Tension Leg Platform Model Research Articles

Study on dynamic responses of hydraulic pneumatic tensioner for a TLP under hydraulic component failure

This paper takes the hydraulic pneumatic tensioner as the research object. In order to study the dynamic response of hydraulic pneumatic tensioner, a coupled model of tension leg platform was established. Three kinds of sea conditions including first-order irregular wave, second-order wave and freak wave are simulated. The dynamic response of the hydraulic pneumatic tensioner is analyzed under the action of different pressure accumulators. It is found that the greater the pressure of accumulator, the smaller the displacement of hydraulic cylinder and the greater the tension under the action of the second-order wave force and the freak wave force. In the second-order waves, the average drift force has great influence on the amplitude of displacement and tension of hydraulic cylinder, while the second-order sum frequency force has great influence on the frequency of displacement and tension of hydraulic cylinder. In addition, the dynamic responses of the tensioner under second-order waves are also studied when one cylinder fails. When one cylinder fails or is in semi-failure state, the hydraulic pneumatic tensioner can still provide a nearly constant tension to the riser. The remaining normal piston cylinders’ tensions will increase, but the total tension will decrease.

An experimental investigation on vortex-induced motion (VIM) of a tension leg platform in irregular waves combined with a uniform flow

Vortex-induced motion (VIM) of a tension leg platform (TLP) is critical to avoid structural and fatigue damages to the tendons and risers. Many previous studies have conducted model tests for TLPs in towing tanks. However, the models were usually established using linear springs to replicate the underwater moorings. In this study, a TLP model with full-water-depth tendons and top tension risers (TTRs) was developed. A series of model tests were conducted in a large-scale wave tank at three incidence angles (θ=0°, 22.5°, and 45°) for the reduced velocity ranging from 4 to 11.5 and two extra cases of flow combined with irregular waves. The maximum transverse amplitude of 0.32D was observed at a reduced velocity of 7.0 when θ=0°. On the other hand, the in-line and yaw motion experienced increasing amplitudes with reduced velocities and no lock-in region was identified. The most significant responses of transverse and yaw motion appeared at 0° and 22.5° headings, respectively, while no critical heading was recognized for surge motion. The competing mechanics between VIM and the wave-induced motion were revealed in the cases of combined flows and irregular waves. The tensions in tendons and TTRs that result from VIM were found as significant as those induced by wave loads, which highlights the necessity of considering VIM in the design of TLP. Furthermore, the result showed that the VIM amplitudes were considerably suppressed under the nonlinear stiffness and larger damping provided by the tendons and TTRs.

Numerical Study of Uncoupled and Coupled TLP Models

Abstract In this paper, two analysis models for tension leg platform (TLP) are proposed based on different simulation methods of the tendons for studying the TLP motion responses in waves. In the uncoupled analysis model, the tendon is simplified as a spring, and the restoring forces matrix is derived with the consideration of the influence of the coupled effect of horizontal offset and vertical setdown of the platform. In the coupled model, the axial and transverse vibration’s coupled effect has been considered for the establishment of the vibration equations for the tendons, and the finite difference method is used to solve the vibration equations. The time-domain coupled motion model of the platform and the mooring system is established based on the interaction forces between the tendons and the platform. The coupled and uncoupled TLP models are compared and analysed to determine their applicability. Compared with the uncoupled TLP model, the coupled TLP model has greater accuracy and a wider application range, and the effects of second-order wave force on the platform responses, horizontal offset, and vertical subsidence are analysed.

The TLP 2-DOF as an alternative model for extreme wave application

Tension Leg Platform (TLP) is an offshore platform structure used for deep-sea oil and gas exploration. The main structure of the TLP consists of a deck, pontoon, mooring system, and foundation. TLP operates in a balance of buoyancy, structural weight, and mooring tension. The problem is the construction of TLP in the deep sea, where sometimes extreme waves appear could damage the TLP structure. This paper proposes a new model of TLP that is more stable to extreme waves. The method is to separate the mass of the deck and the mass of the pontoon into two flexible parts, which are connected by a cantilever spring system. Thus the TLP motion becomes two degrees of freedom (TLP 2-DOF). Using the dynamic vibration absorber (DVA) method, the ratio of the deck mass, pontoon mass, and spring stiffness are adjusted so that the primary mass movement is minimal. Furthermore, the ratio of the amplitude of the deck movement as the primary mass to the wave amplitude is analyzed, which is known as the operator response amplitude (RAO). The results showed that the TLP 2-DOF model was more stable. As an illustration, at resonance conditions, this model can reduce RAO to about 67%.

Investigation of tendon dynamics effects on tension leg platform response in random seas

This study investigated tendon dynamics effects of tension leg platform models with tendons modelled by finite element spring and beam elements for uncoupled and coupled tension leg platform in random waves and current environment. The purpose of the study is to proffering numerical solution of single mathematical equation of motion for fully integrated-coupled tension leg platform floater with tendons model. Structural modelling of complete tension leg platform is achieved with the help of ABAQUS/Standard finite element tools which is incorporated with ABAQUS/Aqua module for the application of hydrodynamic loadings on the partially submerged tension leg platform hull and fully submerged platform tendons. For the uncoupled tension leg platform model, weight, inertia, hydrodynamic force and damping forces are ignored on the tendons modelled with springs, while the stiffness of the tendons is considered as a static restoring force. The coupled tension leg platform model had all the forces applied on the tendons modelled with beam elements. Conclusively, modelling and analysis of the tension leg platform as uncoupled and coupled models have expanded our understanding to know that surge motion response is fairly predicted by the two models, however, heave and pitch motions, and variations in tendon tension differ significantly; hence, coupled tension leg platform model is recommended. The influence of a removed tendon due to accident or maintenance on the tension leg platform motions is also reported.

Experimental investigation of wave-in-deck impact events on a TLP model

Model tests were conducted to investigate the global response of a conventional tension leg platform (TLP) due to wave-in-deck loads associated with extreme wave events in irregular long-crested waves of a cyclonic sea state. The experimental setup was designed to allow for the simultaneous measurement of wave surface elevations, rigid body motions, tendon tensions, as well as the pressure distribution at the model's deck underside. The obtained results demonstrated the variability of all the measurements and provided insights into the effect of wave-in-deck loads on the platform behaviour, tendon tensions and slamming pressures and showed qualitative correlations between these parameters. Based on the repeated tests in several events with different wave parameters, general observations and conclusions were made with respect to the platform dynamics during the deck impact, tendon tensions, slack tendon situations, tendon ringing and local impact pressures. The results of this study could be used for calibrating computational fluid dynamics (CFD) tools.

Analysis of a preliminary configuration for a floating wind turbine

There are many theoretical analyses and experimental studies of the hydrodynamics for the tension leg platform (TLP) of a floating wind turbine. However, there has been little research on the arrangement of the TLP\'s internal structure. In this study, a TLP model and a 5-MW wind turbine model as proposed by the Minstitute of Technology and the National Renewable Energy Laboratory have been adopted, respectively, to comprehensively analyze wind effects and wave and current combinations. The external additional coupling loads on the TLP and the effects of the loads on variables of the internal structure have been calculated. The study investigates preliminary layout parameters-namely, the thickness of the tension leg body, the contact mode of the top tower on the tension leg, the internal stiffening arrangement, and the formation of the spoke structure-and conducts sensitivity analyses of the TLP internal structure. Stress is found to be at a maximum at the top of the tension leg structure and the maximum stress has low sensitivity to the load application point. Different methods of reducing maximum stress have been researched and analyzed, and the effectiveness of these methods is analyzed. Filling of the spoke structure with concrete is discussed. Since the TLP structure for offshore wind power is still under early exploration, arrangements and the configuration of the internal structure, exploration and improvements are ongoing. With regard to its research and analysis process, this paper aims to guide future applications of tension leg structures for floating wind turbine.

Experimental investigation of extreme wave impacts on a rigid TLP model in cyclonic conditions

This paper describes a series of model tests of a rigidly mounted tension leg platform (TLP) subjected to extreme wave events corresponding to long-crested irregular wave trains of a 10,000-year cyclonic sea state. The experimental setup was instrumented to simultaneously measure wave surface elevations in the vicinity of the model, global wave impact forces and local pressure distribution on the underside of the model's topside deck. Model accelerations were also monitored for each wave impact event so that the inertial force due to structural dynamic response could be identified. The deck–column intersection areas were found to experience large wave-in-deck slamming pressures, in particular around the aft columns. A reduction of the deck clearance was found to increase the magnitude of the global horizontal forces; however, the global vertical forces and local wave-in-deck slamming pressures did not follow this trend.

English

The harvesting of wind energy is expected to increase greatly in the future because of its stability, abundance, and renewability in large coastal states such as China. The floating support structure will likely become the major structural form for wind turbines in the future due to its cost advantages when the water depth reaches 50 m. The 5MW wind turbine model from National Renewable Energy Lab (NREL) and the modified tension leg platform model proposed by Harbin Institute of Technology (HIT) were applied to certain sea conditions in the South China Sea in order to consider the effects of external load coupling actions. In this study, the internal force, mooring system force, as well as the acceleration, displacement and velocity of the floating structure of the modified HIT Tension Leg Platform (HIT-TLP) were calculated. During this process, the physical parameters of its tension leg structure at a specific frequency domain were obtained to find the technical reserves for its practical application in the future.

Green water velocity due to breaking wave impingement on a tension leg platform

The present study employed the image-based bubble image velocimetry (BIV) technique to investigate the flow kinematics of a plunging breaking wave impinging on a geometry-simplified, unrestrained tension leg platform (TLP). A high-speed camera was used to record images for the BIV velocity determination for both fluid and structure velocities. The plunging breaker was generated using a wave focusing method, and repeated measurements were acquired to calculate the mean flow and turbulence intensity using ensemble averaging. BIV measurements were performed on two perpendicular planes: side view and top view. The flow measurements were compared with those of a similar experiment on a fixed structure by Ryu et al. (Exp Fluids 43(4):555–567, 2007a). The maximum velocity occurred in the run-up stage with a magnitude reaching 2.8C with C being the phase speed of the breaking wave. The dominant velocities for three distinct phases—impingement, run-up, and overtopping—are very close to those found on the fixed structure. Turbulence intensity was also examined, and the ratio among the three components was quantified. Ryu et al. (Appl Ocean Res 29(3):128–136, 2007b) reported that Ritter’s dam-break flow solution agrees surprisingly well with the measured green water velocity on the fixed structure to a certain degree. The present study followed the same approach and confirmed that Ritter’s solution is also in excellent agreement with the green water velocity on the unrestrained TLP model. Based on the self-similar behavior, the prediction equation proposed by Ryu et al. (2007a) was used to model the green water velocity distribution. The results show that the prediction equation is applicable to not only a fixed structure, but also the unrestrained TLP for green water velocity caused by extreme waves.

Dynamic response mitigation of floating wind turbine platforms using tuned liquid column dampers.

In this paper, we experimentally study and compare the effects of three combinations of multiple tuned liquid column dampers (MTLCDs) on the dynamic performance of a model floating tension-leg platform (TLP) structure in a wave basin. The structural stability and safety of the floating structure during operation and maintenance is of concern for the performance of a renewable energy device that it might be supporting. The dynamic responses of the structure should thus be limited for these renewable energy devices to perform as intended. This issue is particularly important during the operation of a TLP in extreme weather conditions. Tuned liquid column dampers (TLCDs) can use the power of sloshing water to reduce surge motions of a floating TLP exposed to wind and waves. This paper demonstrates the potential of MTLCDs in reducing dynamic responses of a scaled TLP model through an experimental study. The potential of using output-only statistical markers for monitoring changes in structural conditions is also investigated through the application of a delay vector variance (DVV) marker for different conditions of control for the experiments.

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.

Comparison of linear and nonlinear responses of a compliant tower to random wave forces

A vertical member of a compliant offshore structure is modeled as a beam undergoing both bending and extension. The beam has a point mass and is subjected to a point axial load at the free end. The equations of motion for the axial and transverse displacements are nonlinear and coupled. A linear tension model is derived as a special case of the nonlinear coupled model with negligible axial displacement. The responses are obtained numerically for both models. A quarter of the International Ship and Offshore Structures Congress (ISSC) tension leg platform model is used as a numerical example. The free and forced responses obtained using the nonlinear coupled modeled are compared to those of the linear tension model.

Hydrodynamic response of tension-leg platforms—A model

The tension-leg platform (TLP) is a hybrid type of offshore platform, fixed for the vertical degrees of freedom and compliant for the horizontal degrees of freedom. The platform structure is connected to the sea-bed foundation by vertical moorings (tension legs) which are kept in tension under all conditions by the excess buoyancy over weight of the platform. Hydrodynamic analysis is an important stage of the design of these structures; model tests are usually done for this purpose. This paper gives the details of an experimental study to analyze the behavior of a 1:50 TLP model subjected to the action of regular and irregular waves. Platform motions in surge, sway and heave directions were measured using accelerometers. Tether tensions were measured using specially developed load cells. Details of instrumentation, testing and analysis to obtain response amplitude operators (RAOs) are explained. The connection of tether to the column, arrangement for adjusting pretension from top, connection of load cell to the tether and provision for changing the angle of incidence of waves are some special features of this experimental study. The work reported will serve as a useful practical guide for similar types of experimental study. In addition, the conclusions on the comparison of RAOs of the model for regular and irregular waves will be useful in planning hydrodynamic model tests.

Response of a Simple Tension Leg Platform Model to Wave Forces Calculated at Displaced Position

A simple single-degree-of-freedom model of a tension leg platform is used to assess the reliability of the common practice of calculating wave-induced forces at the undisplaced position of offshore structures. This assessment is conducted in conjunction with the Morison equation based modeling of the wave-induced forces on slender structural members. It is shown by numerically integrating the equation of motion that the calculation of wave forces on the displaced position of the structure introduces a steady offset component in the structural response. This is valid for either deterministically or stochastically described wave fields. Several parameter studies are conducted. Furthermore, reliable approximate analytical deterministic and stochastic solution techniques are developed which conform to and, in fact, predict the conclusions drawn from the results of the numerical studies.

Response Analysis of Vertical Pipe Structures Subjected to Irregular Waves

The deformations and bending stresses of a mooring leg and a riser pipe of an assumed tension leg platform model are analyzed by two kinds of analytical methods under the condition of the severe irregular waves. One is a simulation in the time domain in which the drag term of the Morison's equation are left as the nonlinear term. The other is a linear response analysis in the frequency domain which is based on the linearization of the non linear drag term. From the comparison of the power spectral density functions and the related statistics derived from the two kinds of methods, the applicability and characteristics of the simpler latter method are discussed. In addition to these, the effects of bending on the structural responses of the mooring legs and risers of the tension leg platforms are studied.