Articulated tower motions have been characterized by rigid body mode of vibrations falling in the... more Articulated tower motions have been characterized by rigid body mode of vibrations falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of a multi hinged articulated tower platform to random wind and wave forces are presented in this paper. The wave forces on the submerged elements of the tower are calculated by using Morison’s Equation. The fluctuating wind is modeled with Ochi and Shin wind gust spectrum. The effect of wave age (young, intermediate and fully developed waves) on the wind gust spectrum is incorporated by adopting the wave age dependent Volkov, and wave age independent Charnock sea surface roughness models. The response of the tower is determined by a time domain iterative method. An example of results demonstrates the clear effect of wave age on the nonlinear dynamic response on the system. The mean wind modifies the mean position of the surge response to the positive side, causing an offset. Moreover, for high mean w...
ABSTRACT Wind and wave loadings have a predominant role in the design of offshore structures in g... more ABSTRACT Wind and wave loadings have a predominant role in the design of offshore structures in general, and articulated tower in particular for a successful service and survival during normal and extreme environmental conditions. Such towers are very sensitive to the dynamic effects of wind and wind generated waves. The exposed superstructure is subjected to aerodynamic loads while the submerged substructure is subjected to hydrodynamic loads. Articulated towers are designed such that their fundamental frequency is well below the wave frequency to avoid dynamic amplification. Dynamic interaction of these towers with environmental loads (wind, waves and currents) acts to impart a lesser overall shear and overturning moment due to compliance to such forces. This compliancy introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of articulated towers. Prediction of the nonlinear behaviour of these towers in the harsh ocean environment is difficult. However, simplified realistic mathematical models are employed to gain an important insight into the problem and to explore the dynamic behaviour. In this paper, various modeling approaches and solution methods for articulated towers adopted by past researchers are reviewed. Besides, reliability of articulation system, the paper also discussed the design, installation and performance of articulated towers around the world oceans.
Journal of Wind Engineering and Industrial Aerodynamics, 2017
This paper deals with the dynamic behavior of a double-hinged articulated tower to wave alone, an... more This paper deals with the dynamic behavior of a double-hinged articulated tower to wave alone, and correlated wind and waves. The analysis includes the nonlinearities due to nonlinear drag force, fluctuating buoyancy, variable added mass and instantaneous tower orientation. The fluctuating wind load is modeled by Ochi and Shin spectrum, while the wave load is characterized by Pierson-Moskowitz (P-M) spectrum. The nonlinear dynamic equation of motion is derived by Hamilton's principle. The equations of motion are solved in time domain by using Wilson-θ method. Power spectral density function (PSDF) of surge, tilting motion, hinge shear and bending moment are presented under high, moderate and low sea states. Studies of correlated wind and waves are found to be imperative for double hinged articulated towers to serve and survive in the extreme ocean environment. The response PSDF highlights the wind induced dynamic responses of the tower.
ABSTRACT Articulated offshore tower with universal joints in the intermediate level leads to a mu... more ABSTRACT Articulated offshore tower with universal joints in the intermediate level leads to a multi-hinged configuration that can be used for a variety of deep water application. They are flexibly linked to the sea-bed by a universal joint and comply with the oscillatory environmental loads causing large fluctuating seismic demands at the articulating joints. This paper investigates the dynamic response and the reliability assessment of articulated joint (s) of such structures under seismic sea environment. The analysis includes the influence of sea bed shaking on the water-particle kinematics by using Californian earthquakes. The sea state is characterized by DNV version of Pierson Moskowitz spectrum. The dynamic equation of motion is derived using Lagrangian approach, taking into the account the nonlinearities associated with structure and loads. A limit-state function for seismic demand for a universal joint has been derived. Using the derived limit-state function and the responses obtained after time-domain seismic analysis, reliability assessment of the articulated joint has been carried out, using efficient MPP-based probabilistic methods. Design point, important for probabilistic design of articulated joint, located on the failure surface has been worked out. Stochastic sensitivity analysis has been performed to assess the relative importance of design parameter on the stochastic response of articulated joint.
Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant... more Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant. ALP motions have time periods falling in the wind excitation frequency range due to their compliant behaviour. This paper deals with the dynamic behavior of a double hinged ALP subjected to low-frequency wind forces with random waves. Nonlinear effects due to variable submergence, fluctuating buoyancy, variable added mass, and hydrodynamic forces are considered in the analysis. The random sea state is characterized by the Pierson-Moskowitz (P-M) spectrum. The wave forces on the submerged elements of the platform's shaft are calculated using Morison's Equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind load has been estimated using Ochi and Shin wind velocity spectrum for offshore structures. The nonlinear dynamic equation of motion is solved in the time domain by the Wilson-θ method. The wind-structure interactions, along with the effect of various other parameters on the platform response, are investigated. The effect of offset of aerodynamic center (A.C.) with the center of gravity (C.G.) of platform superstructure has also been investigated. The outcome of the analyses indicates that low-frequency wind forces affect the response of ALP to a large extent, which otherwise is not enhanced in the presence of only waves. The mean wind modifies the mean position of the platform surge response to the positive side, causing an offset. Various power spectral densities (PSDs) under high and moderate sea states show that apart from the significant peak occurring at the two natural frequencies, other prominent peaks also appear at very low frequencies showing the influence of wind on the response.
An Articulated loading platform (ALP) is one of the compliant offshore structures connected to th... more An Articulated loading platform (ALP) is one of the compliant offshore structures connected to the sea-bed through a universal joint. This joint is the most vulnerable part of the tower as it is subjected to fluctuating shear stresses. The crucial reliability assessment of articulated joint is highly significant as its failure leads to the failure of the entire structure. In the present study, reliability of articulated tower joints is investigated with respect to the potential fatigue failure due to fluctuating hinge shear. The dynamic analysis of double hinged articulated tower under wind and waves is carried out in time domain. The response histories so obtained are employed for the study of fatigue and fracture reliability analysis. The nonlinearities due to time-wise variation of submergence, buoyancy, added mass and hydrodynamic loading have been taken into account in the derivation of equation of motion. The limit state function is established in terms of random variables for cumulative fatigue damage using S-N curve approach and fracture mechanics approach. In order to evaluate the fatigue reliability, Advanced First Order Reliability Method (FORM) and Monte Carlo Simulation method are used. Survivability of the tower hinges during normal and extreme ocean environments are evaluated. To study the influence of various random variables on joint reliability, sensitivity analysis is carried out. It is found that fracture mechanics approach is found to yield a lower fatigue life as compared to the S-N curve approach.
Articulated tower motions have been characterized by rigid body mode of vibrations falling in the... more Articulated tower motions have been characterized by rigid body mode of vibrations falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of a multi hinged articulated tower platform to random wind and wave forces are presented in this paper. The wave forces on the submerged elements of the tower are calculated by using Morison’s Equation. The fluctuating wind is modeled with Ochi and Shin wind gust spectrum. The effect of wave age (young, intermediate and fully developed waves) on the wind gust spectrum is incorporated by adopting the wave age dependent Volkov, and wave age independent Charnock sea surface roughness models. The response of the tower is determined by a time domain iterative method. An example of results demonstrates the clear effect of wave age on the nonlinear dynamic response on the system. The mean wind modifies the mean position of the surge response to the positive side, causing an offset. Moreover, for high mean w...
ABSTRACT Wind and wave loadings have a predominant role in the design of offshore structures in g... more ABSTRACT Wind and wave loadings have a predominant role in the design of offshore structures in general, and articulated tower in particular for a successful service and survival during normal and extreme environmental conditions. Such towers are very sensitive to the dynamic effects of wind and wind generated waves. The exposed superstructure is subjected to aerodynamic loads while the submerged substructure is subjected to hydrodynamic loads. Articulated towers are designed such that their fundamental frequency is well below the wave frequency to avoid dynamic amplification. Dynamic interaction of these towers with environmental loads (wind, waves and currents) acts to impart a lesser overall shear and overturning moment due to compliance to such forces. This compliancy introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of articulated towers. Prediction of the nonlinear behaviour of these towers in the harsh ocean environment is difficult. However, simplified realistic mathematical models are employed to gain an important insight into the problem and to explore the dynamic behaviour. In this paper, various modeling approaches and solution methods for articulated towers adopted by past researchers are reviewed. Besides, reliability of articulation system, the paper also discussed the design, installation and performance of articulated towers around the world oceans.
Journal of Wind Engineering and Industrial Aerodynamics, 2017
This paper deals with the dynamic behavior of a double-hinged articulated tower to wave alone, an... more This paper deals with the dynamic behavior of a double-hinged articulated tower to wave alone, and correlated wind and waves. The analysis includes the nonlinearities due to nonlinear drag force, fluctuating buoyancy, variable added mass and instantaneous tower orientation. The fluctuating wind load is modeled by Ochi and Shin spectrum, while the wave load is characterized by Pierson-Moskowitz (P-M) spectrum. The nonlinear dynamic equation of motion is derived by Hamilton's principle. The equations of motion are solved in time domain by using Wilson-θ method. Power spectral density function (PSDF) of surge, tilting motion, hinge shear and bending moment are presented under high, moderate and low sea states. Studies of correlated wind and waves are found to be imperative for double hinged articulated towers to serve and survive in the extreme ocean environment. The response PSDF highlights the wind induced dynamic responses of the tower.
ABSTRACT Articulated offshore tower with universal joints in the intermediate level leads to a mu... more ABSTRACT Articulated offshore tower with universal joints in the intermediate level leads to a multi-hinged configuration that can be used for a variety of deep water application. They are flexibly linked to the sea-bed by a universal joint and comply with the oscillatory environmental loads causing large fluctuating seismic demands at the articulating joints. This paper investigates the dynamic response and the reliability assessment of articulated joint (s) of such structures under seismic sea environment. The analysis includes the influence of sea bed shaking on the water-particle kinematics by using Californian earthquakes. The sea state is characterized by DNV version of Pierson Moskowitz spectrum. The dynamic equation of motion is derived using Lagrangian approach, taking into the account the nonlinearities associated with structure and loads. A limit-state function for seismic demand for a universal joint has been derived. Using the derived limit-state function and the responses obtained after time-domain seismic analysis, reliability assessment of the articulated joint has been carried out, using efficient MPP-based probabilistic methods. Design point, important for probabilistic design of articulated joint, located on the failure surface has been worked out. Stochastic sensitivity analysis has been performed to assess the relative importance of design parameter on the stochastic response of articulated joint.
Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant... more Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant. ALP motions have time periods falling in the wind excitation frequency range due to their compliant behaviour. This paper deals with the dynamic behavior of a double hinged ALP subjected to low-frequency wind forces with random waves. Nonlinear effects due to variable submergence, fluctuating buoyancy, variable added mass, and hydrodynamic forces are considered in the analysis. The random sea state is characterized by the Pierson-Moskowitz (P-M) spectrum. The wave forces on the submerged elements of the platform's shaft are calculated using Morison's Equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind load has been estimated using Ochi and Shin wind velocity spectrum for offshore structures. The nonlinear dynamic equation of motion is solved in the time domain by the Wilson-θ method. The wind-structure interactions, along with the effect of various other parameters on the platform response, are investigated. The effect of offset of aerodynamic center (A.C.) with the center of gravity (C.G.) of platform superstructure has also been investigated. The outcome of the analyses indicates that low-frequency wind forces affect the response of ALP to a large extent, which otherwise is not enhanced in the presence of only waves. The mean wind modifies the mean position of the platform surge response to the positive side, causing an offset. Various power spectral densities (PSDs) under high and moderate sea states show that apart from the significant peak occurring at the two natural frequencies, other prominent peaks also appear at very low frequencies showing the influence of wind on the response.
An Articulated loading platform (ALP) is one of the compliant offshore structures connected to th... more An Articulated loading platform (ALP) is one of the compliant offshore structures connected to the sea-bed through a universal joint. This joint is the most vulnerable part of the tower as it is subjected to fluctuating shear stresses. The crucial reliability assessment of articulated joint is highly significant as its failure leads to the failure of the entire structure. In the present study, reliability of articulated tower joints is investigated with respect to the potential fatigue failure due to fluctuating hinge shear. The dynamic analysis of double hinged articulated tower under wind and waves is carried out in time domain. The response histories so obtained are employed for the study of fatigue and fracture reliability analysis. The nonlinearities due to time-wise variation of submergence, buoyancy, added mass and hydrodynamic loading have been taken into account in the derivation of equation of motion. The limit state function is established in terms of random variables for cumulative fatigue damage using S-N curve approach and fracture mechanics approach. In order to evaluate the fatigue reliability, Advanced First Order Reliability Method (FORM) and Monte Carlo Simulation method are used. Survivability of the tower hinges during normal and extreme ocean environments are evaluated. To study the influence of various random variables on joint reliability, sensitivity analysis is carried out. It is found that fracture mechanics approach is found to yield a lower fatigue life as compared to the S-N curve approach.
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