposed, and a friendly simulation model based on the Automated Dynamic Analysis of Mechanical Systems (ADAMS) is built to test and validate the mathematical model for the swell compensator. The factors influencing the dynamic behavior of the TSHD suction pipe system, such as the motion of the vessel in an unquiet situation with different water current velocities, seabed profiles, seabed soil hardness and the forces acting on the suction pipe system, have been taken into consideration. The simulation results show that they fit in with the operating practice qualitatively.     

基于PID反馈的两级超低频隔振系统理论分析及仿真

建立两级隔振系统的运动模型,推导系统的闭环传递函数并对传递函数进行分析。结果表明：1）系统采用PID反馈较当前超长弹簧中采用的PD反馈具有更好的等效阻尼系数调节功能。2）系统的等效固有周期主要受反馈比例系数kp影响,kp越大,等效固有周期越长,隔振效果越好;kp选定后,积分系数ki和微分系数kd共同影响系统的阻尼,且ki占主导地位,决定系统的稳定性。通过仿真验证了闭环传递函数分析结果的准确性,且仿真结果表明,系统在1 Hz处的隔振效果可达-70 dB。     

Study on Dynamic Response of the ＂Dualistic＂ Structure Rock Slope with Seismic Wave Theory

Currently, scant attention has been paid to the theoretical analysis on dynamic response mechanism of the ＂Dualistic＂ structure roek slope. The analysis presented here provides insight into the dynamic response of the ＂Dualistie＂ structure rock slope. By investigating the principle of energy distribution, it is shown that the effect of a joint plays a significant role in slope stability analysis. A dynamic reflection and transmission model （RTM） for the ＂Dualistic＂ structure rock slope and explicit dynamic equations are established to analyze the dynamic response of a slope, based on the theory of elastic mechanics and the principle of seismic wave propagation. The theoretical simulation solutions show that the dynamic response of the ＂Dualistic＂ structure rock slope （soft-hard） model is greater than that of the ＂Dualistic＂ strueture rock slope （hard-soft） model, especially in the slope crest. The magnifying effect of rigid foundation on the dynamic response is more obvious than that of soft foundation. With the amplitude increasing, the cracks could be found in the right slope （soft-hard） crest. The crest failure is firstly observed in the right slope （soft-hard） during the experimental process. The reliability of theoretical model is also investigated by experiment analysis. The conclusions derived in this paper could also be used in future evaluations of Multi-layer rock slopes.     

Numerical investigation of vortex-induced vibrations for flow past a circular cylinder

The objective of the present investigation is to study the vortex-induced vibrations （VIV） for flow past a circular cylinder. The turbulent flow is simulated by using a 2-D standard k-ε model incorporating the finite volume method （FVM） and the Semi-Implicit Method for the Pressure Linked Equations （SIMPLE） algorithm on non-orthogonal boundary-fitted collocated grids. The wall boundaries are approximated with wall functions. In the numerical cases, the turbulent wake patterns are studied by plotting the streamlines and the turbulent kinetic energy contours. The pressure distributions are investigated. Analyses of the vortex-induced force coefficients and the structural vibrations are carried out. The variations of the Strouhal number with the Reynolds number and of the vortex-induced force coefficients with the reduced velocity are obtained. The results show that this numerical approach is feasible and efficient in investigating the VIV problem for a circular cylinder.     

Damage localization of marine risers using time series of vibration signals

Based on dynamic response signals a damage detection algorithm is developed for marine risers. Damage detection methods based on numerous modal properties have encountered issues in the researches in offshore oil community. For example, significant increase in structure mass due to marine plant/animal growth and changes in modal properties by equipment noise are not the result of damage for riser structures. In an attempt to eliminate the need to determine modal parameters, a data-based method is developed. The implementation of the method requires that vibration data are first standardized to remove the influence of different loading conditions and the autoregressive moving average (ARMA) model is used to fit vibration response signals. In addition, a damage feature factor is introduced based on the autoregressive (AR) parameters. After that, the Euclidean distance between ARMA models is subtracted as a damage indicator for damage detection and localization and a top tensioned riser simulation model with different damage scenarios is analyzed using the proposed method with dynamic acceleration responses of a marine riser as sensor data. Finally, the influence of measured noise is analyzed. According to the damage localization results, the proposed method provides accurate damage locations of risers and is robust to overcome noise effect.     

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Calculation of maximum allowable free span length and safety assessment of the DF1-1 submarine pipeline

The DF1-1 submarine pipeline was investigated using a dual-frequency side-scan sonar and a swath sounder system. More than a hundred scour pits under the pipeline were found, most of which have caused the span of the pipeline to increase and threatened its safety. The maximum allowable free span length (MAFSL) of the pipeline was determined through the limitations regarding maximum allowable stress under static or quasi-static loads and the onset of Vortex Induced Vibrations (VIV) under different hydrodynamic actions. The results show that the MAFSL under static conditions is 56 m. However, the MAFSLs are 30 m and 20 m under ordinary weather conditions and hurricane-induced currents for the 100-year return period, respectively, to avoid VIV as calculated by using the highest safety class factor. It is suggested that spanning pipelines longer than 20 m should be supported. Additionally, eight successive spans which may also threaten the pipeline were proposed. The most hazardous scour pits are along the pipeline section from KP42 to KP51.     

Hydroelastic Analysis of a Very Large Floating Structure Edged with a Pair of Submerged Horizontal Plates

This paper is concerned with the hydroelastic problem of a very large pontoon-type floating structure(VLFS) edged with a pair of submerged horizontal plates, which is a combination of perforated and non-perforated plates attached to the for-end and back-end of the VLFS. For the hydroelastic analysis, the fluid is assumed to be ideal and its motion is irrotational so that a velocity potential exists. The VLFS is modeled as an elastic plate according to the classical thin plate theory. The fluid-structure interaction problem is separated into conventional hydrodynamics and structure dynamics by using modal expansion method in the frequency-domain. It involves, firstly, the deflection of the VLFS, which is expressed by a superposition of modal functions and corresponding modal amplitudes. Then the boundary element method is used to solve the integral equations of diffraction and radiation on the body surface for the velocity potential, whereas the vibration equation is solved by the Galerkin’s method for modal amplitudes, and then the deflection is obtained by the sum of multiplying modal functions with modal amplitudes. This study examines the effects of the width and location of the non-perforated horizontal plates on the hydroelastic response of the VLFS, then the performance of perforated plates is investigated to reduce the motion near the fore-end of the VLFS. Considering the advantages and disadvantages of submerged plates without and with cylindrical holes, we propose a simple anti-motion device, which is a combination of a pair of perforated and non-perforated plates attached to the for-end and back-end of the VLFS. The effectiveness of this device in reducing the deformation and bending moment of the VLFS has been confirmed, and is compared with the results in cases without and with the submerged horizontal plates by the analysis in this paper.     

区域地震地质灾害损失分析系统的开发

介绍基于地理信息系统（GIS）的区域地震地质灾害损失分析系统的开发。顾及不同用户的需求，资料搜集精度可调以适应不同经济水平，设计一组可以模拟减灾效果的功能模块，发展了基于反应谱的建筑物破坏模型，并考虑了场地条件对地震动的影响。