Experimental Study on Vortex-Induced Vibration of Rough Risers with Coupling Interference Effect Under Side-by-Side Arrangement

A vortex-induced vibration(VIV)experiment of rough risers with coupling interference effect under a side-by-side arrangement was carried out in a wave-current combined flume.The roughness of the riser was characterized by arranging different specifications of surface attachments on the surface of the riser.Rough risers with three different roughnesses were arranged side by side with smooth risers to explore the VIV response of the riser under the combined action of roughness and interference effect,and to reveal the coupling mechanism between roughness and interference effect.The experimental results show that,compared with that of a smooth riser,the VIV of a rough riser under the coupling interference effect has a wider"lock-in"region,and the displacement decreases more significantly at a high reduced velocity,which is more likely to excite higher-order modes and frequency responses.In addition,the displacement response and frequency response of the smooth riser are not significantly affected by wake interference from the rough riser,which is caused by the decrease of the wake region due to the delay of the boundary layer separation point of the rough riser.

Sparse Modal Decomposition Method Addressing Underdetermined Vortex-Induced Vibration Reconstruction Problem for Marine Risers

When investigating the vortex-induced vibration(VIV)of marine risers,extrapolating the dynamic response on the entire length based on limited sensor measurements is a crucial step in both laboratory experiments and fatigue monitoring of real risers.The problem is conventionally solved using the modal decomposition method,based on the principle that the response can be approximated by a weighted sum of limited vibration modes.However,the method is not valid when the problem is underdetermined,i.e.,the number of unknown mode weights is more than the number of known measurements.This study proposed a sparse modal decomposition method based on the compressed sensing theory and the Compressive Sampling Matching Pursuit(Co Sa MP)algorithm,exploiting the sparsity of VIV in the modal space.In the validation study based on high-order VIV experiment data,the proposed method successfully reconstructed the response using only seven acceleration measurements when the conventional methods failed.A primary advantage of the proposed method is that it offers a completely data-driven approach for the underdetermined VIV reconstruction problem,which is more favorable than existing model-dependent solutions for many practical applications such as riser structural health monitoring.

Vibration Control of A Flexible Marine Riser System Subject to Input Dead Zone and Extraneous Disturbances

An observer-based adaptive backstepping boundary control is proposed for vibration control of flexible offshore riser systems with unknown nonlinear input dead zone and uncertain environmental disturbances.The control algorithm can update the control law online through real-time data to make the controller adapt to the environment and improve the control precision.Specifically,based on the adaptive backstepping framework,virtual control laws and Lyapunov functions are designed for each subsystem.Three direction interference observers are designed to track the timevarying boundary disturbance.On this basis,the inverse of the dead zone and linear state transformation are used to compensate for the original system and eliminate the adverse effects of the dead zone.In addition,the stability of the closed-loop system is proven by Lyapunov stability theory.All the system states are bounded,and the vibration offset of the riser converges to a small area of the initial position.Finally,four examples of flexible marine risers are simulated in MATLAB to verify the effectiveness of the proposed controller.

Dynamic Analysis of A Deepwater Drilling Riser with A New Hang-off System

The safety of risers in hang-off states is a vital challenge in offshore oil and gas engineering.A new hang-off system installed on top of risers is proposed for improving the security of risers.This approach leads to a challenging problem:coupling the dynamics of risers with a new hang-off system combined with multiple structures and complex constraints.To accurately analyze the dynamic responses of the coupled system,a coupled dynamic model is established based on the Euler-Bernoulli beam-column theory and penalty function method.A comprehensive analysis method is proposed for coupled dynamic analysis by combining the finite element method and the Newmarkβmethod.An analysis program is also developed in MATLAB for dynamic simulation.The simulation results show that the dynamic performances of the risers at the top part are significantly improved by the new hang-off system,especially the novel design,which includes the centralizer and articulation joint.The bending moment and lateral deformation of the risers at the top part decrease,while the hang-off joint experiences a great bending moment at the bottom of the lateral restraint area which requires particular attention in design and application.The platform navigation speed range under the safety limits of risers expands with the new hang-off system in use.

Radial Non-uniformity Index Research on High-density,High-flux CFB Riser with Stratified Injection

A high-density, high-flux circulating fluidized bed (CFB) riser (100 mm in ID and 10.614 m in height) was applied in a wide range of operating conditions (with solid fluxes up to 400 kg/m2s and superficial gas velocities up to 12 m/s) to examine its radial non-uniformity dynamics. The solids holdup was determined through the use of a fiber-optic probe at 11 axial levels. The results indicated that under all operating conditions, the high superficial gas velocity and low solid fluxes maintained a low radial non-uniformity index (RNI). The high-density/flux CFB riser had several unique characteristics, so that the peak of the radial solids holdup profile occurred at a position with r/R=0.8. The RNI and solids holdup at the cross-sectional position had a good logarithmic relationship at the low-density condition (with a mean solids holdup of <0.2), and the RNI decreased when the mean solids holdup exceeded 0.2. Investigation of the dynamics of stratified injection revealed that the feed ratio had an important effect on Gs and on solids holdup distribution. A novel "<" shaped axial solids holdup profile was found. Gs decreased sharply when the up-flow feed ratio exceeded 0.5, and RNI was lowest when the up-flow feed ratio was 1.

Mapping the Wall-Region Dynamics of High-Flux Gas-Solid Riser Using Scaling Regions from the Solid Concentration Time Series

An experimental study of the gas-solid flow dynamics in the high-flux CFB riser was accomplished by analysing the scaling regions from solid concentration signals collected from a 76 mm internal diameters and 10 m high riser of a circulating fluidized bed (CFB) system. The riser was operated at 4.0 to 10.0 m/s gas velocity and 50 to 550 kg/m2s solids flux. Spent fluid catalytic cracking (FCC) catalyst particles of 67 μm mean diameter and 1500 kg/m3 density together with 70% to 80% humid air was used. Solid concentration data were analysed using codes prepared in FORTRAN 2008 to get correlation integrals at different embedding dimensions and operating conditions and plot their profiles. Scaling regions were identified by visual inspection method and their location on planes determined. Scaling regions were analysed based on operating conditions and riser spatial locations. It was found that scaling regions occupy different locations on the plane depending on the number of embedding dimensions and operating conditions. As the number of embedding dimensions increases the spacing between scaling regions decreases until it saturates towards higher embedding dimensions. Slopes of scaling regions increases with embedding dimensions until saturation where they become constant. Slopes of scaling regions towards the wall decrease while the number of scaling regions for a particular profile increases. The span of the scaling region is wider at the initial values of hyperspherical radius than its final values. The scaling regions in some flow development sections show multifractal behaviour for each embedding dimension which manifests into visible basin which is defined in this study as multifractal basin. Further, the end points of the scaling region for each correlation integral profile differ from each other as the embedding dimension changes. This study suggests that identification of scaling region by visual inspection method is useful in understanding the gas-solid flow dynamics in the High-Flux CFB riser system. Further studies are recommended on risers of different diameters and heights operated at low and high solid fluxes and different gas velocities for comparison or usage of time series of different signal types like pressure fluctuations.

Analysis of Flow Dynamics in the High-Flux Gas-Solid Riser Using Trajectory Distances across Attractors Reconstructed from Solid Concentration Signals

The study of the entrance and wall dynamics of a high-flux gas-solid riser was conducted using trajectory distances of the reconstructed attractors from solid concentration signals collected from a 76 mm internal diameters and 10 m high riser of a circulating fluidized bed (CFB) system. The riser was operated at 4.0 to 10.0 m/s gas velocity and 50 to 550 kg/m2s solids flux. Spent fluid catalytic cracking (FCC) catalyst particles with 67 μm mean diameter and density of 1500 kg/m3 together with 70% to 80% humid air was used. Solid concentration data were analyzed using codes prepared in FORTRAN 2008 to get trajectories of the reconstructed attractors and their distances apart. Trajectory distances were found to increase from the centre towards the wall indicating the expansion of the attractor. The probability density function (PDF) of the trajectory distances changes from single peak at the centre to multiple peaked profiles in the wall region. Multiple peaked profiles indicate multifractal flow behaviours. Cumulative distribution functions (CDF) of the trajectory distances changes from single S-shaped at the centre to multiple S-shaped profiles in some locations of the wall region indicating multifractal flow behaviours. The PDF distribution of these distances at the entrance section and in the wall region forms different types of statistical distributions showing differences in gas-solid flow structures in various spatial locations of the wall region and the entrance sections. Most of the distributions at the centre fall under the Gumbel max distribution for all flow development sections of the riser, especially at air velocities of 5.5 m/s and 8 m/s showing uniform flow structures. Further, it was found that increase of the number of the phase space reconstruction embedding dimension increases the trajectory distances between the state vectors leading to the expansion of the attractor.

Mapping Correlation Dimension along the Wall Region of a High-Flux Gas-Solid Riser Using Embedded Solid Concentration Time Series

Analysis of the entrance and wall dynamics of a high-flux gas-solid riser was conducted using embedded solid concentration time series collected from a 76 mm internal diameter and 10 m high riser of a circulating fluidized bed (CFB) system. The riser was operated at 4.0 to 10.0 m/s air velocity and 50 to 550 kg/m2s solids flux of spent fluid catalytic cracking (FCC) catalyst particles with 67 μm mean diameter and density of 1500 kg/m3. Data were analyzed using prepared FORTRAN 2008 code to get correlation integral followed by determination of correlation dimensions with respect to the hyperspherical radius and their profiles, plots of which were studied. It was found that correlation dimension profiles at the centre have single peak with higher values than the wall region profiles. Towards the wall, these profiles have double or multiple peaks showing bifractal or multifractal flow behaviors. As the velocity increases the wall region profiles become random and irregular. Further it was found that, as the height increases the correlation dimension profiles shift towards higher hyperspherical radius at the centre and towards lower hyperspherical radius in the wall region at r/R = 0.81. The established method of mapping correlation dimension profiles in this study forms a suitable tool for analysis of high-flux riser dynamics compared to other analyses approaches. However, further analysis is recommended to other gas-solid CFB riser of different dimensions operated at high-flux conditions using the established method.

A New Simulation Model for Recoil Analysis of Deep-Water Drilling Risers After Emergency Disconnection

The recoil response of a deep-water drilling riser following an ED(Emergency Disconnection)scenario is a transient and sensitive process.The recoiling displacement of the riser is the resultant of recoil motion and axial stretch.How-ever,it is typically represented by one variable in recoil simulations.As axial deformation is quite small compared with axial motion in the recoil process,it inevitably introduces numerical errors(i.e.,a large number annihilating a small number).Thus,it is hard to perform a quantitative analysis of axial deformation,although a consensus initial deformation is essential for recoil dynamics.Moreover,the triggered axial natural modes have never been examined before.In this study,the recoil response is decomposed into two parts:recoil motion and axial deformation,and a novel model is developed by Galerkin method.It has demonstrated that the initial stretch has a significant effect at the initial stage in recoil.The existing models underestimate the effects of axial deformation.The new model can capture information of triggered natural modes and figure out the modes undergoing dynamic compression.This study can be beneficial to overpull setting,determination of ED time and anti-recoil control optimization.

Abandonment and Recovery Operation of Steel Lazy-Wave Riser in Deepwater by Controlled Vessel and Cable Velocity Rate

Evaluation of abandonment and recovery operation of steel lazy-wave riser in deepwater is presented in this paper.The calculation procedure includes two single continuous SLWR and cable segments, which are coupled together to form the overall mathematical model. Then the equilibrium equations of SLWR and cable are established based on minimum total potential energy principle. The coupled equations are discretized by the finite difference method and solved by Newton-Raphson technique in an iterative manner. The present method is validated by well-established commercial code OrcaFlex. Recovery methods by considering different ratios of vessel’s moving velocity to cable’s recovery velocity are evaluated to optimize the abandonment and recovery operation. In order to keep the tension more stable during the recovery process, the rate ratio before leaving the seabed is increased, and the rate ratio after leaving the seabed is reduced.

Linear analysis of the dynamic response of a riser subject to internal solitary waves

The flow field induced by internal solitary waves(ISWs)is peculiar wherein water motion occurs in the whole water depth,and the strong shear near the pycnocline can be generated due to the opposite flow direction between the upper and lower layers,which is a potential threat to marine risers.In this paper,the flow field of ISWs is obtained with the Korteweg-de Vries(Kd V)equation for a two-layer fluid system.Then,a linear analysis is performed for the dynamic response of a riser with its two ends simply supported under the action of ISWs.The explicit expressions of the deflection and the moment of the riser are deduced based on the modal superposition method.The applicable conditions of the theoretical expressions are discussed.Through comparisons with the finite element simulations for nonlinear dynamic responses,it is proved that the theoretical expressions can roughly reveal the nonlinear dynamic response of risers under ISWs when the approximation for the linear analysis is relaxed to some extent.

Experimental Study on Vortex-Induced Vibration of Marine Riser Model with Coupling Interference Effect Under Combined Internal and External Flow

A partition model of interference efficiency was constructed to study the coupling interference effect under combined internal and external flow.The concept of“internal flow efficiency”,“velocity ratio”and“interference efficiency”were introduced to quantify the effect of internal flow and interference,and reveal the coupling mechanism among internal flow,external flow and interference effect.The results showed that the dynamic response of risers under variable angles was significantly different after considering the effect of internal flow.When the external flow velocity was smaller than 0.25 m/s,the vibration of risers was promoted by the internal flow.With the increase of external flow velocity,the effect of internal flow was weakened and the dynamic response of riser mainly depended on the external flow and interference effect.Under the effect of different internal flow,the interference efficiency had similar change trend.The interference effect amplified the complex secondary flow effect inside the riser,making the dynamic response of riser complex and random.In this paper,the overlap area and subdivision criterion of interference effect were constructed within the range of experimental velocity ratio,and the change curve of interference efficiency was obtained with an average meaning,which may have important practical meaning.

Dynamic Analysis of A Deep-Water Compliant Vertical Access Riser with A Variable Length During Installation

Compliant vertical access risers(CVAR)have broad application prospects in deep-water oil and gas transportation.However,the mechanical behaviors of the CVAR with a variable length during installation remains unclear.To address this issue,based on the flexible segment method,a model of CVAR with a variable length during installation is established in this study,which is verified by the comparison with commercial software.Then,the mechanical behaviors of CVAR during installation are investigated.The results reveal that the CVAR configuration is significantly affected by the buoyancy blocks.The streamwise displacement of CVAR increases with the increase of current velocity.When the BOP weight is insuffcient,obvious upbending is observed in the lower region and transition region,leading to local compression.When the platform moves in the opposite direction to the current,the maximum stress is larger than that of the scenario when the platform moves in the same direction as the current.

Deformation and Stress Analysis of the Deepwater Steel Lazy Wave Riser Subjected to Internal Solitary Waves

The dynamic response of the steel lazy wave riser(SLWR)subjected to the internal solitary wave is a key to assessing its application feasibility.The innovation of this paper is to study the dynamic response properties of the SLWR with large deformation characteristics under internal wave excitation.A numerical scheme of the SLWR is constructed using the slender-rod theory,and the internal solitary wave(ISW)with a two-layer seawater model is simulated by the extended Korteweg-deVries equation.The finite element method combined with the Newmark-βmethod is applied to discretize the equations and update the time integration.The ISW excitation combined with vessel motion on the dynamic deformation and stress of the SLWR is investigated,and extensive simulations of the ISW parameters,including the interface depth ratio and density difference,are carried out.Case calculation reveals that the displacement of the riser in the lower interface layer increases significantly under the ISW excitation,and the stresses at a part of both ends grow evidently.Moreover,the mean value of riser responses under a combination of vessel motion and ISW coincides with the ISW-induced ones.Furthermore,the dynamic responses along the whole riser,including the displacement amplitudes,bending moment amplitudes,and stress amplitudes,almost increase with the increase in interface depth ratios and density differences.

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.

深水隔水管-防喷器组耦合系统动力学建模与仿真验证

在深水钻井过程中,隔水管与防喷器组系统是整个作业环节的重要一环。由于在作业过程中面临着诸多风险,因此,建立精确的隔水管-防喷器组耦合系统力学分析模型,准确分析其力学行为和性能,对指导钻井作业安全进行具有重要意义。目前大多只关注于对隔水管的独立建模而忽视防喷器组的潜在影响,这将导致所建模型与实际情况存在差异,使得隔水管系统的力学特性无法得到精确分析。提出了隔水管与防喷器组刚柔耦合概念,推导耦合系统动能、势能,采用拉格朗日方法建立理论模型,用Newmark-β直接积分法对动力学模型进行数值计算。采用ADAMS软件进行了仿真建模,开展不同工况条件下的动态响应对比分析。结果表明,理论模型得到的隔水管横向位移包络线、弯矩包络线、隔水管中部节点横向位移时程曲线和底部弯矩时程曲线与ADAMS仿真结果吻合良好,从而表明了该理论模型的重要性,可为我国隔水管的设计和分析提供参考和支持。

管卡位置对海管立管的敏感性分析

以海洋平台海管立管为研究对象,考虑实际海况和立管实际结构特点,研究多层立管之间及多层立管与平台之间的受力耦合作用。通过有限元分析软件,建立具有典型代表性的海洋多层立管数值模型。针对管卡位置对海管立管的结构影响,基于管卡位置对海管立管的结构强度和疲劳寿命进行分析。根据分析结果,对管卡的安装位置进行优化,为现场作业提供理论指导,进一步为海上工程设计和施工提供科学依据和安全保障。

基于拉法尔喷管的新型放空立管可行性研究

针对传统放空立管介质出口速度小、扩散高度低等缺点。文章基于理论分析和模拟的手段对新型放空立管进行可行性研究。结果表明,放空介质动能来源于进出口压差及介质焓值,放空过程是一个压降、温降过程;通过对新型放空立管合理设计,在不改变进出口条件下,可比传统立管出口速度提高37.5%~41.7%,提高可燃气体的扩散高度,降低对地面工作人员的潜在危害。该新型放空立管切实可行,可为今后放空立管的设计提供新的思路。

平台运动对柔性立管涡激振动影响的实验研究

基于自循环实验水槽开展了存在平台纵荡和横荡运动时的悬链线型柔性立管涡激振动响应测试,模型立管的长径比为125,测试的平均约化速度范围为4.40~39.33.利用激光位移传感器和高速摄像机分别监测了平台运动和立管振动位移,从响应频率、振幅和振动形态等方面分析了平台运动对立管振动的影响.根据立管振动的主导频率与平台运动的主导频率是否相同,区分了强耦联和弱耦联现象,定义了强耦联的立管长度,其在立管振动的模态过渡组次相对较短.立管振动与平台运动存在两个方向均耦联、仅单一方向耦联和不耦联3种情形.其中,立管平面外振动受平台运动的影响较平面内振动小.以立管瞬时形态的波腹个数变化量化了模态竞争和平台运动对振动形态的影响.通过对比过滤平台固有频率前后的立管振幅、振形与主导频率的时空分布,用强耦合的时间权重和空间长度量化了平台对立管振动耦合的影响,发现高约化速度时,平台运动抑制了立管空间的频率竞争,也抑制了立管平面内与平面外振动的耦合响应.

深水钻井隔水管横向振动振型分析

为了分析深水钻井隔水管的横向振动固有频率和振型,综合考虑深水隔水管受到的顶部张紧力、海洋环境载荷、钻井平台漂移等作用力,借助微元法建立受力分析模型。根据连续梁理论和隔水管单元的动力学微分方程,确定其无阻尼自然振动基本方程,并进一步剖析建立隔水管横向振动固有频率微分方程和模态振型基本方程,然后进行数值求解。使用控制变量法逐步分析隔水管长度、张力比、自身材质对隔水管固有频率的影响和变化规律。通过MATLAB软件表现出不同阶数的振动振型,结果表明:控制张力比、合理选择隔水管类型能够有效提高固有频率,避免与波流力形成共振,进而保证深水钻井作业安全。此外,隔水管的振型并不是标准正弦曲线,振型会受时间的影响不断变化。