Tension Leg Platform Research Articles

Optimization design of multiple tuned mass dampers for semi-submersible floating wind turbine

Compared with spar-buoy and tension-leg platform floating offshore wind turbines (FOWTs), the flexibilities in operating water depth promote generalized applications of semi-submersible FOWT. In this study, a passive structural control strategy of a tuned mass damper (TMD) design method for a semi-submersible FOWT was implemented to mitigate FOWT responses under combined winds and waves. A reduced coupled model of a semi-submersible FOWT with TMDs mounted in the nacelle and platform was established based on D'Alembert's principle. Sequentially, the parameters of the reduced coupled model were estimated using the Leven-Marquardt (LM) algorithm, and the simulation accuracy was validated by comparison with the related fully coupled model in OpenFAST. To minimize the standard deviation of tower-top displacement, the design of the TMD was optimized by genetic algorithm (GA) based on the reduced coupled model, and three different TMD configurations were proposed. Furthermore, the effectiveness of the optimized TMDs was evaluated based on a fully coupled model in OpenFAST under typical sea states. The optimized nacelle and platform TMDs could effectively suppress the tower fundamental and platform pitch frequencies, respectively. Moreover, improved mitigation effects were observed using the optimized multiple TMDs, which comprise local dampers installed in the nacelle and platform.

Design of adaptive continuous barrier function finite time stabilizer for TLP systems in floating offshore wind turbines

This essay discusses the finite-time stabilization of Tension Leg Platforms (TLP), one of the most considerable offshore floating wind turbine systems. The most critical issue in installing such turbines is controlling and balancing them on the water. In order to stabilize the TLP systems in floating offshore wind turbines, an adaptive terminal sliding mode controller is designed. Because the discontinuous signal function is one of the most important disadvantages of common sliding mode controllers, which causes an undesirable chattering phenomenon in the control law, the suggested approach introduces a new Lyapunov candidate function based on the adaptive continuous barrier function technique, so that the planned control law on TLP systems is obtained continuously and smoothly. The mentioned improved control method uses Lyapunov's stability theory and satisfies the convergence of states around the switching surface in a finite time. In this way, the proposed method removes the chattering phenomenon and operates with higher accuracy than the conventional controllers. In addition, the unknown bound of external disturbance is estimated using the adaptive control scheme. Finally, based on the simulation results, it can be seen that all the state variables of the TLP system are asymptotically stable in the presence of external disturbance and converge to zero, which indicates the efficiency of the proposed technique for stabilizing the tension leg platforms in offshore floating wind turbines.

Comparative study of TLP and ETLP performance in Caspian Sea environment using numerical method

This paper presents a numerical analysis to compare a Conventional TLP (CTLP) and an Extended TLP (ETLP) performance in marine environmental conditions. Both platforms have 12 tethers installed in a water depth of 700 m located in the Caspian Sea under irregular JONSWAP waves, wind and current loads. A coupled BEM-FEM numerical time and frequency domain analysis was performed to estimate motions and mooring line cumulative fatigue damage of two kinds of tension leg platforms. Besides, power spectra density (PSD) of heave motion, which is the most important displacement in the vertical plane for TLPs, under different wave periods at a wave angle of 30 deg was investigated. Finally, to have an in-detailed overlook, surge, heave and pitch RAO of platforms in critical wave periods under wave angles from 0 to 45 deg were studied. Model results showed that the CTLP demonstrate larger mooring fatigue damage and therefore less safety against wave, wind and current loads. However, perturbed waves induce more perturbance around the ETLP than the CTLP which in turn needs a proper control system to damp it. In addition, the obtained results show that in the Caspian Sea condition, mainly the ETLP demonstrates a smaller heave, surge and pitch motions than those of the CTLP under various wave periods. Altogether these results show that using a control system ETLP with respect to the CTLP demonstrates more efficient performance.

Stability Analysis of a TLP with Inclined Tension Legs under Different Marine Survival Conditions

To verify that inclined tension legs can improve the stability of the tension leg platform, this paper established the dynamic equation of a tension leg platform (TLP) under marine environmental loads by using the modified Morrison equation considering the influence of ocean currents on wave forces. Additionally, the velocity and acceleration of random wave water particles were simulated via the JONSWAP spectrum. In addition, a three-dimensional model of a tension leg platform with inclined tension legs was established by AQWA, and its dynamic responses under variable survival conditions were compared and analyzed. The results showed that the surge and heave were more sensitive to the sea current, while the pitch was more sensitive to the wind. There is a significant difference in tendon tensions between the atypical TLP with inclined tension legs established in this study and the typical International Ship and Offshore Structures Committee (ISSC) TLP.

Market Needs, Opportunities and Barriers for the Floating Wind Industry

This paper reviews the status of floating wind energy expansion, market needs, opportunities, and barriers. Even more expensive than many other generation technologies currently, the floating wind will contribute to the decarbonization of Europe. This document assesses the market strategies available to develop floating wind farms in Europe. The study includes four main phases in addition to the overview of the current state-of-the-art: a technology review, market outlook, opportunities, and commercialization barriers. During its development, the offshore wind has moved from experimentation to a final design (Semisubmersible/barge, Tension Leg Platform, and Spar).

Coupled Dynamic Response of an Offshore Multi-Purpose Floating Structure Suitable for Wind and Wave Energy Exploitation

This paper presents the coupled analysis performed in the frequency and time domain, considering a multi-purpose floating structure suitable for offshore wind and wave energy source exploitation. The floating structure encompasses an array of hydrodynamically interacting Oscillating Water Column (OWC) devices, moored through tensioned tethers as a Tension Leg Platform (TLP), and supports a 10 MW Wind Turbine (WT). The analysis is built to incorporate properly the solutions of the diffraction and the pressure- and motion-dependent radiation problems around the floating structure, the mooring lines characteristics, the OWCs characteristics, and the aerodynamics of the WT by accounting for the aerodynamic modelling of the rotor, the elastic modelling of the turbine components, namely the blades, the drive train, and the tower. Numerical results are obtained through the developed analysis methods, presenting the fundamental hydrodynamic properties of the platform, as well as the ultimate and fatigue loads at several locations on the structure expected to be imposed on it over its lifetime. The effect of the installation sites (i.e., environmental excitation) on the lifetime loads is investigated by comparing the estimated loads at various Mediterranean Sea and North Sea locations. Finally, the OWC effect, due to the air pressure oscillation inside the wave energy converters, is discussed, highlighting its significant influence on the hybrid system’s loading conditions.

Numerical Assessment of a Tension-Leg Platform Wind Turbine in Intermediate Water Using the Smoothed Particle Hydrodynamics Method

The open-source code DualSPHysics, based on the Smoothed Particle Hydrodynamics method for solving fluid mechanics problems, defines a complete numerical environment for simulating the interaction of floating structures with ocean waves, and includes external libraries to simulate kinematic- and dynamic-type restrictions. In this work, a full validation of the SPH framework using experimental data available for an experimental test campaign on a 1:37-scale floating offshore wind turbine tension-leg platform (TLP) is presented. The first set of validation cases includes a surge decay test, to assess the quality of the fluid–solid interaction, and regular wave tests, which stimulate the mooring system to a large extent. During this phase, tendons (tension legs) that are simulated by MoorDyn+ are validated. Spectral comparison shows that the model is able to capture the surge and pitch dynamic amplification that occurs around the resonant fundamental mode of vibration. This work concludes with a numerical investigation that estimates the response of TLP under extreme events defined using multiple realizations of irregular sea states; the results suggest that the tendon loads are sensitive to the sea-state realization, providing maximum tendon peak forces in a range of ±10% about the mean. Furthermore, it is shown that the load pattern that forms from considering the relative position of the tendons to the incident wave direction leads to higher forces (≈20%).

Feasibility analysis for floating offshore wind energy

PurposeThe assessment of the economic feasibility of floating offshore wind farms (FOWFs) plays an important role in the future possible spreading of this challenging technology in the wind power industry. The use of specific economic analyses is fundamental to point out the potential of FOWFs and to sustain their technical value. Within this topic, the implementation of the FOWF life cycle cost model and producibility analysis in a geographic information system is developed, with the aim of carrying out a feasibility analysis at the territorial scale, for different types of floater. Moreover, a simplified model for a quick life cycle cost assessment is proposed and calibrated.MethodsThe available cost model is first validated comparing the costs of FOWFs based on different floaters (Semi-Submersible Platform—SSP, Spar Buoy—SB and Tension Leg Platform—TLP) with corresponding results available in the literature. Then, it is implemented in QGIS to be used for territorial-scale analyses and sensitivity analyses of the cost parameters. A feasibility analysis is developed through the main financial parameters. Finally, the results are then used to calibrate a simplified version of the cost model that depends on three main parameters, namely distance to shore, distance from the port of operation and bathymetry.Results and discussionThe FOWF cost values are found to be in good agreement with those coming from analytical methods similar to the one from the authors. However, some discrepancies with those based on average costs are observed. Then, the results of the sensitivity analysis are presented as life cycle cost maps, giving an overall picture of the variation of the total cost of FOWF installations on a reference domain. The results show that among the three types of floaters considered here, the SSP proved to be the most promising one, giving lower costs than the SB and the TLP. Moreover, a good agreement between the results in terms of total cost of FOWFs calculated with the analytical and simplified models for SSPs, SBs and TLPs is observed. Finally, the feasibility analysis showed that the financial parameters are more influenced by the wind speed than by the cost of the farm.ConclusionsThe paper aims to provide guidance on how to carry out feasibility analyses of a specific site for FOWF installation, thus supporting decision-making procedures. The approach and the results presented here are meant for use in the early stage of the decision-making process, as a tool for the assessment of the economic feasibility of FOWFs installation.

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.

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.

Surge Response Analysis of the Serbuoys-TLP Tension Leg Platform under the Action of Wave–Current Coupling

A new type of tension leg platform (TLP) connected to a series of buoys (Serbuoys-TLP) has been proven to effectively suppress the surge response of the platform during wave conditions. However, in the complex marine environment, it is more relevant to study its motion response to the action of waves and currents. Considering the tension tendon as a lumped mass model, a DUTMST 2.0 time-domain simulation program was written, based on MATLAB, which can accurately calculate the surge response of the Serbuoys-TLP under wave–current coupling conditions. The suppression efficiency of the Serbuoys-TLP on the surge response was analyzed under different current velocities and wave parameters, and the results showed that the suppression efficiency by the Serbuoys-TLP of surges was higher under the action of waves and currents compared with the action of waves. In addition, the surge response of the platform under the two conditions of wave–current combination and wave–current coupling was also investigated, where wave–current coupling considers the effect of the current’s velocity on the wave period, while the wave–current combination does not consider it, which means that the wave and current are linearly superimposed. The results show that the surge response of the platform will be overestimated without considering the coupling effect of waves and currents. The effect of wave–current coupling has a greater impact on the surge response of the Serbuoys-TLP than that of conventional TLP. Therefore, in the design of new floating structures, the motion performance in response to the effect of wave–current coupling should be paid full attention.

Floating wind turbines: marine operations challenges and opportunities

Abstract. The global floating offshore wind energy industry is rapidly maturing, with several technologies having been installed at pilot and demonstration scales. As the industry progresses to full array-scale deployments, the optimization of marine activities related to installation, operation and maintenance, and decommissioning presents a significant opportunity for cost reduction. This paper reviews the various marine operations challenges towards the commercialization of floating wind in the context of spar-type, semi-submersible and tension leg platform (TLP) technologies. Knowledge gaps and research trends are identified along with a review of innovations at various stages of development, which are intended to widen weather windows, reduce installation costs, and improve the health and safety of floating-wind-related marine operations.

An experimental study on characteristics of relative wave elevations around a tension leg platform in irregular waves

In this study, a series of model tests were performed to study the relative wave elevation responses around a tension leg platform (TLP) in extreme waves. It is well known that a high relative wave elevation causes a horizontal wave impact or a wave-in-deck impact which can result in a serious damage on both fixed and floating offshore structures. In particular, tension leg platform can be exposed to severe wave-in-deck impact events due to nonlinear wave amplifications inside the multiple columns. Irregular wave tests including the waves of 100-year return periods were carried out at Ocean Engineering Basin of KRISO. Focus is on the nonlinear characteristics of the relative wave elevations. Probability of exceedance as well as time series for relative wave elevation are suggested and discussion is made on the effect of location, wave and current. Also, wave-in-deck events are described based on the model test observations.

Probabilistic multilevel robustness assessment framework for a TLP under mooring failure considering uncertainties

In recent years, mooring failure is one of the prone accidents of floating production system (FPS). And it may lead to catastrophic consequences, due to lack of structural robustness, the ability to withstand progressive failure. During their operational life, Tension leg platforms (TLPs), as a kind of FPS, are exposed to stochastic loading (e.g., wind, wave and current) and perform stochastic dynamic response; on the resistance side, moorings also behave in a stochastic way. In this piece of research, a probabilistic multilevel robustness assessment framework is proposed to effectively evaluate robustness of TLP under mooring failure. For comprehensive assessment, the final robustness curve is obtained by weighting three kinds of major progressive failure modes, that is, progressive tendons failure, excessive offset and capsize. To fully consider uncertain factors, the proposed method includes probabilistic model of response based on hydrodynamic analysis, probabilistic model of ocean environment and probabilistic model of resistance. An illustrative example is developed as a walk-through of proposed methodology, discussing possible outcomes. The results indicate that combinations of failure tendons, classified by tendons’ relative position and their relationships with loading direction, would greatly influence structural robustness; besides, upgrade of tendons couldn't comprehensively improve robustness.

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.

Wave-induced Force Dynamics Analysis of Tension Leg Platform

This work analyses the dynamic response of Tension Leg Platform, TLP under sea wave induced forces with the aid of fluid dynamics modified Morison equation, single degree of freedom mass spring damper theory, and the Runge-Kutta ode45. Specifically, we employ the modified Morison equation to calculate the sea wave forces acting on a cylinder hull of the TLP. Two types of sea wave characteristics are analysed including sea waves in the South China Sea to compute the waves loading on the hull. Evaluated results are incorporated into the equation of motion of the platform, modeled as a single degree of freedom mass-spring-damper system to obtain the platform displacement at x-axis direction. The results showed that the dynamic response of the platform under the influence of sea wave A exhibits a displacement of 0.02 m in the direction of wave propagation parallel to the x-axis of the platform. Meanwhile, sea wave B manifests a magnitude at least twenty times larger compared to sea wave A, resulting in 0.5 m displacement in the same axis direction. We further examine the consequence of velocity profile of sea waves on displacement and time taken for a complete vibrational cycle. A parameter-fed CFD simulation with Star-CCM+ shows clear dynamic response of the TLP when acted upon by sea wave A. Obtained results indicate the importance of materials selection for construction of the hull tendons based on the motion of the hull and gives a fair estimate for cyclic loading on the tendons throughout the life cycle of the platform.

A Type-2 Fuzzy Controller for Floating Tension-Leg Platforms in Wind Turbines

This paper proposes a type-2 fuzzy controller for floating tension-leg platforms in wind turbines. Its main objective is to stabilize and control offshore floating wind turbines exposed to oscillating motions. The proposed approach assumes that the dynamics of all units are completely unknown. The latter are approximated using the proposed Sugeno-based type-2 fuzzy approach. A nonlinear Kalman-based algorithm is developed for parameter optimization, and linear matrix inequalities are derived to analyze the system’s stability. For the fuzzy system, both rules and membership functions are optimized. Additionally, in the designed approach, the estimation error of the type-2 fuzzy approach is also considered in the stability analysis. The effectiveness and performance of the proposed approach is assessed using a simulation study of a tension leg platform subject to various disturbance modes.

Concurrent wind, wave and current loads on a monopile-supported offshore wind turbine

Offshore wind energy harnessing is a major source of renewable energy. From the design perspective, it is crucial to properly determine environmental load effects on offshore wind turbines concurrently subjected to wind, wave and currents as reflected in a realistic offshore environment. Accordingly, the focus of the present study is on addressing this issue in the form of small-scale laboratory experiments in the Wind-Wave-Current Tank (WWCT) at Newcastle University, UK, using a scaled model of an offshore wind turbine. The hydrodynamic surge force exerted by currents represents a major contribution to the overall average loads. The waves are the main source of the unsteady loads. Higher amplitude waves cause an increase in the relative standard deviation of the integral loads, an effect which is more pronounced at greater wind and current velocities. While increasing the wind raises the dynamic loads at the natural frequency of the wind turbine, i.e., inertial loads, whereas currents introduce loads, which occur particularly at low frequencies due to nonlinear hydrodynamics, both observations were made in the absence of waves. If the waves are also present, the dynamic loads on the offshore wind turbines are primarily at the wave frequency, i.e. wave-induced dynamic loads on the offshore wind turbines are predominant. Higher modes of the surge and sway force fluctuations are more pronounced when all three load components act concurrently than is the case when the current or the wind acts in isolation. These trends may be further amplified in the dynamic response of floating wind turbines as noted in previous computational studies involving tension leg platforms.

Extreme riser experimental loads caused by sea currents in the Gulf of Eilat

Risers are widely used in offshore industry and are subjected to dynamic environmental loads. Robust prediction of extreme riser external loading under the action of sea currents and subsequently VIV (vortex-induced vibrations) is an important safety concern for various offshore installations, for example TLP (Tension Leg Platform) and FPSO (Floating Production Storage and Offloading Unit). Excessive loading acting on riser may occur during offshore operations, posing an operational risk. In this paper, experimental results were used to study hydrodynamic current loads, acting on a riser under actual sea conditions.This paper presents methodology for estimating extreme loads, based on laboratory measurements. The ACER (averaged conditional exceedance rate) method has been briefly presented. Suggested methodology provides accurate extreme value predictions, utilizing available measured data efficiently. In this study estimated extreme value prediction, corresponding to a relatively large return period, was obtained by ACER method, based on the in situ sea current environmental statistics input. Based on the overall performance of the suggested method, it was concluded that ACER method can incorporate environmental input and provide robust and accurate prediction based on experimental measurements.Data de-clustering issue has been briefly addressed. Paper highlights ACER method ability to have environmental statistical distribution as an input, as required in long term statistical analysis. Described approach may be well used at the riser design stage, while defining optimal riser parameters that would minimize potential riser damage.

Investigation on a Large-Scale Braceless-TLP Floating Offshore Wind Turbine at Intermediate Water Depth

Tension leg platform (TLP) is a cost-effective and high-performance support structure for floating offshore wind turbine (FOWT) because of its small responses in heave, pitch, and roll with the constraint of the tendons. China, as the largest market of offshore wind energy, has shown a demand for developing reliable, viable floating platform support structures, especially aiming at the intermediate water depth. The present paper described a newly proposed 10-MW Braceless-TLP FOWT designed for a moderate water depth of 60 m. The numerical simulations of the FOWT are carried out using the coupled aero-hydro-servo-elastic-mooring calculation tool FAST. The measured wind and wave data of the target site close to the Fujian Province of China were used to evaluate the performance of the FOWT under the 100-, 50-, 5-, and 2-year-return stochastic weather conditions. The natural periods of the platform in surge, sway, heave, pitch, roll, and yaw were found to be within the range recommended by the design standard DNV-RP-0286 Coupled Analysis of Floating Wind Turbines. The largest surge of the water depth ratio among all the load cases was 15%, which was smaller than the admissible ratio of 23%. The tower top displacements remained between −1 m and 1 m, which were at a similar order to those of a 10-MW monopile-supported offshore wind turbine. The six tendons remained tensioned during the simulation, even under the operational and extreme (parked) environmental conditions. The Braceless-TLP FOWT showed an overall satisfying performance in terms of the structural stability and illustrates the feasibility of this type of FOWT at such a moderate water depth.