Nonlinear dynamics and impact load in float-over installation

A time-domain 3 Degrees of Freedom model is developed to investigate nonlinear dynamics and impact loads during float-over installations, which generally involve multi-body interactions between wave-induced vessel motions and nonlinear constraint components. By replacing the time-consuming convolution in calculating the retardation function, a more efficient method, i.e. state-space model, is applied to evaluate part of the radiation force. The established model, incorporating the multi-body interactions, is applied to study the nonlinear impact on Leg Mating Unit (LMU) by considering the sway, heave and roll motions of the float-over system. The structural characteristics are considered when modelling the characteristics of LMU. The dynamic behaviors of a given system is investigated in the form of bifurcation diagrams, along with impact map, amplitude spectrum and power spectral density (PSD). It is found that bifurcation phenomena, or a large angle of docking cone could dominate the installation due to the increased impact loads.     

Local discontinuous Galerkin method for far-field underwater explosion shock wave and cavitation

The shock wave and cavitation are main effects in the far-field underwater explosion, which could cause serious damage to marine structures. In this paper, the fluid mechanical behavior of blast load is described by the propagation of pressure wave. The acoustic pressure caused by far-field explosion is determined by solving the wave equation, where a strongly discontinuous axisymmetric numerical model is established with the local discontinuous Galerkin (LDG) method. The model can calculate the dynamic pressure in the fluid field and capture the high-resolution shock wave. The pressure cutoff model is employed to deal with the cavitation effect due to the reflection of the shock wave. The numerical model is verified by comparing with the analytical solution of the cavitation effect near the structure in one dimension. With the same mesh discretization, the present model shows higher precision than the results calculated by the acoustic finite element method. In addition, the propagation of shock wave in the cylindrical water column is studied. Finally, the formation, growth and collapse of the cavitation region near the free surface are simulated. The LDG model can remove the spurious oscillations behind the shock front and it’s more accurate than the results of the acoustic finite element method, in terms of capturing the sharpness of shock wave and calculating the shock and cavitation loading. And the present model can be applied to calculate the structural damage caused by shock wave in three dimensions.     

混合破坏模式在自升式钻井平台插桩中的应用

自升式钻井平台插桩深度分析要求高,难度大,可检验性非常强。常规计算模式主要为“穿刺破坏模式”和“排挤破坏模式”。对于某些地质条件复杂的井场,按这两种模式分析计算的结果仍存在个别预测不准的情况。因此,如何提高钻井平台插桩分析的准确性,是目前需要攻克的一个技术难点。在总结工程插桩实践经验的基础上,提出了一种新的分析模式,即“混合破坏模式”。以南海一个工程实例进行具体分析,该井场应用“混合破坏模式”分析的插桩结果与实际结果较为吻合。并对数个复杂井场的实际插桩结果进行了统计分析,总结了桩端土体破坏模式的影响因素及“混合破坏模式”的特点,为在复杂地层中进行插桩分析提供了参考。     

An Integrated Approach to Fatigue Life Prediction of Whole System for Offshore Platforms

The failure of one or even more components usually does not lead to the collapse of the whole structure. Most of the analysis of fatigue is centered on only a single component which the researchers are interested in or much attention should be paid to. However, the collapse of a structure is the result of failure of a series of components in a specific order or path. This paper proposes an integrated approach to fatigue life prediction of whole structural system for offshore platforms, mainly describing the basic principles and prediction method. A method is presented for determining the failure path of the whole structure system and calculating the fatigue life in the determined failure path. The corresponding final collapse criteria for the whole structure system are discussed. A simple method of equivalent fatigue stress range calculation and a mathematical model of structural component fatigue life estimation in consideration of sea wave and sea ice loads are provided. As an application of the propo     

Geometrical aspects of a sonar-based navigation system for underwater vehicles

A novel self-contained navigation system has been devised for underwater vehicles operating in and around offshore installations. This system matches data from a sector-scanning sonar device to a computer model of the installation. The paper begins by highlighting the existing approaches to subsea navigation before outlining the main features of the proposed system. It then concentrates on a key component of this system which is a method for calculating the position and heading of an underwater vehicle navigating in the vincinity of tubular steel structures. An iterative solution method is presented which incorporates six degree of freedom vehicle motions and this is verified in a series of laboratory experiments with various arrangements of structural members and using a commercial sonar device. The key features, applications and performance of this method are discussed. The main conclusion is that the proposed method for calculating the position and heading of an underwater vehicle contributes towards achieving an accurate and reliable subsea navigation capability.     

A linear model for the structure of turbulence beneath surface water waves

The structure of turbulence in the ocean surface layer is investigated using a simplified semi-analytical model based on rapid-distortion theory. In this model, which is linear with respect to the turbulence, the flow comprises a mean Eulerian shear current, the Stokes drift of an irrotational surface wave, which accounts for the irreversible effect of the waves on the turbulence, and the turbulence itself, whose time evolution is calculated. By analysing the equations of motion used in the model, which are linearised versions of the Craik–Leibovich equations containing a ‘vortex force’, it is found that a flow including mean shear and a Stokes drift is formally equivalent to a flow including mean shear and rotation. In particular, Craik and Leibovich’s condition for the linear instability of the first kind of flow is equivalent to Bradshaw’s condition for the linear instability of the second. However, the present study goes beyond linear stability analyses by considering flow disturbances of finite amplitude, which allows calculating turbulence statistics and addressing cases where the linear stability is neutral. Results from the model show that the turbulence displays a structure with a continuous variation of the anisotropy and elongation, ranging from streaky structures, for distortion by shear only, to streamwise vortices resembling Langmuir circulations, for distortion by Stokes drift only. The TKE grows faster for distortion by a shear and a Stokes drift gradient with the same sign (a situation relevant to wind waves), but the turbulence is more isotropic in that case (which is linearly unstable to Langmuir circulations).     

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。     

北部湾典型海岛生态系统服务价值空间异质性对比研究

基于北部湾海域典型海岛高分1号数据,借助于CART决策树方法,获取了广西北部湾两个典型海岛的景观格局数据,并采用生态系统服务价值计量模型、空间自相关模型及地统计学中的变异函数模型对其生态系统服务价值进行了空间异质性分析。结果表明:团和岛单位面积生态系统服务价值是七星岛的1.08倍,但其变异系数低于七星岛;七星岛由于其地质背景为泥沙岛,景观格局受到人类活动以及自然环境的强烈干扰,趋向于复杂化和破碎化;而团和岛由于所处的外部环境以及自身的地质特征,景观格局呈现出整体斑块化分布状态;团和岛的景观格局主要受到结构性因素的控制,而七星岛则以随机性因素控制为主,选用变程和Moran’s指数来表征其复杂化和破碎化,来决定其优先开发时序,可对前者进行适当开发,而对后者适当进行生态保护;对受到结构性因素影响的岛屿在进行开发时,时序的识别选用变程和Moran’s指数来表征,若变程和Moran’s指数越小,说明海岛整体的景观分布格局趋向于复杂化和破碎化,反之生态系统较好,可进行优先开发。同时本文构建了北部湾海岛生态系统异质性评估与自然资源管理框架,并提出了海岛管理的建议与对策。     

海洋磁力仪拖体入水深度试验与分析

拖体入水深度是水下拖曳作业的主要参数,其取决于拖体自身重量、拖缆长度和船速。通过G882TVG海洋磁力仪阵列的拖曳试验,分析了船速、配重、拖缆长度的相互关系,导出拖体入水深度计算模型,对类似设备的拖曳参数确定具有借鉴意义。     

海洋钻井平台购置项目经济风险评估

以某海洋钻井公司购置自升式深水钻井平台为例，建立了海洋平台购置项目经济风险评估的综合概率模型。考虑未来的平台购置价格、作业租金、年营运天数、年维修费、日耗油量、还贷资金利率等多种因素的影响，对项目投资的经济评价指标进行了风险计算。通过主要参数的灵敏度分析，使得决策指标更趋于客观实际。     

深水脐带缆结构阻尼的数值与试验研究

深水动态脐带缆内部结构复杂,其阻尼特性对脐带缆的舞动以及疲劳损伤有着重要的影响。脐带缆在和流体相互作用过程中,脐带缆内部构件可能会产生滑动,进而影响脐带缆的结构阻尼,这也使得真实海况中脐带缆的舞动和疲劳是一项复杂而值得深入研究的课题。为验证有限元模型能够很好地模拟脐带缆的结构阻尼特性,在考虑内部结构单元间摩擦的基础上,利用ANSYS建立了用于中国南海某油田的脐带缆多层滑移接触摩擦模型。同时,采用自由衰减法对该动态脐带缆开展了原型试验,测得了该缆的结构阻尼比与自振周期。通过对比有限元模型计算结果与原型试验结果发现：数值模型获得的该脐带缆的结构阻尼值与试验值非常接近,这为工程中获得脐带缆结构阻尼比的值提供了好的方向。     

Numerical simulations and model tests of the mooring characteristic of a tension leg platform under random waves

Analyzing the dynamic response and calculating the tendon tension of the mooring system are necessary for the structural design of a tension leg platform （TLP）. The six-degree-of-freedom dynamic coupling responses and the mooring characteristics of TLP under random waves are studied by using a self-developed program. Results are verified by the 1：40 scaling factor model test conducted in the State Key Laboratory of Ocean Engineering at Shanghai JiaoTong University. The mean, range, and standard deviation of the numerical simulation and model test are compared. The influences of different sea states and wave approach angles on the dynamic response and tendon tension of the mooring system are investigated. The acceleration in the center and corner of the deck is forecasted.     

Mechanical Behavior of Light-Weight Soil Under Consolidated Drained Shear Condition

To reveal the influence of material composition on mechanical properties of light-weight soil, stress-strain -volumetric strain characteristics and Poisson's ratio of mixed soil were researched by consolidated drained shear tests. The results show that light-weight soil is a kind of structural soil, so its mechanical properties are affected by mixed ratio and confining pressure, and mixed soil possesses structural yield stress. When confining pressure is less than the structural yield stress, strain softening occurs; when confining pressure is more than the structural yield stress, strain hardening is observed. There are two kinds of volume change behavior: shear contraction and shear dilatancy. Shear dilatancy usually leads to strain softening, but there isn't an assured causal relationship between them. Poisson's ratio of mixed soil is a variational state parameter with the change of stress state, it decreases with increased confining pressure, and it increases with increased stress level. When axial strain is near 5%, Poisson’ ratio is gradually close to a steady value. The main range of Poisson's ratio is 0.25～0.50 when confining pressure changes from 50 to 300 kPa. When unconfined compressive strength of mixed soil is less than 328 kPa, its stress-strain-volumetric strain characteristics can be predicted very well by Duncan-Chang model (E-B model). However, when the range of unconfined compressive strength is [328 kPa, 566 kPa], the model can't predict stress-strain characteristics accurately when confining pressure is under 200 kPa, and it also can't predict the strong shear dilatancy phenomenon of mixed soil under low confining pressure.     

用直接法分析超大型浮体的水弹性响应

探讨了浮舟桥型超大型浮体结构的水弹性响应分析问题。将超大型浮体结构简化成弹性平板模型,用压力分布法计算流体压力,用直接法计算流体-结构系统,给出了它们的数学计算模型。计算表明本计算方法和程序是正确的,并能保证充分的精度,进而计算了更大尺度的超大型浮体,分析了波长、波向等对响应振幅的影响。     

黄河口泥沙异重流的数值模拟

将已被广泛应用的一维泥沙异重流控制方程推广到平面二维情形 ,构成一组四方程模型 ,并对方程中重要参数的取值作了讨论。而后针对黄河口泥沙异重流的发生和演变受潮相控制的特殊性对其进行数值模拟。在泥沙异重流的基本控制方程中加入了潮汐影响项 ,采用 ADI法对黄河口的潮流控制方程与有潮汐影响的泥沙异重流控制方程进行半耦合求解。模拟结果与实测结果基本吻合     

Time-Dependent Deformation Behavior of Jiangsu Marine Clay

In this article, the mechanical behavior of a Jiangsu marine clay was investigated by drained triaxial tests, traixial rheological tests, and one-dimensional compression and swelling tests. A visco-plastic model, the Bingham model combining two yield surfaces model, was proposed to describe the time-dependent deformation behaviors of the marine clay. The governing equation of Biot's consolidation theory for the visco-plastic soil is solved using a finite element code which incorporates the visco-plastic model. Using the finite element method, settlements of a typical embankment on the Lianxu expressway in China are calculated. Settlement calculations using the visco-plastic model are in agreement with the measured settlements in the field. The results demonstrate that the visco-plastic model is appropriate for calculating the visco-plastic deformations of Jiangsu marine clay. Theoretical and experimental studies show that the visco-plastic deformation of Jiangsu marine clay is substantial.     

Experimental investigation of the system behaviour of a model three-legged jack-up on clay

Results from combined loading experiments dedicated to evaluating the capacity and stiffness of a single footing have significantly improved the understanding of the response of circular shallow foundations. However, due to experimental practicalities, little corroborative evidence is available to confirm predicted behaviour when multiple footings act within a structural system. This paper addresses this concern by presenting the results of a series of experimental tests on a three-legged model jack-up unit founded on soft, heavily overconsolidated clay. The model adopted geometric features representative of a large prototype rig by appropriately scaling leg height, leg separation and flexural stiffness, as well as the ‘spudcan’ footing diameter and profile. Pushover events were considered by subjecting the model jack-up to monotonic horizontal loading at hull level. By measuring loads and displacements at the hull and at each spudcan, the system behaviour of the jack-up could be analysed. Nine separate tests were performed, revealing how load orientation, leg length and preload ratio changed the system capacity and affected the load–displacement paths of the jack-up and spudcan system. The test results are compared with those derived from single spudcan experiments, and are interpreted within the combined load yield surface approach gaining acceptance within the offshore industry.     

About applying linear structural method on ocean data: Adjustment of satellite wave data

Linear structural relations express the linear relationship between random variables when each of the variables under consideration is subjected to inherent variability and measurement error. The linear structural method is more general than the classical regression, which is often used incorrectly in calibration of altimeter data or validation of wave model results. In this work the linear structural method is elaborated and applied for the adjustment of TOPEX/Poseidon (T/P) data by means of in situ measured wave data in the Aegean Sea. In addition, a comparison of the buoy and T/P data with the WAM wave model predictions is performed revealing an underestimation trend of the model.     

Structural Types of Sea Embankment and Their Stability Analysis

In this paper based on investigation on the structural types of sea embankments in thesoutheast coastal area of China,as well as the related tidal stages,waves and strength of marine soils,thefinite element method(FEM)calculations for seepage flow stability,including the overall stability againstsliding and local stability of sloping surface under the action of tidal stage and waves are carried out.Acomparison of the computational results of FEM for single circular slip,composite circular slip andgeogrid reinforcement against sliding shows that for calculation of stability against sliding of marine softsoil foundation it is even more reasonable to use the composite circular slip.The stability of sloping typesea embankment against sliding is slightly better than that of the vertical face type;for the combination ofthree defences(wave,scour and seepage defences)the sea embankment structural type of a compositecross section with a platform should be a good one,but it still should be suitable to local condit     

老平台新增井槽技术的应用与发展

老油田的二次开发难度较大,为充分利用老平台设备资源,解决现有海上平台存在的预留井槽不足的问题,曾提出并成功应用了内挂、外挂、新增桩腿外挂等增加井槽的方式,但井槽增加的方位和数量受到限制。结合两平台短距离连接和修井机双向滑移避让钻井船技术,提出了新的修井机共用方式。通过采用双向滑轨实现了修井机在新建平台和老平台间的往复滑移。此技术节约了开发投资,为今后类似项目提供参考和借鉴。