Random dynamic response of a tethered buoyant platform production riser

A new normal mode spectral analysis method is presented for calculating r.m.s. riser deflections, bending stresses and lower ball joint angles. Forces on the riser consist of: (a) non-linear fluid drag taking account of the relative velocity due to tethered buoyant platform (TBP) motion, riser elastic deflection and wave induced fluid velocity, (b) wave induced fluid acceleration, (c) inertia forces due to TBP acceleration, and (d) buoyancy. The non-linear fluid drag forces are linearized using Tung and Wu's approximation based on the r.m.s. relative fluid velocity and current. A wide range of results is presented for risers in water depths up to 1000 m and it is observed that 6 normal modes are sufficient for calculating bending stresses. A static analysis is also presented for bending stresses due to wave and current induced drag forces and riser offset.     

Stochastic response of compliant platforms to irregular waves

Response of a compliant platform to irregular waves is determined using finite element method. The tower is idealized by 2-D beam elements with an elastic support at the guy lines location. The flexural characteristics of the beam correspond to the four corner members of the trusses. The guying system is modelled by an axial element with linear load deformation characteristics. A computer program based on the linearized Morison's equation and the linear (Airy) wave theory, is developed to calculate the total force based on the storm wave height data at different levels of the structure. The response of the structure to random waves is based on the spectral approach. The direct and cross spectral densities of the generalized wave forces are determined and used to obtain the spectral densities of the generalized modal coordinates and mean square response at each level. Possible extension of the method is indicated to compute the evolutionary response to nonstationary wave forces.     

Coupled dynamics of tethered buoy systems

The three-dimensional coupled behavior during the interaction of buoys with their mooring systems is numerically analyzed. A time-domain model was developed to predict the response of a tethered buoy subject to hydrodynamic loadings. External loadings include hydrodynamic forces, tethers tensions, wind loadings and weight. System nonlinearities include large rotational and translational motions, and non-conservative fluid loadings. The mooring problem is formulated as a combined nonlinear initial-value and two-point-boundary-value problem which is directly integrated both in time and space. Buoy equations of motion are derived using small Eulerian angles. Coupling between rotational and translational degrees of freedom is included and coupling between the buoy and cable is effected by adopting the buoy equations of motion as boundary conditions at one end for the mooring problem. Numerical examples are provided to validate the formulation and solution technique; predicted responses of three types of buoy (sphere, spar, and disc) are compared with experimental results.     

Large angular motions of tethered surface buoys

A three-dimensional coupled analysis of the interaction of a floating buoy and its mooring is studied. External loads include hydrodynamic forces, tether tensions, wind loads and system weight and buoyancy. Nonlinearities include large rotational and translational motions and non-conservative fluid loads. The mooring problem is formulated as a nonlinear two-point-boundary-value-problem. At each instant in time, the mooring problem is solved by direct integration using a successive iterative algorithm to satisfy boundary conditions. Buoy kinetic and kinematic equations are derived assuming large angles represented by Euler parameters. Coupling between the buoy and the mooring is enforced by matching the velocities of the tether and buoy at the attachment point. A predictor-corrector coupling algorithm is used with multiple sizes of time steps used to provide stability for the separate mooring and buoy models. Numerical results are compared to experimental responses of three types of buoys (sphere, spar and disc) subject to both regular and irregular waves.     

Passive control of offshore jacket platforms

The wave-induced dynamic force is one of the most important excitations to be dealt with in the design of offshore structures. In order to perform a reliable design of an offshore structure, it is important to obtain an exact evaluation of its dynamic response but also to examine the ways of reducing the response. This paper presents the response of offshore jacket platforms installed with energy dissipation devices such as viscoelastic, viscous and friction dampers under wave loading. The offshore jacket platforms are modeled as multi-degrees-of-freedom system provided with dampers at each floor location. The wave forces are modeled as per Morison's equation. The governing equations of motion of the jacket platform with dampers are derived and their solution in the frequency domain is presented. The uni-directional random wave loading is expressed by the Pierson-Muskowitz spectrum. The response of the jacket platform with viscoelastic, viscous and friction dampers is compared with the corresponding response without dampers in order to investigate the effectiveness of the passive control systems. It is observed that the additional dampers add substantial damping to structure and thus favorably control the response of platform structure. Among the various energy dissipation devices used for study, the viscoelastic dampers perform better in comparison to the other dampers. This is due to the fact that the added viscoelastic dampers contribute to increased viscous damping as well as lateral stiffness which reduces the response of the offshore jacket platforms significantly.     

导管架海洋平台沉降监控

导管架海洋平台在长期使用中存在着桩基础沉降的问题。海上固定式结构物由于在海面上很难找到固定的参考点,因此很难采用常规土木工程中的沉降测量方法对沉降问题进行测量。根据土壤的压缩性质以及桩基沉降给平台带来的危害程度,制定了沉降的三级监控方法。通过对平台上部总荷载、单桩荷载以及不均匀沉降的监测来监控平台的沉降情况,并在渤海某平台上进行了实际应用。通过采用光纤光栅等先进的监测手段以及合理的监测方案,实现了导管架平台沉降的三级监控,实际应用效果良好。     

拖曳系统拖曳动态过程仿真分析

对于海洋缆索系统,论文针对传统有限段法的不足,提出改进的缆索有限段法,缆索离散为若干弹性缆段组成的多柔体系统,根据缆索的特点选择适当的参考系和广义速率,引入有限元法中的形函数描述段内各点位移,根据Kane方程推导改进缆索有限段模型的运动方程。基于改进的缆索有限段法,提出了模拟拖曳缆索释放一回收过程的变拓扑结构模型,即用可变长度缆段长度的变化和缆段数量的改变建模缆索的释放和回收过程。文中对一海洋拖曳系统进行了动力学仿真,与海洋试验结果比较验证了模型的正确性。     

Wave attenuation characteristics of a tethered float system

Wave attenuation characteristics of a tethered float system have been investigated for various wave heights, wave periods, water depths, depths of submergence of floats and float sizes. As the floats are similar in size and shape, only a single tethered spherical float is considered for the theoretical analysis. Float motion is determined through the dynamical equation of motion, developed for a single degree of freedom. From incident and transmitted wave powers, transmission coefficients are computed. The results show that transmission coefficient does not vary with changes in wave height or water depth. When depth of submergence of float increases, wave attenuation decreases, showing that the system performs well when it is just submerged. As float velocity decreases with increase in float size, transmission coefficient increases with increase in float size. The influence of wave period on wave attenuation is remarkable compared to other parameters. The effect of drag on wave attenuation is studied for varying drag coefficient values. Theoretical results are compared with experimental values and it is found that theory overestimates wave attenuation which may probably be due to various linearisations involved in the theoretical formulation.     

Practical stochastic fatigue analysis of offshore platforms

A method for stochastic fatigue analysis of offshore platforms is presented. The method accounts for the nonlinearity in the drag loading term, and for the systematic deviation from a Gaussian process for a platform response. The method is based on an assumed form of the stress response process at a hot spot. A number of full scale measurements for elements in fixed platforms and for marine risers show very good agreement with this assumed form. A conventional spectral fatigue approach with stochastic linearization of the drag term in the basis for the new method, which uses results from linearizations in two or more sea states. The method is easily implemented in standard computer programs presently used within the offshore oil industry for fatigue analysis.     

海洋平台随机冰力谱的识别

将海冰对结构的作用看作是随机激励,利用海洋平台原型结构冰激振动的实测响应数据,用逆虚拟激励法(1PEM)识别了平台的冰载荷谱,结果表明逆虚拟激励法识别效果满足工程实际的需要,有较好的应用前景。     

海洋平台结构形状与拓扑优化设计

以实际环境荷载作用下的实际海洋导管架平台结构为对象,研究了海洋平台结构在风,流,海冰等多荷载模式和多荷载工况下的优化设计的理论方法和应用技术,建立了结构尺寸优化,形状优化和拓扑优化问题的统一模型。计算结果反映出形状优化比尺寸优化所得设计更优,而拓扑优化的设计效果最好。不同的荷载工况对应的最优拓扑形式不同,因此在实际设计中应考虑实际的荷载工况,选取不同的拓扑形式。     

新型深水半潜式生产平台发展综述

干式采油树具有修井方便、操作成本低、停机时间短和流动安全性好等优势,然而由于它要求平台具有较低的垂荡运动幅度,目前仅有张力腿平台和Spar平台能够支持干式采油树.相比这两种平台,半潜式生产平台不受水深限制,可变载荷大,且可实现整体拖航和上船体码头安装,但运动幅度相对较大.近年来,海洋工程界致力于通过改善半潜式平台的运动性能,使其可支持干式采油树.介绍了深水油气开发领域中可采用干式采油树的几种新型半潜式生产平台概念,分别从结构型式、安装方法和性能特征等方面进行了综述,总结了新型平台的技术发展趋势以及所带来的新问题,最后提出了我国发展新型半潜式生产平台的目标和需要解决的问题.     

Wave interaction with tension leg platforms

The design of deep water offshore platforms requires the analysis of wave-structure interaction phenomena which have not been as critical for shallower water platform designs. In the case of tension leg platforms (TLPs) interaction phenomena such as wave run-up on the vertical legs and the amplification of the waves beneath the deck are major design considerations. The research investigation reported here focuses on a series of small scale wave tank tests on four column TLP models examining these phenomena. The role of vertical leg spacing and comparative tests of the TLP models with and without pontoons was investigated. As the vertical legs were moved closer an increase in wave run-up and a shifting of the incident wave period corresponding to the maximum wave upwelling were noted. Comparisons with wave measurements for single cylinders from previous experimental studies and the TLP configurations used in this study are presented. A design formula for estimating wave run-up on TLPs is suggested based upon these experiments. The wave run-up on a leg directly in the wake of another leg is presented. A comparison of the wave upwelling measurements with previously published numerical results are discussed. A wave uplift force model which allows for the inclusion of the experimentally obtained wave upwelling measurements is presented and discussed with regard to the design specification of platform deck elevation.     

Surface buoyant plumes from melting icebergs in the Labrador Sea

Canada׳s Department of Fisheries and Oceans (DFO) conducts annual surveys in the Labrador Sea along the repeat hydrography line AR7W. The occupation of the AR7W line in May 2013 was followed by the experiment aimed at resolving the imprint of melting drifting icebergs on the upper layer thermohaline characteristics in the Labrador Sea. We present high-resolution observations around two icebergs conducted with the towed undulating platform Moving Vessel Profiler (MVP). The first iceberg drifted in relatively warm water of Atlantic origin (~2.5–3.1 °C) off Greenland, while the second iceberg was on the Labrador shelf in cold water below 0 °C. Both icebergs had a lengthscale of O(100 m). In both cases surface buoyant plumes fed by melt water and attached to the iceberg were observed. The plumes were evident in the anomalous thermohaline characteristics of the seawater. Their density anomalies were sufficiently strong to produce visible frontal structures, which imply a development of the intrinsic dynamics associated with a plume. The first plume formed over a time interval of ~10 h, while the second plume formed over several days and extended for more than 1 km (tenfold the iceberg׳s size). Strong vertical displacements of the pycnocline were observed near the second iceberg. They are interpreted as the internal wave wake. This interpretation is based on the temporal scale of these oscillations (local buoyancy frequency), as well as on the spatial orientation of these waves with respect to the iceberg drift relative to the pycnocline. The observed internal waves partially overlapped with the plume and affected its structure. The saline seawater splashing by swell contributed to the surface melting of the icebergs. Scaling analysis of the second plume suggests that it could be in the “rotational” dynamic regime with recirculating anticyclonic flow.     

Time-domain simulation of large-amplitude response of floating platforms

Two numerical simulation models to predict large-amplitude motions of floating platforms are presented. The first method is based on the application of the relative-velocity formulation of Morison's equation for force calculations. The second method developed in this work uses the three-dimensional potential theory in time domain. In this method, both the Froude-Krylov and scattering forces are calculated by considering finite wave amplitude effects in random waves. The effect of various nonlinearities on the low-frequency motions and high-frequency tether-tension response of a tension leg platform are studied using these simulation models in conjunction with power spectral methods. The presence of current and the nonlinear drag force are observed to have a significant effect on the low-frequency motions and tether tensions.     

赤潮藻类非线性动力学模型的随机分岔

选取典型赤潮藻类的硅藻和甲藻,建立了一个随机非线性动力学模型,运用随机平均法结合现代非线性动力学理论对模型进行了化简,运用拟不可积 Hamilton 系统的相关理论和 Oseledec 乘性遍历定理求解了模型的最大 Lyapunov 指数,得到了模型随机稳定性的条件;利用动态系统不变测度的 Lyapunov 指数分析了模型的随机分岔,得到了系统发生随机D-分岔时参数的取值条件,结果表明,系统在随机因素作用下变得更敏感、更不稳定.