Finite Element Analysis of Deformed Legs of Offshore Platform Structures

The element stiffness matrix of the equivalent beam or pipe element of the deformed leg of the platform is derived by the finite element method. The stresses and displacements of some damaged components are calculated, and the numerical solutions agree well with those obtained by the fine mesh finite element method. Finally, as an application of this method, the stresses of some platform structures are calculated and analyzed.

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 shallow- water 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.

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

This paper presents the Hill instability analysis of Tension keg Platform （TLP） tether in deep sea. The 2-D nonlinear beanl 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 envirotnnental 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 TI2 tether subjected to the combined platfolna surge and heave motions.

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.

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 （TIP） 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 TIP（s） combined with FPSO has been chosen to efficiently develop the deepwater fields. The TIP with a Tender Assisted Drilling system significantly reduces the payload of the platform and reduces the investment in the TIP system substantially. This opens the door for many new deepwater field developments to use the tension leg platform. The advantage of the TIP combined with a TAD system is more significant when several TIPs are used for the continuous development of the field. One of the applications for the TIP 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 TIP structure for those marginal fields due to the large amount of initial investment. The TIP 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 TIP 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.

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.

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.

A Single Mooring System with Sag-Extensibility and Flexural Rigidity Applied to Offshore Platform

Floating platform system has been extensively used in ocean exploitation, particularly for a tension-leg platform （TLP） system in deep water. Most of the TLPs are multi-mooring systems, where multi-joints are connected to the tension-legs so that the platform is not allowed to twist freely and may subject to enormous force induced by large incident waves in the weak-direction of the structure. This study aims to exploit a single moored offshore platform system that may attract less force and can be operated with less effort. In our analysis, in addition to mechanical properties of the tether, two important properties are also taken into consideration for the single mooring tether with expanded cross sectional dimension and utilization of stronger material, namely, the sag-extensibility and the flexural rigidity. Finally, the dynamic structural behavior produced by the mechanical effects on the new system is investigated and compared with that of traditional design while the wave-structure interactions of large body are also accounted for. Our study finds that the neglect of sag-extensibility or the flexural rigidity of large, strong mooring cable may result in a conservative but not necessarily safe design.

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

Transportation of tension leg platform （TLP） structures for a long distance has always been associated with the use of a heavy semi-transport vessel. The requirements of this type of vessel are always special, and their availability is limited. To prepare for the future development of South China Sea deepwater projects, the China Offshore Oil Engineering Corporation has recently built a heavy lift transport vessel-Hai Yang Shi You 278. This semi-submersible vessel has a displacement capacity of 50k DWT, and a breath of 42 meters. Understanding the vessel＇s applicability and preparing it for use in future deepwater projects are becoming imminent needs. This paper reviews the current critical issues associated with TLP transportation and performs detailed analysis of the designed TLP during load-out and transportation. The newly built COOEC transportation vessel HYSY 278 was applied to dry transport of the TLP structure from the COOEC fabrication yard in Qingdao to an oil field in South China Sea. The entire process included the load-out of the TLP structure from the landsite of the fabrication yard, the offloading and float-on of the platform from the vessel, the dry transport of the TLP over a long distance, and the final offloading of the platform. Both hydrodynamic and structure analysis were performed to evaluate the behavior of the transport vessel and TLP structure. Special attention was paid to critical areas associated with the use of this new vessel, along with any potential limitations. The results demonstrate that HYSY 278 can effectively be used for transporting the structure with proper arrangement and well-prepared operation. The procedure and details were presented on the basis of the study results. Special attention was also given to discussion on future use based on the results from the analysis.

Response-based Analysis for Tension Leg Platform

The typical industry practice for Tension Leg Platform （TLP） design focuses on a conventional short-term design recipe, which assumes that an N-year design environment leads to an N-year response. In the response-based design method, the TLP is designed to withstand N-year responses rather than respond to N-year environmental conditions. In this paper, we present an overview and a general procedure for the response-based design method and use a case study to compare the critical TLP responses between the two methods. The results of our comparison show that the conventional short-term design method often contains an element of conservatism and that the response-based design method can reduce the design conditions and thereby achieve cost savings.

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.

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 theo- retical 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 displace- ment, 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. Furthermore, numerical results are presented, which illustrate that nonlinearities exert a significant influence on the dynamic responses of the TLP.

NONLINEAR COUPLED RESPONSE OF TENSION LEG PLATFORM

A dynamic response analysis of tension leg platform (TLP) to deterministic first order wave forces is presented, considering coupling between various degrees of freedom surge, sway, heave, pitch, roll and yaw. The analysis duly considers nonlinearities produced due to changes in cable-tension and due to nonlinear hydro-dynamic drag forces. The wave forces on the elements of the pontoon structure are calculated using Airy's wave theory and Morison's equation. The nonlinear equation of motion is solved in the time domain by Newmark's β-method. With the help of proposed analysis, some example problems are solved in order to investigate the effects of different important factors that influence the response of TLP.

Hydrodynamic Comparison of a Semi-submersible,TLP,and Spar:Numerical Study in the South China Sea Environment

The South China Sea contains tremendous oil and gas resources in deepwater areas. However, one of the keys for deepwater exploration, the investigation of deepwater floating platforms, is very inadequate. In this paper, the authors studied and compared the hydrodynamics and global motion behaviors of typical deepwater platforms in the South China Sea environment. The hydrodynamic models of three main types of floating platforms, e.g. the Semi-submersible, tension leg platform （TLP）, and Truss Spar, which could potentially be utilized in the South China Sea, were established by using the 3-D potential theory. Additionally, some important considerations which significantly influence the hydrodynamics were given. The RAOs in frequency domains as well as global motions in time domains under time-varying wind, random waves, and current in 100-y, 10-y, and 1-y return period environment conditions were predicted, compared, and analyzed. The results indicate that the heave and especially the pitch motion of the TLP are favorable. The heave response of the Truss Spar is perfect and comparable with that of the TLP when the peak period of random waves is low. However, the pitch motion of Truss Spar is extraordinarily lar^er than that of Semi-submersible and TLP.

Aerodynamic Effects on TLP Type Wind Turbines and Predictions of the Electricity They Generate

This research proposes a new offshore wind energy generation system that uses a tension leg platform (TLP) and describes experiments performed on a TLP type wind turbine in both waves and wind. The following conclusions can be made from the results of this research. 1) In the case of coexisting wave-wind fields, the wind effect stabilizes the pitch motion. 2) The wind effect decreases vibration of the mooring lines when waves and wind coexist. In particular, the springing (2nd or 3rd order force) also decreases in this field. 3) It can be estimated that the reduction in the rate of generation of electrical power can be up to about 6% as a result of the heel angle. In addition, the annual amount of electricity generated was estimated along with the utilization factor based on the experimental results.

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.