Truss Spar Platform Research Articles

Sensitivity Study on Influencing Factors of Coupled Motion Characteristics for Truss Spar

In the absence of mooring condition, the heave and pitch coupled equations are established considering the effect of static stability and wave surface elevation for a truss spar platform. The influence of the change of the pitch period and the external excitation force (combined wave loads and wind loads together) on the stability of the heave-pitch coupling motion is studied by means of numerical iteration method. The results show that when the wave frequency is close to the natural heave frequency besides the heave-pitch frequency ratio is close to 2: 1, both of the amplitude of heave and pitch motion are increased considering the wind loads. It is about 12 percent increase for heave motion, while 8 percent for pitch motion. Meanwhile, there is a penetration phenomenon between heave and pitch. And the wind load play a more important role in pitch motion, the characteristic frequency of pitch motion will have an obvious change between 1/2 times sub harmonic motion frequency component and 1 time forced motion frequency component considering the wind load.

Assessment of ultimate strength of a typical truss Spar using nonlinear finite element analysis

This paper investigates the ultimate strength of a typical truss Spar platform subjected to characteristic hydrodynamic loads using nonlinear finite element analysis with ANSYS. Material nonlinearities are taken into account without giving consideration to geometrical nonlinearities and geometrical imperfections. Main concerns of ultimate global strengths analysis are initial failure place and final collapse states of the whole platform. It can be found that final collapse modes of Spar platform are closely related to corresponding characteristic hydrodynamic responses. Simulation results are discussed and checked by DNV rules. According to failure mechanism of the object platform, ultimate limit state formulations are developed. Performing progressive collapse analysis of truss spar can provide a better understanding of structure behaviour of typical truss Spar under extreme wave conditions. This paper is hopeful to be a good reference to the ultimate limit state of typical truss Spar platforms under characteristic wave load.

Mooring System Optimisation and Effect of Different Line Design Variables on Motions of Truss Spar Platforms in Intact and Damaged Conditions

This paper presents the effect of mooring diameters, fairlead slopes and pretensions on the dynamic responses of a truss spar platform in intact and damaged line conditions. The platform is modelled as a rigid body with three degrees-of-freedom and its motions are analysed in time-domain using the implicit Newmark Beta technique. The mooring restoring force-excursion relationship is evaluated using quasi-static approach. MATLAB codes DATSpar and QSAML, are developed to compute the dynamic responses of truss spar platform and to determine the mooring system stiffness. To eliminate the conventional trial and error approach in the mooring system design, a numerical tool is also developed and described in this paper for optimising the mooring configuration. It has a graphical user interface and includes regrouping particle swarm optimisation technique combined with DATSpar and QSAML. A case study of truss spar platform with ten mooring lines is analysed using this numerical tool. The results show that optimum mooring system design benefits the oil and gas industry to economise the project cost in terms of material, weight, structural load onto the platform as well as manpower requirements. This tool is useful especially for the preliminary design of truss spar platforms and its mooring system.

Motion response analysis of large-scale structures with small-scale cylinders under wave action

The influence of small-scale cylinders is always ignored in classical models for hydrodynamic calculations of truss spar platforms and other large-scale structures in published research. This study is focused on this influence on the motion response under the wave action so that the response can be forecast more accurately. A numerical model is proposed to calculate the wave-body interactions under the second-order frame by utilizing the time-domain higher-order boundary element method. The wave loads on small-scale cylinders are calculated by the Morison formula, in which the wave field disturbed by a large-scale structure is taken into account. Three-dimensional elastic rod theory is chosen to analyze the cable force to a platform, and the motion equation for the mooring system and hull are solved simultaneously using the Newmark-β method. A moored truss spar platform is modelled, and the numerical results demonstrate that the existence of small-scale cylinders may lead to a decrease of the motion response of the platform, while the natural frequency will move to a lower frequency. This phenomenon explains why small-scale cylinders, which can be modelled as truss spar play an important role in damping and change the added mass of the platform. Therefore, in order to get more accurate results for motion response, the wave loads of small-scale components cannot be ignored.

Dynamic coupling analysis for the design of hybrid mooring lines of truss spar in deep sea

Truss spar platform is one of the most optimal solutions to the oil and gas exploration. Owing to the severe environment, the design of mooring lines is vital for the platform. As traditional steel chain is very heavy, synthetic rope is attractive for its lightness. Considering the dynamically coupled interaction between the hull of truss spar and mooring lines, the dynamic coupling analysis is performed to get the motion and line tension responses. The dynamic stiffness of synthetic rope is modelled as upper-lower critical stiffness model. The upper critical stiffness is used to calculate the motions, while the lower critical stiffness is for the line tension. Results show that the hybrid mooring lines could secure the platform sufficiently. Higher pre-tension could improve the stiffness of the system, make full use of the mooring lines and limit the motions. From the responses, we can conclude that the energy dissipation is located in the low frequency and middle frequency. The above observations made based on a truss spar may have important implications for other floating structures moored by a synthetic rope mooring lines.

Nonlinear coupled dynamics analysis of a truss spar platform

Accurate prediction of the offshore structure motion response and associate mooring line tension is important in both technical applications and scientific research. In our study, a truss spar platform, operated in Gulf of Mexico, is numerically simulated and analyzed by an in-house numerical code ‘COUPLE’. Both the platform motion responses and associated mooring line tension are calculated and investigated through a time domain nonlinear coupled dynamic analysis. Satisfactory agreement between the simulation and corresponding field measurements is in general reached, indicating that the numerical code can be used to conduct the time-domain analysis of a truss spar interacting with its mooring and riser system. Based on the comparison between linear and nonlinear results, the relative importance of nonlinearity in predicting the platform motion response and mooring line tensions is assessed and presented. Through the coupled and quasi-static analysis, the importance of the dynamic coupling effect between the platform hull and the mooring/riser system in predicting the mooring line tension and platform motions is quantified. These results may provide essential information pertaining to facilitate the numerical simulation and design of the large scale offshore structures.

Evaluation of the dynamic responses of truss spar platforms for various mooring configurations with damaged lines

Safe mooring for any platform is of utmost importance and concern because of the number of accidents as well as lack of knowledge due to uncertainties in the mooring lines behavior. This has led researchers to investigate the possible effect on the responses of a floating platform due to mooring line damages. Hence, this paper addresses the responses of a truss spar platform for various symmetric and asymmetric mooring configurations when one and two lines attached are damaged. To investigate this effect, the platform has been modeled as a three degree-of-freedom rigid structure and its motions are analyzed in time-domain. The force–excursion relationship of mooring lines is determined using quasi-static approach. It is observed that the effect of damaged mooring lines on symmetric and asymmetric configurations is nearly same. The migration distance of platform is considerably significant due to the damaged lines. It is more than thrice when two lines are damaged compared to one damaged line condition for symmetric and asymmetric configurations. However, the dynamic motions are not significantly affected.

Effects of Water Inside Semiclosed Moon Pool on the Hydrodynamic Coefficients and Heaving Damping of a Truss Spar Platform

The hydrodynamic coefficients and heaving damping of a truss spar platform were studied by the computational fluid dynamics (CFD) method with considering the semiclosed moon pool. A partial model was established to investigate the added mass and viscous damping of platform heaving; a full model and a simplified model were established to study the nonlinear damping of platform heaving. The influences of opening ratios of moon pool on the added mass and viscous damping of platform heaving were analyzed. It is found that the natural period and the heaving added mass of the platform increase if the semiclosed moon pool is considered. Different opening ratios of the moon pool will lead to different heaving damping of the platform and dynamic pressure acting on the guide plate of the moon pool. In practice, an optimal opening ratio of the moon pool can be designed. The numerical results were verified by comparing with the results of model experiments.

Identification of slow drift motions of a truss spar platform using parametric Volterra model

Identification of the first and second-order surge motion transfer functions of a truss spar platform from model test data is presented in this paper. The identification is carried out by estimating the time-varying kernels coefficients of a second-order Volterra model. The coefficients are estimated using proposed method, named particle swarm optimization based Kalman smoother (PSO-KS). System input–output data for identification process are wave height and surge motion from a scaled 1:100 model of a prototype truss spar. The applicability of proposed method is assessed numerically and experimentally under unidirectional long-crested random waves. The results show that the linear and quadratic frequency response functions (LFRF and QFRF) as well as the wave and low frequency responses of a truss spar platform can be well identified either in time or frequency domains. The LFRF and QFRF have high resolution so that evolution of the nonlinear wave interactions can be revealed.

The nonlinear bifurcation and chaos of coupled heave and pitch motions of a truss spar platform

This paper presents the results from a numerical study on the nonlinear dynamic behaviors including bifurcation and chaos of a truss spar platform. In view of the mutual influences between the heave and the pitch modes, the coupled heave and pitch motion equations of the spar platform hull were established in the regular waves. In order to analyze the nonlinear motions of the platform, three-dimensional maximum Lyapunov exponent graphs and the bifurcation graphs were constructed, the Poincare maps and the power spectrums of the platform response were calculated. It was found that the platform motions are sensitive to wave frequency. With changing wave frequency, the platform undergoes complicated nonlinear motions, including 1/2 sub-harmonic motion, quasi-periodic motion and chaotic motion. When the wave frequency approaches the natural frequency of the heave mode of the platform, the platform moves with quasi-periodic motion and chaotic motional ternately. For a certain range of wave frequencies, the platform moves with totally chaotic motion. The range of wave frequencies which leads to chaotic motion of the platform increases with increasing wave height. The three-dimensional maximum Lyapunov exponent graphs and the bifurcation graphs reveal the nonlinear motions of the spar platform under different wave conditions.

Investigation on the dynamic responses of a truss spar platform for different mooring line groups

The dynamic responses of any floating platform are dependent on the mass, stiffness and damping characteristics of the body as well as mooring system. Therefore, it is very essential to study the effect of individual contributions to the system that can finally help to economise their cost. This paper focuses on the effect of mooring stiffness on the responses of a truss spar platform, obtained by different grouping of lines. The study is part of our present researches on mooring systems which include the effect of line pretension, diameter and azimuth angles. The platform is modelled as a rigid body with three degrees-of-freedom and its motions are analyzed in time-domain using the implicit Newmark Beta technique. The mooring lines restoring force-excursion relationship is evaluated using a quasi-static approach. It is observed that the mooring system with lines arranged in less number of groups exhibits better performance in terms of the restoring forces as well as mean position of platform. However, the dynamic motions of platform remain unaffected for different line groups.

Coupling response of heave and moonpool water motion of a truss Spar platform in random waves

This paper presents the heave responses and the moonpool water motions of a truss Spar platform with semi-closed moonpool in random waves. A 2-DOF (degree of freedom) coupling dynamical equations of the platform heave and vertical motions of the moonpool water are derived. The linear wave theory is used to simulate the random waves. The response statistical values and the power spectrums are calculated to analyze the mutual influences between the platform heave and the moonpool water motions for different opening ratios of the moonpool. The effect of coupling parameters on the platform heave and the moonpool water motions are analyzed. The results show that motions of the moonpool water significantly affected the platform heave when the characteristic wave period is far away from the natural period of the platform heave, and different moonpool opening ratios lead to different heave amplitudes of the platform. In the actual design, an optimized moonpool opening ratio can be designed to reduce heave motions of the platform.

Effect of mooring line configurations on the dynamic responses of truss spar platforms

Considering the cost impact of mooring lines on the overall project, it is essential to find the best possible mooring configuration for a given platform and metocean data, which reduces the motion responses of the platform to an acceptable level. In this paper, the effect of symmetric and asymmetric mooring configurations in terms of line azimuth angles on the platform responses is studied. A truss spar platform is modelled as a rigid body with three degrees-of-freedom and the dynamic response analysis of the platform is performed in time-domain by computing the wave kinematics and water wave force using linear Airy wave theory and modified Morison equation, respectively. The responses of the platform are found to be not significantly affected by changing mooring line configurations in terms of azimuth angles. However, the initial offset of the platform is significantly affected with mooring line configurations.

Analysis of heave motions of a truss spar platform with semi-closed moon pool

This paper presents the heave motion of a truss Spar platform in regular waves considering the dynamic coupling between the motions of the platform and water in the moon pool. For a spar platform with semi-closed moon pool, water can flow freely through the guide plate at bottom of the moon pool. The mass of water inside the moon pool is comparable to the platform mass itself if the top tension riser system is used. Then the effect of the moon pool water on the motions of the platform should not be ignored. In the study presented in the paper, a 2-DOF (Degree of Freedom) dynamic coupling equations of the heave motions of the spar platform and the vertical motions of water in the moon pool were derived considering that the moon pool is totally closed, or 30% or 70% open. The results show that motions of water in the moon pool significantly affect the heave motions of the spar platform. Parametric study was also carried out to find the effect of the ratio of opening of the bottom plate on the coupled motions. Finally the model experiments were carried out to validate the numerical calculations.

Nonlinear Analysis of Stochastic Heave Motion for Truss Spar

Applying the Path Integral Solution Method, the stochastic heave motion of Truss Spar platform was studied in this paper. Taking a platform for example, probability density function of stochastic heave motion amplitude was calculated under the conditions of different wave excitations. The results show that, the changing process of probability density function in the time domain has an obvious periodicity. As the increasing of external excitation, the heave motion amplitude and speed both increases. Meanwhile, as the increasing of excitation amplitude, the probability density function of heave motion has a more pronounced periodicity.

Hydrodynamic response of alternative floating substructures for spar-type offshore wind turbines

Hydrodynamic analyses of classic and truss spar platforms for floating offshore wind turbines (FOWTs) were performed in the frequency domain, by considering coupling effects of the structure and its mooring system. Based on the Morison equation and Diffraction theory, different wave loads over various frequency ranges and underlying hydrodynamic equations were calculated. Then, Response Amplitude Operators (RAOs) of 6 DOF motions were obtained through the coupled hydrodynamic frequency domain analysis of classic and truss spar-type FOWTs. Truss spar platform had better heave motion performance and less weight than classic spar, while the hydrostatic stability did not show much difference between the two spar platforms.

CFD simulation of the vertical motion characteristics of the moonpool fluid for the truss spar

The research purpose of this paper is to estimate the impacts of the parameters of the guide plate on the vertical motion characteristics of the moonpool fluid. With the volume of fluid (VOF) method, three-dimensional models of the moonpool fluid motions of the truss spar platform are established. Simulation results are then presented for the moonpool forced oscillation by employing the dynamic mesh method and user-defined functions in FLUENT. The motions of the moonpool fluid and the loads on the guide plates are obtained for both cases of square-ring and crisscross. The results show that the shape and area of the guide plate at the bottom of the moonpool have a significant impact on the physical parameters of the moonpool, including the load on the moonpool guide plate, motion form of the moonpool fluid and the mass flow rate.

Nonlinear System Identification via Basis Functions Based Time Domain Volterra Model

This paper proposes basis functions based time domain Volterra model for nonlinear system identification. The Volterra kernels are expanded by using complex exponential basis functions and estimated via genetic algorithm (GA). The accuracy and practicability of the proposed method are then assessed experimentally from a scaled 1:100 model of a prototype truss spar platform. Identification results in time and frequency domain are presented and coherent functions are performed to check the quality of the identification results. It is shown that results between experimental data and proposed method are in good agreement.

Stochastic Analysis to Truss Spar Platform in Deep Sea

Truss spar model was tested using regular waves in a wave basin and the responses in surge, heave and pitch were measured. A program was developed to determine the responses using numerical method. This program was run using the model parameters. The results are agreed well with the corresponding results obtained from the test measurements. Then this program was applied to a prototype spar simulated results were compared with the corresponding numerical results and test measurements.

Responses of Concrete Truss Spar Platform in Deep Wave

Concrete truss spar model was tested using regular waves in a wave basin andthe responses in surge, heave and pitch were measured. A program was developed to determine the responses using numerical method. This program was run using the modelparameters. The results are agreed well with thecorresponding results obtained from the test measurements. Then thisprogram was applied to a prototype sparsimulated results were compared with the corresponding numericalresults and test measurements.