Modeling touchdown point oscillation and its relationship with fatigue response of steel catenary risers

The riser-seabed interaction resulting in a trench formed in the touchdown zone (TDZ) of steel catenary risers (SCR) has a significant influence on accumulated fatigue damage. Several studies have used different trench modeling approaches to investigate the trench effect on fatigue. However, contradictory observations have been reported with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue. In this study, the significance of trench geometry in fatigue damage evaluation was investigated. Using analytical and numerical approaches, a meaningful relationship was observed between the trench slope in different zones and the peak fatigue damage. A new set of rules was proposed for the qualitative assessment of the overall trend of trench effect on the variation of fatigue damage. The proposed assessment rules were validated by performing comprehensive numerical fatigue analysis. A comparison with samples of published experimental and numerical studies was also completed. It was observed that depending on the direction of the low-frequency vessel excursions, the peak fatigue damage may increase towards the near offsets and decrease towards the far vessel offset. This implied that the case dependency of the trench effect on fatigue response in different geographical locations with various environmental loads was a potential source for the contradictory results reported in previously published studies.     

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.     

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.     

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。     

Parametric interpolation method for probabilistic fatigue analysis of steel risers

Slender marine structures such as mooring lines and risers are susceptible to failures due to stress variations coming from environmental actions. Wave, wind and current are random phenomena, and consequently the most adequate methodology to estimate the fatigue damage accumulation on these structures is the probabilistic fatigue analysis. In practice, the estimation of fatigue life requires the numerical simulation of a huge number of loading cases to compute the multi-dimensional integral of the total fatigue damage.This paper presents an efficient approach to compute the long-term fatigue damage of marine structures. The proposed method needs only a few number of numerical simulations to estimate the structure fatigue life. It uses a parametric interpolation procedure to evaluate the fatigue damage for any individual short-term condition (sea state) required in the calculation of the multi-dimensional integral. In this way, the total number of short-term structural analyzes is considerably reduced.The effectiveness and accuracy of the proposed method is compared to the full direct integration by means of two comprehensive examples. The first studied case is an idealized theoretical model of a single-degree-of-freedom (SDOF) system under wave loading, and the second one is a Steel Catenary Riser (SCR) connected to a FPSO (Floating, Production, Storage and Offloading floating unit).     

A finite differences formulation for the linear and nonlinear dynamics of 2D catenary risers

A finite differences (FD) solution method is proposed for the numerical treatment of the dynamic equilibrium problem of 2D catenary risers. The method is based on the so-called Box approximation, which in the scope of the present contribution is applied to the complete nonlinear model as well as to the reduced linearized formulation. The application of the Box method transforms the original governing systems into convenient sets of algebraic equations, which in turn are solved efficiently by the relaxation method. Extensive numerical calculations are presented that describe the dynamic behaviour of the structure and evaluate the amplification in loading due to the dynamic components. The effect of the geometric nonlinearities is assessed through comparative calculations that concern both mathematical formulations examined in the present, i.e. the complete nonlinear, and the reduced linearized model. Special attention is paid to the heave excitations as they amplify significantly the magnitudes of the loading components.     

深海钢悬链立管触地点动力响应分析

研究深海钢悬链立管(SCR)在海洋环境载荷作用下的动力响应.利用非线性弹簧模拟立管与海床触地点的耦合模型,通过模态分析得到钢悬链立管的动力特性参数;时域动力响应分析获得不同工况下触地点及典型部位的位移、弯矩和应力时程.比较分析表明:浮体垂荡运动对触地点的应力状态影响较大,触地点附近存在钢悬链立管动力响应过程中的位移极值点和弯矩极值点.所提方法为触地点区域模拟分析提供了新思路,给出的分析结论对钢悬链立管设计有一定借鉴意义.     

On the long-term response of marine structures

This paper addresses some important issues related to the estimation of long-term extreme responses of marine structures. Several convolution models to establish the long-term distribution of a marine structure response parameter are available in the literature. These methods are typically based either on all short-term peaks, all extreme short-term peaks or all short-term upcrossing rates. The main assumptions and simplifications of the five models most usually found in the literature are discussed in this paper. A linear single-degree-of-freedom (SDOF) system along with a bi-lognormal probability model for significant wave heights and zero-crossing wave periods have been used for numerical tests. An improved approach to efficiently evaluate the long-term convolution integrals is also proposed in this paper. It is shown that a combination of the Inverse First Order Reliability Method (IFORM) and an Importance Sampling Monte Carlo Simulation (ISMCS) approach can be used to obtain a very good result for the exact solution of long-term integrals.     

A simulation study on the uncertainty of environmental contours due to sampling variability for different estimation methods

Environmental contours are often used in design of marine structures to identify extreme environmental conditions that may give rise to extreme loads and responses. Recently, attention has been given to the fact that different methods exist for establishing such contours, and that in some cases significant differences may be obtained from the various methods.In this study, another aspect of the uncertainty related to the calculation of environmental contours is addressed, namely the uncertainty due to sampling variability when environmental contours are constructed based on metocean data of finite sample size. The uncertainty of environmental contours for the joint distribution of significant wave height and wave period for different sample sizes (10, 25 and 100 years of data) are investigated considering different underlying datasets and for different estimation methods for the joint distribution. Both cases where samples are drawn from a known joint distribution of wave height and periods and cases where samples are drawn from a real hindcast dataset and fitted to the joint distribution are considered. The uncertainty of the estimated contours is quantified and discussed in light of differences that can be anticipated from the different methods used to calculate the contours. Moreover, the potential bias from assuming different estimation methods is illustrated.     

半潜平台总体强度评估的谱分析法和设计波法对比

采用谱分析方法对半潜平台进行总体强度评估,验证了谱分析方法的可操作性。利用谱分析法计算平台关键连接部位处的应力水平,并与采用设计波法得到的应力结果进行比较,讨论两种方法之间的相关性。计算表明,谱分析法与设计波法存在一定的相关性,谱分析法的计算结果相对准确可靠,设计波法的计算精度依赖于选取的特征载荷工况,且目前规范推荐的载荷工况不够全面。本研究成果可为类似平台的总体强度分析提供参考。     

宁波镇海泥螺山北侧区域建设用海平面设计

围填海工程的平面设计关系到海洋资源的合理利用,也直接关系到用海项目的生态环境影响,国家海洋局对此提出了指导性意见,而在高含沙量海域,如何应用"意见"提供的3种平面设计方式值得思考。本文结合宁波镇海泥螺山北侧区域建设用海规划,对高含沙量海域围填海项目平面设计问题进行了有益的探讨。分析表明,"意见"倡导的3种平面设计方式并不适合于泥螺山北侧围填海建设,通过结合当地实际情况,提出了3种平面设计方案,从环境、经济和技术等方面综合比较,得出推荐方案。     

张力腿平台水下丛式井口圆形布局方法研究

张力腿平台(TLP)水下丛式井口布局设计关系到立管服役期间的安全性,是含有丛式井口平台设计过程中考虑的主要因素。根据TLP立管安装、钻井以及防碰等要求,考虑尾流效应对丛式立管的影响,研究水下丛式井口布局设计准则,对丛式井口—立管系统进行碰撞分析以确定水下井口间距阈值,提出水下丛式井口圆形布局方法和相应的布局设计流程。通过算例对计算方法进行了具体运用。研究表明:与目前采用的"等边三角形网格"方法相比,水下丛式井口圆形布局方法可允许水下整体基盘安装位置误差达到水下井口间距阈值的20%,立管的最大倾角可达到0.5°,安装作业窗口增加1倍以上。本方法对水下整体基盘安装位置误差的容错能力强,能有效扩大丛式立管钻井和安装作业窗口,提高丛式立管作业安全性,研究结果可为TLP丛式井口的设计提供参考。