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.     

Prediction of the dynamic response of a slender marine structure under an irregular ocean wave using the NARX-based quadratic Volterra series

The time series of the dynamic response of a slender marine structure was predicted in approximate sense using a truncated quadratic Volterra series. The wave-structure interaction system was identified using the NARX (Nonlinear Autoregressive with Exogenous Input) technique, and the network parameters were determined through supervised training using prepared datasets. The dataset used for network training was obtained by nonlinear finite element analysis of the slender marine structure under random ocean waves of white noise. The nonlinearities involved in the analysis were both large deformation of the structure under consideration and the quadratic term of the relative velocity between the water particle and structure in the Morison formula. The linear and quadratic frequency response functions of the given system were extracted using the multi-tone harmonic probing method and the time series of the response of the structure was predicted using the quadratic Volterra series. To check the applicability of the method, the response of a slender marine structure under a realistic ocean wave environment with a given significant wave height and modal period was predicted and compared with the nonlinear time domain simulation results. The predicted time series of the response of structure with quadratic Volterra series successfully captured the slowly varying response with reasonably good accuracy. This method can be used to predict the response of the slender offshore structure exposed to a Morison type load without relying on the computationally expensive time domain analysis, especially for screening purposes.     

深水钻井隔水管紧急脱离后的反冲响应

深水钻井中,在恶劣海况或浮式平台定位失效等情况下,需紧急断开隔水管与水下防喷器的连接。隔水管突然脱离后会加速向上反冲,如果反冲作用控制不当,可能威胁钻井平台、隔水管体和水下井口的安全。介绍了国外隔水管紧急脱离与反冲响应的研究情况,分析了隔水管紧急脱离的原因与程序、脱离后的反冲过程、反冲控制方式和成功脱离标准、反冲响应的分析方法。建议:由于南海深水海况复杂,作业者需制定紧急脱离的预案和作业规程,并进行紧急脱离操作培训;由于隔水管紧急脱离后的反冲响应涉及多个非线性瞬态过程,研究者应该开展隔水管紧急脱离测试和反冲响应模拟计算,开发隔水管反冲响应分析软件。     

考虑浮体升沉及张紧环运动深海立管固有振动特性研究

研究深海顶张力立管(TTR)的固有振动特性。通过考虑两种不同的分析模型,研究沿水深变化的轴力以及张紧环运动对于立管固有振动频率和振型的影响。考虑立管顶端动边界条件及立管轴力随水深变化,提出了计算深水顶张力立管固有振动频率的方法,给出考虑浮体升沉及张紧环运动时顶张力立管的固有振动特性。结果表明,考虑变轴力和张紧环的影响是计算立管固有频率需要考虑的;立管浮体的运动,导致立管频率显著变化,从而改变立管的共振特性。     

一种新型干树式半潜平台设计

开发了一种新型的深吃水干树式半潜生产平台(TCDD-Semi),该平台可用于中国南海深水区域的油气田开发。它的一个显著特点是立柱的截面形状呈现为渐变形式,而非传统的等截面形式。立柱的底部最大,向上逐渐变小,再结合适当的下浮体设计,能够为平台在服役前期及在位状态时提供足够的浮力以及稳性。最重要的是这种新型立柱的设计能够明显地减小平台在恶劣海况时的垂向运动,从而使在平台上布置具有干式采油树的TTR立管系统成为可能。此外,这种渐变式的立柱设计也有利于减弱平台在流作用下的涡激诱导运动(VIM)。分析结果表明这种渐变式立柱半潜平台可以明显降低平台的垂向运动,也可以在码头组装时提供足够的浮力以及在平台下沉过程中提供足够的稳性。平台优越的垂向运动性能可以使顶部张紧式立管的行程范围限制在10.5 m之内(包括平台偏移、垂荡运动、潮汐影响、海床沉降、热膨胀等因素)。分析结果表明平台的垂荡范围、水平偏移、倾侧角度、上部模块重心处的加速度等可以满足设计要求,证明了这种新型干树式半潜平台的可行性。     

A methodology for structural analysis and optimization of riser connection joints

In riser structural systems, the region where the riser is connected to the platform is critical due to the development of high stress levels. To reduce the stress concentration in this region, bend stiffeners and stress joints are used in order to provide a gradual stiffness transition between the riser and the platform. The present paper presents an optimization design approach for bend stiffeners and stress joints, using a slender beam procedure for the connection joint analysis and an evolution strategies multi-objective optimization algorithm. This approach produces similar results when compared to a finite element analysis of the complete riser, but with significant reduction of computational costs. Numerical examples are presented, showing the efficiency and robustness of the suggested methodology.     

A practical approach to predicting cross-flow and in-line VIV response for deepwater risers

In this study, a practical model is proposed to predict cross-flow (CF) and in-line (IL) vortex-induced vibrations of a flexible riser in time domain. The hydrodynamic force as a function of non-dimensional amplitude and frequency is obtained from the forced vibration experimental data of a two-dimensional cylinder. An empirical nonlinear damping model is used to simulate the hydrodynamic damping outside the experiment's range. Coupling effect of CF and IL-VIV is taken into account by implanting a magnification model for the IL hydrodynamic force associated with CF amplitude, and by increasing the non-dimensional amplitude corresponding to the IL hydrodynamic coefficient in the second excitation region. The experimental models of flexible riser under the uniform and sheared current are simulated to validate the proposed model. The predicted displacement, curvatures, excited modes and fatigue damage show reasonable agreement with the measured data.     

国外深水铺管方法与铺管船研究现状及发展趋势

深水铺管船是深水油气田开发的主要施工装备,它担负着浮式生产平台的安装、海底管线的铺设以及立管系统安装任务。通过对国外主要深水铺管船和重点工程项目的分析,简要介绍了国外深水铺管船的基本特征、性能和设备能力;重点阐述了深水铺管船的关键设备及其对施工能力的影响;最后,对国外深水铺管船的发展趋势作了进一步的分析。为我国的深水铺管船研发提供有益的参考。     

Three-dimensional Lump-Mass formulation of a catenary riser with bending, torsion and irregular seabed interaction effect

A three-dimensional Lump-Mass formulation of a catenary riser, capable of handling irregular seabed interaction, with bending and torsional stiffness is presented in this paper. This formulation uses only three degrees of translational freedom and one independent torque variable for each computational node. The generality of the present formulation permits static and dynamic analyses of a wide range of offshore-related slender structure systems such as mooring cables, rigid and flexible risers as well as submarine pipelines. Four sets of results are presented for (i) a hanging catenary, (ii) as (i) but subjected to end torsion, (iii) a wire, chain and spring buoy mooring and (iv) a steel catenary riser on an irregular seabed.     

Reliability analysis of wake-induced collision of flexible risers

Collision between risers is an important design and operational concern, especially in deep water since the probability of collision tends to increase as the riser length increases. Riser collision is due to the joint effects of many processes, i.e. environmental loads, hydrodynamic interference and surface floater motions and the most of them are stochastic processes. This paper provides an approach for estimating the failure probability of riser collision by considering these joint effects and their stochastic nature. Firstly, a procedure for establishing the distribution function of the extreme minimum relative distance between two risers is introduced based on simulation tools and statistical data. Numerical simulation is performed to compute the minimum distance between risers for a given duration. Repeated simulations are applied so that the extreme value distribution can be established. Secondly, reliability analysis is performed by considering the uncertainties of input parameters related to environmental loads and riser system. The collision probability is calculated based on both the First/Second Order Reliability Method and the Monte Carlo simulation techniques.