Tension Leg Platform System Research Articles

A new quasi-dynamic method for the prediction of tendon tension of TLP platform

The assessment of tendon mechanics for the tension leg platform (TLP) is crucial to the safety of the whole TLP system. The tendon design includes nonlinear mechanical characteristics such as set-down and various dynamic effects such as slow drift and springing. In this study, a new quasi-dynamic method dedicated to the dynamic tension of tendon and global motion of TLP is established by employing two primary assumptions, i.e. the separation of the hull’s horizontal and vertical response, and the separation of low-frequency, wave-frequency, and high-frequency excitations due to waves. An analytical tendon model is proposed, and the simultaneous time history of tendon tension can be obtained through the post-treatment after carefully handling the hull’s various dynamic responses. A remarkable efficiency is achieved since the time-consuming convolution integral, finite element model of the tendon, etc., are avoided. A comprehensive validation regarding the TLP system, global motion, and tendon tension is conducted by the comparisons with model test measurement and fully coupled analysis using OrcaFlex. An excellent agreement is achieved no matter the cycles and phases in the time series or the extremes in the statistics. The study has proven the quasi-dynamic method a reliable approach with satisfactory accuracy and superb efficiency, which should be a useful method in the design of the TLP platform.

Stabilization Control for Strict-Feedback Nonlinear Systems With Time Delays

In this article, the output-feedback control problem for a class of nonlinear time-delay systems has been considered. Unmeasured system states and the time-varying delays which appear in all state variables bring great challenges to the controller design. Novel Lyapunov–Krasovkii functionals with control gains are proposed for a delay-independent controller. The system is proved to be asymptotically stable driven by the controller with suitable design parameters. Finally, we apply the new control scheme to a two-stage chemical reactor system and the time-delay tension leg platform system, the effectiveness of the control scheme is illustrated in the simulation results.

Numerical and Experimental Study on the Dynamics of the Tendon/Top Tension Riser System of a Tension-Leg Platform

In this study, the dynamics of the tendon/top tension riser (TTR) system of a tension-leg platform (TLP) are investigated through an experiment and by using absolute nodal coordinate formulation (ANCF). First, the model test of the TLP system is conducted in the water tank of Harbin Engineering University to examine the motion response of the TLP and the dynamic response characteristics of the tendon and TTR. The test scale ratio is set to 1: 66.3. Then, on the basis of the ANCF, the stiffness, external load, and mass matrices of the element are deduced to establish the motion equation of the tendon/riser. Finally, the static and dynamic characteristics of the tendon/TTR system of TLP are analyzed systematically by using the ANCF method. The results are compared with commercial software and test results. The motion response of tendon/TTR is affected by the TLP movement and environmental load simultaneously. The analysis proves the effectiveness and accuracy of the ANCF method despite the low number of riser units, suggesting the superiority of the ANCF method for calculating the dynamics of tendon/riser in the field of ocean engineering.

On the hydrodynamic responses of a multi-column TLP floating offshore wind turbine model

The offshore floating wind turbine system is a key piece of equipment for harvesting wind energy from the ocean field. However, the structural safety analysis and design of such equipment largely require a suitable platform to accurately obtain the dynamic characteristics. In the present study, an attempt was made to investigate the dynamic responses of the proposed 5-MW WindStar multi-column tension-leg-platform (TLP) system under different ocean environment conditions. Typical motion responses of the WindStar system, including the surge and pitch, and a series of measurements obtained from wave basin tests, such as nacelle acceleration and tendon tension, were compared with those of simulated results from the FAST code under four hydrodynamic load conditions. Additionally, the second-order wave load was incorporated with the first-order wave load, so as to allow for accurate evaluation of the high-frequency response of the floating offshore wind turbine (FOWT). Further, the effect of high-order wave loads on the high-frequency responses of the TLP FOWT was elucidated. The second-order frequency wave load was found to be a significant factor in determining the motion responses of the WindStar TLP system, and the results from FAST were found to agree well with those of the experiment. The coupled method of experiments and numerical analysis can be effectively used to understand the high-frequency dynamic responses of TLP FOWT systems, such as the WindStar TLP.

Design, analysis and horseshoes chaos control on tension leg platform system with fractional nonlinear viscoelastic tendon force under regular sea wave excitation

In this paper, the dynamic response of a Tension Leg Platform (TLP) system with fractional nonlinear viscoelastic tendon force under regular sea wave is investigated. Analytical and numerical methods are employed to analyse the effect of the fractional viscoelastic parameter, the tendon viscosity coefficient and the number of tendons on the amplitude of the system. It is found that, when the tendon viscosity coefficient and the number of tendons increase, the amplitude of vibration decreases. We also show that, increase of the fractional order derivative also contributes to decrease the unstable range of amplitude. Nevertheless, beyond a certain value of the fractional parameter, we rather observe an increase in amplitude. In other hand, Melnikov technique is used to derive the analytical criterion for the appearance of the heteroclinic chaos in the system. Analytical prediction is tested against numerical simulations based on the basin of attraction. It is found that, the appearance of horseshoes chaos depend of the fractional viscoelastic parameter, the tendon viscosity coefficient and the number of tendons.

Experimental investigation of the vortex-induced motion of tension leg platform system with suppression system under current

The vortex-induced motion (VIM) of a tension leg platform (TLP) and vortex-induced vibration (VIV) of tension legs and risers occur only under the action of water flow. The VIM results in a high amplitude motion in the cross-flow (CF) direction, and the VIV leads to fatigue failure of the tension legs and risers. A three-dimensional (3D) physical model investigation was conducted on the overall dynamic response of the platform and acceleration at different positions of the tension legs in the in-line (IL) and CF directions. Different lengths of the splitter plate behind the TLP columns were observed, and the suppression effect of the VIM at six flow velocities was investigated. The experimental results indicated the obvious VIM phenomenon of the platform without the splitter plate, and multi-frequency time-varying coupling characteristics were observed when the heading angle was 45°. Furthermore, the results revealed that the VIM phenomenon of the platform was evidently suppressed, in amplitude and frequency, and the force coefficients are reduced significantly, after the splitter plates were arranged behind each column of the platform. However, the splitter plate slightly affected the VIV of the tension legs.

Coupled dynamic response of a tension leg platform system under waves and flow at different heading angles: An experimental study

A Tension leg platform (TLP) exhibits different movement characteristics under waves or flow. A three-dimensional (3-D) experimental investigation of the coupled dynamic response of a TLP system under different environmental conditions is presented in this paper. The TLP system includes platform, tension legs and risers. Regular and irregular waves, flow, and a combination of irregular waves and flow are employed as the environmental conditions. The influence of different heading angles on the motion responses in the in-line (IL) and cross-flow (CF) directions are presented. The motion spectra, wavelet transform, and motion trajectories are employed to analyze the time histories. Statistical analysis of motion characteristics of TLP under different environmental conditions is presented here and the most disadvantage heading angle is recommended. With an increase of reduced velocity, the flow-induced motion (FIM) of the platform first increases and then decreases, and multi-frequency time-varying coupling characteristics are presented in the cases of large reduced velocity. The FIM in this experiment and that in the towing tank are compared simultaneously. In addition, the influence of free surface waves on the FIM has been illustrated through qualitative and quantitative analysis.

Transient response of a TLP-type floating offshore wind turbine under tendon failure conditions

Among various types of floating offshore wind turbines (FOWTs), the tension leg platform (TLP) floating wind turbines have relatively small motions due to stiff tendons. Similar to TLP applications in the offshore industry, tendon failure may lead to deteriorated stability and large transient responses, which should be considered as part of accidental limit state (ALS) checks at the design stage of the TLP FOWTs. This paper takes the WindStar TLP system as a representative and investigates the transient effects of one-tendon failure on the system responses. A coupled numerical model is first established using the aero-hydro-servo-elastic simulation tool FAST. Subsequent numerical simulations of tendon failure are carried out to consider different tendon breakage and turbine shutdown scenarios in both operational and parked conditions. Response statistics of key design parameters including platform motion, nacelle acceleration, and tendon tension are analyzed. The results indicate that several transient responses of the FOWT under operational conditions may be even higher than the extreme values under 50-year conditions. Our analyses also reveal that the shutdown strategies, if not applied appropriately, may have limited effects in reducing the responses of the FOWT, thus in certain scenarios threaten the safety of the tendon closest to the broken tendon if no other actions are taken. For the considered TLP FOWT, the ALS design requirements dominate over the ultimate limit state requirements. These findings can be relevant to the ALS design of TLP FOWTs.

Study on the static behavior of installing a deep-water drilling riser on a production platform

Riser interference caused by wake effects between adjacent risers severely threatens the safety during riser installation process on a Dry-Tree Tension Leg Platform (TLP) system. Due to the ignorance of interference effect, the static deflection of the drilling riser during installation is highly over-estimated in previous work. To obtain a static behavior of a drilling riser during installation, this paper built a dual-riser model where two risers are arranged in tandem and considered as Bernoulli beam subjected to steady ocean current. The modified Huse model is applied to study the interference between object risers. Then, the structural equation is solved by Finite-Difference-Method (FDM). Case studies show that an upstream production riser can largely reduce the deflection of a downstream hung-off drilling riser, hence increasing the probability of the collision of two risers. The influence of water depth, riser wall thickness, Blowout Preventer (BOP) weight, etc. are also studied. Finally, the current condition based weather windows with respect to wind and tidal current velocity are obtained corresponding to different operation conditions. Results show that installing a drilling riser with lower BOP weight in shallow water is most feasible for multi-riser TLP systems. Moreover, drilling riser installation operability can be improved by increasing the top tension of the production riser.

Hydrodynamic performance of a floating offshore OWC wave energy converter: An experimental study

To investigate the dynamic response of a floating moored oscillating water column device under realistic sea states, a focused wave technique was developed in this study to generate user-defined wave trains in a wave basin equipped with a piston-type wavemaker. The experimental setup allowed for simultaneous measurements of the designed focused wave trains, internal chamber wave elevations and air pressure, dynamic tendon response and the model's global motion responses. Based on the experimental results it was found that to accurately replicate the theoretical wave time series, a minimal focal reconstruction of 200 components was required which resulted in a focused wave regeneration accuracy greater than 93% with strong repeatability. The tested model showed typical motion responses to that of generalised tension leg platform systems with significant surge offsets along with stiff heave and pitch motions. Applying vertical tendons to the system allowed for the floating offshore device to be exposed to deeper waters, and hence the larger energy source associated with deep water ocean waves.

Mooring analysis for a whole TLP with TTRs under tendon one-time failure and progressive failure

In deepwater oil and gas field development, a Tension Leg Platform (TLP) consists of a hull, tendons, and top tension risers (TTR). When one or more tendons failure due to extreme sea state, a TLP will be under the working condition which is very different from its design condition, showing structural instability and transient effect. To enhance the safety and global stability of whole TLP system, obtaining failure consequences is necessary. By 3D potential flow theory and panel element method, a series of mooring analyses are performed for the target TLP to predict its coupled hydrodynamic response after tendon one-time failure and, in particular, during progressive failure of several tendons. Different directions of environment load that including extreme wind, wave and current and tendon failure positions are considered. In the results, the motion response of the hull, the force from residual tendons and TTRs are presented.

LMI-based robust control of floating tension-leg platforms with uncertainties and time-delays in offshore wind turbines via T-S fuzzy approach

This paper proposed the stabilization control of the tension leg platform (TLP), which is one of the most common types of offshore floating wind turbines. The main problem which exists in the installation of this kind of turbines is how to control and float them on water owing to the collision of turbulent waves of the sea. For this purpose, to design the controller on mentioned system, first a nonlinear function is introduced as a candidate of Lyapunov; then, it is proved if TLP system is stable using robust control in the presence of uncertainties, time delays and disturbances. Then, phase Takagi-Sugeno (T-S) model as a controller is provided for the mentioned system. In this case, using linear matrix inequalities (LMIs), the required feedback efficiency of the fuzzy controller can be obtained. Finally, by simulating in Simulink-MATLAB environment, it can be seen that not only the controller design is performed more easily, but all the state variables are asymptotically stable.

Coupled motion analysis of a tension leg platform with a tender semi-submersible system

In this study, an experimental and numerical analysis have been carried out to investigate the coupled behavior of a Tension Leg Platform (TLP) combined with tender semi-submersible platform with focus on the multi-modal behaviors. Free decay tests were conducted for the TLP and semi-submersible, which showed that the TLP and semi-submersible system had complex coupled behaviors with multiple natural mode frequency components. To investigate this behavior rigorously, an eigenvalue analysis was applied. The natural modes and periods of the multi-body system were identified from the eigenvalue analysis. A selected set of model experiments was compared with corresponding numerical simulations which showed a good agreement.

Structural system simulation and control via NN based fuzzy model

This paper deals with the problem of the global stabilization for a class of tension leg platform (TLP) nonlinear control systems. It is well known that, in general, the global asymptotic stability of the TLP subsystems does not imply the global asymptotic stability of the composite closed-loop system. Finding system parameters for stabilizing the control system is also an issue need to be concerned. In this paper, we give additional sufficient conditions for the global stabilization of a TLP nonlinear system. In particular, we consider a class of NN based Takagi-Sugeno (TS) fuzzy TLP systems. Using the so-called parallel distributed compensation (PDC) controller, we prove that this class of systems can be globally asymptotically stable. The proper design of system parameters are found by a swarm intelligence algorithm called Evolved Bat Algorithm (EBA). An illustrative example is given to show the applicability of the main result.

Extreme Response Prediction of a Multi-Column Tension-Leg-Type Floating Wind Turbine under Operating Conditions

The extreme response of a multi-column tension-leg platform (TLP) floating wind turbine (the WindStar TLP system) is analyzed here under operating conditions. Coupled dynamic response simulations are performed using FAST, a fully coupled aero-hydro-servo-elastic time-domain simulation tool. A land-based system (a U.S. National Renewable Energy Laboratory offshore 5 MW baseline wind turbine installed on land) is quantitatively compared with the WindStar TLP system. Extreme loads with different return periods are obtained by statistical load extrapolation. Peaks are extracted using peak-over-threshold analysis with time separation between peaks. The WindStar TLP system shows almost similar mean generator power output to the land-based system under operating conditions owing to its small dynamic motion responses to environmental loads. It shows greater characteristic load (blade 1 out-of-plane moment at the blade root and tower base fore-aft bending moment) under operating conditions than does the land-based system, with the characteristic load ratio reaching approximately 1.2. These results will help identify the extreme response differences between the two systems, and thus aid the development of relevant modifications to the initial WindStar TLP design.

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Preliminary Assessment of the Importance of Platform–Tendon Coupling in a Tension Leg Platform

This paper presents performance metrics that can be used to evaluate the response sensitivity of a tension leg platform (TLP) to its tendons. An uncoupled TLP model ignores the intrinsic dynamics and environmental loads on the cables by treating each tendon as an ideal massless spring. A coupled TLP system, in contrast, considers the effects of distributed mass and drag along the tendon. Under certain operating conditions, an uncoupled dynamics model can produce results comparable to its coupled counterpart. This paper defines the conditions under which it is acceptable to model a TLP tendon as a linear spring, as opposed to one that considers the cable dynamics. The analysis is performed in the frequency domain and, for generality, the results are nondimensionalized. The findings indicate that a more elaborate set of conditions than the platform–to–cable mass ratio must be satisfied for the two models to provide similar results. To conclude this study, two simulations are performed and compared against the performance metrics derived in this paper.

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