钢悬链式立管管土耦合作用的吸力效应研究

钢悬链式立管（steel catenary riser,简称SCR）的流线段敷设在海床上,在浮体运动和环境荷载作用下管线作拔出海床的上升运动时,软质海床的黏性性质将阻碍管线的拔出而表现出吸力效应。吸力的大小与管线的拔出速度和管土循环作用次数、土的重塑时间等相关。基于现有试验数据拟合得到吸力数值模型,用于改进立管动力分析程序,研究立管拔出速度和管径等对触地区吸力分布、动态响应和疲劳寿命的影响。结果表明：海床土吸力对立管触地区应力特别是弯曲应力的影响较大,二者的变化趋势相似;管径的影响主要体现在贯入深度与管径的相对大小,同一贯入速度下,管径越小则相对贯入深度越大,拔出位移与吸力也会越大,反之则越小;立管的拔出速度是影响海床吸力最大值和拔出位移的主要因素,土吸力和拔出位移随拔出速度的增大而增大,导致触地区的疲劳损伤加剧。因此,探究管土耦合作用的吸力效应及其对SCR触地区疲劳损伤的影响,可为SCR与复杂海床相互作用及工程应用提供重要参考。     

深水SCR触地区管-土相互作用试验研究进展

与海床的相互作用是钢悬链线立管特有的性质,是传统立管发展中不曾遇到的问题,也是控制立管疲劳寿命的主要因素,特别是触地区出现的峰值弯曲应力和管土接触模型的不确定,成为SCR触地区研究的难点。由于立管与海床的作用机理非常复杂,因此管土相互作用模型的提出大多是建立在模型试验的基础上。通过对国内外深水SCR触地区管土相互作用试验研究案例的分析,简要介绍了SCR触地点的动态响应取决于一系列参数,如海床土刚度、加载状况、立管的提升速度、土的重塑时间和土吸力等,并设计了一套三维管-土相互作用的试验装置,能够为深水钢悬链线立管触地区管-土相互作用的试验研究提供参考。     

平台运动激励下钢悬链式立管触地点动态分析

平台运动和管土相互作用是引起钢悬链式立管(Steel Catenary Riser,SCR)触地点动态响应和疲劳损伤的关键因素。基于竖向土抗力-埋深曲线和侧向库伦摩擦双线模型模拟SCR触地区管土作用,并考虑竖向海床刚度的退化,研究平台三维运动激励下SCR触地区的动态响应与疲劳寿命,相对于简单的单向运动激励和仅考虑竖向管土作用,更符合工程实际。计算结果表明:①三维管土作用下,平台垂荡运动对立管触地区的影响最大,纵荡运动次之,横荡的影响最小;②平台三维运动的耦合,在一定程度上可降低立管触地区的动力响应和疲劳损伤,若仅考虑垂荡运动,结果偏于保守;③非线性海床模型相对于线弹性海床,引起的立管触地区的应力幅值、位移幅值显著增加,虽然贯入位移较小,但土抗力较大,选用线弹性海床的计算误差较大。平台运动和海床强度对SCR触地点动态响应和疲劳损伤的影响较大,因此在SCR响应分析与设计中,准确选择海床模型和平台运动方式,对于预测立管的疲劳寿命具有重要意义。     

非线性管土作用下钢悬链式立管动力响应分析

钢悬链线立管(SCR)在上部浮体运动和波流荷载激励下会与海床土相互作用,传统的线性海床模型假定荷载位移关系是线性的,没有考虑管土相互作用的非线性过程和海床土吸力的影响,本文基于大挠度柔性索理论的钢悬链式立管动力分析程序CABLE3D,将立管受到线性海床的弹性支撑力扩充为立管受到的海床垂向力,充分考虑管土非线性相互作用,并考虑海床土吸力对钢悬链式立管触地点区域的影响,开发出新的动力分析程序。程序采用非线性有限元方法对控制方程进行离散,时域内采用Newmark-β法,求解给定上部浮体运动条件下,SCR的动力响应,通过算例对线性刚度海床和非线性刚度海床进行对比,并分析了不同海床刚度对SCR触地点动力响应和疲劳损伤的影响。结果表明:非线性海床刚度模型比线性海床刚度模型更接近真实的管土作用过程;在非线性海床刚度模型下,海床土刚度越大,SCR触地点区域垂向位移响应越小,应力幅值越大,疲劳损伤越严重。     

考虑非线性管-土接触模型的钢悬链线立管触地区动态曲率分析

钢悬链线立管(SCR)具有特殊的结构型式,循环载荷作用下,由于海床模型的不确定性易导致触地区产生较高的弯曲应力,引发疲劳损伤。基于非线性P-y曲线管-土接触模型,运用大挠度曲线梁模型来模拟SCR与海床土的相互作用,研究SCR在浮体二维运动和海床作用下,触地区的动态曲率变化情况。由计算结果可知:1)由于在立管的有效张力中考虑局部曲率的影响,导致立管触地区的有效张力显著增加,并产生较高的弯矩; 2)动态分析中,分别运用线弹性海床和非线性海床模型,研究立管触地区的相对曲率随相对时间的变化曲线,表明非线性海床将使触地区的相对曲率具有明显的非线性,且有多个峰值,变化幅度较大,并出现反向曲率; 3)垂荡运动比纵荡对曲率的影响大,且运动幅值越大,影响越明显。     

非线性海床土对钢悬链式立管触地点动力响应和疲劳损伤影响分析

基于钢悬链式立管(SCR)动力分析程序CABLE3D,采用大挠度柔性梁理论建立SCR的运动方程,将线性海床模型扩展为考虑海床土吸力的非线性海床模型,采用非线性有限元方法对控制方程进行离散,时域内积分采用Newmark-β法,开发出新的计算程序。通过算例分析上部浮体垂荡运动幅值、海床土剪切强度、海床土剪切强度梯度对SCR触地点区域动力响应和疲劳损伤的影响。分析结果表明:SCR触地点区域动力响应和疲劳损伤对上部浮体垂荡运动幅值和海床土剪切强度的变化较为敏感,疲劳损伤在触地点区域最大,远大于悬垂段和流线段,在设计过程中应采取一定的加强措施。     

浮体二阶运动下钢悬链式立管触地区动态响应分析

浮体运动是引起钢悬链式立管(steel catenary riser,简称SCR)动态响应和疲劳损伤的关键因素,目前研究SCR问题时,为简化计算往往仅考虑平台一阶运动,忽略二阶运动影响。而实际上不同浮体结构的二阶运动响应特征明显,拟以SCR服役张力腿平台(tension leg platform,简称TLP)为例,探讨浮体二阶运动对SCR触地区动态响应的影响。建立考虑海床刚度退化的管土作用模型以改进现有的CABLE3D RSI程序,通过编写程序接口,将有限元分析得到的平台实际运动响应导入,研究平台不同运动作用下SCR触地区的位移、动力响应及疲劳分布情况。根据波流作用方向将TLP二阶慢漂运动分为近端和远端漂移两种工况,发现二阶运动下立管与海床的作用范围会增大,且触地区不仅发生高频小幅振荡运动,同时伴随低频大幅运动响应;平台远端漂移时,管内张力敏感程度高,而近端漂移时触地区的弯矩显著增大,都会不同程度提高触地区的疲劳损伤率。研究可为服役不同浮体的SCR响应预测与疲劳分析提供参考和借鉴。     

考虑海床土吸力的SCR-Spar整体波浪响应分析

研究海床土吸力对深海钢悬链线立管(SCR)与Spar平台整体波浪响应的影响。分别采用大挠度曲线梁模型、弹性地基梁模型模拟SCR的悬垂段和流线段,考虑SCR与Spar的动力耦合效应,提出整体分析法,并基于锚链/SCR分析程序Ca-ble3D开发V-Cable3D。考虑海床土吸力影响,时域动力响应分析获得一海况下SCR顶点和触地点的位移、张力、弯矩和应力时程。比较分析表明:SCR顶点和触地点附近分别存在波浪响应过程中的张力最大值和弯矩最大值,吸力对这两个特征量以及立管应力状态影响较大。提出的整体分析法为SCR波浪响应分析方法提供了新思路,对SCR与海床的相互作用分析有一定的参考意义。     

钢悬链式立管与非线性海床土相互作用分析方法研究

对基于大挠度柔性梁理论的立管动力分析程序CABLE 3D改编,将原程序中立管受到线性海床的弹性支撑力扩充为立管受到的海床垂向力充分考虑管土非线性相互作用,使新程序中立管与海床土的相互作用遵循p-y曲线。采用伽辽金方法在空间内离散立管的动态方程,最终采用Newmark-β法进行时域内迭代求解。利用改编后的新程序分别研究了立管与线性海床土和非线性海床土相互作用的对比以及不同垂荡幅值情况下立管的动态响应。研究表明,非线性海床土能够更加准确地模拟真实的管土相互作用,触地点区域的节点会经历不同的管土相互作用过程。     

钢悬链线立管触地点区域管土相互作用的有限元分析

由于钢悬链线立管具有非线性特性,而海床土体又是软黏土,因此钢悬链线立管触地区域的管土的相互作用十分复杂。根据国外相关试验数据,采用ANSYS中的非线性弹簧单元模拟海床土体,考虑海床土体刚度退化和土吸力对管道的作用,建立海底管道拟静力有限元计算模型,计算分析管道与海床土体的相互作用,并探讨管道触地点区域关键点在顶端升沉运动下弯矩的变化规律,为进一步研究SCR与海床的动力相互作用提供参考。     

The influence of nonlinear hysteretic seabed interaction on slug-induced stress oscillations in steel catenary risers

Steel catenary risers (SCRs) are usually cost-effective solutions in the development of offshore fields and the transferring of the hydrocarbons from the seabed to the floating facilities. These elements are subjected to the fatigue loads particularly in the touchdown zone (TDZ), where the oscillating SCR is exposed to cyclic contact with the seabed. The slug-induced oscillation is a significant contributor to the fatigue loads in the TDZ. The cyclic seabed soil softening under the wave-induced riser oscillations and the gradual penetration of the SCR into the seabed are widely accepted to have a significant influence on SCR fatigue performance. However, this has never been investigated for slug-induced oscillations due to the lack of integrated access to comprehensive numerical models enabling the simulation of the riser slugging and nonlinear hysteretic riser-seabed interaction at the same time. In this paper, an advanced interface was developed and verified using the multi-point moving tie constraint in order to examine the influence of cyclic seabed soil softening on slug-induced oscillations of SCR. The interface was integrated with a pre-developed user subroutine for modeling of the nonlinear hysteretic riser-seabed interaction and incorporated into a global SCR model in ABAQUS. A comprehensive parametric study was conducted to investigate the influence of slug characteristics and nonlinear seabed soil model on slug-induced, wave-induced, and combined wave/slug induced oscillations of SCR in the TDZ. It was observed that the nonlinear seabed model could significantly affect the embedment of the SCR into the seabed under the slug-induced oscillations and consequently improve the fatigue life. The developed user interface was found to be a strong framework for modeling riser slugging.     

Time domain prediction approach for cross-flow VIV induced fatigue damage of steel catenary riser near touchdown point

Previous steel catenary riser (SCR) models targeted for VIV prediction are truncated at touchdown point (TDP) where simple constrain and rotation stiffness are generally applied. In this study, a time domain approach accounting for the SCR–soil interaction is proposed to predict the cross-flow (CF) VIV induced fatigue damage of a SCR near TDP. The hydrodynamic force is simulated based on the forced vibration test data as a function of the non-dimensional amplitude and frequency, and an empirical damping model. When the non-dimensional frequency associated with the calculated frequency falls in the excitation region, the natural frequency closer to the frequency corresponding to the maximum excitation force is taken to be the dominant frequency, and applied to obtain the excitation force. The SCR–soil interaction model takes into account the trench shape, and the mobilization and release of the soil suction. Fatigue damage is linearly accumulated by using the rain-flow counting methodology. To validate the proposed models, simulation for a riser model test is carried out, and the envelopes of RMS displacement, curvature, and fatigue damage are compared. Further works focus on the sensitivity of VIV induced fatigue damage near TDP to the seabed parameters, such as mudline shear strength, shear strength gradient and soil suction, and some conclusions are obtained.     

Trenching effects on dynamic behavior of a steel catenary riser

The fatigue life of a steel catenary riser (SCR) near its touch-down zone (TDZ) is substantially affected by its interaction with the seabed. Therefore, accurate estimate of the fatigue life of a SCR requires the understanding and realistic modeling of this interaction. The interaction depends on several factors, such as soil properties, riser characteristics, and the development of trenching at the seabed. Existing approaches for modeling the seabed in interaction with a SCR approximate the behavior of the seabed soil by linear or nonlinear spring and dashpot, which represent the stiffness and damping of the soil, respectively. However, these approaches do not account for certain phenomena resulting from the plastic deformation of soil, such as trenching development at the seabed. In this study, a more realistic approach is developed for simulating the interaction between a SCR and the seabed. In addition to the use of a realistic P–y curve (where P stands for the supporting force of the seabed and y for the vertical penetration of the riser into the seabed) to simulate the soil deformation during its interaction with the riser, it considers the development of a trench caused by continuous impact of a riser on the seabed and then its feedback effect on the variation of the bending moment along the riser. It is found that the trenching development on the seabed may decrease the maximum variation of bending moment of a riser near its TDZ. Since the variation of bending moment dictates the fatigue damage to the SCR, the results based on this approach indicate that the trenching development at the seabed may increase the fatigue life of the SCR and hence it may have important application to the design of a SCR.     

Cross-Flow VIV-Induced Fatigue Damage of Deepwater Steel Catenary Riser at Touch-Down Point简

A prediction model of the deepwater steel catenary riser VIV is proposed based on the forced oscillation test data, taking into account the riser-seafloor interaction for the cross-flow VIV-induced fatigue damage at touch-down point （TDP）. The model will give more reasonable simulation of SCR response near TDP than the previous pinned truncation model. In the present model, the hysteretic riser-soil interaction model is simplified as the linear spring and damper to simulate the seafloor, and the damping is obtained according to the dissipative power during one periodic riser-soil interaction. In order to validate the model, the comparison with the field measurement and the results predicted by Shear 7 program of a full-scale steel catenary riser is carried out. The main induced modes, mode frequencies and response amplitude are in a good agreement. Furthermore, the parametric studies are carried out to broaden the understanding of the fatigue damage sensitivity to the upper end in-plane offset and seabed characteristics. In addition, the fatigue stress comparison at TDP between the truncation riser model and the present full riser model shows that the existence of touch-down zones is very important for the fatigue damage assessment of steel catenary riser at TDP.     

钢悬链线立管与海床相互作用小尺寸三维试验研究

围绕钢悬链线立管(SCR)与海床的相互作用,在水箱内开展三维试验研究,研究在不同模拟运动激励下SCR触地点的应力状态。针对当前模拟试验中,全尺寸试验耗资巨大,且试验环境难以控制,缩尺试验大多模拟立管二维运动等现状,提出一套简单易行的三维管土作用试验装置,通过横向、纵向、垂向三个方向轨道位置的合理布置,使得立管可在单向、二维耦合和三个方向同时运动,对模型立管的顶端、底端的边界条件进行处理,通过驱动器在顶端施加位移,模拟在周期运动作用下,立管触地区与土的相互作用,在三维空间内研究立管的力学特性。由此指导立管的整体设计与分析,对保证SCR在深水油气开采中的安全可靠性,具有非常重要的意义。     

Performance of non-linear seabed interaction models for steel catenary risers,part II: global response

Fatigue response of steel catenary risers (SCR) in the touchdown zone (TDZ) is significantly affected by riser-seabed interaction. Non-linear hysteretic riser-seabed interaction models have been recently developed to simulate the SCR cyclic embedment into the seabed. Despite the advancements achieved in the prediction of non-linear hysteretic riser-seabed interaction, several inconsistencies have been recently identified in the nodal performance of some of the popular models. These limitations need to be resolved by proposing new models or improving the existing models. However, it is necessary to evaluate the influence of the identified shortcomings of the existing models on the global performance of the riser. In this paper, the influence of nodal inconsistencies observed in a popular riser-seabed interaction model on the global performance of the riser was comprehensively examined in the TDZ. The riser embedment profile, cyclic contact stress, contact stress envelop, mean shear force, cyclic bending moment, and consequently the cumulative fatigue damage was investigated. The study showed that the soil model overestimates the riser embedment and other global responses. Recommendations were made to overcome the identified shortcomings of the existing models in future developments.     

Centrifuge modelling of SCR vertical motion at touchdown zone

Steel catenary risers (SCR) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep waters. Waves and currents induce motions of the structure and the risers. The repeated motions of the risers at the touchdown zone in turn induce loads on the seabed soil and might eventually cause fatigue damage to the risers. The analysis of riser fatigue damage is heavily dependent on the soil model. Soil behaviour at touchdown zone such as soil remoulding, stiffness degradation and deformation of the seabed at the touchdown zone further complicate the accurate assessment of riser fatigue damage, which is currently not appropriately quantified in existing design methods. This paper presents centrifuge model tests simulating the repeated vertical movement of a length of riser on clay seabed with increasing undrained shear strength with depth. During the tests, the pipe was subject to cyclic motion over fixed vertical displacement amplitude from an invert embedment of 0.5-3.5 pipe diameters into the soil. The test results show a significant progressive degradation of soil strength and diminution of excess pore water pressure with increasing number of riser penetration/uplift cycle. In view of the different types of environment loadings experienced by floating platforms and various soil conditions, tests were also conducted to investigate the effect of soil strength, riser displacement rate and loading mode on riser-soil interaction during repetitive penetration/uplift motion of the riser.     

Experimental Investigation on Vortex-Induced Vibration of Steel Catenary Riser

Steel catenary riser(SCR) is the transmission device between the seabed and the floating production facilities. As developments move into deeper water, the fatigue life of the riser can become critical to the whole production system, especially due to the vortex-induced vibration(VIV), which is the key factor to operational longevity. As a result, experimental investigation about VIV of the riser was performed in a large plane pool which is 60 m long, 36 m wide and 6.5 m deep. Experiments were developed to study the influence of current speed and seabed on VIV of SCR. The results show that amplitudes of strain and response frequencies increase with the current speed both in cross-flow(CF) and in-line(IL). When the current speed is high, multi-mode response is observed in the VIV motion. The amplitudes of strain in IL direction are not much smaller than those in CF direction. The seabed has influence on the response frequencies of riser and the positions of damage for riser.