Tension Leg Research Articles

Construction and installation engineering for floating wind turbines

The construction and installation engineering of floating offshore wind turbines is important to minimize schedules and costs. Floating offshore wind turbine substructures are an expanding sector within renewable power generation, offering an opportunity to deliver green energy, in new areas offshore. The floating nature of the substructures permits wind turbine placement in deep water locations. This paper investigates the construction and installation challenges for the various floating offshore wind types. It is concluded that priority areas for project management and design engineers minimising steel used in semi submersible construction, reducing the floating draft of Spars and for Tension Leg Platforms developing equipment for a safe installation. Specifically tailored design for construction and installation includes expanding the weather window in which these floating substructures can be fabricated, transported to and from offshore site and making mooring and electrical connection operations simpler. The simplification of construction methodology will reduce time spent offshore and minimise risks to installation equipment and personnel. The paper will include the best practice for ease of towing for offshore installation and the possible return to port for maintenance. The construction and installation process for a floating offshore wind turbine varies with substructure type and this will be developed in more detail in the paper. Floating offshore wind structures require an international collaboration of shipyards, ports and construction vessels, though to good project management. It is concluded that return to port for maintenance is possible for semi submersibles and barges whereas for Spars and TLP updated equipment is required to carry out maintenance offshore. In order to facilitate the construction and to minimize costs, the main aspects have to be considered i.e., the required construction vessel types, the distance from fit-out port to site and the weather restrictions.

Dynamic analysis of a multi-column TLP floating offshore wind turbine with tendon failure scenarios

This paper presents the design and dynamic analysis of a multi-column tension leg platform floating offshore wind turbine (TLP FOWT) with broken tendons. The proposed concept is based on a conventional Tri-Floater platform configuration but is especially optimized for an intermediate water depth of 60 m. Tendons are simulated to be broken at a specific time of interest. The hydrodynamic parameters and motion responses of the TLP FOWT have been calculated using the hydrodynamic analysis software WAMIT and ANSYS/AQWA. These parameters are then used in the simulation of coupled responses of the FOWT by the National Renewable Energy Laboratory's analysis suite of tool, FAST. The tendon failure has been investigated to examine the system performance under the accidental limit states (ALS) described in the design code DNV-RP-0286.A series of dynamic analysis under operational and extreme environmental conditions (with 12 different load cases) are carried out with the combination of wind and waves. The results demonstrate that the turbine is able to remain highly stable during the 3-h simulations conducted. The platform's response amplitude operators (RAOs) evaluated show good stability in heave and pitch. The tendon failures affected the FOWT's heave, pitch and roll motions, hence its natural frequencies, and the tension in the non-broken tendons. The average tensile force in the line adjacent to the broken line is found to be doubled, while the safety factors as specified in DNV-OS-E301 are remained within the recommended value. This paper provides further insights into the hydrodynamics of the multi-column TLP FOWT, with the aim to provide new information to the offshore wind research community.

Effect of the delay between the detection of vibration and the action of tendons on the dynamics response of tension leg platform (TLP) under sea waves excitation

In this study, the dynamic response of the tension leg platform (TLP) under sea waves excitation is investigated. One establishes the analytical framework consisting of mathematical modeling of TLP taking into account the tendons and the delay. We analyse the stability and determine the physical characteristics of tendon system that allow the system to be always stable. Conditions on the space parameters of the system for which harmonic, subharmonic, superharmonic, combination resonants states are obtained using the multiple time scales method. The results show that the stability area of the system decreases when the delay increases and increases when the damping coefficient increases. Furthermore, increasing the time- delay only increases the value of the maximum amplitude response of the system. However, reasonable selection of the system parameters can effectively reduce the level of vibration of the system.

Hydrodynamic characteristics of two side-by-side flexible cylinders with different diameters experiencing flow-induced vibration

Slender flexible cylindrical structures, such as marine risers and the tendons of tension leg platforms, have been widely used in offshore engineering. This paper mainly investigates the hydrodynamic characteristics of two flexible cylinders with unequal diameters in a side-by-side arrangement subjected to flow-induced vibration (FIV). A series of model tests are carried out in a towing tank. The aspect ratios and mass ratios of the two flexible cylinders are 350/181 and 1.90/1.47, respectively. A diameter ratio of 0.5 and a centre-to-centre spacing ratio of 6.0 are selected for the experimental tests. The experimental results of two cases, namely, those of an unequal-diameter cylinder system and an equal-diameter cylinder system, are compared and discussed. With the comparison of the hydrodynamic coefficients of large cylinder #1 and small cylinder #2 in the unequal-diameter cylinder system and small cylinder #3 in the equal-diameter cylinder system, the FIV features of two side-by-side unequal-diameter cylinders are systematically explored. The in-line (IL) and cross-flow (CF) vibration responses and hydrodynamic characteristics are specifically investigated. The wake effect caused by the vibration of the cylinders is more obvious and important for small cylinder #2 in the unequal-cylinder system. The lift coefficient CLy and fluctuating force coefficient CCF_rms of small cylinder #2 in the CF direction are obviously higher than those of the isolated cylinder in the larger reduced velocity range (Vrs = 7.52–17.54). The added mass coefficient Cax, varying drag coefficient CDx and fluctuating force coefficient CIL_rms in the IL direction decrease significantly when reduced velocity Vrs increases from 22.54 to 25.05. Compared with the two side-by-side cylinders in the equal-diameter cylinder system, the two cylinders in the unequal-diameter cylinder system exhibit a stronger interaction under the initial spacing ratio (S/d = 6.0).

Influence of second-order wave force and focusing position on dynamic responses of tension leg platform under a freak wave

As a semicompliant and semirigid column group structure, both second-order sum-frequency and diff-frequency wave forces and focusing positions are important for tension leg platforms (TLPs) under freak waves. The double-wave train superposition model is widely used to generate freak waves; however, multiple trials are inevitable for freak wave generation owing to the uncertainty in parameter selection, which may decrease the simulation efficiency. In this study, the effects of the energy ratio of the focused wave train, number of constituent waves, and parameters of the JONSWAP wave spectrum on the generation probability of freak waves are studied, and the generation probability of the freak wave is fitted by a Γ cumulative probability density function. The advantage of the fitted formula is that the generation probability of a freak wave is quantified, and an appropriate number of constituent waves can be determined for a given energy ratio of the focused wave to increase the simulation efficiency. Furthermore, the dynamic responses of a TLP in a freak wave considering the influences of second-order wave forces and focusing positions are simulated and discussed. Two main findings are that the second-order diff-frequency wave force may cause large-amplitude low-frequency motion in the surge direction and set-down motion in the heave direction, which are 3.71 and 4.08 times the first-order surge and heave responses, respectively, whereas the second-order sum-frequency wave force may cause high-frequency oscillation in the pitch direction, the amplitude of which is 3.9 times the amplitude of the first-order pitch response. The response of the TLP is significantly increased under the freak wave, especially when the second-order wave force is considered. The apparent focusing position of the maximum response changes with the spectral peak period owing to the nonrepeatability of the freak wave and phase difference of the columns. The maximum predicted errors of surge, heave, and pitch motion reached 42.0%, 41.1%, and 98.9%, respectively, when the freak wave acts on the front column, center of the platform, and back column, which may significantly underestimate the dynamic response. Therefore, the effects of the focusing position should be considered when calculating the response of a TLP under a freak wave.

Bottom-supported tension leg towers with inclined tethers for offshore wind turbines

Offshore wind with high energy density and low turbulence has attracted the global scientific community towards the development of the Offshore Wind Turbines (OWTs). Today, most of the shallow water sites are already occupied, and the industry is looking to install turbines at the deep water sites (water depth > 50m). However, an OWT support structure designed with the conventional concepts becomes massive and expensive in deeper waters. In contrast, floating OWTs are more expansive than the bottom supported ones, and considered suitable for a water depth of 60 m or more. A novel support structure concept termed a ‘bottom supported tension leg tower’ (BSTLT) has been proposed recently and the possibility of using bottom supported structures in deeper waters is shown. In this paper, an extension of the BSTLT concept has been achieved using inclined tethers with a monopile. The preliminary technical feasibility including the sensitivity of the natural frequencies and dynamic analyses considering the in-place operating and the extreme design conditions has been studied for 50 m water depth. The results show that the proposed concept can be utilized to make the monopile based support structures more cost-efficient for the water depth stated above.

The effect of time-varying axial tension on VIV suppression for a flexible cylinder attached with helical strakes

Flexible cylindrical structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), usually vibrate under the combined excitation of vortex shedding and time-varying axial tension. Helical strakes are widely used to suppress vortex-induced vibration (VIV). The VIV suppression effectiveness of helical strakes for a flexible cylinder under axial tension excitation is still unknown. Model tests were conducted to investigate the effect of time-varying axial tension on VIV suppression for a flexible cylinder fitted with helical strakes. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant axial tension) and four tension frequency ratios (fv/f1 = 0.5, 1.0, 2.0 and 4.0, where fv is the frequency of the varying tension and f1 is the fundamental natural structural frequency) were considered. The vibration displacements were reconstructed using the measured strains by the modal analysis method. The dominant mode, response frequency and displacement amplitude were determined to show the influence of axial excitation on the dynamic characteristics of the flexible cylinder attached with helical strakes. When the reduced velocity is low, the second-order mode vibrations of the straked cylinder with axial excitation are excited. Due to the axial excitation, the straked cylinder has large cross-flow (CF) and in-line (IL) displacements at low reduced velocities. The displacement contours in the case of fv/f1 = 1.0 are remarkably different from those in other cases. When the reduced velocity increases, the effect of vortex shedding is enhanced. Both fv induced by axial excitation and the fn induced by vortex shedding are prominent, meaning that both vortex shedding and axial excitation affect the vibration of the straked cylinder. Under axial excitation, the IL displacements of the straked cylinder are enlarged, especially in the case of fv/f1 = 1.0. Axial excitation can reduce the VIV suppression efficiency of helical strakes.

Review on Dynamics of Offshore Floating Wind Turbine Platforms

This paper presents a literature review of the dynamics of offshore floating wind turbine platforms. When moving further offshore, there is an increase in the capacity of wind power. Generating power from renewable resources is enhanced through the extraction of wind energy from an offshore deep-water wind resource. Mounting the turbine on a platform that is not stable brings another difficulty to wind turbine modeling. There is a need to introduce platforms that are more effective to capture this energy, because of the complex dynamics and control of these platforms. This paper highlights the historical developments and progresses in the design of different types of offshore floating wind turbine platforms needed for harvesting the energy from offshore winds. The relative advantages and disadvantages of the platform types with the design challenges are discussed. The major types of floating platforms included in this study are tension leg platform (TLP) type, spar type, and semisubmersible type. This study reviews the previous work on the dynamics of the floating platforms for a single turbine and multiple turbines under various operating environmental conditions. The numerical methods to analyze the aerodynamics of the wind turbine and hydrodynamics of floating platforms are discussed in this paper. This paper also investigates the performance of analytical wake loss models of Jensen, Larsen, and Frandsen that can provide guidelines for using these wake models in future applications. There are still a lot of challenges that need to be addressed to study the accurate behavior of floating platforms operating under combined wind–wave environmental conditions. With the current technological advancements, the offshore floating multi-turbine platform can be a potential solution to harness the abundant offshore wind resource. Based on this literature review, recommendations for future work are suggested.

Seismic analysis of a model tension leg supported wind turbine under seabed liquefaction

The paper presents numerical analyses of a pile supported Tension Leg Platform (TLP) wind turbine, during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when subsoil liquefaction is extensive, leading to degradation of the pseudo-static safety factor against pile pullout well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance. Pile head displacements develop steadily both during shaking and dissipation, but only while the static pullout safety factor remains less than unity. Due to the high tensional stiffness of the tendons relative to the buoyancy stiffness of the platform considered in this study, the pile head pullout is mostly transmitted to the platform with a relatively small part corresponding to reduction of tendon elongation. The resulting loss of buoy stability and tendon pretension may prove detrimental for the short-term operation of the platform. However, except possibly from the rare scenario of concurrent extreme environmental loads, both effects are recoverable, as they are unlikely to cause irreparable damage to the foundation and the superstructure.

Stochastic Analysis of Short-Term Structural Responses and Fatigue Damages of A Submerged Tension Leg Platform Wind Turbine in Wind and Waves

Stochastic Analysis of Short-Term Structural Responses and Fatigue Damages of A Submerged Tension Leg Platform Wind Turbine in Wind and Waves

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.

Wind resource assessment uncertainty for a TLP-based met mast

The current work presents wind resource assessment results of a complete one-year offshore measurement campaign in the Aegean Sea, at 65 m deep waters, with a floating tension leg platform (TLP) equipped with a met mast reaching the height of 44 m above mean sea level (AMSL). Wind conditions uncertainty is evaluated according to the existing standards and guidelines and the additional uncertainty, due to the platform motion, is addressed with two different methods: First by performing wind tunnel experiments, in which the wind sensors are submitted to various controlled motions and second by comparing results of the onboard lidar to a nearby onshore one. Both methods converge that the motion effects are minimal and the additional uncertainty of the mean values due to the TLP motion is ≤1%.

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.

An experimental study on transporting a free-float capable tension leg platform for a 10 MW wind turbine in waves

The paper presents the towing tests of CENTEC-TLP, a state-of-the-art free-float capable tension leg platform supporting a 10 MW wind turbine. The platform's design process and the overall dynamic behaviour was previously described in the literature. This work focuses on the platform's transportation stage, which is designed to have a shallow draft and certain characteristics resembling blunt bodies such as barges. The hydrodynamic behavior of this hull form in calm water and waves is investigated experimentally. The results are compared with similar relevant data available in the literature. As for added resistance in waves, the influence of wave characteristics and speed are evaluated. It is found that some of the cases display a reduction of average resistance in waves compared to calm water. Finally, the dynamic behaviour of the system in two relevant sea states is assessed with reference to risk of excessive motions during transport.

Performance based design of Tension Leg Platforms under seismic loading and seabed liquefaction: A feasibility study

The response of pile supported Tension Leg Platforms during seismic loading and seabed liquefaction is analyzed numerically, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo-static factors of safety against pullout failure of the pile well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking due to subsoil liquefaction, but fully recovers during the following excess pore pressure dissipation phase and may even exceed the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the finite time period when the static pullout factor of safety of the pile remains less than unity. Thus, final pile head displacements may increase considerably when this time period is elongated. Due to the generally high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout is mostly transmitted to the platform, with only a small percentage corresponding to reduction of tendon elongation. For the cases examined herein, the potential loss of platform buoyancy and tendon pretension may prove detrimental but is unlikely to threaten the safety and functionality of the platform. On the other hand, initially minor tilting of the platform (e.g. due to long period lateral loading and differential tendon pretension) may be magnified during seismic loading above allowable limits.

Whole-body energy transfer strategies during football instep kicking: implications for training practices

ABSTRACT Knowledge of whole-body energy transfer strategies during football instep kicking can help inform empirically grounded training practices. The aim of this study was thus to investigate energy transfer strategies of 15 semi-professional players performing kicks for speed and accuracy. Three-dimensional kinematics and GRFs (both 1000 Hz) were incorporated into segment power analyses to derive energy transfers between the support leg, torso, pelvis and kick leg throughout the kick. Energy transferred from support leg (r = 0.62, P = 0.013) and torso (r = 0.54, P = 0.016) into the pelvis during tension arc formation and leg cocking was redistributed to the kick leg during the downswing (r = 0.76, P < 0.001) and were associated with faster foot velocities at ball contact. This highlights whole-body function during instep kicking. Of particular importance were: (a) regulating support leg energy absorption, (b) eccentric formation and concentric release of a ‘tension arc’ between the torso and kicking hip, and (c) coordinated proximal to distal sequencing of the kick leg. Resistance exercises that replicate the demands of these interactions may help develop more powerful kicking motions and varying task and/or environmental constraints might facilitate development of adaptable energy transfer strategies.

Numerical and Experimental Study on Dynamic Response Mitigation of Tension Leg Platform Using Tuned Mass Damper

_ Response amplitude mitigation of the offshore structures like tension leg platform(TLP) is important since these structures are always exposed to environmental loads such as waves, and in the case of TLP, reduction in response amplitude of platform causes reduction in stress range in tendons; this would increase the fatigue life of tendons, and therefore, increases the structural safety. Also providing stable conditions for machinery and crew increases the efficiency and functionality of the platform. This article thus aims to investigate the possibility and effectiveness of applying tuned mass damper (TMD) as a passive structural control system to suppress the surge motion of TLP that is exposed to wave load. Both numerical and experimental studies were carried out to assess the performance of the TMD. A close agreement is obtained between the numerical simulations and experimental results. The results of numerical and experimental investigations in this study indicate that applying the TMD, tuned to the surge natural frequency of the platform or frequencies close to the surge natural frequency of the platform, doesn’t have efficiency in reducing the surge responses of TLP in the range of probable waves in seas and oceans. Introduction Tension leg platform (TLP) is one type of the offshore structures used for exploitation of petroleum from deep seas and oceans. This is a type of positively buoyant floating platform, which means that the buoyancy caused by the hull of the TLP in operational draught is more than the total weight of the platform. This excess buoyancy is used to impose initial pretension in straight vertical mooring lines called tendons. TLP is moored to the seabed using the group of tendons at its every corner. One of the features of tendons is that this type of mooring lines have high axial stiffness. Because of that, tendons restrict the vertical motion (heave) and vertical in-plane rotations (pitch and roll) of the platform, but show flexibility in horizontal motions (surge and sway) and horizontal in-plane rotation (yaw), therefore, TLP can be considered laterally compliant because of significant range of displacements in these degrees of freedom.

Implementation and evaluation of control strategies based on an open controller for a 10 MW floating wind turbine

The reliability assessment concerning the drivetrain system is important for integrated dynamic analysis of large-scale floating wind turbines (FWTs). An open, modular, and adaptable baseline wind turbine controller is implemented and evaluated in this paper to work with the DTU 10 MW reference wind turbines supported by a proposed Tension Leg Platform (TLP). Higher natural frequency of the controller can account for the coupling effects between the blade pitch control and the platform motions that contributing to poor performances of the FWT and negative damped pitch motions. Through simulations by FAST code, the baseline controller is evaluated by comparing the conventional pitch-to-feather strategy and the active pitch-to-stall strategy. The controller is detuned with different control frequencies and the active stall control strategy is tailored for the proposed TLPFWT. The results suggest that system instabilities induced by higher control frequency decreases fast as the growth of wind speed and the stall controller can lead to around twice platform motions and structure force as large as baseline controller in a wide range of frequency, whereas the rotor performance is fine. The DRC working with FAST proves applicable and different control algorithms and the integrated dynamic effects with other floating foundations can be achieved.

Nonlinear hydrodynamic characteristics of multi-body platform system

Along with the technology development of ocean resources, offshore platforms are gradually becoming larger and more complex. Recent development of the oil and gas field in the deep-water region often involves multiple floating platforms adjacent to each other to perform more complex functions for oil and gas production. This paper describes the investigation carried out on the dynamic responses of a two platforms system containing a Tension Leg Platform (TLP) and a tender assisted drilling (TAD) with a flexible connection between the two platforms. The mooring lines and tendons are taken into consideration in the coupled analysis of the multi-body platform’s system. The motion responses and wave load characteristics on the two platforms in multi-body coupled model are investigated in the numerical simulation. Compared to the situation of the two platforms in isolation, it is revealed that the motion responses for the two platforms in coupled model are altered by the combined effects of the platforms’ interaction and constrain by the connection. The interaction between platforms can increase both the first- and second-order wave force on platforms in the arrangement direction of two platforms. Quantitative analysis demonstrates their relative importance and thus provides much-needed guidance in practical design of the coupled system.

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%.