Comprehensive study on the results of tension leg platform responses in random sea

Compliant offshore structures are used for oil exploitation in deep water. Tension leg platform (TLP) is a suitable type for very deep water. The nonlinear dynamic response of TLP under random sea wave load is necessary for determining the maximum deformations and stresses. Accurate and reliable responses are needed for optimum design and control of the structure. In this paper nonlinear dynamic analysis of TLP is carried out in both time and frequency domains. The time history of random wave is generated based on Pierson-Moskowitz spectrum and acts on the structure in arbitrary direction. The hydrodynamic forces are calculated using the modified Morison equation according to Airy’s linear wave theory. The power spectral densities (PSDs) of displacements, velocities and accelerations are calculated from nonlinear responses. The focus of the paper is on the comprehen-sive interpretation of the responses of the structure related to wave excitation and structural characteristics. As an example a case study is investigated and numerical results are discussed.

Dynamic Analysis of Tension Leg Platform for Offshore Wind Turbine Support as Fluid-Structure Interaction

Tension leg platform (TLP) for offshore wind turbine support is a new type structure in wind energy utilization.The strong-interaction method is used in analyzing the coupled model,and the dynamic characteristics of the TLP for offshore wind turbine support are recognized.As shown by the calculated results:for the lower modes,the shapes are water's vibration,and the vibration of water induces the structure's swing;the mode shapes of the structure are complex,and can largely change among different members;the mode shapes of the platform are related to the tower's.The frequencies of the structure do not change much after adjusting the length of the tension cables and the depth of the platform;the TLP has good adaptability for the water depths and the environment loads.The change of the size and parameters of TLP can improve the dynamic characteristics,which can reduce the vibration of the TLP caused by the loads.Through the vibration analysis,the natural vibration frequencies of TLP can be distinguished from the frequencies of condition loads,and thus the resonance vibration can be avoided,therefore the offshore wind turbine can work normally in the complex conditions.

Parametric Study on the Behavior of An Innovative Subsurface Tension Leg Platform in Ultra-Deep Water

This study focuses on a new technology of Subsurface Tension Leg Platform(STLP),which utilizes the shallowwater rated well completion equipment and technology for the development of large oil and gas fields in ultra-deep water(UDW).Thus,the STLP concept offers attractive advantages over conventional field development concepts.STLP is basically a pre-installed Subsurface Sea-star Platform(SSP),which supports rigid risers and shallow-water rated well completion equipment.The paper details the results of the parametric study on the behavior of STLP at a water depth of 3000 m.At first,a general description of the STLP configuration and working principle is introduced.Then,the numerical models for the global analysis of the STLP in waves and current are presented.After that,extensive parametric studies are carried out with regarding to SSP/tethers system analysis,global dynamic analysis and riser interference analysis.Critical points are addressed on the mooring pattern and riser arrangement under the influence of ocean current,to ensure that the requirements on SSP stability and riser interference are well satisfied.Finally,conclusions and discussions are made.The results indicate that STLP is a competitive well and riser solution in up to 3000 m water depth for offshore petroleum production.

Parametric Dimensional Analysis on the Structural Response of An Innovative Subsurface Tension Leg Platform in Ultra-Deep Water

The innovative Subsurface Tension Leg Platform(STLP), which is designed to be located below Mean Water Level(M.W.L) to minimize direct wave loading and mitigate the effect of strong surface currents, is considered as a competitive alternative system to support shallow-water rated well completion equipment and rigid risers for large ultra-deep water oil field development. A detailed description of the design philosophy of STLP has been published in the series of papers and patents. Nonetheless, design uncertainties arise as limited understanding of various parameters effects on the structural response of STLP, pertaining to the environmental loading, structural properties and hydrodynamic characteristics. This paper focuses on providing quantitative methodology on how each parameter affects the structural response of STLP, which will facilitate establishing the unique design criteria as regards to STLP. Firstly, the entire list of dimensionless groups of input and output parameters is proposed based on VaschyBuckingham theory. Then, numerical models are built and a series of numerical tests are carried out for validating the obtained dimensionless groups. On this basis, the calculation results of a great quantity of parametric studies on the structural response of STLP are presented and discussed in detail. Further, empirical formulae for predicting STLP response are derived through nonlinear regression analysis. Finally, conclusions and discussions are made. It has been demonstrated that the study provides a methodology for better control of key parameters and lays the foundation for optimal design of STLP. The obtained conclusions also have wide ranging applicability in reference to the engineering design and design analysis aspects of deepwater buoy supporting installations, such as Grouped SLOR or TLR system.

Nonlinear Dynamic Responses of Tension Leg Platform with Slack-Taut Tether

The slack-taut state of tether is a particular adverse circumstance,which may influence the normal operation state of tension leg platform(TLP).The dynamic responses of TLP with slack-taut tether are studied with consideration of several nonlinear factors introduced by large amplitude motions.The time histories of stresses of tethers of a typical TLP in slack-taut state are given.In addition,the sensitivities of slack to stiffness and mass are investigated by varying the stiffness of tether and mass of TLP.It is found that slack is sensitive to the mass of TLP.The critical curved surfaces(over which indicates the slack) for the increase of mass are obtained.

Tension leg platform project execution

The first floating platform concept design work for South China Sea is undergoing in DMAR's office now. This tension leg platform has potential to become the first advanced floating production platform project. Project execution is always a challenge for floating system. This paper focuses on the critical elements of project execution for tension leg platform,and studies potential implications to future oil and gas exploration in South China Sea. There are many factors affecting successful execution of floating system project,including technical issues, engineering management,interface management,etc. There are also failure examples of project execution in the industry. The author has participated 28 large detailed projects and has gained extensive experience on floating projects,with ample hands-on project experiences. A detailed tension leg platform project study example and discussions in depth are presented for future project execution in China deepwater development.

Nonlinear Random Motion Analysis of A Tension Leg Platform Consider- ing the Set-down Motion of A Floating Body

The dynamic analysis of a Tension Leg Platform(TLP) in random wave is investigated by considering the set-down of a floating body.The nonlinear restoring stiffness is derived with the set-down motion of a floating body and the coupled motion of the tension leg and platform and the differential equations of the motion are established.The study focuses on the influence of the set-down motion on the nonlinear response of the platform.By considering different significant wave heights and currents,motion responses of the platform are calculated and compared.The analysis shows that the set-down motion significantly increases the heave motion with low frequency and the equilibrium position of the heave motion with the set-down motion is much lower than that without set-down motion.The results in this paper indicate that the set-down motion has a major impact on the safety of the platform inproduction operation,and it is also a threat to the strength of tension legs and risers.

Application of Tension Leg Platform Combined with Other Systemsin the Development of China Deepwater Oil Fields

The development of deepwater oil fields has reached a new stage with the dramatic increase in water depth and the recent increasing demands of the economic development in the filed. The use of a Tension Leg Platform (TLP) combined with other systems, such as Floating Production Storage and Offloading (FPSO) system, Floating Production Unit (FPU) system, Tender Assisted Drilling (TAD) system, etc., has drawn the industry attention and increased significantly in the past few years. For the areas lacking of pipeline system, the use of TLP(s) combined with FPSO has been chosen to efficiently develop the deepwater fields. The TLP with a Tender Assisted Drilling system significantly reduces the payload of the platform and reduces the investment in the TLP system substantially. This opens the door for many new deepwater field developments to use the tension leg platform. The advantage of the TLP combined with a TAD system is more significant when several TLPs are used for the continuous development of the field. One of the applications for the TLP with a tender assisted drilling system can be in the development of an offshore marginal field. Owing to the increase of water depth, the conventional fixed platform model for the exploration of those fields becomes uneconomical. It also would be too expensive to use a large TLP structure for those marginal fields due to the large amount of initial investment. The TLP system with tender assisted drilling can be used to develop those fields economically. There are many marginal fields in China offshore, especially in shelf areas. The application of this field developing model, combined with the existing field developing experience in China, will open the door for many marginal field developments.This paper will review the application of the combined TLP system through some examples of completed/ongoing projects, and major technical issues encountered in those practices. The potential application of this technology in China deepwater development will be discussed in the end.

An Approximate Method for the Surge Response of the Tension Leg Platform

The solution for the Duffing equation in a nonlinear vibration problem is studied in this paper. Clearly, in the case of the perturb parameter being a larger value, the traditional perturbation method is no longer valid but the Homotopy Perturbation Method(HPM) is applicable usually.HPM is used to solve the weak and strong nonlinear differential equations for finding the perturbed frequency of the response. The obtained frequencies via HPM and the approximate method have good accordance for weak and strong nonlinear differential equations. Additionally, the calculated responses by use of the approximate method are compared with the responses obtained from the Numerical method in the time history of the response and phase plane.The results represent good accordance between them.

Vortex-Induced Vibration of a Tension Leg Platform Tendon:Multi-Mode Limit Cycle Oscillations

This paper studies the application of mathematical models to analyze the vortex-induced vibrations of the tendons of a given TLP along the Indian coastline, by using an analytical approach, using MATLAB. The tendon is subjected to a steady current load, which causes vortex-shedding downstream, leading to cross-flow vibrations. The magnitude of the excitation(lift and drag coefficients) depends on the vortex-shedding frequency. The resulting vibration is studied for possible resonant behavior. The excitation force is quantified empirically, the added mass by potential flow hydrodynamics, and the vibration by normal mode summation method. Non-linear viscous damping of the water is considered. The non-linear oscillations are studied by the phase-plane method, investigating the limit-cycle oscillations. The stable/unstable regions of the dynamic behavior are demarcated. The modal contribution to the total deflection is studied to establish the possibility of resonance of one of the wet modes with the vortex-shedding frequency.

Response Analysis of Tension-Based Tension Leg Platform Under Irregular Waves

Tension Leg Platform(TLP) is a hybrid structure used as oil drilling and production facility within water depths of 1200 m. The extension of this TLP concept to deeper waters is a challenge and warrants for some innovative design concepts. In this paper, a relatively new concept of TLP which is christened as Tension-Based Tension Leg Platform(TBTLP) and patented by Srinivasan(1998) has been chosen for study. Response analysis of TLP with one tension base under irregular waves for three different sea states has been performed using hydrodynamic tool ANSYS? AQWA?. Results are reported in terms of RAOs, response spectrums for surge, heave and pitch degrees of freedom from which spectral statistics have been obtained. The statistics of TBTLP have been compared with TLPs(without tension base) for two different water depths to highlight the features of the new concept. The effect of viscous damping and loading effects on the RAOs are also investigated.

Application of a Newly Built Semi-submersible Vessel for Transportation of a Tension Leg Platform

紧张腿站台(TLP ) 的交通组织因为长距离总是与一个重半运输容器的使用被联系了。这类容器的要求总是是特殊的，并且他们的可获得性是有限的。为华南海深海的工程的未来发展准备，中国近海油工程公司最近造了重电梯运输 vessel-Hai 杨实你 278。这个半能沉入水中的容器有 42 米的 50k DWT，和呼吸的一个排水量能力。理解容器适用性并且在未来为使用准备它深海的工程正在成为逼近的需要。这份报纸考察与 TLP 交通联系的当前的关键问题并且在装载外面和交通期间执行设计 TLP 的详细分析。新建 COOEC 交通容器 HYSY 278 被使用在华南在 Qingdao 从 COOEC 制造院子弄干 TLP 结构的运输到一块油地海。全部进程从制造院子，从容器，在长距离上的 TLP 的干燥运输，并且平台的最后的卸载卸载和平台的漂浮上的 landsite 包括了 TLP 结构的装载外面。水动力学和结构分析被执行评估运输容器和 TLP 结构的行为。特殊注意对与这个新容器的使用联系的批评区域被给予，与任何潜在的限制一起。结果证明 HYSY 278 能有效地被用于与合适的安排和准备得好的操作搬运结构。过程和细节根据学习结果被介绍。特殊注意也从分析基于结果被给未来使用的讨论。

A Stability Criterion for Time-Delay Tension Leg Platform Systems Subjected to External Force

Stability analysis plays a central role in nonlinear system theory and engineering application.Over the past few years,the stability analysis of fuzzy systems has been proposed and there are many successful applications in practical engineering.Therefore,in this paper firstly proposed is the stability analysis on oceanic structure by fuzzy models.In the present study,Takagi-Sugeno(T-S) fuzzy model is proposed for a time delay tension leg platform(TLP) system subjected to an external wave force.In terms of stability analysis,linear matrix inequality(LMI) conditions are derived via Lyapunov theory to guarantee the stability of the TLP system.

Response-based Analysis for Tension Leg Platform

为一个常规短期的设计配方的紧张腿站台(TLP ) 设计焦点的典型工业实践，它假设 N 年设计环境导致 N 年回答。在基于反应的设计方法， TLP 被设计承受 N 年回答而非对 N 年环境条件作出回应。在这份报纸，我们在场为基于反应的设计方法和使用的概述和一个一般过程比较在二个方法之间的批评 TLP 回答的案例研究。我们的比较的结果证明常规短期的设计方法经常包含保守主义的一个元素并且基于反应的设计方法能减少设计条件并且从而完成费用积蓄。

Governing equations and numerical solutions of tension leg platform with finite amplitude motion

It is demonstrated that when tension leg platform (TLP) moves with finite amplitude in waves, the inertia force, the drag force and the buoyancy acting on the platform are nonlinear functions of the response of TLP. The tensions of the tethers are also nonlinear functions of the displacement of TLP. Then the displacement, the velocity and the acceleration of TLP should be taken into account when loads are calculated. In addition, equations of motions should be set up on the instantaneous position. A theoretical model for analyzing the nonlinear behavior of a TLP with finite displacement is developed, in which multifold nonlinearities are taken into account, i.e., finite displacement, coupling of the six degrees of freedom, instantaneous position, instantaneous wet surface, free surface effects and viscous drag force. Based on the theoretical model, the comprehensive nonlinear differential equations are deduced. Then the nonlinear dynamic analysis of ISSC TLP in regular waves is performed in the time domain. The degenerative linear solution of the proposed nonlinear model is verified with existing published one. Purthermore, numerical results are presented, which illustrate that nonlinearities exert a significant influence on the dynamic responses of the TLP.

Numerical and experimental study of internal solitary wave loads on tension leg platforms

With the large-scale density stratified tank and the numerical flume proposed,series of numerical cases in line with the experiments are carried out to investigate the interaction between the tension leg platforms(TLPs)and the internal solitary waves(ISWs).The waveforms,and the loads and the torques on the TLP obtained by the experiments and the simulations agree well with each other.Experimental results show that the amplitudes of the dimensionless horizontal force and torque linearly increase with the dimensionless amplitude,while that of the vertical force increases in a parabolic curve.Besides,the numerical results indicate that the horizontal and vertical forces on the TLP due to the ISWs can be divided into three components,namely,the wave pressure-difference forces,the viscous pressure-difference forces,and the frictional force that is negligible.The wave pressure-difference forces are always the major constituents.However the viscous pressure-difference component is unimportant,it is negligible as compared with the vertical forces.

Effects of Second-Order Sum-and Difference-Frequency Wave Forces on the Motion Response of a Tension-Leg Platform Considering the Set-down Motion

This paper presents a study on the motion response of a tension-leg platform(TLP) under first-and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function(QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.

Investigation of Pitch Motion Portion in Vertical Response at Sides of a Tension-Leg Platform

腱垂直地系在因此，物理腱压力的深理解要求的紧张腿平台(TLP ) 腱的全部的轴的变丑的决心，它是举，沥青，和澎湃的回答的联合。TLP 的侧面的方面的垂直运动被结合巨浪并且组成沥青运动的部分。腱被连接到 TLP 的方面；因此，侧面的方面的全部的排水量与腱和全部的轴的压力的全部的变丑有关。因此，在 TLP 的方面调查全部的垂直反应是必要的。在自由的各种各样的度之间联合没在反应振幅操作员(RAO ) 被考虑。因此在频率领域分析，估计的垂直 RAO 是不完全的。在时间领域，另外，在 TLP 的中心的仅仅举运动典型地被学习；这个问题需要被处理。在这份报纸，我们在频率和时间领域在 TLP 的方面在垂直反应调查沥青运动的部分。数字结果在一些时期范围在 TLP 的边的垂直运动表明沥青运动的重要效果。

Hill Instability Analysis of TLP Tether Subjected to Combined Platform Surge and Heave Motions

This paper presents the Hill instability analysis of Tension Leg Platform(TLP)tether in deep sea.The 2-D nonlinear beam model,which is undergoing coupled axial and transverse vibrations,is applied.The governing equations are reduced to nonlinear Hill equation by use of the Galerkin's method and the modes superposition principle.The Hill instability charted up to large parameters is obtained.An important parameter M is defined and can be expressed as the functions of tether length,the platform surge and heave motion amplitudes.Some example studies are performed for various environmental conditions.The results demonstrate that the nonlinear coupling between the axial and transverse vibrations has a significant effect on the response of structure.It needs to be considered for the accurate dynamic analysis of long TLP tether subjected to the combined platform surge and heave motions.

考虑桩基错动的深水张力腿平台动力响应研究

由于地震、断层、滑坡和沉降等原因,张力腿平台(tension leg platform, TLP)下端的桩基可能发生错动。考虑平台本体有限位移、六自由度运动耦合、瞬时位置和瞬时湿表面、自由表面效应等非线性因素,建立TLP系统的耦合动力学方程。采用变步长龙格库塔算法编写了数值计算程序,得到了波浪作用下平台本体六个自由度运动响应和系泊张力响应。通过文献对比,验证了理论模型和计算程序的正确性。基于南海流花油田一年一遇的设计工况,计算得到桩基下沉0.1 m时16个工况下平台系统的动力响应的时间历程。以正常工况为基准,对比了不同工况中平台本体六个自由度运动、系泊张力的幅值和标准差,研究结果表明:桩基下沉对平台的六自由度响应和张力腿张力幅值产生明显的影响,但不加剧运动响应和张力响应的波动;桩基下沉会破坏平台系统原有的对称性,引起垂荡位移的较大恶化;桩基下沉会引起张力腿张力大幅增大,存在张力腿断裂和系泊失效的风险。