Dynamic Properties of Steel Catenary Riser near Touchdown Point Under Coplanar Vessel Heave and Vortex Induced Vibration

The present study establishes a simple numerical model for the coupled response of a steel catenary riser(SCR) subjected to coplanar vessel motion and vortex-induced vibration(VIV). Owing to the large deflection of the SCR, the geometric nonlinearity is considered in this model. The hydrodynamic force comprises the excitation force and hydrodynamic damping, where the excitation force that only exists when the non-dimensional frequency is located in the lock-in range, is associated with the VIV. The hydrodynamic force model is validated based on the published VIV test data.As for the seabed resistance at the touchdown zone(TDZ), integrated with an initial seabed trench, the hysteretic feature is modeled. Based on the model, the study emphasizes on the coupled response characteristics near the touchdown point(TDP) induced by coplanar vessel heave and VIV, and analyzes the sensitivity of the coupled response to the heaving amplitude and frequency. It is found that with the increase of the heave amplitude and frequency, the VIV can be obviously mitigated, but the heave-related response in the coupled analysis seems to be close to that in the heave-only simulation. Finally, the fatigue damage near TDP is parametrically investigated based on the separate analysis and the coupled analysis. The results demonstrate that the coupled effect plays a significant role in the fatigue assessment near TDP. Besides, the proportion of the coupled effect accounting for the total fatigue damage decreases with the increasing seabed stiffness, while increases with the increasing seabed trench depth.

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.

Dynamic Response Study of Steel Catenary Riser Based on Slender Rod Model

A numerical model of the steel catenary riser(SCR) is built based on the slender rod model. The slender rod model,which describes the behavior of the slender riser in terms of the center line position, can solve the geometrical nonlinearity effectively. In a marine environment, the SCR is under the combined internal flow and external loads,such as wave and current. A general analysis considers only the inertial force and the drag force caused by the wave and current. However, the internal flow has an effect on the SCR; it is essential to explore the dynamic response of the SCR with the internal flow. The SCR also suffers the lift force and the fluctuating drag force because of the current. Finite element method is utilized to solve the motion equations. The effects of the internal flow, wave and current on the dynamic response of the SCR are considered. The results indicate that the increase of the internal flow density leads to the decrease of the displacement of the SCR, while the internal flow velocity has little effect on the SCR. The displacement of the SCR increases with the increase of the wave height and period. And the increasing wave period results in an increase in the vibration period of the SCR. The current velocity changes the displacements of the SCR in x-and z-directions. The vibration frequency of the SCR in y-direction increases with the increase of the current velocity.

Fatigue Characteristic Analysis of Deepwater Steel Catenary Risers at the Touchdown Point

This paper presents fatigue characteristic analysis of a deepwater steel catenary riser （SCR） under ambient excitations. The SCR involves complex nonlinear dynamic behaviors, especially at the touchdown point （TDP） where the riser first touches the seafloor. Owing to the significant interaction with soil, the touchdown zone is difficnlt to be modeled. Based on Lumped-Mass method and P-y curve, nonlinear springs are used to simulate the SCR-seabed coupled interaction. In case studies, an SCR＇s dynamic features have been obtained by transient analysis and the structure fatigue assessment has been carried out by S-N approach. The comparative analysis shows that the TDP is the key location where soil-riser interaction rises steeply and minimum fatigue life occurs. Parameters such as ocean environment loads, vessel motions, riser material and geometric parameters are discussed. The results indicate that the vessel motion is the principal factor for the structure fatigue life distribution.

Multiaxial Fatigue Analyses of Stress Joints for Deepwater Steel Catenary Risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser （SCR） in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types： Tapered Stress Joint （TSJ） and Sleeved Stress Joint （SSJ）, and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs： for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

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

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.

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.

Synergetic Analysis and Possible Control of Vortex-Induced Vibrations in a Fluid-Conveying Steel Catenary Riser

This work aimed to demonstrate possibilities for both active and passive control of the vortex-induced vibration and fatigue life of steel catenary risers via an analysis of the self-organization and evolution of the structural vibration based on synergetic theory. An analysis of the complex interrelated and synergistic relationship between the order parameter and the fast variable was performed, and the master equation of the nodal displacements was established as the order parameter for the evolution of the riser's structural vibration. Passive control methods include modifying the structure's elastic modulus, the internal fluid velocity, the top tension and the structural damping ratio, while an active control involves adjusting the external flow rate. Optimized parameters were obtained by analyzing the non-steady state solution of the master equation. The results show that the fatigue life greatly increases as the riser's elastic modulus decreases. In contrast, the fatigue life decreases with an increase of the internal fluid velocity. With an increase of the top tension, the vibration amplitudes and the number of modes may decrease, resulting in fewer bending stress cycles and a longer fatigue life. Furthermore, the structural damping ratio should be as large as possible. Finally, an active and passive control of the riser structure's response to vortex-induced vibration and its fatigue life can be achieved by carefully modifying the parameters mentioned above. The results may provide a theoretical framework for engineering practice concerning the design and control of steel catenary riser structures which are affected by vortex-induced vibration.

Fatigue Analysis of Steel Catenary Risers Based on a Plasticity Model

在链状的起床人设计是精确地在触地地区评估疲劳损坏的钢的最关键的问题。在触地地区的起床人土壤抵抗的适当建模能由优化设计导致重要费用减小。这份报纸论述能被使用数字地模仿起床人土壤相互作用并且评估动态回答和钢的疲劳损坏的一个粘性模型在触地地区的链状的起床人。利用模型，众多的起床人土壤元素被纳入钢链状的起床人有限元素，各个沿着起床人触地地区在模仿本地基础抑制。起床人土壤相互作用粘性模型在许可的联合装载表面以内说明行为。模型将在这份报纸被代表，允许简单数字实现。更重要地，它能在钢的结构的分析以内被合并有有限元素方法的链状的起床人。模型的适用性理论上被解释，结果通过申请被显示出到一近海 8.625 钢链状的起床人例子。班机橡皮起床人土壤模型的疲劳分析结果也被显示出。根据二个模型的比较结果，粘性框架的疲劳生活分析结果是合理的，起床人土壤相互作用的水平效果能被包括。

Multiaxial Fatigue Analysis on Reeled Deepwater Steel Catenary Risers with Girth Weld Defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers（SCRs） using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis（FEA）, we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.

Random Vibration of Steel Catenary Riser Conveying Fluid Under Wave Excitation

This paper presents a frequency domain approach for the calculation of the random response of fluid-conveying steel catenary risers under random wave force.The partial differential equations of motion of the steel catenary riser under a combination of internal flow and random wave excitation are established based on a series of earlier publications.The mass matrix,stiffness matrix,damping matrix and wave loading for steel catenary riser are derived in frequency domain by using Hamilton's principle.Analysis of free vibrations is then carried out to investigate the effect of flow velocity on natural frequency.By further introducing the pseudo-excitation method,the dynamic analysis of the steel catenary riser subject to wave excitation is performed in frequency domain to see how the flow velocity affects the bending moment response of the steel catenary riser.The parametric studies on the example steel catenary riser show that flow velocity may decrease the natural frequencies and increase the dynamic response of the steel catenary riser.Moreover,the dynamic stability of fluid-conveying steel catenary risers is investigated and the critical fluid velocity is identified.

深水SCR立管柔性连接接头弯曲刚度研究

钢悬链立管(SCR)柔性接头研制技术在国外发展已比较成熟,国内独立开发的相关案例较少,及较少柔性接头的设计制造能力。考虑到立管顶端与平台之间的连接部位要承受较大的交变弯矩和轴向拉力等复杂载荷作用,研究主要针对12-inch钢悬链线SCR立管的柔性连接接头进行稳态热传导分析以及各工况条件下的强度分析,对有限元模型进行建立并计算。结果表明,柔性接头有限元模型幸存受损和水压试验过程中,螺栓应力值和主体应力值均在许用应力值以下,柔性接头具有很强的可靠性及抗疲劳性。

深水钢悬链线立管(SCR)的设计与研究进展

随着深海油气开发的不断发展,钢悬链线立管(SCR)已经成为广泛使用的一种深水立管形式。目前已经被应用到多种海洋平台,如TLP平台,半潜平台,FPSO等。本文介绍了钢悬链线立管在海洋油气开发中的重要性,分析了钢悬链线立管设计时的关键影响因素及其主要步骤,进行了钢悬链线立管分析的主要理论介绍,包括静态受力分析及运动方程等,就钢悬链线立管现今的主要发展研究作出了主要的阐释。最后,就钢悬链线立管面临的问题及其未来的发展作出了展望,并提出了相应的建议。

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。

基于OrcaFlex的钢悬链立管疲劳损伤预报

涡激振动是钢悬链立管疲劳损伤的重要诱导因素之一,因此涡激振动分析在钢悬链立管设计中占据重要地位。OrcaFlex在海洋工程领域功能强大、应用广泛,能够提供两种涡激振动疲劳损伤预报方法,分别是基于模态叠加法的频域预报方法和基于Iwan-Blevins尾流振子模型的时域预报方法。OrcaFlex的涡激振动时域预报模型只考虑了横流向涡激振动造成的疲劳损伤,基于OrcaFlex的时域预报方法开发了双自由度涡激振动尾流振子模型的接口,考虑了浮式平台运动对钢悬链立管的涡激振动激励作用,实现了钢悬链立管双自由度涡激振动疲劳损伤预报。

深海SCR触地管道与土体水平向作用离心模型试验研究

针对深海钢悬链线立管系统流线段与土体相互作用问题,采用离心模型试验方法开展管道多次水平向运动循环试验,揭示了考虑淤泥层的管土相互作用机制及破土抗力发挥规律。结果表明:首次推管可以发现,由于软弱淤泥层的存在,土体抗力曲线均随位移呈现硬化型曲线,在滑移距离为0.1 D左右时达到破土状态,安装工况、水压试验工况和操作工况对应的破土抗力分别为3.45 kN、4.28 kN和3.78 kN,在破土抗力后管道持续下扎,土拱不断扩大,破土抗力以后的侧向抗力持续增加,呈现重管特征,当最大位移(X/D=2.5)时对三个工况的侧向抗力分别为9.07 kN、12.07 kN和9.97 kN;随后的往复推管过程中可以发现,随着首次推管淤泥层消散,管道接触土体强度增强,管土作用曲线呈轻管特征,在滑移距离为(0.7~1)D时出现峰值抗力,随着往复推管次数的增加,由于土拱范围的累积增大,峰值土体抗力逐渐增加,平均增长幅度不超过25%。

深水SCR X65管线钢环缝无衬垫全自动GMAW焊接接头疲劳行为

对钢悬链线立管X65管线钢进行无衬垫全自动熔化极气体保护焊(Gas Metal Arc Welding,GMAW),然后对焊接接头开展疲劳试验。疲劳试验载荷值根据API RP 2A-WSD的规定进行设定,为了弥补小尺寸试件焊接残余应力的释放,对焊接后的管子测试残余应力,将残余应力作为平均应力进行考虑。在疲劳试验后,采用扫描电镜进行疲劳断口分析。试验结果表明,X65管线钢环缝无衬垫全自动GMAW焊接接头在高、中、低3个应力水平下的疲劳寿命最低值和平均值均高于API RP 2A-WSD标准的要求,疲劳断裂以盖面焊缝焊趾断裂为主。疲劳断口分析结果表明,部分试件盖面焊缝焊趾处存在微咬边缺陷,根部焊缝存在微小未熔合缺陷。

CT-Spar平台及其系泊与立管系统优化

以CT-Spar平台及其系泊与立管系统为对象,识别出平台垂荡板数目、张紧式系泊缆预张力、钢悬链线立管上浮子位置及浮力大小为CT-Spar平台-系泊-立管系统设计时的关键参数。基于SESAM运用频域分析、时域耦合分析等手段,以系统运动响应及受力为指标,对这些关键参数进行分析与优化,结果表明,垂荡板数目由4调整为3、张紧式系泊缆预张力为1800 kN、3600 kN的浮子安装在距钢悬链线立管距顶端悬垂长度段的60%处的参数设计是适应1500 m深海风-浪-流复杂动力环境的优选方案。对该方案在极端动力条件及平台系统部分破损状态下进行时域耦合分析,发现该方案也能满足石油平台安全相关规范要求,并显著改善系泊缆、立管受力状态。

基于共旋坐标法的钢悬链线立管管土作用数值分析

为分析钢悬链立管(SCR)在顶部浮体小范围垂降、侧向漂移运动下的立管触地段垂向、侧向管土作用,将立管初始贯入海床及侧向移动过程作为准静态问题,在共旋坐标框架内建立三维有限梁单元模型,并耦合p-y加载曲线模拟立管初始贯入海床过程,并对比了线性、双线性及非线性管土模型在计算管土响应上的差异,分析了立管顶部水平牵引载荷、海流作用及土体抗剪强度对立管触地段垂向管土作用的影响,进一步研究了浮体小范围漂移工况下的侧向牵引力、土体特性参数对立管触地段侧向、垂向组合的管土作用影响。结果表明:在共旋坐标框架下,非线性管土模型对立管贯入海床的响应模拟更准确;顶部水平牵引力及海流流速增大,均会导致立管贯入海床深度及垂向土阻力减小;土体抗剪强度增大导致立管贯入深度减小但垂向及侧向土阻力增大;顶部浮体侧向牵引力增大致使立管触地段的侧向位移及土阻力增大,但垂向土阻力减小;侧向摩擦系数的增大可显著提高立管触地段的侧向稳定性。本文所述共旋坐标法利用刚体与变形解耦的特性高效求解了立管全局运动及触地段的管土作用,为立管与海床作用研究提供了参考。

深海钢悬链线立管(SCR)安装强度分析

在研究深水钢悬链立管(SCR)安装方法基础上,结合实际工程安装经验,提出了深水SCR在J-lay这种铺管方式下的安装方案,应用专业立管安装分析软件Orcaflex对各个安装过程进行实例分析,对安装过程中立管的安装强度进行校核。