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.     

新型多功能干树储油半潜平台研发

针对凝析油气田开发了一型适应我国南海环境条件的深水多功能干树半潜平台,平台运动性能优良,可实现干树开采、回接水下井口、油气生产处理、凝析油存储、钻修井等功能。平台由上部组块、主船体、细长柔性连接结构、垂荡板、顶张紧式立管(TTR立管)系统、系泊系统构成,结构特点为采用细长柔性连接结构连接主船体与箱型垂荡板。垂荡板采用箱型结构,承受作业吃水压力,可实现平台与箱型垂荡板的整体拖航,平台安装简易,细长柔性连接结构可在位维修或更换。平台立柱内设置储油舱存储天然气凝析油,解决凝析油存储外输问题。平台采用成熟系统系统集成,运动性能优良,海上施工风险可控,未来有望成为一种新的深水油气田开发平台型式。     

Coupling Effects of A Deep-Water Drilling Riser and the Platform and the Discharging Fluid Column in An Emergency Disconnect Scenario

As drilling operations move into remote locations and extreme water depths, recoil analysis requires more careful considerations and the incidence of emergency disconnect is increased inevitably. To accurately capture the recoil dynamics of a deep-water riser in an emergency disconnect scenario, researchers typically focus on modelling the influential subsystems (e.g., the tensioner, the mud discharge and seawater refilling process) which can be solved in the preprocessing, and then the determined parameters are transmitted into an existing global riser analysis software. Distinctively, the current study devotes efforts into the coupling effects resulting from that the suspended riser reacts the platform heave motion via the tensioner system in the course of recoil and the discharging fluid column follows the oscillation of the riser in the mud discharge process. Four simulation models are established based on lumped mass method employing different formulas for the top boundary condition of the riser and the discharging flow acceleration. It demonstrates that the coupling effects discussed above can significantly affect the recoil behavior during the transition phase from initial disconnect to the final hang-off state. It is recommended to develop a fully- coupled integrated model for recoil analysis and anti-recoil control system design before extreme deep-water applications.     

The effect of nonlinear multi-contact coupling with gap between risers and guide frames on global spar motion analysis

Nonlinear multi-contact coupling between vertical risers and guide frames inside the spar moon-pool is studied. The existing numerical model for hull/mooring/riser coupled dynamics analysis treats riser as an elastic rod truncated at the keel (truncated riser model), and the risers are free to slide in vertical direction with constant tension, while restricted in horizontal direction. The truncated riser model neglects the portion of the riser inside the moon-pool and tends to overestimate the spar pitch motion. In the newly developed model, the risers are extended through the moon-pool with realistic boundary conditions at multiple guide frames, and thus additional contact forces and moments on the spar hull are considered. The gap effects between the riser buoyancy-cans and riser guide frames are also modeled using three different types of gap springs. Their different dynamic characteristics are extensively studied. The new riser model also considers the Coulomb damping between buoyancy-cans and riser guide frames and also allows the impact-like contact force calculation on risers for ensuring fatigue analysis.     

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。     

柔性管道骨架层压溃失效机理和安全评价方法研究进展

非黏结柔性管道作为深水油气开发领域的关键设备,是连接海底井口和海洋平台的主要纽带。海洋油气开发水深已经超过了3 000 m,高静水压力是深水软管设计和安全评价的主要挑战,明确深水柔性管道压溃失效机理,并准确地预测柔性管道的压溃压力可以为柔性管道结构设计和安全评价提供依据,柔性管道的压溃压力与骨架层的几何形状以及骨架层之间复杂的相互作用有关,准确高效地预测压溃压力变得极具挑战性。针对柔性管道失效机理研究和压溃压力预测方法进行了总结,指出当前柔性管道压溃失效分析中薄弱的环节,为我国非黏结柔性管道的结构设计和安全评价提供参考。     

基于双缸耦合原理的阀控式能量回收系统原理设计与仿真分析

针对小规模海水淡化需要解决能量回收装置的能回效率与空间问题,将能量回收装置与压力提升泵进行集成一体化设计,研究了一种双缸耦合阀控式能量回收系统。系统两液压缸内活塞各由一个电动推杆推动,叠加高压浓海水压力实现压力交换,同时进行活塞推力的耦合控制,避免了压力交换过程中活塞的压力脉冲波动,保障了反渗透膜工作压力稳定;基于小型海水淡化装置,根据压力能流反渗透工艺节点压力流进行了计算分析,得出电动推杆推力压力0.5 MPa左右的补偿压力曲线。通过AMESim进行液压仿真分析,结果表明在系统回收率为30%,高压浓海水进口压力为4.8 MPa,增压海水压力4.6 MPa的情况下,通过压力补充增压海水压力可增压到5.0 MPa,反渗透膜压力与流量波动较小,满足反渗透海水淡化的压力需求。     

Numerical Investigation on Severe Slugging in A Catenary Riser

A mathematical model is presented to study the main characteristics of severe slugging in a downward inclined pipeline followed by a catenary riser. In this model, both simplified transient model and phase distribution model are included so that the flow characteristics of each stage for severe slugging can be accurately reproduced, especially for blowout stage. The results show that the flow features of severe slugging can be simulated by the proposed mathematical model. A good agreement between the experimental data and the numerical results is observed. The model can predict the transient fluctuation of many respects, such as the superficial gas velocity at the bottom of the riser and the average velocity at the outlet of the riser.     

Effect of fluid column dynamics on longitudinal vibrations of an ultra deepwater riser covered by an orifice plug

The paper studies longitudinal vibrations of an ultra-deepwater drilling riser whose bottom end is disconnected from the sea floor and covered by a plug (a blind one or the one with a hole). An elastic shell is used as a model of a riser pipe. The fluid column motion in the riser is described by the Navier–Stokes equation for a compressible fluid. Losses of fluid pressure during its flowing through a hole in the plug are taken into account. Solution of the riser equations is carried out in the frequency domain. Analysis of the effect which the riser length, wave period and height, and the plug-hole diameter on the amplitude of the riser's tension vibrations is conducted. A riser manufactured of strong aluminium alloys, some of prospective materials for ultra deepwater drilling, was considered as a prototype. It has been shown that risers 2000–4000 m long, plug-hole, and a riser 6000 m long with plug-hole of 0.087 m in diameter can withstand wave height of >15 m throughout the frequency range.     

玻纤增强柔性管等效简化模型研究

玻纤增强柔性管作为一种新型海底油气输送管道,具有比强度高、柔度大和抗腐蚀性强等特点,因此在深海油气开发中具有非常广阔的应用前景。玻纤增强柔性管主要由内衬层、增强层和外保护层组成,其中增强层的等效模拟是玻纤增强柔性管设计成功与否的关键。根据玻纤增强柔性管的结构特征和材料特性,选取了四种不同的等效简化模型,对比研究了玻纤增强柔性管在轴向拉压荷载、弯曲荷载以及内压荷载作用下的力学性能。将不同简化模型的计算结果与相应的试验数据进行对比,进行等效模型的优选。研究结果表明,在内压载荷和弯曲载荷作用下,基于Halpin-Tsai模型数值结果与试验结果最为接近。在轴向载荷作用下,采用分离式模型或回形模型计算精度更高,若材料达到屈服状态时,则建议采用分离式模型进行模拟。     

Experimental Investigation of Vibration Response of A Free-Hanging Flexible Riser Induced by Internal Gas-Liquid Slug Flow

The vibration response of a free-hanging flexible riser induced by internal gas-liquid slug flow was studied experimentally in a small-diameter tube model based on Froude number criterion. The flow regime in a curved riser model and the response displacements of the riser were simultaneously recorded by high speed cameras. The gas superficial velocity ranges from 0.1 m/s to 0.6 m/s while the liquid superficial velocity from 0.06 m/s to 0.3 m/s. Severe slugging type 3, unstable oscillation flow and relatively stable slug flow were observed in the considered flow rates. Severe slugging type 3 characterized by premature gas penetration occurs at relatively low flow rates. Both the cycle time and slug length become shorter as the gas flow rate increases. The pressure at the riser base undergoes a longer period and larger amplitude of fluctuation as compared with the other two flow regimes. Additionally, severe slugging leads to the most vigorous in-plane vibration. However, the responses in the vertical and horizontal directions are not synchronized. The vertical vibration is dominated by the second mode while the horizontal vibration is dominated by the first mode. Similar to the vortex-induced vibration, three branches are identified as initial branch, build-up branch and descending branch for the response versus the mixture velocity of gas-liquid flow.     

基于OLGA的海底管道水合物的生成和影响因素分析

海底管道是海洋油气输送的重要纽带。为解决海底管道面临的水合物生成和堵塞问题,文章结合海底多相流管道水合物生成的数学模型,采用OLGA对海底管道不同含水率、气油比和流量下水合物的生成情况进行数值模拟。研究结果表明：在某海底管道的工艺参数条件下,水合物生成风险随着含水率和流量的增大而降低,随着气油比的增大而增加;海底水平管路和立管都有可能生成水合物,尤其立管常是水合物最大生成量的位置;模拟结果可为海底管道水合物的防治和保障海底管道的安全运行提供参考。     

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.     

Physical modelling of chemical compaction,overpressure development,hydraulic fracturing and thrust detachments in organic-rich source rock

Geological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. To investigate processes of chemical compaction, overpressure development and hydraulic fracturing, we have developed new techniques of physical modelling in a closed system. During the early stages of our work, we built and deformed models in a small rectangular box (40 × 40 × 10 cm), which rested on an electric flatbed heater; but more recently, in order to accommodate large amounts of horizontal shortening, we used a wider box (77 × 75 × 10 cm). Models consisted of horizontal layers of two materials: (1) a mixture of equal initial volumes of silica powder and beeswax micro-spheres, representing source rock, and (2) pure silica powder, representing overburden. By submerging these materials in water, we avoided the high surface tensions, which otherwise develop within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. During heating, the basal temperature of the model surpassed the melting point of beeswax (∼62 °C), reaching a maximum of 90 °C. To investigate tectonic contexts of compression or extension, we used a piston to apply horizontal displacements.In experiments where the piston was static, rapid melting led to vertical compaction of the source layer, under the weight of overburden, and to high fluid overpressure (lithostatic or greater). Cross-sections of the models, after cooling, revealed that molten wax had migrated through pore space and into open hydraulic fractures (sills). Most of these sills were horizontal and their roofs bulged upwards, as far as the free surface, presumably in response to internal overpressure and loss of strength of the mixture. We also found that sills were less numerous towards the sides of the box, presumably as a result of boundary effects. In other experiments, in which the piston moved inward, causing compression of the model, sills also formed. However, these were thicker than in static models and some of them were subject to folding or faulting. For experiments, in which we imposed some horizontal shortening, before the wax had started to melt, fore-thrusts and back-thrusts developed across all of the layers near the piston, producing a high-angle prism. In contrast, as soon as the wax melted, overpressure developed within the source layer and a basal detachment appeared beneath it. As a result, thin-skinned thrusts propagated further into the model, producing a low-angle prism. In some experiments, bodies of wax formed imbricate zones within the source layer.Thus, in these experiments, it was the transformation, from solid wax to liquid wax, which led to chemical compaction, overpressure development and hydraulic fracturing, all within a closed system. According to the measurements of overpressure, load transfer was the main mechanism, but volume changes also contributed, producing supra-lithostatic overpressure and therefore tensile failure of the mixture.     

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。     

Experiments with scale models of oil collectors for subsea well blowouts

Laboratory experiments on conically shaped oil collectors beneath a marine riser are described. The collection concept involves oil, water and gas entering the collector and being driven into a separating sytem by gas-lift. A parametric study involving various collector shapes, heights above the wellhead, and gas, oil and water flow rates was carried out. The important dimensionless variables were identified and quantified with the conclusion that effective collection is possible if the collector height is sufficiently small and the ratio of water pumped to gas flow is sufficiently large. Increased collector heights can be accommodated at the expense of requiring increased water flows by use of larger diameter risers. The amount of gas required for most efficient oil collection is found to be much less than is expected to come from most blowouts when a single collector and riser system is used. This difficulty can be greatly diminished by the use of a specially designed gas-separating collector which passes most of the gas to the surface through a riser separate from the one which carries the liquids.     

非平坦海床下缓波型立管转移过程中的参数分析

随着海上油气田探索及开发水域不断加深,通常会遇到有坡度的海床,这对缓波型立管的安装转移提出了更高的要求。基于虚功原理和最小势能原理,建立了缓波型立管的力学分析模型。基于该模型在非平坦海床条件下对缓波型立管转移过程做参数分析,研究海床坡度角、安装船张力、浮筒段长度、船与平台的水平间距等参数对缓波型立管转移过程的影响。研究发现：随着海床坡度角的增大,立管转移过程中的拱弯弯曲程度加剧,上拱点、下垂点均上移,悬挂点张力及最大剪力均减小;在非平坦海床下,随着影响参数的改变,立管上拱点、下垂点和悬挂点张力变化的拐点和趋势不同。该结论为在非平坦海床海洋环境下缓波型立管的设计与应用提供一定的参考。     

Behavior of unbonded flexible risers subject to axial tension

Owing to nonlinear contact problems with slip and friction, a lot of limiting assumptions are made when developing analytical models to simulate the behavior of an unbonded flexible riser. Meanwhile, in order to avoid convergence problems and excessive calculating time associated with running the detailed finite element （FE） model of an unbonded flexible riser, interlocked carcass and zeta layers with complicated cross section shapes are replaced by simple geometrical shapes （e.g. hollow cylindrical shell） with equivalent orthotropic materials. But the simplified model does not imply the stresses equivalence of these two layers. To solve these problems, based on ABAQUS/Explicit, a numerical method that is suitable for the detailed FE model is proposed. In consideration of interaction among all component layers, the axial stiffness of an eight-layer unbonded flexible riser subjected to axial tension is predicted. Compared with analytical and experimental results, it is shown that the proposed numerical method not only has high accuracy but also can substantially reduce the calculating time. In addition, the impact of the lay angle of helical tendons on axial stiffness is discussed.     

Numerical Prediction of Vortex Induced Vibrations on Top Tensioned Riser in Consideration of Internal Flow

In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the vortex-induced vibration （VIV） of top tensioned riser （Till）, the differential equation is derived based on work-energy principles and the riser near wake dynamics is modeled by Facchinetti＇ s wake oscillator model. Then Galerkin＇ s finite element approximation is implemented to derive the nonlinear matrix equation of the coupled equations and file corresponding numerical programs are compiled which solve the coupled equations directly in the time domain. The comparison of the predicted results with the recent experimental results and the prediction of SHEAR7 is performed. The results show the validity of the proposed method on the prediction of VIV of deep water risers. The effect of internal flow on the dynamic characteristics and dynmnic response of the riser is analyzed and several valuable conelusions are drawn.     

新型深水钻井隔水管悬挂系统设计

南海海域台风频发,对于深水浮式钻井平台,常规避台模式要求回收所有隔水管,但是在深水、超深水海域,这种应对台风的方案有其局限性。据测算,水深超过1 500 m时,台风准备时间(回收所有隔水管、处理井口的作业时间)超过4天,而目前天气预报的水平只能提前4～5天,因此常规避台难度极大,经常会遭遇到还没有起完隔水管,台风已经逼近平台位置的情况,悬挂隔水管成为一种不得不面对的防台避台方案。硬悬挂和软悬挂是目前悬挂隔水管两种常见的模式,由于软悬挂能够补偿和缓解钻井船升沉施加在悬挂隔水管上的动态载荷,因此业内普遍认为软悬挂模式在提高悬挂隔水管对海洋环境的适应能力方面具有一定的优势,但是常规的软悬挂模式作业程序复杂,而且实施过程中存在伸缩节、张力器液缸等相关设备冲程超标的风险。通过对隔水管悬挂状态下横向和轴向的动力响应进行分析,确定了限制隔水管悬挂安全的主要因素,针对隔水管应力超标和转角过大、干涉等风险,设计了一种具有加装扶正器的锥形悬挂短节;针对悬挂隔水管的轴向压缩风险,研制了一种具有补偿功能的隔水管悬挂装置,降低了悬挂隔水管动态载荷产生轴向压缩和动态应力超标的风险。定量评估表明使用新型隔水管悬挂系统,可以将常规的悬挂窗口由不足10年一遇波流环境条件提高到满足100年一遇的波流环境条件,大大提高了悬挂隔水管的安全性。