Critical Top Tension for Static Equilibrium Configuration of A Steel Catenary Riser

This paper aims to present the critical top tension for static equilibrium configurations of a steel catenary riser（SCR） by using the finite element method. The critical top tension is the minimum top tension that can maintain the equilibrium of the SCR. If the top tension is smaller than the critical value, the equilibrium of the SCR does not exist. If the top tension is larger than the critical value, there are two possible equilibrium configurations. These two configurations exhibit the nonlinear large displacement. The configuration with the smaller displacement is stable, while the one with larger displacement is unstable. The numerical results show that the increases in the riser＇s vertical distances, horizontal offsets, riser＇s weights, internal flow velocities, and current velocities increase the critical top tensions of the SCR. In addition, the parametric studies are also performed in order to investigate the limit states for the analysis and design of the SCR.     

Analytical solution of mean top tension of long flexible riser in modeling vortex-induced vibrations

The vortex-induced vibration (VIV) of flexible long riser with combined in-line and cross-flow motion has been studied using a wake oscillator in this paper. The analytical solution of mean top tension of long flexible riser is evaluated and compared with experimental results, and good agreement is observed to verify its validity. Then the nonlinear coupled dynamics of the in-line and cross-flow VIV of a long tension-dominated riser were analyzed through wake oscillator model with the consideration of variation of the mean top tension. The in-line and cross-flow resonant frequencies, lift and drag coefficients, dominant mode numbers, amplitudes and instantaneous deflections are reported and compared with experimental results, and excellent agreements are observed. The comparison of mode numbers between the calculation with and without consideration of variation of mean top tension shows that the proposed analytical solution of the mean top tension can produce a better prediction of multi-mode VIV.     

Hydrodynamic Response of A Fully Coupled TLP Hull-TTR System with Detailed Modeling of A Hydraulic Pneumatic Tensioner and Riser Joints

Tension Leg Platform(TLP) in deepwater oil and gas field development usually consists of a hull, tendons, and top tension risers(TTRs). To maintain its top tension, each TTR is connected with a tensioner system to the hull. Owing to the complicated configuration of the tensioners, the hull and TTRs form a strong coupled system. Traditionally,some simplified tensioner models are applied to analyze the TLP structures. There is a large discrepancy between their analysis results and the actual mecha...     

An inverse problem methodology for multiple parameter estimation in bend stiffeners

The flexible riser top connection is a critical region for lifetime assessment due to large tension/curvature variations and modeling uncertainties. The bend stiffener polyurethane mechanical response not only presents a nonlinear loading rate and temperature dependency but is also subjected to weather ageing during operation, which may affect its mechanical behavior over time. The top tension, employed for riser local cross-section stress calculation, is usually obtained from global dynamic analyses performed under selected environmental conditions, if direct measurement is not available. As a consequence, both the bend stiffener effect on the curvature distribution and the top tension time series present inherent uncertainties for riser lifetime (re)assessment. In the present work, a proposed monitoring approach composed by gyrometers installed along flexible riser/bend stiffener top connection system length combined with an inverse problem methodology is numerically investigated to estimate the following parameters: (i) polyurethane hyperelastic response and (ii) effective top tension. The top connection system is modeled using a large deflection beam bending model and the parameters are estimated using a damped least-square minimization approach with the Levenberg–Marquardt algorithm. For the preliminary feasibility investigation, the gyrometer experimental data is numerically estimated through Monte Carlo simulations. A case study is carried out to investigate the influence that the number of sensors, sensors arrangement, loading conditions and top connection model have on the inverse parameters estimation. The results indicate that the proposed monitoring approach and inverse parameter estimation methodology may effectively reduce flexible riser lifetime calculation uncertainties.     

超深水钻井作业隔水管顶张力确定方法

对于深水和超深水钻井作业,确定钻井隔水管系统顶张力是钻前设计非常重要的工作。研究三种隔水管系统顶张力确定方法,分别是理论方法、基于隔水管系统底部残余张力方法和基于下放隔水管系统的最大钩载方法。算例计算与超深水钻井实践对比表明,在相同的隔水管系统配置下,三种方法计算结果都接近于实际钻井作业时的顶张力设定值。但研究认为,基于下放最大钩载的顶张力计算方法简单实用,推荐作为优选方法。     

Dynamic Characteristics of Marine Risers Conveying Fluid

The lateral vibration differential equation for a marine riser conveying fluid is derived by useof the small deflection theory.and the effect of internal flow velocity and top tension on the natural fre-quency of the riser is studied by use of FEM.At the same time,the preliminary relationship between thenatural frequency and riser span under different internal flow velocities is obtained,the effect of riser sup-ports on the vibration frequency is computed.It is found that the natural frequency of the marine riser in-creases with the increase of top tension.however decreases with the increase of internal flow velocity.In ad-dition,the frequency decreases drastically with the increase of riser span.     

海洋环境荷载下输液立管的静、动力特性研究

考虑管内流动流体和管外海洋环境荷载共同作用 ,建立海洋立管侧向运动微分方程。用Hermite插值函数离散 ,在微机上编写海洋立管静、动力分析程序 ,通过计算分析研究管内流体对立管侧向变形和应力的作用 ;另外 ,探讨管内流体的流动速度和立管顶端的预张力对立管动力特性的影响。结果表明 ,立管变形和应力均随管内流体流动速度增加而增大 ,同时内流速度的增大会降低立管的固有频率 ,但适当增大立管顶端预张力会抵消内流流速增加引起的固有频率下降。     

深海采矿车布放回收中脐带缆轴向张力响应特性研究

在深海采矿车布放回收中,准确预报脐带缆在底部自由悬垂边界条件和顶部波浪载荷随机激励等共同影响下的动力响应是关键点之一。基于三维势流理论与集中质量法,结合水面支持船运动和海流联合激励,对深海布放回收中脐带缆的顶端张力等力学特性进行研究。结果表明：在布放回收过程中,轴向张力在结构载荷中占主要成分;由波浪引起的轴向张力极值与均值差距最大可达到54%;轴向张力的频谱存在双谱峰,分别为 0.14 Hz的主峰和0.10 Hz 的次峰;当浪向角为180°时,轴向张力处于峰谷。     

Study on key performance parameters of hydro-pneumatic tensioner for top tensioned riser

In this paper an improved mathematical model of a hydro-pneumatic tensioner (HPT) system for top tensioned riser (TTR) is derived by consideration of friction, mass of piston and piston rod, tension loss in hydraulic oil piping, and compressibility of hydraulic oil. The vertical motion of the riser string is also considered. Subsequently, the proposed detailed model and the conventional simplified model are comparatively studied. Finally, the tension characteristic and performance parameters of the HPT are analyzed based on the proposed model. Results show that the conventional simplified model indeed overestimates the tension of tensioner as it neglects some performance parameters resulting in tension loss. The diameters of piston and piston rod and the initial pressure of high pressure (HP) gas have the most significant influence on the tension of the tensioner. The initial volume of HP gas and the initial pressure of low pressure (LP) gas also have some impact. The influence of the initial volume of LP gas and the inner diameter and length of the piping is relatively small. The research has some reference value for HPT and TTR design.     

Transient effects of tendon disconnection of a TLP by hull–tendon–riser coupled dynamic analysis

This paper deals with a numerical study of the transient effect of tendon disconnection on global performance of an extended tension leg platform (ETLP) during harsh environmental conditions of Gulf of Mexico (GoM). The ETLP has twelve tendons with twelve production top-tensioned risers (TTRs) and one drilling riser. The risers are attached by hydro-pneumatic tensioners. A time-domain nonlinear global-motion-analysis program for floating hulls coupled with risers/mooring lines is developed to model the transient effects associated with tendon disconnection at the top or the bottom. The sudden disconnection of one or more tendons causes the change of stiffness and natural periods, the imbalance of forces and moments of the total system, and possibly large transient overshoots in tension at the moment of disconnection. The breakage at the top and the unlatch at the bottom also make different impacts on the system. The transient responses and tensions are compared and discussed in the viewpoint of the robustness of the system. The survivability of a TLP with the loss of one or two tendons by accident during a lesser-than-extreme environment can also be checked by this kind of time-domain simulation technique.     

Time series prediction of mooring line top tension by the NARX and Volterra model

To minimize the computation burdens of long-term analysis, the mooring line top tensions under various short-term sea states were predicted using the pre-calculated nonlinear time domain analysis results of some selected sea states. A nonlinear autoregressive with an exogenous input (NARX) technique was introduced with a finite nonlinear memory length to predict the top-tension of a mooring line of the box-type floating production storage and offloading (FPSO). The NARX was designed in such a way that it takes the floater’s motions as its input and produces the mooring line top tension as an output. After training the NARX using the pre-calculated time series of the motions and tensions of some selected sea states, the tension time histories of different sea sates were predicted and compared with the direct time domain analysis results. In addition, to explore the nonlinearity of the system and its contribution to the response, the transfer functions of different orders were extracted after expanding the NARX equation using a Taylor series expansion. The nonlinear contributions coming from both the 3^rd and 5^th order were significant.     

A Singular Perturbation Method for Parametric Investigation on J-lay Installation of Deepwater Pipelines

The maximum bending moment or curvature in the neighborhood of the touch down point (TDP) and the maximum tension at the top are two key parameters to be controlled during deepwater J-lay installation in order to ensure the safety of the pipe-laying operation and the normal operation of the pipelines. In this paper, the non-linear governing differential equation for getting the two parameters during J-lay installation is proposed and solved by use of singular perturbation technique, from which the asymptotic expression of stiffened catenary is obtained and the theoretical expression of its static geometric configuration as well as axial tension and bending moment is derived. Finite element results are applied to verify this method. Parametric investigation is conducted to analyze the influences of the seabed slope, unit weight, flexural stiffness, water depth, and the pipe-laying tower angle on the maximum tension and moment of pipeline by this method, and the results show how to control the installation process by changing individual parameters.     

深海采矿系统长距离垂直输运管道力学性能研究

深海采矿系统中的长距离垂直输运管道在工作中具有复杂的流固耦合力学特性,在生产作业中其结构安全性和可靠性面临严峻考验。建立深海采矿系统从海底到水面的完整模型,采用集中质量法对于其中的长距离垂直输运管道的环境载荷进行了研究,并将计算结果与模型试验测量值进行对比验证,重点分析管道顶部轴向张力和剪力的大小关系以及变化规律。分析结果表明:在深海采矿系统的长距离垂直输运管道中,轴向张力在结构载荷占主要成分,由波浪引起的结构轴向张力增加会达到管体自重的38%～48%。因此,选择合适的作业工况对降低结构载荷,加强结构安全性能有很大帮助。     

深水悬链线式脐带缆总体动力响应特性研究

对某采用悬链线布局的深水脐带缆进行结构分析,得到了脐带缆的整体结构响应,包括脐带缆初始线形、有效张力和弯曲曲率的分布特性。同时对脐带缆悬链线布局下的关键参数进行了参数敏感性分析,得到了顶部悬挂角、管内流体密度以及顶部浮体位移对脐带缆结构响应的影响。这些研究结果可以为深水脐带缆的悬链线布局设计提供参考。     

基于有限差分法的复合系泊链缆动力模型及其验证

具有链—缆—链结构的复合系泊链缆因其相对于全钢链质量和成本上的优势而在深水系泊中得以广泛应用。基于细长杆理论采用有限差分法建立了可以考虑链—缆—链结构的复合系泊缆数值模型,将其应用于不同工况下全钢链和复合链缆运动的数值模拟中,并开展了验证。首先,将单根钢链顶张力数值模拟结果与不同工况下的模型试验结果进行了对比,验证了数值预报程序应用于全钢链的准确性。然后,对于复合系泊链缆开展了静刚度和动刚度迭代数值模拟,并将模拟结果同已发表文章中的算例结果进行比较,验证了该数值模型在复合链缆模拟上的准确性。发现对于单根钢锚链的验证,激励半径越大,激励周期越小,一个周期内顶张力幅值及其极差越大,钢链运动就越剧烈。对于链—缆—链式复合系泊链缆的验证,发现静刚度迭代中数值模拟结果与算例结果差异较小;对于动刚度迭代,除个别大幅慢漂工况外,两者有较高的吻合;且激励周期越小,激励半径越大,复合系泊链缆顶张力越大,弹性模量越小,运动越剧烈。对于聚酯缆刚度的敏感性分析,发现改变动刚度经验公式参数的情况下,杨氏模量的静刚度迭代和动刚度迭代结果误差分别最大达到了60.81%和68.21%,因此合成纤维材料特性对复合系泊链...     

基于神经网络优化算法的海洋动态缓波型立管设计

柔性立管连接海上平台和水下生产系统,缓波型构型可以减轻其受到的顶部张力和疲劳损伤,触地点处系链对于固定水下立管起着重要作用,然而加装系链的缓波型立管系统设计更为复杂。通过缓波型基本理论计算出合理的立管初始状态,之后使用立管分析软件OrcaFlex,在充分考虑环境因素、船体结构、立管材料和线型以及重力块、浮力块、系链等配置参数的情况下,建立缓波型立管系统有限元模型,通过静动态分析验证其满足设定的关于立管构型、张力、弯曲半径,系链张力和FPSO偏移量5个约束条件。结合神经网络优化算法和遗传算法制定出针对文中立管系统的优化算法并通过MATLAB编程将浮力块数量优化至最少,之后基于L-M算法构建神经网络模型,通过迭代训练提升精度,得到最终参数优化结果。通过对比优化前后静动态分析结果可知：优化后浮力块数量大幅减少,立管最大有效张力大幅减小,而悬挂段最低点深度有一定程度的增大,整体构型更趋向于合理的缓波型构型。     

Coupling of linear waves and a hybrid porous TLP

An analytical solution has been developed in this paper to quantify the flow field and the surge motion of a porous tension leg platform with an impermeable top layer induced by linear waves. The porous layer of the TLP is considered to be anisotropic but homogeneous. The nonlinear form drag in the porous layer is replaced by a linear drag according to Lorentz’s hypothesis of equivalent work. The convergence of the series solution is verified. The dependence of the flow field, the surge motion of the platform, and its resonant frequency on wave and structure properties has been studied.     

Superpile system: A feasible alternate foundation for tension leg platforms in deep water

Abstract

This article discusses a possible alternate foundation system for a tension leg platform (TLP) in deep water. It was developed through Tecnomare S.p.A. for Agip S.p.A. during a feasibility study of a site located in the southern Adriatic Sea having a water depth of 827 m. The system is a combination of a pile‐gravity foundation, with relatively short (?20 m) steel piles of very large diameter (6 to 12 m) called “superpiles,”; which are closed at the top and open at the bottom. The superpiles are installed in soft soil under the effect of self‐weight and active suction. Permanent tension of the TLP tendons is equilibrated by self‐weight only; tension due to wave action is equilibrated by the weight of the soil inside the superpiles. In fact, a pulsating tension applied to the superpiles generates a suction in the pore water that tends to keep the soil plug inside the cylinder and prevents the cylinder as a whole from being extracted from the (impervious) foundation soil.

The results of analyses of the installation and in service behavior of superpiles are described on conventional principles of soil mechanics and related to a typical profile at the Adriatic site. Installation problems are briefly discussed; then the stability in service is examined to evaluate the capacity of superpiles in compression and tension and the associated factors of safety. Finally, considering the pore pressure gradient generated by the applied tension inside the soil plug, the flow rate and related deformations during storms and during the entire expected life of the TLP are evaluated.

The proposed superpile system appears to be feasible, simple to install, and economical. Further study and experimental research are justified to optimize the system.     

Free vibrations of three-dimensional extensible marine cables with specified top tension via a variational method

This paper presents a model formulation that can be used for analyzing the three-dimensional vibration behaviours of an inclined extensible marine cable. The virtual work-energy functional, which involves strain energy due to axial stretching of the cable and virtual work done by external hydrostatic forces is formulated. The coupled equations of motion in the Cartesian coordinates of global systems are obtained by taking into account the difference between Euler’s equations and equilibrium equations. The method of Galerkin finite element is used to obtain the mass and stiffness matrices which are transformed into the local coordinate systems. Then the eigenvalue problem is solved to determine its natural frequencies and corresponding mode shapes. The model formulation developed herein is conveniently applied for the cases of specified top tension. The numerical investigations are carried out to demonstrate the validity of the model and to explore in details the influence of various parameters on the behaviours of marine cables. Results for the frequency avoidance phenomenon, maximum dynamic tension and coupled transverse mode shapes are presented and discussed.     

Marine riser emergency disconnection analysis using scalar elements for tensioner modelling

This work proposes nonlinear parallel spring-damper scalar elements in series with a rigid beam element to represent drilling riser tensioner behavior during an emergency disconnection scenario in global riser analysis. The software package used was Orcaflex. This is a simpler approach than the more complex, full hydraulic-pneumatic-mechanic dynamic simulation modelling of the tensioning system, commonly used in literature, which can be time consuming and numerically more sensitive. The model can also be used to represent top tension variation in global riser analysis, allowing for more complex and precise simulations, especially for fatigue life and overpull estimations. The results show adequate agreement with the researched bibliography.