Response Of Spar Research Articles

Impacts of Mooring-Lines Hysteresis on Dynamic Response of Spar Floating Wind Turbine

Floating wind turbines often experience larger-amplitude motions caused by wind and ocean wave loads, while mooring-lines, such as catenary and taut mooring-lines, make the structure configurations along with an analysis of the global response more complicated compared to a fixed support foundation. Moreover, the restoring performance of dynamic mooring-lines exhibits a significant hysteresis behavior, and this hysteresis behavior may have profound impacts on the structural response of floating wind turbines under environmental loads. In this study, using the coupled finite element method, a dynamic simulation model is developed to study the motion responses of a spar floating wind turbine under consideration of mooring-lines hysteresis. In order to consider large-amplitude motion and nonlinear behaviors of catenary mooring-lines, a FEM (finite element method) model is developed based on a combination of 3D nonlinear beam elements and the super-element approach, and the interaction between mooring-lines and seabed is also included. Using our FEM numerical simulations, firstly, the restoring performance of mooring-lines and its hysteresis behavior are studied. Then, the motion responses, e.g., the displacements of the spar float undergoing various wave loads, are examined. The numerical results show that: the restoring stiffness of mooring-lines exhibits significant hysteresis behavior, and the restoring force is directionally dependent. Due to the hysteresis of restoring performance, for a case of regular wave conditions, little change of the spar surge in a steady-state is seen; however, for a case of extreme wave loads, the motion response gets about 14.4% smaller, compared with the quasi-static cases.

Hydrodynamic Response of Floating Coupled Spar in Deep Sea

Various advanced floating structures were proposed and developed till date due to their cost effectiveness and productiveness in deep and ultra-deep water exploration. Spar has been seen as a latest and productive alternative amongst the all type of floating offshore platform. However, the precise behavior of fully coupled action of the platform in deeper water is still a great issue to be forecasted in actual sea environment. Dynamic analysis for fully coupled 3D model of floating spar platform structure can be a resourceful tool to predict the responses, where primary assemblage of Spar platform and mooring lines are considered in a joined compact coupled system. To define accurately the interaction between the Spar and mooring lines, coupled dynamic analyses are appropriate to get responses in deep sea. Considering the coupling effect of the platform and its mooring system, hydrodynamic analyses of a coupled Spar has been carried out in time domain. Also, Response Amplitude Operator (RAO) has been studied of every motion and mooring tension with Fast Fourier Transform method (FFT). The numerical simulation and motion investigations are completed with the commercial finite element (FE) package ABAQUS/AQUA. The purpose of this paper is to investigate the response of fully coupled Spar platform under regular ocean environment. Tension to the deformable mooring lines is equally distributed. Wave loading acts simultaneously on the rigid spar hull and deformable mooring line. Rigid body resonant motions influence the dynamic motion responses of Spar. Damping of the mooring line in coupled Spar-mooring system decreases the top tension of its mooring system after certain wave duration. The wave induced effects have been evaluated and discussed to investigate their interaction with the floating spar platform. RAO of the motion responses and mooring tension are consistence to predict motion of the Spar platform. It shows energy concentrate peaks of the floater's response with the frequency.

Semi-active control of vibrations of spar type floating offshore wind turbines

A semi-active algorithm for edgewise vibration control of the spar-type floating offshore wind turbine (SFOWT) blades, nacelle and spar platform is developed in this paper. A tuned mass damper (TMD) is placed in each blade, in the nacelle and on the spar to control the vibrations for these components. A Short Time Fourier Transform algorithm is used for semi-active control of the TMDs. The mathematical formulation of the integrated SFOWT-TMDs system is derived by using Euler-Lagrangian equations. The theoretical model derived is a time-varying system considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar, mooring system and the TMDs, the hydrodynamic effects, the restoring moment and the buoyancy force. The aerodynamic loads on the nacelle and the spar due to their coupling with the blades are also considered. The effectiveness of the semi-active TMDs is investigated in the numerical examples where the mooring cable tension, rotor speed and the blade stiffness are varying over time. Except for excessively large strokes of the nacelle TMD, the semi-active algorithm is considerably more effective than the passive one in all cases and its effectiveness is restricted by the low-frequency nature of the nacelle and the spar responses.

Effect of Viscous Damping on Hydrodynamic Response of Spar with Heave Plate

In the present paper, effect of viscous damping on the heave response of spar is studied using WAMIT software for spar with single and two heave plates. For spar with single heave plate, a plate is attached at the keel of the spar and the diameter of the heave plate is varied. Diameter ratio (i.e., ratio of diameter of heave plate to diameter of spar – Dp/Ds) of 1.1 to 1.4 is considered here. For spar with two heave plates, one plate is fixed at the keel of the spar and other plate is placed at some distance from the keel. Diameter of the two plates is same and diameter ratio of 1.3 is taken for these plates. Spacing between the plates is varied and relative spacing (i.e., ratio of spacing between the two heave plates to diameter of heave plate - L/Dp) of 0.1 to 0.4 is considered. WAMIT software is based on potential theory so it considers only radiation damping. To account the viscous damping in WAMIT analysis, viscous damping values obtained from the literatures are used as an external damping matrix in WAMIT software. Increase in diameter ratio and relative spacing increases the heave added mass and damping of the spar. Hence, heave response of spar reduces with increase in diameter of the heave plate and spacing between the two heave plates. It is found that heave viscous damping reduces the heave response of spar with heave plate significantly. It is suggested that viscous damping should be included in WAMIT software for the reduction of heave response of spar with heave plate. Diameter ratio of 1.2 to 1.3 and relative spacing of 0.2 to 0.3 are recommended for significant heave motion reduction.

Semi-analytical numerical approach for the structural dynamic response analysis of spar floating substructure for offshore wind turbine

A semi-analytical numerical approach for the effective structural dynamic response analysis of spar floating substructure for offshore wind turbine subject to wave-induced excitation is introduced in this paper. The wave-induced rigid body motions at the center of mass are analytically solved using the dynamic equations of rigid ship motion. After that, the flexible structural dynamic responses of spar floating substructure for offshore wind turbine are numerically analyzed by letting the analytically derived rigid body motions be the external dynamic loading. Restricted to one-dimensional sinusoidal wave excitation at sea state 3, pitch and heave motions are considered. Through the numerical experiments, the time responses of heave and pitch motions are solved and the wave-induced dynamic displacement and effective stress of flexible floating substructure are investigated. The hydrodynamic interaction between wave and structure is modeled by means of added mass and wave damping, and its modeling accuracy is verified from the comparison of natural frequencies obtained by experiment with a 1/100 scale model.

Hydrodynamic Responses of Spar Hull with Single and Double Heave Plates in Random Waves

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing <TEX>$L_d/D_d$</TEX> (<TEX>$D_d$</TEX>-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.

STRUCTURAL BEHAVIOUR OF FULLY COUPLED SPAR–MOORING SYSTEM UNDER EXTREME WAVE LOADING

Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme wave loading at severe deep sea. Morison's equation has been used to calculate the wave forces. Spar responses and mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state, it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability of a spar platform for deep water harsh and uncertain environmental conditions.

Experimental and Numerical Investigation on Hydrodynamic Response of Spar with Wind Turbine under Regular Waves

The necessity of offshore wind turbines to generate wind power is multifold and well known. The focus of offshore based wind farms are mainly due to abundant availability of unobstructed wind power and land use restriction. Therefore, the development of floating type supports for offshore wind turbines is getting more attention. In this paper the experimental and numerical investigation on the motion response of Spar supporting the wind turbine under regular waves is presented. The experiments were conducted in three stages viz. (a) Spar with stationary wind turbine under regular waves. b) Spar with rotating wind turbine under calm sea conditions. c) Spar with rotating turbine under regular waves. Numerical simulation of the Spar response due to incident waves and the wind turbine torque is carried out using AQWA software. The results obtained from experiments are then compared with that obtained from numerical analysis. The measured Response Amplitude Operator (RAO) compares well with that obtained from numerical analysis. The comparison of measured response and simulated response for wind turbine rotation case shows reasonable match. The combined effect of wind turbine rotation and incident wave on the hull response is 10%, 16% and 32% higher than that of the response due to incident wave alone for heave, surge and pitch responses respectively.

Experimental and Numerical Investigation of Hydrodynamic Response of Spar with Wind Turbine under Random Waves

In this paper, experimental and numerical investigation on the motion response of Spar with wind turbine subjected to random waves are presented and discussed. The studies were carried out for three different cases: (a) Spar with wind turbine subjected to waves when the turbine blades are in stationary condition; (b) Spar with wind turbine in operating condition under calm sea state; (c) Spar with wind turbine subjected to waves when the turbine blades are in operating condition. The numerical investigation was carried out using hydrodynamic software Ansys AQWA. The influence of the turbine blade rotation on the motion response of the Spar was investigated and the responses of the system under random waves were presented in terms of statistical values. The experimental results and the results obtained from numerical investigation matches reasonably well. The motion response of the system has increased by about 30%, 20% and 25% for surge, heave and pitch respectively, for turbine blades in rotating condition when compared with Spar with wind turbine in stationary condition.

Hydrodynamic Response of Spar with Single and Double Heave Plates in Regular Waves

Hydrodynamic Response of Spar with Single and Double Heave Plates in Regular Waves

Impact of spar-nacelle-blade coupling on the edgewise response of floating offshore wind turbines

The impact of spar-nacelle-blade coupling on edgewise dynamic responses of spar-type floating wind turbines (S-FOWT) is investigated in this paper. Currently, this coupling is not considered explicitly by researchers. First of all, a coupled model of edgewise vibration of the S-FOWT considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar and mooring system, the hydrodynamic effects, the restoring moment and the buoyancy force is proposed. The aerodynamic loads are combined of a steady wind (including the wind shear) and turbulence. Each blade is modeled as a cantilever beam vibrating in its fundamental mode. The mooring cables are modeled using an extended quasi-static method. The hydrodynamic effects calculated by using Morison's equation and strip theory consist of added mass, fluid inertia and viscous drag forces. The random sea state is simulated by superimposing a number of linear regular waves. The model shows that the vibration of the blades, nacelle, tower, and spar are coupled in all degrees of freedom and in all inertial, dissipative and elastic components. An uncoupled model of the S-FOWT is then formulated in which the blades and the nacelle are not coupled with the spar vibration. A 5MW S-FOWT is analyzed by using the two proposed models. In the no-wave sea, the coupling is found to contribute to spar responses only. When the wave loading is considered, the coupling is significant for the responses of both the nacelle and the spar.

NON-LINEAR DYNAMIC ANALYSIS OF COUPLED SPAR PLATFORM

Spar platforms are treated as cost-effective and resourceful type of offshore structure in deep water. With increasing depth there are significant changes in its structural behaviour due to coupling of spar hull-mooring line along with radical influence of mooring line damping. So these phenomena should be precisely counted for accurate motion analysis of spar mooring system. In present study, spar platform are configured as a single fully coupled integrated model in ABAQUS/AQUA. Non-linear dynamic analysis in time domain is performed adopting Newmark-β automatic time incrementation technique. Non-linearities due to geometric, loading and boundary conditions are duly considered. Displacement and rotational responses of spar and mooring tensions are obtained during long-duration storm. spar responses get significantly modified and mean position of oscillations gets shifted after longer wave loading. The surge, heave and pitch responses are predominantly excited respectively. The energy contents of PSDs of these responses reduce considerably after long wave loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines. The pitch response is fairly sensitive to the wave loading duration. After long duration of storm the wave frequency response increases. However, low frequency and wave frequency responses may simultaneously occur due to synchronising sea states.

Hydrodynamic response of spar and semi-submersible interlinked by a rigid yoke – Part I: regular waves

Experimental and numerical investigations on the hydrodynamic response of a spar and semi-submersible linked system under regular waves (Part I) and random waves (Part II) are presented. These studies were performed for two cases: the first case with the spar alone and the second case with a semi-submersible linked to the spar. The influence of the semi-submersible on the motion response of the spar and its mooring system are evaluated and presented together with acceptance criteria used in the industry. Experimental results are compared with numerical results and found to be acceptable.

Effect of hull geometry on the hydrodynamic response of spar in regular waves

The hydrodynamic response of the floating production systems, especially deep draft spars, is of major concern in the offshore industry for its influence on the selection of suitable drilling and production equipment. A number of response reduction methods are being investigated in view of better motion response and improved design methods. In the present study, experimental and numerical investigations were carried out on spar hulls with different shapes to obtain the motion response characteristics and to compare them with that of the classic spar. The spar hulls were designed for a common topside weight of 10,000 tonnes with a displacement of 65,000 tonnes. Experiments were carried out on 1:100 scaled models of spar with circular disk, enlarged base, eight-cell spars and six-cell spars for a water depth of 300 m and numerically validated using ANSYS AQWA software package. Experimental and numerical values showed good agreement especially at lower wave periods. Among the cell spars, the six-cell spar showed better motion response characteristics than the eight-cell spar, both in heave and surge, with limited difference in pitch. The spar with enlarged base was found to be more stable in pitch and less stable in heave than the spar with circular disc at keel.

Hydrodynamic response of spar and semi-submersible interlinked by a rigid yoke–Part II: random waves

Experimental investigations on the hydrodynamic response of a spar and semi-submersible linked system under random waves are presented in this paper. Responses under regular waves were presented in Part I. These studies were performed for two cases: in the first case with the spar alone and the second case with a semi-submersible linked to the spar. The influence of the semi-submersible on the motion response of the spar and its mooring system are evaluated and presented together with acceptance criteria used in the industry. The responses are lower than allowable acceptance criteria limits, and the system is found to be stable in all mooring configurations both in survival and operating conditions.

Effect of Damaged Mooring Line on Response of Spar with Wind Turbine

Spar platforms have several advantages for deploying wind turbines in offshore for depth beyond 120 m. The merit of spar platform is large range of topside payloads, favourable motions compared to other floating structures and minimum hull/deck interface. This paper addresses the effect of mooring line damages in responses of spar platform subjected to regular waves. A 1:100 scale model of the spar with taut (intact), taut (damaged) and slack (intact) mooring line configuration was studied in the wave basin (30 × 30 × 3 m) in Ocean Engineering Department of IIT Madras. The heave and surge accelerations along with mooring line tension was measured and used. The surge and heave RAO comparison for all three mooring line conditions shows that the effect of damaged mooring line in surge response is negligible whereas in heave response, taut (damaged) will behave similar to slack (intact) condition. The normalized mooring line tension comparison between taut intact and taut damaged configuration is also presented.

Numerical Study on Hydrodynamic Behavior of Deepwater Spar Platform with Different Mooring Configurations

The motion performance of Spar platform and dynamic characteristics for the mooring lines under different mooring configurations have been studied both in static analysis and coupled dynamic analysis. First, 3D hydrodynamic finite element model is built and the effects of the mooring system are taken into account by giving the specified pre-tension, angle and stiffness of the mooring lines on the fairleads. And hydrodynamic analysis of Spar platform is performed by the way of utilizing potential flow theory in frequency domain in order to calculate the hydrodynamic coefficients. Then, static analysis is applied to obtain restoring stiffness curves for the mooring system, structure displacements and mooring line tensions etc.. At last, coupled time domain analysis of the motion response of Spar is conducted for the coupled system and the dynamic tensions of mooring lines are calculated. The research results can be served as a reference for the selection and the performance study for mooring systems during preliminary design.

파랑 중 계류된 스파 플랫폼의 시간영역 해석

The Spar platform with deep draft is characterized as effective structure in extreme wave condition, which has larger natural period than that of waves in sea. In this paper, the time domain simulation of motion responses of Spar with catenary mooring line is presented in irregular waves. The memory effect is modeled by added mass at infinite frequency and convolution integrals in terms of wave damping coefficients. The added mass, wave damping coefficients and wave exciting forces are obtained from three-dimensional panel method in the frequency domain. The motion equations are consisted of forces for inertia, memory effect, hydrostatic restoring, wave exciting and mooring line. The forces of mooring line are modeled as quasi-static catenary cable.

Three-dimensional analysis of guyed logging spars

Trees are commonly guyed and used as spars to increase the capacity of cable logging systems in mountainous terrain. Spar response to loading is complex because of nonlinearities due to cable behavior, beam-column loading of the spar, and the geometric effects of large spar deflections. A numerical procedure has been developed to determine spar deformation and the resulting state of stress in the spar and the guylines due to the application of static loads. The Newton-Raphson algorithm is utilized to determine spar position which is denned by force equilibrium. A numerical damper is required to assure convergence. The problem is formulated using the finite element procedures where the spar is treated as an assemblage of three-dimensional beam elements, the guylines are modeled as nonlinear springs, and spar base fixity is provided by a linear rotational spring. The details of the solution method and an example problem are presented.