Tension Leg Research Articles

Analytical investigation of nonlinear heave-coupled response of tension leg platform

Having deep view in structural response of tension leg platform is important issue not only for response analysis but also for engineering design. Coupling between surge and heave motions of tension leg platform is such a problem. Here, tension leg platform motions are considered only in surge and heave degrees of freedom without pitch effect. The coupled term of heave is a nonlinear differential equation. Because the focus of this article is on this term, therefore, Duffing equation of motion in the surge direction is linearized. The wave forces are calculated using Airy’s wave theory and Morison’s equation, ignoring the diffraction effects. Current force also can be very important in dynamic analysis of tension leg platform. Because it affects the term of heave that is coupled with surge. It is shown that the effect of surge motion coupling on heave motion is very important in large displacement of surge motion in many sea states. The main result is that the coupling effects appeared in some frequencies such as heave and surge frequency, twice the frequency of wave, twice the natural surge frequency, and summation and difference of frequency of wave and surge frequency.

Suppression of vortex-induced vibrations using flexible shrouding—An experimental study

A number of offshore compliant structures such as risers, mono-columns, tension leg platforms (TLPs), spars, etc., which are deployed in the offshore field are subjected to vortex-induced vibration (VIV). The harsh environmental conditions often lead to failure of these structures well before their design life period resulting in fatal accidents. To minimize VIV, several investigations were conducted by other researchers. The present study focuses on the experimental investigations of an elastically mounted circular cylinder shrouded with net substructure called as Ventilated Net (VN) to suppress VIV. The VN is an omnidirectional, economical, customizablenet substructure comprising of flexible hollow tubes in a systematic arrangement. This device could be retrofitted to the offshore structures/risers, to attenuate VIV, thereby preventing the structure from fatigue failure. Experimental investigations are conducted to address the reduction of VIV and drag forces acting on the oscillating cylinder shrouded with VN of various configurations. The experiments are performed in towing tank facility in Department of Ocean Engineering, IIT Madras, India. The primary objective of the present investigation is to quantify the VIV amplitudes by employing VN shrouding the cylinder, at high Re regime, ranging from (0.22–2.50) × 105. The effect of spaces between the flexible hollow tubes and shrouding radii of VN around the cylinder are also addressed in this article. From the measurements, it is observed that cylinder with VN of dense mesh at a radial spacing of twice the diameter of the bare cylinder, suppresses VIV by 98% and drag force by 40% at Re of 1.2 × 105.

Multibody Dynamics of a Floating Wind Turbine Considering the Flexibility Between Nacelle and Tower

Stability and dynamic modeling of the floating wind turbine (FWT) is a crucial challenge in designing of the type of structures. In this paper, the tension leg platform (TLP) type FWT is modeled as a multibody system considering the flexibility between the nacelle and tower. The flexibility of the FWT is modeled as a torsional spring and damper. It has 6 degrees of freedom (DOFs) related to the large-amplitude translation and rotation of the tower and 4 DOFs related to the relative rotation between the rotor-nacelle assembly and the tower. First, the nonlinear equations of motion are derived by the theory of momentum cloud based on the conservation of momentum. Then, the equations of motion are solved and the system is simulated in MATLAB. Moreover, the effect of flexibility between the nacelle and tower is investigated via the dynamic response. The stability of the system in three different environmental conditions is studied. Finally, the spring and damping coefficients for the system response to get near to instability are determined, by which the critical region is defined. The simulation results demonstrate the importance of the flexibility between the nacelle and tower on the overall behavior of the system and its stability.

Risk Assessment and Reduction for an Innovative Subsurface Well Completion System

In recent years, many oil and gas fields have been discovered in ultra-deep sea (UDS). Some of these fields are evaluated to have no commercial value if existing oil field development approaches are used, especially while the oil prices remain low. A new alternative field development solution, termed as Subsurface Well Completion (SWC) system, is proposed with the aim to produce oil and gas in a cost-effective manner in UDS. This system primarily consists of four parts: a tethered subsurface platform, the rigid riser, SWC equipment and flexible jumper. Obviously, central to the evaluation and application of the new SWC technology is the inherent risk relative to acceptance level. In particular, an uncontrolled release of hydrocarbons to sea, which may lead to catastrophical consequences involving personnel risk, environmental damage and economic losses, is a main contributor to the total risk and of great concern to the offshore petroleum industry. As for the new SWC system, any failure will not be a direct source of risk for the personnel on the surface installation due to its offset feature. In this context, this paper proposes a quantitative risk assessment (QRA) framework to assess such uncontrolled releases to sea with regard to the SWC system for an oil field in the production phase based on the new Subsurface Tension Leg Production (STLP) facility. According to the QRA results presented in this paper, the identified scenarios representing uncontrolled releases to sea are subsea wellhead leaks, rigid riser leaks, subsurface wellhead leaks, releases from X-mas tree and flexible jumper leaks. Among these scenarios, subsea wellhead is found to be the high-risk area. Compared with the established risk acceptance criteria (RAC), the environmental risk levels for the subsea wellhead’s leak lie within the As Low As Reasonably Practicable (ALARP) region while other risks are all below ALARP limits, which means that there is a need for improved consideration of the existing design with regard to the subsea wellhead area, and the corresponding risk reduction measures are proposed. Furthermore, the sources and effects of uncertainties are reviewed and sensitivity studies are carried out to illustrate the effect of some of the important assumptions in the risk model. It can be found that some assumptions made are conservative or optimistic while others are unknown. However, the final QRA results can be regarded as somewhat conservative. This paper concludes that the new SWC technology has a distinct advantage with respect to the leakage duration time in UDS, and thus mitigates the environmental and commercial impacts to a large extent. Besides, relaxed design requirements for the X-mas tree and flexible jumper can be accepted. It is also concluded that there are no serious and major commercial losses for all the identified accidental release scenarios, which is of great importance and attractiveness to oil producers.

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.

A numerical study of dynamic response of crane semi-submersible along TLP in tender-assisted drilling operation

ABSTRACTDynamic response for tender-assisted drilling is crucial due to the safety concern and impact on operability. Notably, the hydrodynamic interaction between two free floating units is believed as the main factor that threatens the operation. However, this urgent issue has not been thoroughly studied yet. We aim at establishing an accurate and efficient numerical model to compute the coupled dynamic response between crane SEMISUB and alongside tension leg platform (TLP). In terms of crane and lifting, an ideal pendulum model is employed; the lifting cargo is assumed as an independent mass-body in the air. The important physical variables e.g. the load motion at boom tip location, swing angle and tension are computed. The impact of cargo weight and sling length are investigated parametrically through sensitivity study. The time-domain analysis is performed in order to study the response under irregular waves, and the standard deviation as well as the most probable maximum value by Weibull distribution fitting are obtained.

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.

Health monitoring of tension leg platform using wireless sensor networking: experimental investigations

Process of deploying wireless sensor network in offshore structures for structural health monitoring is presented in this paper. Offshore structures are subjected to environmental loads with high uncertainties that are capable of causing damage during their service life. Preventive measures to safeguard the offshore platforms from such damages are not only innovative but also inevitable. Structural damages can be readily identified by monitoring their response behavior deploying dense array of sensors. Deploying wireless structural health monitoring (SHM) system ensures reduced installation time, compact design and cost reduction in comparison to that of the traditional wired systems. Wireless sensor network architecture for monitoring offshore platforms, proposed in the present study will acquire data using network of sensor nodes and transmits to the base station for post-processing. Present study compares the structural responses of offshore tension leg platform under different postulated failure cases through experimental investigations on scaled model. Acquired data are also hosted in the webpage after analyzing and post-processing. The alert monitoring system, which is an integral part of SHM triggers alert messages is an indigenous design.

A Parametric Study on the Innovative Coupling Arm Connecting the Coupled TAD-TLP

Two body marine systems, like a Tender Assisted Drilling (TAD) vessel coupled with a floating Dry-Tree Unit (DTU), have become very common in offshore operations. One of the unavoidable challenge we have to cope with is the connection between the TAD and DTU should make sure the TAD does not drift away from the platform and also avoid the possible collision in case of a harsher environment. The objective of this study is to understand the hydrodynamic interactions between the two coupled floating bodies and improve the devising of the innovative connection system. In this study, an innovative rigid connection system, the coupling arm is applied to connect the TAD and a DTU, in this case, a Tension Leg Platform (TLP). The whole system is modelled by the commercial software HARP. A comprehensive parametric study on the pretension and the nominal length of the coupling arm is carried out. The hydrodynamic analysis of the coupled TAD-TLP system elucidates the interactions between the two bodies. The chosen combination of the coupling arm pretension and the nominal length will determine the required stroke range and maximum forces needed to design the innovative coupling arm for safe operations.

Study on a Floating Point Absorber Type Wave Energy Converter with One Line Tension Leg Mooring

Motions and load characteristics of a floating point absorber type wave energy converter (WEC) were investigated by tank test considering operation modes. Four typical operation modes of a movable float, namely fixed mode, free mode, controlled mode without power generation, and controlled mode with power generation, were examined by the WEC model with an AC servo motor and a rack and pinion mechanism as a power take-off system in the tank test. The test results show that the operation modes have little effects on surge and pitch motions of the WEC. The measured load, such as mooring tension and axial force on the WEC, on the contrary, were strongly affected by the operation modes. The measured bending moment of the WEC, however, was affected by the WEC surge and pitch motions rather than the operation modes. Additionally, a numerical simulation using a commercial code, the Orcaflex, was conducted, and the simulation results were compared with measured ones. The WEC motions and mooring tension of the simulation results were comparable to those of measured ones in regular waves. The simulated power spectrum density of surge, pitch and mooring tension qualitatively matched measured ones, but they slightly underestimated measured ones at peak wave frequency in irregular waves.

Multi-objective optimization of Tension Leg Platform using evolutionary algorithm based on surrogate model

An Innovative Tension Leg Platform (TLP) Optimization Program, called ITOP, has been developed to solve the multi-objective optimization problem for TLP. We first examine the hydrodynamic behavior of a base TLP for wave headings between 0∘ and 45∘. The numerical results show that the maximum heave and surge motion responses occur in 0∘ wave heading in long-crest waves. It is found that the dynamic tension of No. 8 tendon is larger than the other tendons and reaches its maximum in 45∘ wave heading. It can be attributed to the fact that heave and pitch motions are almost out of phase for wave periods between 10 and 15 s. Because the maximum wave elevation occurs near the northeast column and the vertical motion is very small, the minimum airgap occurs there. Moreover, a surrogate model based on radial basis function (RBF) has been built and adopted to estimate the hydrodynamic performance of TLP. A multi-objective evolutionary algorithm, Non-dominated Sorting Genetic Algorithm II (NSGAII), is employed to find the Pareto-optimal solutions. By comprehensive and systematic computations and analyses, it is revealed that the maximum dynamic tension shows positive correlation with pontoon height and width, but negative correlation with hull draft, column spacing, and column diameter. The most efficient modification strategy for design is proposed to reduce the maximum dynamic tendon tension. According to the strategy, the column spacing, draft, and column diameter should be increased in sequence. By applying this strategy, the maximum dynamic tendon tensions can be reduced while the total weight of the platform is minimized as much as possible.

A research note on a study of motion characteristics of tension leg spar

The present research note, reports a study on the dynamic motion analysis of a tension leg spar (TLS). In the research note the time domain analysis is done using USFOS *TM software for a tension leg spar supporting a ‘National Renewable Research Laboratory (NREL)’ 5MW wind turbine at an operating depth of 320m to determine the dynamic responses of tension leg spar subjected to sea state 11 ( =15m and =16sec). Motion characteristics such as surge, pitch and heave as well as the combined effect of the above are discussed at length. Non-linear effect is introduced in the spar due to the tension leg. The nonlinear effects include combined-frequency effect such as double natural pitch and surge frequencies as well as sum of frequencies. The reported study is only limited to wave induced motions and no aerodynamic forces are taken into account. Hydrodynamic loads are based on a combination of the Morison formula and the pressure integration method. The response for the base case is compared with available result from the literature. Furthermore, to achieve a proper understanding of the response of the floating concept, parametric studies are carried out by varying parameters such as draft, pretension and position of ‘center of gravity (COG)’. The results highlight the variation in platform responses caused by changing these parameters. The analyses are carried out for combination of parameters in order to find a configuration having the least response for the above sea state. Keywords: Tension leg spar, Coupled motion characteristics, Time domain analysis, Dynamic response, Parametric study. Cite this Article Yadav K, Vamshikrishna Domala, Sharma R. A Research Note on a Study of Motion Characteristics of Tension Leg Spar. Journal of Offshore Structure and Technology . 2017; 4(3): 1–20p.

Vortex-induced vibration of a tension leg platform tendon: Multi-mode limit cycle oscillations

This paper studies the application of mathematical models to analyze the vortex-induced vibrations of the tendons of a given TLP along the Indian coastline, by using an analytical approach, analyzed using MATLAB. The tendon is subjected to a steady current load, which causes vortex-shedding downstream, leading to cross-flow vibrations. The magnitude of the excitation (lift and drag coefficients) depends on the vortex-shedding frequency. The resulting vibration is studied for possible resonant behavior. The excitation force is quantified empirically, the added mass by potential flow hydrodynamics, and the vibration by normal mode summation method. Non-linear viscous damping of the water is considered. The non-linear oscillations are studied by the phase-plane method, investigating the limit-cycle oscillations. The stable/unstable regions of the dynamic behavior are demarcated. The modal contribution to the total deflection is studied to establish the possibility of resonance of one of the wet modes with the vortex-shedding frequency.

Effects of second-order wave forces and aerodynamic forces on dynamic responses of a TLP-type floating offshore wind turbine considering the set-down motion

A coupled 6 degree of freedom (DOF) aero-hydro-restoring model is developed to study the dynamics of a tension leg platform (TLP)-type offshore wind turbine. This model includes second-order wave loads, pitch control strategies, and effects of platform motion on aerodynamic performance. The second-order wave force is calculated using the full-field quadratic transfer function. The coupled effects of the horizontal motions (such as surge, sway, and yaw motions) and the set-down motion are taken into consideration through the nonlinear restoring matrix. Different load scenarios are chosen to simulate the platform's dynamic response and the rotor's aerodynamic performance in the time domain. The analysis shows that second-order wave forces will induce slow-drift and springing resonances. Due to the large motion in surge, a significant set-down motion which is an important part of heave motion is induced by the second-order difference-frequency force. When different wind types are chosen, the aerodynamic damping effect is found in the slow-drift motion in surge and the springing motion in pitch. On the other hand, low-frequency set-down motion is excited and the heave response is amplified by the wind. Compared to the steady wind, the effect of aerodynamic damping is reduced under the turbulent wind. Slow-drift frequency, wave frequency, and springing frequency are all observed in the aerodynamic results.

Parametric study on the behavior of an innovative subsurface tension leg platform in ultra-deep water

This study focuses on a new technology of Subsurface Tension Leg Platform (STLP), which utilizes the shallowwater rated well completion equipment and technology for the development of large oil and gas fields in ultra-deep water (UDW). Thus, the STLP concept offers attractive advantages over conventional field development concepts. STLP is basically a pre-installed Subsurface Sea-star Platform (SSP), which supports rigid risers and shallow-water rated well completion equipment. The paper details the results of the parametric study on the behavior of STLP at a water depth of 3000 m. At first, a general description of the STLP configuration and working principle is introduced. Then, the numerical models for the global analysis of the STLP in waves and current are presented. After that, extensive parametric studies are carried out with regarding to SSP/tethers system analysis, global dynamic analysis and riser interference analysis. Critical points are addressed on the mooring pattern and riser arrangement under the influence of ocean current, to ensure that the requirements on SSP stability and riser interference are well satisfied. Finally, conclusions and discussions are made. The results indicate that STLP is a competitive well and riser solution in up to 3000 m water depth for offshore petroleum production.

The Impact of Extreme Wave Events on a Fixed Multicolumn Offshore Platform

This paper presents an experimental and numerical investigation into the magnitude and distribution of the hydrodynamic loads affecting a fixed multicolumn offshore platform (rigidly mounted tension leg platform) when subjected to extreme wave events. All wave load components, including wave-in-deck slamming pressures, were predicted using a commercial computational fluid dynamics (CFD) code STAR-CCM+ and compared against experimental measurements. Slamming pressures were calculated using both data obtained locally at discrete points and globally averaged over the whole exposed area of the deck. In all simulated cases, the deck area exposed to a wave-slamming event was found to be in contact with a water–air mixture with a significant proportion of air phase. It was concluded that the slamming pressure data for the exposed area provided better insights into the pressure changes due to air compressibility and its content.

16.01: Bjørnafjorden suspension bridge TLP concept

ABSTRACTThe Bjørnafjorden TLP (Tension Leg Platform), a multi span suspension bridge solution, is one of the possible fjord crossings options considered as part of the ambitious “Ferry‐free Coastal Highway E39” under development in Norway. The original design concept for Bjørnefjorden was very much similar to that of a conventional suspension bridge using central locks between main cable and bridge deck to distribute movements equally to the bridge ends. However since the bridge is of the multi‐span type, the inherent flexibility of the pylons caused a severe of in‐plane stability of the original system leading to large vertical displacement of the deck. To mitigate this effect top cables have been introduced between pylon tops preventing these from excessive longitudinal displacements and thereby also limiting the vertical deck displacement from traffic load. In this paper the different concept challenges for the Bjørnafjorden TLP solution is outlined.The suspension bridge has a total of four pylons – a northern and southern pylon of concrete located near shore, while two central steel pylons are supported by TLP's at 550m and 450m depth. The TLP's, as well known from off‐shore industry, consist of pre‐tensioned steel pipes anchored in the seabed and at the surface connected to a large steel “floaters” onto which the towers are placed. The main spans are 1300–1400m and the bridge has an overall length from anchorage to anchorage of about 5200m.Due to the long tethers of the TLP the central pylons are relatively unrestrained to move up to 20m horizontally and thus in order to achieve sufficient stability of the bridge system a special feature in form of top cables are introduced. The top cables connect the tower top of all four pylons and are anchored to the bridge anchor blocks together with the main cable.The bridge structural system is the outcome of innovative thinking and a fine example on how a few unconventional features can make a very conventional bridge system work in challenging conditions. As the bridge is floating it is essential that the structure is light and therefore the primary structures are in steel.

Experimental and numerical investigations on the hydrodynamic performance of a floating–moored oscillating water column wave energy converter

This paper presents an experimental and numerical hydrodynamic performance assessment of a 1:50 scale model offshore floating–moored Oscillating Water Column (OWC) wave energy converter. The device is a tension–leg structure with four vertical mooring lines. The performance of the OWC device was investigated for several design parameters including regular and irregular wave conditions of different heights and periods, power take–off (PTO) damping and mooring line pre–tension. A 3D Computational Fluid Dynamics (CFD) model using RANS–VOF approach was constructed and validated against experimental results for regular waves showing good agreement. It was found that the hydrodynamic efficiency of the floating–moored OWC device follows the same general trend as a fully–constrained (fixed) model, but the addition of surge motion in the floating device improved the energy production efficiency over a broader bandwidth around the chamber resonance. Increasing the incoming wave height resulted in a higher efficiency for low–frequency waves, but noticeable reductions in the efficiency were observed in the intermediate– and high– frequency zones. The effectiveness of utilizing offshore OWC devices in deep–water was demonstrated by increasing the extracted energy by a maximum of 7.7 times and 5.7 times when regular and irregular wave heights were doubled, respectively. Decreasing the mooring line pre–tension slightly increases the energy extraction efficiency in the intermediate–frequency zone.

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.

Numerical Simulation of the Influence of Platform Pitch Motion on Power Generation Steadiness in Floating Offshore Wind Turbines

Offshore wind energy is a valuable renewable resource that is inexhaustible, strong, and consistent. To reduce cost and improve energy production efficiency, future trends are moving towards wind turbines in deep water, which use floating platforms such as tension leg platforms, barges, and semi-submersible designs. Compared to fixed based substructures, these floating platforms are in a state of constant motion which affects the power generation steadiness. The resulting complex dynamic behavior might compromise their efficiency and reduce their nominal life. The complex analysis of floating wind turbines requires computer tools that couple all the different components to represent the complete dynamic response. One such tool, developed by the National Renewable Energy Laboratory, is the aero-hydro-servo-elastic tool FAST. In this work, simplified models are used for three platform types, and the results are compared with FAST as a way of understanding the essential dynamics. Secondly, using FAST, the influence of platform pitch motion on the steadiness of power generation is examined. This analysis is done for all three platforms for a constant above rated wind speed and above average wave load. Results demonstrate that the power output fluctuation depends on the platform type and blade pitch motion. The effect of platform pitch on the steadiness of power output is only apparent under large oscillating pitch motion, where recurring power drops are observed. The semi-submersible design performs well with relatively steady power output, while the barge design has the most unsteady output, as a result of its susceptibility to typical wave loads.