Effect of current on the dynamic response of a bi-articulated offshore tower

Wind and wave loadings have a predominant role in the design of articulated offshore towers for its successful service and survival. Such towers are very sensitive to the dynamic effect of environmental loads. The compliant nature of these towers with environmental loads introduces geometric nonlinearity due to large displacements, which becomes an important consideration in the analysis of these towers. This article deals with the dynamic response of a bi-articulated offshore tower to wind, wave, and current forces. The exposed portion of the tower is subjected to the action of wind, while the submerged portion is acted upon by random wave and current forces. Wind load is modeled by Ochi and Shin spectrum, while the wave load is characterized by Pierson–Moskowitz spectrum. The nonlinear dynamic equations of motion are derived by Hamilton’s principle. Response of the tower is determined by a time domain iterative (Wilson-θ) method. Power spectral density of important parameters such as surge, tilting motion, hinge shear, and bending moment are presented under high and low sea states. It is observed that response of tower due to current modifies the peak energy of power spectral density functions.

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Damage Identification in Plate-like Structures using Bending Moment Response Power Spectral Density

Structural Health Monitoring ◽

10.1177/1475921707072059 ◽

2007 ◽

Vol 6 (1) ◽

pp. 5-24 ◽

Cited By ~ 13

Author(s):

W. L. Bayissa ◽

N. Haritos

Keyword(s):

Spectral Density ◽

Power Spectral Density ◽

Damage Identification ◽

Bending Moment ◽

Power Spectral

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Dynamic Response of an End-Supported Pontoon Bridge due to Wave Excitation: Numerical Predictions versus Measurements

Shock and Vibration ◽

10.1155/2021/8842812 ◽

2021 ◽

Vol 2021 ◽

pp. 1-18

Author(s):

Knut Andreas Kvåle ◽

Ole Øiseth

Keyword(s):

Dynamic Response ◽

Spectral Density ◽

Power Spectral Density ◽

Limit State ◽

Ultimate Limit State ◽

Linear Potential ◽

Beam Model ◽

Numerical Predictions ◽

Power Spectral ◽

Standard Deviations

Herein, numerical predictions of the dynamic response of an existing floating pontoon bridge are compared with the measured dynamic response. Hydrodynamic coefficients that describe the fluid-structure interaction and the wave transfer functions are obtained by applying linear potential theory. The results obtained from the hydrodynamic analysis are combined with a beam model of the bridge in a finite element method (FEM) framework to enable stochastic response prediction through the power spectral density method. The standard deviations of the predicted accelerations are compared with the standard deviations of the measured accelerations, and the overall quality of the prediction model is discussed. Predictions with sea states related to the serviceability limit state (SLS) and ultimate limit state (ULS) conditions used in design of the bridge are emphasized. To investigate the behaviour more in depth, a measurement segment is chosen and predictions of the displacement response power spectral density due to excitation characterized by the recorded sea surface elevation are compared with those obtained from the corresponding response measurements. A decent agreement is obtained for both cases when using the model as it is and with waves as the only excitation source, but significant discrepancies are present, in particular, for the torsional components. By including preliminary contributions from wind action and relying on a model optimized against measured modal parameters, a satisfactory agreement is obtained. The effect on the response of an uncertain structural damping is also quantified and concluded to be significant within realistic damping levels.

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Analysis of Offshore Guyed Tower Platforms to Wind and Wave Forces

Journal of Energy Resources Technology ◽

10.1115/1.2795045 ◽

1998 ◽

Vol 120 (4) ◽

pp. 256-262 ◽

Cited By ~ 2

Author(s):

R. S. Bisht ◽

T. K. Datta ◽

A. K. Jain

Keyword(s):

Dynamic Response ◽

Spectral Density ◽

Power Spectral Density ◽

Wind Velocity ◽

Parametric Study ◽

Response Analysis ◽

Wave Forces ◽

Wind Force ◽

Displacement Response ◽

Power Spectral

The fundamental frequency of guyed tower platforms is designed to be low, and, therefore, it is susceptible to low-frequency excitation caused by fluctuating components of the wind velocity. The dynamic response of a simplified model of the Lena guyed tower is obtained for the fluctuating component of the wind force and the wave force. Both wind velocity and water particle kinematics due to wave motion are modeled as stationary random processes and are simulated from their respective power spectral density functions. The response analysis is performed by an iterative frequency domain procedure which duly considers the nonlinearities produced due to the drag effect and the nonlinear guyline resistance. The responses are also obtained in the presence of current. A parametric study is conducted to evaluate the relative contribution to the responses made by the wind force. The parametric study also illustrates how the current modifies the overall dynamic response. It is shown that the wind force significantly influences the first mode displacement response of the tower; the bending moment and shear force are not much affected by it. The current velocity tends to modify the peaks of the power spectral density function of the displacement response.

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