Parametric Instability Analysis of Deepwater Top-Tensioned Risers Considering Variable Tension Along the Length

Parametric instability of a riser is caused by fluctuation of its tension in time due to the heave motion of floating platform. Many studies have tackled the problem of parametric instability of a riser with constant tension. However, tension in the riser actually varies linearly from the top to the bottom due to the effect of gravity. This paper presents the parametric instability analysis of deepwater top-tensioned risers(TTR) considering the linearly varying tension along the length. Firstly, the governing equation of transverse motion of TTR under parametric excitation is established. This equation is reduced to a system of ordinary differential equations by using the Galerkin method. Then the parametric instability of TTR for three calculation models are investigated by applying the Floquet theory. The results show that the natural frequencies of TTR with variable tension are evidently reduced, the parametric instability zones are significantly increased and the maximum allowable amplitude of platform heave is much smaller under the same damping; The nodes and antinodes of mode shape are no longer uniformly distributed along the axial direction and the amplitude also changes with depth, which leads to coupling between the modes. The combination resonance phenomenon occurs as a result of mode coupling, which causes more serious damage.     

A nonlinear vortex induced vibration model of marine risers

With the exploitation of oil and gas in deep water, the traditional vortex induced vibration (VIV) theory is challenged by the unprecedented flexibility of risers. A nonlinear time-dependent VIV model is developed in this paper based on a VIV lift force model and the Morison equation. Both the inline vibration induced by the flow due to vortex shedding and the fluid-structure interaction in the transverse direction are included in the model. One of the characteristics of the model is the response-dependent lift force with nonlinear damping, which is different from other VIV models. The calculations show that the model can well describe the VIV of deepwater risers with the results agreeing with those calculated by other models.     

Combined action of uniform flow and oscillating flow around marine riser at low Keulegan-Carpenter number

With the increase of petroleum and gas production in deep ocean, marine risers of circular cylinder shape are widely used in the offshore oil and gas platform. In order to research the hydrodynamic performance of marine risers, the dynamic mesh technique and User-Defined Function （UDF） are used to simulate the circular cylinder motion. The motion of a transversely oscillat-ing circular cylinder in combination of uniform flow and oscillating flow is simulated. The uniform flow and oscillating flow both are in x direction. SIMPLE algorithm is used to solve the Navier-Stokes equations. The User-Defined Function is used to control the cylinder transverse vibration and the inlet flow. The lift and drag coefficient changing with time and the map of vorticity isolines at different phase angle are obtained. Force time histories are shown for uniform flow at Reynolds number （Re） of 200 and for the com-bination of uniform and oscillating flows. With the increase of amplitude of oscillating flow in combined flow, the change of lift am-plitude is not sensitive to the the change of cylinder oscillating frequency. Lift amplitude increases with the increase of oscillating flow amplitude in the combined flow, but there is no definite periodicity of the lift coefficient. The drag and inertia force coefficients change when the maximum velocity of the oscillating flow increases in the combined flow. The vortex shedding near the circular cylinder shows different characteristics.     

Investigation of a new vortex-induced vibration suppression device in laboratory experiments

In order to mitigate vortex-induced vibration (VIV) of marine risers, especially to eliminate the phenomenon of frequency ’lock-in’, a new suppression device of crescent-shaped flow spoiler was designed with seven different layout schemes. VIV model tests with six flow levels were conducted in a large wind-wave-current flume. In all cases, vibration responses in both in-line and cross-flow cases were measured. With the installation of suppression devices vibration frequency evolution of a riser was analyzed by Morlet wavelet transform. The principle of VIV suppression was interpreted through vibration characteristics. Fatigue life of the riser was calculated by the Palmgren-Miner rule. Compared with a bare riser, vibration of an outfitted riser with suppression devices disturbed the steady flow, the vibration amplitudes in the two flow directions were reduced, and the riser fatigue life was improved.     

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.     

Suppression of Vortex-Induced Vibration by Fairings on Marine Risers

Vortex-induced vibration is quite common during the operation of offshore risers or umbilical cables,commonly leading to serious damage to risers and reduced service life.Vortex-induced vibration of the offshore risers could be effectively suppressed by fairing devices.In this paper,a newly developed vortex-induced vibration fairing and large eddy simulation model of the FLUENT software were used for numerical analysis,experimental research and stimulating vortex-induced vibration at 0.1–2 ms^-1.The data of the numerical model with fairing was compared and analyzed to study the vortex shedding frequency at different Reynolds numbers and changes in drag and lift coefficients.The displacement state of 12 in risers with and without fairing was experimentally tested using a five degree-of-freedom balance.The vortex-induced vibration effect of the fairing was tested at different velocities.The result shows the drag reduction effect of the fairing is more obvious when the flow velocity is 0.4–1.2 ms^-1 and the maximum drag reduction reaches 55.6%when the flow velocity is 0.6 ms^-1.Additionally,the drag reduction effect was obvious when the flow velocity was greater than 1.3 ms^-1 and less than 0.3.The result indicates that the developed 12 in fairing,with good potential in engineering applications,has good vortex-induced vibration-suppression effects.     

Deep-water riser fatigue monitoring systems based on acoustic telemetry

Marine risers play a key role in the deep and ultra-deep water oil and gas production. The vortex-induced vibration (VIV) of marine risers constitutes an important problem in deep water oil exploration and production. VIV will result in high rates of structural failure of marine riser due to fatigue damage accumulation and diminishes the riser fatigue life. In-service monitoring or full scale testing is essential to improve our understanding of VIV response and enhance our ability to predict fatigue damage. One marine riser fatigue acoustic telemetry scheme is proposed and an engineering prototype machine has been developed to monitor deep and ultra-deep water risers’ fatigue and failure that can diminish the riser fatigue life and lead to economic losses and eco-catastrophe. Many breakthroughs and innovation have been achieved in the process of developing an engineering prototype machine. Sea trials were done on the 6th generation deep-water drilling platform HYSY-981 in the South China Sea. The inclination monitoring results show that the marine riser fatigue acoustic telemetry scheme is feasible and reliable and the engineering prototype machine meets the design criterion and can match the requirements of deep and ultra-deep water riser fatigue monitoring. The rich experience and field data gained in the sea trial which provide much technical support for optimization in the engineering prototype machine in the future.     

Numerical simulation for accuracy of velocity analysis in small-scale high-resolution marine multichannel seismic technology

When used with large energy sparkers, marine multichannel small-scale high-resolution seismic detection technology has a high resolution, high-detection precision, a wide applicable range, and is very flexible. Positive results have been achieved in submarine geological research, particularly in the investigation of marine gas hydrates. However, the amount of traveltime difference information is reduced for the velocity analysis under conditions of a shorter spread length, thus leading to poorer focusing of the velocity spectrum energy group and a lower accuracy of the velocity analysis. It is thus currently debatable whether the velocity analysis accuracy of short-arrangement multichannel seismic detection technology is able to meet the requirements of practical application in natural gas hydrate exploration. Therefore, in this study the bottom boundary of gas hydrates (Bottom Simulating Reflector, BSR) is used to conduct numerical simulation to discuss the accuracy of the velocity analysis related to such technology. Results show that a higher dominant frequency and smaller sampling interval are not only able to improve the seismic resolution, but they also compensate for the defects of the short-arrangement, thereby improving the accuracy of the velocity analysis. In conclusion, the accuracy of the velocity analysis in this small-scale, high-resolution, multi-channel seismic detection technology meets the requirements of natural gas hydrate exploration.     

INTERNAL WAVE EIGENMODES AND THEIR WEAK NONLINEAR RESONANT INTERACTION IN A NONLINEARLY STRATIFIED FLUID- A PRELIMINARY THEORETICAL AND EXPERIMENTAL STUDY

A preliminary theoretical and experimental study was conducted on internal wave modes and their weak nonlinear resonant interaction in a nonlinearly stratified fluid . An asymptotical solution of the modes and a dispersion relation of internal waves in a stratified fluid with density profile similar to that in our experiment were obtained theoretically . The resonant-interaction mechanism to 2nd order approximation is also discussed . The resonant interaction of the 3rd and 4th mode internal waves excited by the unstable 1st mode wave is analyzed on the basis of data obtained by conductivity probes. The resonant-interaction condition, , is examined . It is shown that the resonant instability increases with pycnocline thickness and wave maker driving frequency .     

Numerical analysis of wellbore instability in gas hydrate formation during deep-water drilling

Gas hydrate formation may be encountered during deep-water drilling because of the large amount and wide distribution of gas hydrates under the shallow seabed of the South China Sea. Hydrates are extremely sensitive to temperature and pressure changes, and drilling through gas hydrate formation may cause dissociation of hydrates, accompanied by changes in wellbore temperatures, pore pressures, and stress states, thereby leading to wellbore plastic yield and wellbore instability. Considering the coupling effect of seepage of drilling fluid into gas hydrate formation, heat conduction between drilling fluid and formation, hydrate dissociation, and transformation of the formation framework, this study established a multi-field coupling mathematical model of the wellbore in the hydrate formation. Furthermore, the influences of drilling fluid temperatures, densities, and soaking time on the instability of hydrate formation were calculated and analyzed. Results show that the greater the temperature difference between the drilling fluid and hydrate formation is, the faster the hydrate dissociates, the wider the plastic dissociation range is, and the greater the failure width becomes. When the temperature difference is greater than 7℃, the maximum rate of plastic deformation around the wellbore is more than 10%, which is along the direction of the minimum horizontal in-situ stress and associated with instability and damage on the surrounding rock. The hydrate dissociation is insensitive to the variation of drilling fluid density, thereby implying that the change of the density of drilling fluids has a minimal effect on the hydrate dissociation. Drilling fluids that are absorbed into the hydrate formation result in fast dissociation at the initial stage. As time elapses, the hydrate dissociation slows down, but the risk of wellbore instability is aggravated due to the prolonged submersion in drilling fluids. For the sake of the stability of the wellbore in deep-water drilling through hydrate formation, the drilling fluid with low temperatures should be given priority. The drilling process should be kept under balanced pressures, and the drilling time should be shortened.     

Vector form Intrinsic Finite Element Method for the Two-Dimensional Analysis of Marine Risers with Large Deformations

Robust numerical models that describe the complex behaviors of risers are needed because these constitute dynamically sensitive systems. This paper presents a simple and efficient algorithm for the nonlinear static and dynamic analyses of marine risers. The proposed approach uses the vector form intrinsic finite element (VFIFE) method, which is based on vector mechanics theory and numerical calculation. In this method, the risers are described by a set of particles directly governed by Newton’s second law and are connected by weightless elements that can only resist internal forces. The method does not require the integration of the stiffness matrix, nor does it need iterations to solve the governing equations. Due to these advantages, the method can easily increase or decrease the element and change the boundary conditions, thus representing an innovative concept of solving nonlinear behaviors, such as large deformation and large displacement. To prove the feasibility of the VFIFE method in the analysis of the risers, rigid and flexible risers belonging to two different categories of marine risers, which usually have differences in modeling and solving methods, are employed in the present study. In the analysis, the plane beam element is adopted in the simulation of interaction forces between the particles and the axial force, shear force, and bending moment are also considered. The results are compared with the conventional finite element method (FEM) and those reported in the related literature. The findings revealed that both the rigid and flexible risers could be modeled in a similar unified analysis model and that the VFIFE method is feasible for solving problems related to the complex behaviors of marine risers.     

3种附属管排布方式对主管路水动力特性的影响

【目的】研究均匀流多管束干涉流动下圆柱受迫振动的水动力特性。【方法】基于SSTκ-ω模型,在亚临界雷诺数下(Re=1×10⁵)对多管束共振强迫的涡激振动问题进行二维数值模拟,比较与分析三种典型附属管排布方式对主管路流体动力学特征的影响。【结果与结论】采用模型3下的附属管排布方式可在较大范围的振幅比下(A_y/D=0.1~0.8)有效改善主管路水动力特性:1)有效降低主管路上平均升力系数的幅值;2)抑制在单管路系统中出现的脉动升力系数突变衰减。同时,由于多管束对流动产生干涉效应,主管路上表现的尾迹涡度随着振动幅度的增大而呈现出不同的模式。此外,功率谱密度分析发现,多管束系统相较于单圆柱系统,在频率比为1时,模型2与模型3的共振“锁定”状态得到改善。     

Study of the Sensitive Properties of Marine Gas Hydrate Based on the Prestack Elastic Inversion

Using a bottom simulating reflector(BSR) on a seismic profile to identify marine gas hydrate is a traditional seismic exploration method. However, owing to the abundance differences between the gas hydrate and free gas in different regions, the BSR may be unremarkable on the seismic profile and invisible in certain cases. With the improvement of exploration precision, difficulty arises in meeting the requirements of distinguishing the abundance differences in the gas hydrate based on BSR. Hence, we studied other sensitive attributes to ascertain the existence of gas hydrate and its abundance variations, eventually improving the success rate of drilling and productivity. In this paper, we analyzed the contradiction between the seismic profile data and drilling sampling data from the Blake Ridge. We extracted different attributes and performed multi-parameter constraint analysis based on the prestack elastic wave impedance inversion. Then, we compared the analysis results with the drilling sampling data. Eventually, we determined five sensitive attributes that can better indicate the existence of gas hydrate and its abundance variations. This method overcomes the limitations of recognizing the gas hydrate methods based on BSR or single inversion attribute. Moreover, the conclusions can notably improve the identification accuracy of marine gas hydrate and provide excellent reference significance for the recognition of marine gas hydrate. Notably, the different geological features of reservoirs feature different sensitivities to the prestacking attributes when using the prestack elastic inversion in different areas.     

海底泥底辟(泥火山)对天然气水合物成藏的影响

海底泥底辟(泥火山)与周缘发育的天然气水合物存在着密切的关联,表现在静态要素和动态成藏2个方面。作为一种重要而有效的运移通道类型,泥底辟(泥火山)携带的气体将是天然气水合物的重要气体来源。同时,含气流体沿着泥底辟(泥火山)的上侵,可能会导致上覆地层中温压场和地球化学组分的改变,进而引起天然气水合物稳定带厚度的变化。因此,泥底辟(泥火山)将控制天然气水合物的成藏,如位于构造中心部位的矿物低温热液成藏模式和位于构造边缘的矿物交代成藏模式。另一方面,泥底辟(泥火山)的不同演化阶段将对天然气水合物的形成和富集产生不同的影响。早期阶段,泥底辟(泥火山)形成的运移通道可能并未延伸到天然气水合物稳定带,导致气源供给不够充分;中期阶段,天然气水合物成藏条件匹配良好,利于天然气水合物的生成;晚期阶段,泥火山喷发带来的高热量含气流体引起天然气水合物稳定带的热异常,可能导致天然气水合物的分解,直至泥火山活动平静期,天然气水合物再次成藏。      

Damage localization of marine risers using time series of vibration signals

Based on dynamic response signals a damage detection algorithm is developed for marine risers. Damage detection methods based on numerous modal properties have encountered issues in the researches in offshore oil community. For example, significant increase in structure mass due to marine plant/animal growth and changes in modal properties by equipment noise are not the result of damage for riser structures. In an attempt to eliminate the need to determine modal parameters, a data-based method is developed. The implementation of the method requires that vibration data are first standardized to remove the influence of different loading conditions and the autoregressive moving average (ARMA) model is used to fit vibration response signals. In addition, a damage feature factor is introduced based on the autoregressive (AR) parameters. After that, the Euclidean distance between ARMA models is subtracted as a damage indicator for damage detection and localization and a top tensioned riser simulation model with different damage scenarios is analyzed using the proposed method with dynamic acceleration responses of a marine riser as sensor data. Finally, the influence of measured noise is analyzed. According to the damage localization results, the proposed method provides accurate damage locations of risers and is robust to overcome noise effect.     

Accumulation and exploration of gas hydrate in deep-sea sediments of northern South China Sea

The large deep-sea area from the southwestern Qiongdongnan Basin to the eastern Dongsha Islands,within the continental margin of northern South China Sea,is a frontier of natural gas hydrate exploration in China.Multiform of deep-sea sedimentations have been occurred since late Miocene,and sediment waves as a potential quality reservoir of natural gas hydrate is an most important style of them.Based on abundant available data of seismic,gravity sampling and drilling core,we analyzed the characteristics of seismic reflection and sedimentation of sediment waves and the occurrence of natural gas hydrate hosted in it,and discussed the control factors on natural gas hydrate accumulation.The former findings revealed the deep sea of the northern South China Sea have superior geological conditions on natural gas hydrate accumulation.Therefore,it will be of great significance in deep-sea natural gas hydrate exploration with the study on the relationship between deep-sea sedimentation and natural gas hydrate accumulation.     

基于PID反馈的两级超低频隔振系统理论分析及仿真

建立两级隔振系统的运动模型,推导系统的闭环传递函数并对传递函数进行分析。结果表明：1）系统采用PID反馈较当前超长弹簧中采用的PD反馈具有更好的等效阻尼系数调节功能。2）系统的等效固有周期主要受反馈比例系数kp影响,kp越大,等效固有周期越长,隔振效果越好;kp选定后,积分系数ki和微分系数kd共同影响系统的阻尼,且ki占主导地位,决定系统的稳定性。通过仿真验证了闭环传递函数分析结果的准确性,且仿真结果表明,系统在1 Hz处的隔振效果可达-70 dB。     

Stratigraphic Sequence and Sedimentary Systems in the Middle-Southern Continental Slope of the East China Sea from Seismic Reflection Data: Exploration Prospects of Gas Hydrate

Many evidences for gas hydrate bearing sediments had been found in the continental slope of the East China Sea, such as bottom simulating reflections(BSRs), undersea gas springs, pyrite associated with methane leakage, mud diapirs/mud volcanos, bottom-water methane anomalies and so on. In this study, six key stratigraphic interfaces including T_0(seafloor), T_1(LGM, 23 kyr B.P.), T_2(2.58 Myr), T_3(5.33 Myr), T_4(11.02 Myr) and T_5(16.12 Myr) were identified, and then five third-order sequences of SQIII1 to SQIII5 were divided. However, T5 in southern continental slope is not found, which shows that the middle-northern Okinawa Trough had begun to rift in the early Miocene, earlier than the southern segment. Four system tracts including lowstand systems tract(LST), transgressive systems tract(TST), highstand systems tract(HST) and falling stage systems tract(FSST) are further divided. The marine erosion interface of 11.02 Myr and regressive unconformity interface of 23 kyr B.P. indicate two large-scale sea level drop events in the research area. Seven typical seismic facies identified in the continental slope are continental shelf-edge deltas, littoral fluvial-delta plains, incised channels or submarine canyons, slope fans, submarine fans or coastal sandbars, littoral-neritic finegrained sediments, mud volcanos and some other geological bodies respectively. The minimum water depth for hydrate occurrence in the Okinawa Trough is 630 m, and the thickness of gas hydrate stability zone in continental slope is between 0 and 590 m. The calculated bottom boundary of hydrate stability zone is slightly deeper than BSRs on the seismic sections. The re-depositional turbidite sand bodies, such as canyon channels, slope fans and submarine fans developed in Quaternary strata, are the predominant hydrate reservoirs. According to developing process, the dynamic accumulation of hydrate systems can be divided into three evolutionary stages including canyon erosion and hydrate stability zone migration stage, sediments destabilizing and methane leakage stage, and channel filling and hydrate re-occurrence stage.     

Simulating the effect of hydrate dissociation on wellhead stability during oil and gas development in deepwater

It is well known that methane hydrate has been identified as an alternative resource due to its massive reserves and clean property. However, hydrate dissociation during oil and gas development (OGD) process in deep water can affect the stability of subsea equipment and formation. Currently, there is a serious lack of studies over quantitative assessment on the effects of hydrate dissociation on wellhead stability. In order to solve this problem, ABAQUS finite element software was used to develop a model and to evaluate the behavior of wellhead caused by hydrate dissociation. The factors that affect the wellhead stability include dissociation range, depth of hydrate formation and mechanical properties of dissociated hydrate region. Based on these, series of simulations were carried out to determine the wellhead displacement. The results revealed that, continuous dissociation of hydrate in homogeneous and isotropic formations can causes the non-linear increment in vertical displacement of wellhead. The displacement of wellhead showed good agreement with the settlement of overlying formations under the same conditions. In addition, the shallower and thicker hydrate formation can aggravate the influence of hydrate dissociation on the wellhead stability. Further, it was observed that with the declining elastic modulus and Poisson’s ratio, the wellhead displacement increases. Hence, these findings not only confirm the effect of hydrate dissociation on the wellhead stability, but also lend support to the actions, such as cooling the drilling fluid, which can reduce the hydrate dissociation range and further make deepwater operations safer and more efficient.